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Ord 39-2011 8/2/2011ORDINANCE NO. 39 -2011 Ordinance Amending Article VIII, "Stormwater Management," Of Chapter 106 Of The Code Of Ordinances Of The City Of Wichita Falls; Adopting A Stormwater Design Manual; Providing For Severability; Providing A Penalty Not To Exceed $2,000 Per Violation; And Providing For Codification. WHEREAS, the City Council finds that this ordinance will amend City policies governing the planning, design, construction, operation, and maintenance of stormwater and erosion control facilities and practices, the quantity and quality of stormwater discharge; and WHEREAS, the City Council finds this ordinance allows for development in conjunction with a safe, efficient and effective drainage system in keeping with appropriate engineering practices and regulatory requirements; and WHEREAS, the City Council finds that the regulations established by this ordinance are intended to prevent imminent destruction of property or injury to persons from flooding; and WHEREAS, the City Council further finds that construction standards established herein are for public works located on public lands and easements, which includes land and easements to be dedicated to the public upon the completion of the public works to be constructed thereon; and WHEREAS, the City Council finds that any municipal infrastructure improvements required herein are reasonable in relation to the development permitted hereby, and the developer's portion of costs provided herein do not exceed an amount that is roughly proportionate to the impacts of the permitted developments on the City's drainage system. NOW, THEREFORE, BE IT ORDAINED BY THE CITY COUNCIL OF THE CITY OF WICHITA FALLS, TEXAS, THAT: 1. Amendment of Sections 106 -951 through 106 -985 Article VIII, "Stormwater Management," of Chapter 106 of the Code of Ordinances of the City of Wichita Falls is amended to read as follows: CITY OF WICHITA FALLS CODE OF ORDINANCES CHAPTER 106 - UTILITIES ARTICLE VIII. STORMWATER MANAGEMENT Sec. 106 -951. — Definitions: Sec. 106 -952. — Purpose and Intent: Sec. 106 -953. —Applicability: Sec. 106 -954. — Disclaimer of Liability: Sec. 106 -955. —Administration: Sec. 106 -956. — Stormwater Design Manual: Sec. 106 -957. — Regulatory Permits: Sec. 106 -958. — Maintenance of Facilities: Sec. 106 -959. — Drainage Systems: Sec. 106 -960. — Limitation of Development Runoff: Sec. 106 -961. — Variance, De Minimis Waiver, and Appeal Sec. 106 -962. — Stormwater Pollution Prevention Requirements for Construction Activity and Post Construction Runoff Secs. 106 -963 - 106 -985. - Reserved Sec. 106 -951. Definitions. The following words, terms and phrases, when used in this chapter, shall have the meanings ascribed to them in this section, except where the context clearly indicates a different meaning: 2 year design storm means a rainfall event as documented by actual measurement or by recognized design methods that has a 50% probability of being equaled or exceeded in any given year. 10 year design storm means a rainfall event as documented by actual measurement or by recognized design methods that has a 10% probability of being equaled or exceeded in any given year. 100 year design storm means a rainfall event as documented by actual measurement or by recognized design methods that has a 1 % probability of being equaled or exceeded in any given year. Channel means any natural or artificial stream that conveys water. Channel Improvement refers to the improvement of the flow characteristics of a channel by clearing, excavating, realigning, lining or other means in order to increase its capacity and effectiveness. The term may also be used to refer to channel stabilization. Common Plan of Development means a construction activity that is completed or is planned to be completed in separate stages, separate phases, or in combination with other construction activities. Construction Activity. Activities subject to TPDES Construction Permits. These include construction projects resulting in land disturbance of 1 acre or more. Such activities include but are not limited to clearing and grubbing, grading, excavating, and demolition. Controlled release structure means a facility constructed to regulate the volume or peak discharge that can be conveyed through the structure given certain depths of flow, velocity of approach, and structure geometry. Detention Facilities Agreement means a legal document in recordable form and substance satisfactory to the City of Wichita Falls to be executed by a developer and the City, which describes the duties and responsibilities of a developer to comply with this Ordinance and describes and establishes an easement area and access to the facilities involved. Detention Pond means a basin or pond that is designed to be normally dry and which stores or detains stormwater runoff temporarily after a rainfall event and discharges all of that water over time at a reduced rate from that which would normally have occurred. Developer means a person or business entity that owns or intends to develop a tract or tracts of land and intends to improve the tract or tracts. Improvements may include, but are not necessarily limited to: clearing, grubbing, filling, mining, excavating, re- grading, constructing buildings, other structures, parking, drives, streets, and utilities and /or otherwise preparing the tract or tracts for division or development. Final Drainage Study refers to those final project plans, analyses, studies, and supporting documents as required to obtain City review and approval and to facilitate construction of improvements and facilities necessary to comply with the requirements of this Ordinance. Final Stabilization. A construction site status where any of the following conditions are met: (a) All soil disturbing activities at the site have been completed and a uniform (i.e., evenly distributed, without large bare areas) perennial vegetative cover with a density of at least 70% of the native background vegetative cover for the area has been established on all unpaved areas and areas not covered by permanent structures, or equivalent permanent stabilization measures (such as the use of riprap, gabions, or geotextiles) have been employed. (b) For individual lots in a residential construction site by either: (1) The homebuilder completing final stabilization as specified in condition (a) above; or (2) The homebuilder establishing temporary stabilization for an individual lot prior to the time of transfer of the ownership of the home to the buyer and after informing the homeowner of the need for, and benefits of, final stabilization. If temporary stabilization is not feasible, then the homebuilder may fulfill this requirement by retaining perimeter controls or other best management practices, and informing the homeowner of the need for removal of temporary controls and the establishment of final stabilization. (c) For construction activities on land used for agricultural purposes (e.g. pipelines across crop or range land), final stabilization may be accomplished by returning the disturbed land to its pre- construction agricultural use. Areas disturbed that were not previously used for agricultural activities, such as buffer strips immediately adjacent to surface water and areas that are not being returned to their preconstruction agricultural use must meet the final stabilization conditions of condition (a) above. (d) In arid, semi -arid, and drought- stricken areas only, all soil disturbing activities at the site have been completed and both of the following criteria have been met: (1) Temporary erosion control measures (e.g., degradable rolled erosion control product) are selected, designed, and installed along with an appropriate seed base to provide erosion control for at least three years without active maintenance by the operator, and (2) The temporary erosion control measures are selected, designed, and installed to achieve 70% vegetative coverage within three years. Finish Floor Elevation ( "FFE') means the elevation of the lowest enclosed floor area of a structure as further defined by the National Flood Insurance Program regulations promulgated by the Federal Emergency Management Agency ( "FEMA "). Fully Developed Watershed Conditions refers to that level of development anticipated when all of the land within a watershed is developed to the maximum extent allowable, typically determined by comparing existing development with projected uses of vacant land and nonconforming properties based upon the latest City's zoning or land use plan, whichever is more intense. Gross aggregate area means the area of the development under consideration plus the area of any contiguous land, owned or controlled by the developing entity. A hydrograph is a graphical representation of the flow rate (or discharge) of water over time, consequently, the area under the hydrograph curve describes a volume of water. Several methods are available to construct hydrographs. Maximum Extent Practicable or MEP is what is available and capable of being done after taking into consideration cost, existing technology and logistics in light of overall project purposes (40 CFR 230.10(a)(2)). Municipal Separate Storm Sewer System or MS4 means a conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man -made channels, or storm drains) that are: (a) Owned or operated by the federal government, state, municipality, township, county, district, or other public body (created by or pursuant to state or federal law) including special district under state law such as a sewer district, flood control district or drainage districts, or similar entity, or a designated and approved management agency under section 208 of the act that discharges into surface waters of the state; and (b) Designed or used for collecting or conveying solely storm water, (c) Not a combined sewer, and (d) Not a part of a publicly owned treatment works. National Pollutant Discharge Elimination System ( NPDES) Storm Water Discharge Permit means a permit issued by EPA (or by a State under authority delegated pursuant to 33 USC § 1342(b)) that authorizes the discharge of pollutants to waters of the United States, whether the permit is applicable on an individual, group, or general area -wide basis. Notice of Change or NOC. Written notification to the executive director from a discharger authorized under this permit, providing changes to information that was previously provided to the agency in a notice of intent form. Notice of Intent or NOI means the Notice of Intent that is required by the NPDES Storm Water Multi- Sector General Permit, the EPA Region 6 NPDES Storm Water Construction general permit, or any similar general permit to discharge storm water associated with industrial activity that is issued by the EPA or the TCEQ. Notice of Termination or NOT A written submission to the executive director from a discharger authorized under a general permit requesting termination of coverage. On -Site Storage Facility refers to a stormwater storage facility that is designed for the storage of runoff on and for a particular site or development. An On -Site Storage Facility may be a detention pond that is designed to be normally dry and which stores or detains stormwater runoff temporarily and discharges all of that water over time at a reduced rate from that which would normally have occurred, or a retention pond (a wet bottom pond) that is designed to retain a certain amount or level of water, a conservation pool, and a flood storage pool for storing stormwater runoff on a temporary basis for the purpose of reducing the peak discharge from the basin. Positive and adequate outfall describes the drainage of an area in a manner that will adequately convey and ensure complete removal of all surface water after a rainfall event by means of natural gravity. Preliminary Drainage Study refers to a preliminary plan with supporting documentation which is prepared to demonstrate that a proposed development can, from a conceptual standpoint, properly address the requirements of this Ordinance. The preliminary study is presented to the City's Department of Public Works Engineering Division for conditional approval prior to final design and preparation of construction plans. Retention Pond means a normally wet bottom basin or pond that is designed to retain a certain amount or level of water in a conservation pool above which capacity for an additional flood storage pool is available for storing stormwater runoff and discharges the additional flood storage pool over time at a reduced rate from that which would normally have occurred. Runoff refers to that portion of the precipitation that makes its way toward channels or lakes as surface or subsurface flow. When the term runoff is used alone, surface runoff is usually implied. Sheet flow refers to water, usually stormwater runoff, flowing in a thin layer over the ground surface. Synonymous with overland flow. Stormwater refers to water (or runoff) which results from precipitation that is not absorbed by soil or plant material over a defined area. Stormwater Pollution Prevention Plan or SWP3 means a document which describes the Best Management Practices (BMP) and activities to be implemented by a person or business to identify sources of pollution or contamination at a site and the actions to eliminate or reduce pollutant discharges to Storm water, Storm water Conveyance Systems, and /or Receiving Waters to the Maximum Extent Practicable. Texas Pollutant Discharge Elimination System or TPDES means the program delegated to the State of Texas by the EPA pursuant to 33 USC §1342(b). Topographically Identifiable Watercourse refers to a watercourse or drainage way that is easily identified by a thin, light blue divided line on a USGS topographic map or that is designated as a conveyance of water on the most recent City of Wichita Falls topographic map. TCEQ means the Texas Commission on Environmental Quality or its successor agency. Water Surface Elevation ( 'WSEL') is the elevation of the water surface (or the height, in relation to the National Geodetic Vertical Datum of 1988) resulting from certain rainfall events and frequencies in a drainage facility. Sec. 106 -952. Purpose and Intent. It is the purpose and intent of this ordinance to establish policies to govern the planning, design, construction, operation and maintenance of stormwater and erosion control facilities and stormwater practices; to regulate the quantity and quality of stormwater discharges; and to protect public health, welfare and safety by seeking to minimize loss and damage related to stormwater runoff and flooding within the jurisdiction of the City of Wichita Falls. This ordinance sets forth the minimum requirements necessary to provide and promote a safe, efficient and effective drainage system in keeping with appropriate engineering practices and regulatory requirements and to establish the various public and private responsibilities necessary to achieve that end. Sec. 106 -953. Applicability. This ordinance shall apply to all public drainage facilities; to any public or private commercial or multi - family residential development or redevelopment activity comprising a gross aggregate area of one acre or more; and to any single family residential development comprising a gross aggregate area of two acres or more. The gross aggregate area shall include parcels of common ownership that would be connected were it not for separations by intervening public rights -of -way, alleys or easements. However, with respect to the federally mandated requirements for stormwater discharges from construction activities and post construction runoff as described in Section 106 -962 hereof, this ordinance shall be applicable to any site of one acre or more in size regardless of use. Sec. 106 -954. Disclaimer of Liability. This Article seeks to establish degrees of stormwater conveyance, discharge, erosion control, stormwater quality and flooding protection that are considered reasonable for regulatory purposes and that are based on scientific and engineering considerations. Unanticipated circumstances and unforeseen combinations of rainfall events and conditions can and will occur. Consequently, this Article shall not be construed to imply that designs, construction, and controls approved and installed pursuant to this Article will be free from or will prevent flood damage or loss. Use of or reliance upon this Article and the Stormwater Design Manual shall not create liability on the part of the City of Wichita Falls or the City's officials, representatives or employees. Sec. 106 -955. Administration. The Department of Public Works, under the direction of the Director of Public Works, shall be responsible for the administration of this ordinance including, but not limited to, enacting any procedures, programs, standards, guidelines and criteria necessary to achieve the intent of this ordinance. Sec. 106 -956. Stormwater Design Manual. In designing developments, the Developer shall comply with the " Stormwater Design Manual" (known in this Article as the "Manual "), promulgated by the City's Department of Public Works, which shall contain written policies, standards, technical design criteria, procedures, methodology, details and regulations related to determination, design, review, and approval of stormwater facilities as necessary to implement and comply with the provisions of this ordinance. The Manual shall be the governing document for all design activities related to compliance with this Ordinance. Although the intent of this Manual is to establish uniform design practices, it neither replaces the need for sound engineering judgment nor precludes the use of information not presented. Other accepted engineering procedures may be used to conduct hydrologic and hydraulic studies with prior approval of the Director of Public Works. Sec. 106 -957. Regulatory Permits. It shall be the developer's responsibility to secure all regulatory permits associated with stormwater runoff and the development of drainage improvements. Such permits shall include, but are not limited to: U.S. Army Corps of Engineers 404 Permits, Texas Commission on Environmental Quality ( "TCEQ ") Section 401 permits; Federal Emergency Management Agency floodplain revision permits; U.S. Environmental Protection Agency National Pollutant Discharge Elimination System Permits; TCEQ Texas Pollutant Discharge Elimination System permits, and any required City permits. The developer shall also be responsible to provide any necessary project documentation to the City as required to facilitate the City's meeting its requirements under an existing maintenance agreement with the Corps of Engineers for all drainage work along or entering Holliday Creek. When the City is responsible for the design of a drainage project, the City shall be responsible for all required permits. Sec. 106 -958. Maintenance of Facilities. Improvements and facilities constructed as required by and in accordance with this ordinance shall be maintained as required to ensure that the improvements and facilities continue to properly function and perform as designed. All drainage facilities and improvements dedicated and constructed for public use and located within public ownership or easements shall, after acceptance, be maintained by the City. All improvements and facilities constructed on and /or constructed solely for the purpose of providing for compliance with this ordinance for any non - residential, multi - family or single lot residential development activity shall be maintained by the property owner and this requirement for maintenance shall pass with the chain of title of the property. Such private construction and maintenance responsibilities shall be: defined in a written Detention Facilities Agreement in a form satisfactory to the City; executed by the property owner /developer and the City; and recorded in the Wichita County real property records. Sec. 106 -959. Drainage Systems. (a) Drainage System Criteria. It is the purpose of this section to establish design criteria for various stormwater facilities for the purpose of promoting the health, safety and general welfare of the population by minimizing dangers of flooding to life and property. Storm drainage analysis and design for the City of Wichita Falls shall take into account fully developed watershed conditions unless another standard is specifically authorized elsewhere in this ordinance. Areas within a designated floodplain shall meet the more stringent FFE requirements of City floodplain development regulations. The developer shall comply with the following Drainage System Criteria unless otherwise authorized by the Director in writing: Table 1 — Drainage System Criteria Facility 10 -Year Event 100 -Year Event Streets and Alleys Maximum water depth Maximum water depth shall not within the street or alley exceed the lesser of 18" above the shall not exceed 6" above lowest point of the pavement section the lowest point of the or 8" above the crown of the pavement section. roadway. Flows must remain within City right -of -way OR the developer must provide site or lot grading plans demonstrating that a 6" minimum freeboard is provided between the maximum WSEL and the FFE of adjacent buildings. Enclosed Pipe Sized to convey calculated Sized to convey calculated flows Systems flows from contributing from contributing drainage area(s) drainage area(s) and /or to and /or convey all flows resulting in convey all flows resulting excess depths of flow within streets in excess depths of flow and alleys. A 6" minimum freeboard within streets and alleys. shall be provided between the maximum WSEL and FFE of adjacent buildings. Channels and Convey calculated flows Convey calculated flows from Creek from contributing drainage contributing drainage area(s) with Improvements area(s) and maintain maximum WSEL remaining within velocity requirements per easement and maintain velocity the Stormwater Design requirements per the Stormwater Manual. Design Manual. A 6" minimum freeboard shall be provided between the maximum WSEL and FFE of adjacent buildings. Culverts Upstream headwater not WSEL within easement; not to to exceed top of pavement exceed 12" above the lowest point of or curb at the low point of the pavement section at the low point the crossing of the crossing; and complying with FEMA regulations. A 6" minimum freeboard shall be provided between the maximum WSEL and FFE of adjacent buildings. Bridges Refer to 100 -year Bridges shall pass a 100 -year event requirement with a minimum 12" freeboard between the calculated 100 -year WSEL and the lowest horizontal structural member. Detention or Refer to Limitation of Refer to Limitation of Development Retention Ponds Development Runoff, Runoff, Section 106 -960 Section 106 -960. Dams and Regardless of size and Regardless of size and dam height, Impoundments dam height, all dams must all dams must meet TCEQ meet TCEQ requirements requirements per 30 TAC §299 per 30 TAC §299 (b) Lot -to -Lot Drainage: Existing drainage between developed lots shall be the responsibility of the adjoining property owners. New residential subdivision developments shall drain surface runoff from no more than three residential lots across an adjoining residential lot before conveying runoff into a public right -of -way, easement or drainage system. New commercial developments shall not drain surface runoff onto another adjoining lot unless a means of conveyance is provided within a dedicated drainage easement to reach a public right -of -way, easement, natural topographically identifiable watercourse, or manmade drainage system. (c) Positive and Adequate Outfall: For all developments, whether constructed in whole or in phases, drainage from all improvements shall be conveyed by means of a positive and adequate outfall to the nearest natural topographically identifiable watercourse or manmade drainage system. Drainage improvements required to achieve positive and adequate outfall shall be designed and included in the site development plans submitted for City Engineering approval. On -site drainage easements shall be provided as required for these improvements. The developer and design engineer must consider the impact of drainage flows and discharges onto adjoining landowners. A downstream evaluation may be required to determine if downstream drainage improvements and drainage easements are required. (d) Drainage Easements: The developer shall dedicate drainage easements for all stormwater conveyance systems and improvements constructed pursuant to this ordinance including but not necessarily limited to all inlets, piping, ditches, swales, channels, and detention or retention ponds that are to be dedicated to and accepted by the City and that are not already located within existing City easements or rights -of -way. These easements may be dedicated on a plat of record or provided by separately recorded instrument. (1) Drainage easements shall be a minimum of 15 feet in width and sufficient to include the entire stormwater facility width as designed pursuant to the Stormwater Design Manual plus an additional 10 feet on one side for access and maintenance activities. The additional 10 feet may be omitted for lined or unlined channels that are easily and directly accessible by City crews with maintenance equipment or with side slopes of less than or equal to 4:1. The additional easement width may also be avoided by providing reinforced concrete access ramps to the channel bottom at 4:1 slopes with a 6 foot minimum bottom width and sloped (flared) sidewalls from City rights -of -way at 500' maximum spacings along the length of the drainage facility. (2) All drainage easements shall remain open and unobstructed. Prohibited obstructions include but are not limited to fences, buildings, shrubbery, or any other improvement that may be considered to impede the flow of stormwater or that would prevent City access for maintenance. However, drainage easements extending along the long axis between residential lots for small ( <24 ") drainage pipe systems may be fenced. (3) Without prior written approval of the Director of Public Works , a utility franchisee shall not install utilities in a drainage easement except for essentially perpendicular crossings of underground or overhead utilities. (e) Drainage Studies: Those studies as described hereafter are considered to be in addition to any specialized studies or applications that are required by FEMA or other governmental agency for a particular project. (1) General: The developer shall submit a drainage study for industrial, commercial, or multi - family residential developments comprising a gross aggregate area of one acre or more. Drainage studies shall be submitted for single family residential developments comprising a gross aggregate area of two acres or more. If a drainage study was not completed on an area that was previously platted, a Final Drainage Study will be provided before project plans are approved, unless that requirement is waived by the Director of Public Works. (2) Preliminary Drainage Study: a. Timing: The developer shall demonstrate to the satisfaction of the City that proper drainage pursuant to this Ordinance can be conceptually addressed for a development at the Preliminary Plat stage. A developer shall schedule a conference with the City Engineering Division prior to the submission of a Preliminary Plat and shall then submit a Preliminary Drainage Study which includes a conceptual layout of the proposed storm drainage system, or master drainage plan for the proposed development concurrently with the submission of the preliminary plat. Department of Public Works approval of the Preliminary Plat shall be contingent upon its review and written acceptance of the Preliminary Drainage Study. b. Qualifications To Perform The Study: The Preliminary Drainage Study shall be prepared by a Professional Engineer licensed in the State of Texas with demonstrated knowledge of the study of drainage issues and proficiency with drainage analysis and modeling tools. The Preliminary Drainage Study shall be signed, sealed and dated by the engineer responsible for preparation of the study. c. Requirements: The requirements for a Preliminary Drainage Study shall be as established in the Stormwater Design Manual. (3) Final Drainage Study: a. Timing: The developer shall demonstrate to the satisfaction of the City that drainage analysis and design pursuant to this Ordinance are properly addressed for a development at the Final Plat stage. The developer shall submit a Final Drainage Study for the proposed development concurrently with the submission of the final plat. The Final Drainage Study shall include all appropriate computation and analysis documentation as required by the Stormwater Design Manual. Final Drainage Studies of all proposed improvements or land developments shall clearly identify the on -site and off -site drainage system improvements required to satisfy this Ordinance and demonstrate the upstream and downstream limits to which the improvements or development comply with the design criteria of this Ordinance and the Stormwater Design Manual. The Final Drainage Study shall identify intended phases of development and shall provide that drainage improvements for each phase are sufficient to meet the requirements of this Ordinance should any successive phase not be undertaken. Additionally, the Final Drainage Study shall identify and define the stages within a phased development that mandate certain drainage improvements to be completed in order to continue the development sequence. If the developer did not submit a Final Drainage Study for a proposed development or if development conditions have been modified, a Final Drainage Study shall be provided prior to issuance of a construction permit for the development. The City's Department of Public Works may condition its approval of the Final Plat or issuance of the construction permit on its review and acceptance of the Final Drainage Study. b. Qualifications To Perform The Study: The Final Drainage Study shall be prepared by a Professional Engineer licensed in the State of Texas with demonstrated knowledge of the study of drainage issues and proficiency with drainage analysis and modeling tools. The Preliminary Study shall be signed, sealed and dated by the engineer responsible for preparation of the study. c. Requirements: The requirements for a Final Drainage Study shall be as established in the Stormwater Design Manual. (f) Drainage Improvements Required for Development: (1) Drainage Improvements: All developments shall provide for any new drainage facilities, the improvement of any existing drainage facilities, channel improvements or grading, driveway adjustments, culvert improvements or any other drainage improvement, drainage facility, or work that is necessary to provide for the stormwater drainage requirements specified in the Stormwater Design Manual. (2) Easements: The developer shall dedicate drainage easements across the site that will accommodate fully developed watershed conditions. (3) Construction Required: The developer shall construct drainage improvements that convey stormwater runoff through the proposed development to positive and adequate outfall locations in accordance with the design criteria in the Ordinance and the Stormwater Design Manual. The developer is only required to construct that portion of the drainage system across the site that will convey existing off -site flows and the flows from the proposed site development. Upsizing or phasing of the construction of the drainage systems to accommodate fully developed watershed conditions shall be coordinated with the City. Funding for increased sizing of these facilities may be funded by the City at its discretion subject to funding availability. Sec 106 -960 - Limitation Of Development Runoff. (a) General: This division shall establish policies necessary to limit and restrict the increase in runoff from development activities and /or to provide adequate downstream drainage facilities and conveyances to minimize the effects of runoff and downstream flooding from development. The intent of this section is to focus design attention on the stormwater impact of development on the entire watershed rather than on a single, discrete site. Further standards, technical design criteria, procedures, methodology, details and regulations to limit runoff from development are included in the Stormwater Design Manual. (b) Runoff Release Rate: The controlled release rate of stormwater runoff from development or redevelopment activities shall not exceed predevelopment conditions unless downstream facilities can be demonstrated to be sufficient to convey fully developed 100 year design storm flows from the watershed as provided hereinafter. For the purposes of this division, predevelopment conditions are considered to be those site conditions and characteristics which existed prior to the 1988 adoption of the City's initial detention ordinance and which can be clearly identified on the 1986 aerial photography archived by the City Engineering Division. In certain circumstances where downstream drainage facilities are insufficient to carry the concentrated point flows from a development's on- site storage facilities, downstream channel improvements shall be required or special outflow structures to provide sheet flow release shall also be constructed to mitigate downstream impact. (c) Options: The following options are available to analyze, account for, and reduce the impact of runoff from development on the drainage systems of the City. Engineering analyses for the option chosen for a development shall be performed as required by this Ordinance and in accordance with the Stormwater Design Manual. (1) On -Site Storage With Controlled Release: A developer may limit or reduce stormwater discharge rates from the proposed development to predevelopment conditions as defined herein by providing on -site storage facilities with an appropriately designed, controlled release structure. Design of such on -site facilities shall be designed and submitted in accordance with the Stormwater Design Manual and shall provide for 100 year design storm storage plus a minimum freeboard of six inches above the maximum calculated water surface elevation and a controlled release structure that provides for proper storage and release rates at the 2 year, 10 year and 100 year design storm predevelopment rates. An on- site storage and discharge reduction plan shall be prepared by a competent professional engineer licensed by the State of Texas and shall be submitted to the City Engineering Division for its review and approval. Construction and maintenance responsibilities for these facilities on single lot residential, multi - family residential or non- residential developments shall be defined in a written Detention Facilities Agreement in form satisfactory to the City, executed by the property owner /developer and the City, and recorded in the county real property records. This agreement shall grant the City a non - exclusive easement on and access to the area containing the discharge reduction improvements. For multiple lot single family residential developments, a deed to the City in a form satisfactory to City shall be granted conveying the property on which the on -site storage facilities are located, and an acceptable easement shall be granted to the City as required to provide access to the area on which these facilities are constructed. (2) Downstream Impact Assessment: A downstream impact assessment as described in the Stormwater Design Manual may be performed to document that existing downstream drainage facilities to which the proposed development is to be connected are already sufficient to transport the fully developed 100 year design storm flows, to protect downstream properties from increased flooding, to protect downstream channels from increased erosion potential due to upstream development, and to protect public health and safety. The downstream assessment shall extend from each outfall of the proposed development to a stormwater facility identified by existing plans, FEMA modeling, or other computer models previously prepared and acceptable to the City as having fully - developed flow capacity and shall follow the guidelines of the Stormwater Design Manual as approved on a site -by -site basis by the Director of Public Works. Runoff computations shall be based upon fully developed watershed conditions in accordance with the City's latest land use projections and may include areas outside of the City limits and ETJ. The assessment shall be performed and submitted in accordance with the Stormwater Design Manual. The results of the downstream assessment may indicate that the developer has several options as follows to address compliance with this ordinance. If none of these options are available or acceptable to the developer, then on -site storage with controlled release shall be required. a. No Impact: In those instances where the downstream impact assessment demonstrates to the satisfaction of the Director of Public Works that the proposed development causes no downstream increases in discharge or base flood elevations due to the timing of the hydrographs or that the existing downstream drainage system can convey the fully developed flows from the project without exceeding the requirements of this Ordinance or the Stormwater Design Manual, then no drainage improvements or on -site storage facilities will be required. b. Downstream Improvements: When a downstream impact assessment indicates that downstream facilities do not meet the requirements of this Ordinance and the Stormwater Design Manual, the developer may provide for the proper design and construction of downstream drainage improvements rather than construct on -site storage facilities to properly mitigate the effects of additional runoff. Alternatively, where a downstream drainage improvement project has already been identified and analyzed by the City, with the permission of the Director of Public Works, a developer may participate in the project cost of such downstream improvements to satisfy the requirements of this Section. Financial participation may include a portion or all of the City drainage project cost and /or the cost of drainage easement acquisition for the City's benefit. The amount of financial participation by the developer shall be determined using an acceptable engineer's opinion of the probable construction cost necessary to meet the Ordinance requirements for on -site storage and release, plus engineering design fees, and cost of payment, performance and maintenance bonds. c. Participation in Regional Detention: The developer may identify or participate in a regional storage facility designed for the fully developed watershed conditions for an area of common development. Such a facility shall be designed pursuant to the Stormwater Design Manual and shall be subject to a predetermined facilities agreement in form and substance satisfactory to the City. Sec 106 -961 — Variance, De Minimis Waiver, and Appeal. 1. Variance: A person or entity (an "Applicant ") may request a deviation or variance from the provisions of this ordinance; however, there is no guarantee that such requests will be granted. Requests shall be made in writing to the Director of Public Works, shall include all accompanying data, calculations and documentation necessary for City Engineering staff to properly analyze the request, and will not be permitted unless the following criteria are addressed to the satisfaction of the Director of Public Works: a. The requested variance will not adversely affect conditions either upstream or downstream of the subject property; b. The requested variance will not conflict with any other projects or plans adopted by the City, state, or federal agencies or cause increased flood heights, threats to public safety, additional expenditures of public funds, impairment of the public interest, or conflict with existing local, state or federal laws or regulations; c. The requested variance would not confer upon the Applicant any special privilege not available to others subject to the Ordinance; and d. Not granting the variance would result in exceptional hardship because of circumstances unique to the particular property, the cost of Ordinance compliance not being considered a hardship. 2. Waiver for De Minimis Stormwater Discharge Impact: Certain development projects may be subject to this ordinance as a result of their gross aggregate area; however, the proposed site improvements will result in negligible or very minimal stormwater discharge. In these circumstances, the developer shall have two options to request a waiver of the requirements of this ordinance: a. The developer may request that a stormwater discharge reduction waiver request be prepared and submitted in writing to the City Engineering Division by a professional engineer licensed in the State of Texas. Such a waiver request shall include specific project and site information along with actual engineering calculations to document the request, or b. The developer may schedule, in advance, an appointment with an individual designated by the Director of Public Works within the City Engineering Division to apply for a waiver of this ordinance in person. The applicant shall provide a site plan to scale which indicates the location and size of existing and proposed improvements and major property features along with a copy of the final plat or survey delineating the property boundaries for the proposed development. For either of the above options, the City Engineering Division will take the request presented under advisement and will notify the developer or his consultant in writing as to the City's decision concerning the waiver request. If the waiver request is denied, the developer must address the requirements of this ordinance by use of another option as provided herein. Sec. 106 -962. Stormwater Pollution Prevention Requirements For Construction Activity And Post Construction Runoff. This section applies to all construction sites located within the city limits or otherwise subject to the regulatory authority of the City of Wichita Falls that have discharges from construction site activities disturbing one acre or more, or which are part of a plan of common development that will disturb one acre or more. (a) Submittal of NOI, Small Construction Site Notice, and SWP3: A copy of the Notice of Intent (NOI) and /or the Small Construction Site Notice, and Storm Water Pollution Prevention Plan (SWP3) shall be submitted to the Department of Public Works for review in accordance with NPDES or TPDES permit requirements and this Ordinance at least ten (10) business days prior to the anticipated start of construction activities. Should the SWP3 review process exceed 10 business days, construction activities may commence under the submitted SWP3. Once the plan review is complete, any necessary changes will be made to the SWP3 at the project site. If relevant information provided in the NOI or SWP3 changes, the operator must submit a Notice of Change (NOC) in accordance with the requirements of the TPDES Construction General Permit and update the SWP3. A copy of the NOC shall be provided to the Department of Public Works within 14 days of change. (b) Review and modification of the SWP3: (1) The Department of Public Works retains the authority to request for review any document or plans, including but not limited to a SWP3, spill prevention control plans, hazardous material plans, and waste management documentation from a regulated construction site that the Director of Public Works or designee reasonably believes may affect stormwater discharges to the MS4. (2) The Director of Public Works or designee may require an operator of a regulated facility or construction site to modify its SWP3 if the SWP3 does not comply with the requirements of NPDES or TPDES permit to discharge stormwater associated with the construction activity. (3) The deficiencies in a SWP3 will be communicated in writing and will provide to the operator a reasonable amount of time, not to exceed 30 days, to make necessary changes in the SWP3. (c) Review and modifications of Best Management Practices (BMP): (1) Any person engaged in activities or operation, or owning facilities or property, which will or may result in pollutants entering the MS4 shall implement BMP's to the Maximum Extent Practicable to prevent and reduce such pollutants. The owner or operator of a regulated facility or construction site shall provide reasonable protections from accidental discharge of prohibited materials or other wastes into the MS4. Practices implemented to prevent accidental discharge of prohibited materials or other wastes shall be provided and maintained at the owner or operator's expense. (2) The City of Wichita Falls will maintain a list of approved BMP's as part of the City of Wichita Falls Engineering Standards which are accessible on the City's website. The Department of Public Works may request a demonstration of the effectiveness of implemented BMP's. (3) The Department of Public Works may require an operator of a regulated facility or construction site to modify its BMP's if the BMP's do not provide effective protection from accidental discharge of prohibited materials or other wastes from entering into the MS4. (4) The deficiencies in facilities or construction site's BMP's will be communicated in writing and the operator will be provided a reasonable amount of time, not to exceed 60 days, to make the necessary changes in the BMP's. (d) Access to construction sites: The intent of the facility or construction site inspections shall be to determine compliance with the conditions of the City's MS4 TPDES permit, any applicable NPDES or TPDES general permits and this Ordinance. The facility and construction site owners and operators shall allow the Department of Public Works ready access to applicable sections of public and private premises for the sole purpose of inspections, surveillance, and monitoring for the presence of illegal discharges to the municipal stormwater drainage system, illicit connections to the municipal stormwater drainage system, and assessment of any portions of a regulated construction site influenced by stormwater runoff that may adversely affect the municipal stormwater drainage system. An owner or operator of a facility or construction site shall authorize the Director of Public Works and /or his designee to enter such site at reasonable times during normal business hours to ensure compliance with the provisions of this article. If the Director of Public Works determines that an imminent and substantial danger exists to public health or infrastructure, then the Director or his designees may enter the site without notice to the operator. If owner or operator refuses entry after a request to enter and inspect has been made, the City is hereby empowered to seek assistance from any court of competent jurisdiction in obtaining such entry. (e) Compliance with permit: (1) A construction site shall comply with the requirements of the NPDES and TPDES permits to discharge stormwater associated with the construction site activity. (2) A person commits an offense if the person operates a construction site in violation of the requirements of the NPDES and /or TPDES permit discharge stormwater associated with the construction site activity. (3) Prior to the submittal of a Notice of Termination (NOT), the operator of the construction site shall first notify the Department of Public Works to request an inspection of the construction site for approval of final stabilization. After final inspection approval, the operator of the construction site shall submit to TCEQ and the Department of Public Works a NOT that includes the information required by the TPDES Construction General Permit. (4) The operator shall retain copies of any SWP3 and all reports required by this ordinance or by the TPDES permit for the site and records of all data used to complete the NOI for a period of at least three years from the date that the site meets the definition of final stabilization. (f) Post - Construction Storm Water Control. (1) The purpose of post construction controls is to protect, maintain and enhance the public health, safety, environment and general welfare by establishing minimum requirements and procedures to control the adverse effects of increased post- construction storm water runoff and non -point source pollution associated with new development and redevelopment. i. Establishing design and review criteria for the construction, function, and use of structural storm water Best Management Practices (BMP's) that may be used to meet the minimum post - development storm water management standards; ii. Encouraging the use of better management and low impact site development practices, such as the use of vegetated conveyances for storm water and preservation of green space, buffers and other conservation areas to the maximum extent practicable; and iii. Establishing provisions for the long -term responsibility for and maintenance of structural and nonstructural storm water BMP's to ensure that they continue to function as designed, are maintained appropriately, and pose minimum risk to public safety. (2) Regulated post- construction storm water control. The requirements of this section shall apply to all developments of the City of Wichita Falls and redevelopments within the corporate limits or in the extraterritorial jurisdiction that are equal to or greater than one acre, unless one of the exceptions applies to the development or redevelopment as of the effective date of this ordinance. (3) Exceptions. i. Redevelopment or expansion that results in no net increase in impervious area. ii. Development and redevelopment that decreases predevelopment runoff volumes as defined in Sec. 106 -960. iii. Any new development or redevelopment project that has or will have permit coverage under the Texas Pollutant Discharge Elimination System Industrial Storm Water Permit issued by the Texas Commission on Environmental Quality (TCEQ). (4) Standards for post- construction storm water control. All storm water control measures and storm water treatment practices (BMP's) required under this ordinance shall be evaluated by the Director according to the policies, criteria, and information, including technical specifications, standards and the specific design criteria for required development or redevelopment. All plans shall be submitted in conjunction with the drainage criteria review process. The Director shall determine whether these measures will be adequate to meet the requirements of this ordinance. (g) Administrative Enforcement: (1) Violations. It shall be an offense for an owner or operator of a construction site or a third party performing work on a construction site to violate any of the requirements of this Article, including, but not limited to, the following: i. Conducting any land disturbing or construction activity equal to or greater than one acre without an approved NOI or Small Construction Site Notice and an SWP3 for the location where the violation occurred. ii. Failing to implement the SWP3 to include, but not limited to, installing erosion control devices or maintaining erosion control devices throughout the duration of land disturbing activities, in compliance with the approved SWP3 and BMP's for the location where the violation occurred. iii. Failing to remove off -site sedimentation that is a direct result of land disturbing activities where such off -site sedimentation results from the failure to implement or maintain erosion control devices as specified in an approved SWP3 for the location where the violation occurred. iv. Allowing sediment laden water to flow from a site without being treated through an erosion control device. v. Failing to repair damage to existing erosion control devices, including replacement of existing grass or sod or established final stabilization. (2) Notice of Violation. Written Notice of Violation (NOV) shall be given to the Responsible Party or site operator /owner as identified in the SWP3 for a site. The NOV shall identify the nature of the alleged violation, the action required to obtain compliance with the approved SWP3, and the time to comply. (3) Stop Work Order. If the Director of Public Works, or designee, determines that compliance subsequent to the NOV is not being attained, that a construction site is operating in a dangerous or unsafe manner, or that conditions exist at a construction site that may lead to an illicit discharge, upon written notice of an issuance of a stop work order, such work or conditions shall be immediately terminated or remedied. Written notice shall be provided to the owner, operator, and responsible party of the construction site and shall state the conditions under which work may be resumed. However, where an emergency exists which may result in discharge of hazardous materials or other discharges which pose an immediate injury and harm to property, natural resources, wildlife, or habitat written notice shall be delivered as soon as practical. (4) If the violation has not been corrected within 7 days after the Notice of Violation or, in the event of a valid appeal, within 7 days of the decision of the Director upholding the decision of the Department of Public Works, then representatives of the Department of Public Works may enter upon the subject private property and are authorized to take any and all measures necessary to abate the violation and /or restore the property. It shall be unlawful for any person, owner, agent or person in possession of any premises to refuse to allow the government agency or designated contractor to enter upon the premises for the purposes set forth above. (h) Nuisance, Abatement, and Lien: (1) Failure of a property owner or facility operator to comply with a Notice of Violation issued pursuant to this article is deemed to be furtherance of: An unwholesome situation, ii. A condition that may produce disease, iii. The presence of impure or unwholesome matter, iv. Objectionable, unsightly, and unsanitary matter, and V. A public nuisance. (2) If the owner of property in the municipality does not comply with a municipal ordinance or requirement under this chapter within seven days of notice of a violation, the Department of Public Works may: i. Do the work or make the improvements required; and ii. Pay for the work done or improvements made and charge the expenses to the owner of the property. (3) In performing or contracting for the aforementioned work or improvements, the Department must comply with the procedures established in Texas Health & Safety Code § 342.006, as amended. After following such procedures, the Department may assess the expenses of such work against the real estate on which the work is done or improvements are made in accordance with Texas Health & Safety Code § 342.007, as amended. (i) Penalty and Injunction: (1) Violation of a provision of this Article is declared to be a violation of an ordinance involving health and safety. Accordingly, it shall be punishable by a fine in an amount not to exceed $2,000 per violation and as provided by Section 1 -14 of this Code. (2) If a person has violated or continues to violate the provisions of this ordinance, the City Attorney may petition for a preliminary or permanent injunction restraining the person from activities which would create further violations of this article or compelling the person to perform abatement or remediation of the violation. 2. Stormwater Design Manual The City adopts the Stormwater Design Manual attached hereto. Future editions of the Stormwater Design Manual may be promulgated by the City's Department of Public Works. In a Final Drainage Study submitted prior to September 1, 2014, the developer may calculate the required stormwater detention storage volume by adding 33% to the stormwater detention storage determination calculated from the modified rational hydrograph method of analysis as discussed in "Urban Stormwater Management Special Report No. 49" published by the American Public Works Association. In a Final Drainage Study submitted after September 1, 2014, but before September 1, 2017, a developer may calculate required stormwater detention storage by adding 67% to the stormwater detention storage determination calculated from the modified rational hydrograph method of analysis as discussed in "Urban Stormwater Management Special Report No. 49" published by the American Public Works Association. If a Final Drainage Study is submitted for a development after September 1, 2017, the developer will not be permitted to utilize "Urban Stormwater Management Special Report No. 49" in calculating required stormwater detention storage, and must use the detention calculation storage calculation methodology of the Stormwater Design Manual. 3. If any section, paragraph, clause, or provision of this ordinance shall for any reason be held to be invalid or unenforceable, the invalidity or unenforceability of such section, paragraph, clause, or provision shall not affect any of the remaining provisions of this ordinance. 4. The City Council intends Section 1 of this ordinance to become a part of the Code of Ordinances of the City of Wichita Falls, and the provisions of Section 1 of this ordinance may be renumbered or relettered to accomplish such intention. 5. This ordinance regulates fire safety and public health and sanitation, and shall be punishable by a penalty of up to $2,000 per violation and as provided by Section 1- 14 of the Wichita Falls Code of Ordinances. PASSED AND APPROVED this the 2nd day of August 2011. MAYOR ATTEST: City Clerk STORMWATER DESIGN MANUAL CITY OF WICHITA FALLS, TEXAS PUBLIC WORKS DEPARTMENT 1300 SEVENTH STREET WICHITA FALLS, TEXAS 76301 Approved by: Russell Schreiber, P. E., Public Works Director Effective Date: , 2011 TABLE OF CONTENTS 1.0 INTRODUCTION 1.1 Purpose and Scope 2.0 DEVELOPMENT PROCESS 2.1 Predevelopment Conference 2.2 Concept /Preliminary Study 2.3 Final Study /Construction Plans 2.4 Construction Review and Engineer Certification 2.4.1 Public Drainage Improvements 2.4.2 Privately Maintained Drainage Improvements 3.0 DESIGN CRITERIA 3.1 Hydrology 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.1.6 Hydrologic Methods Rainfall Estimation Rational Method Modified Rational Method SCS Method Snyder's Unit Hydrograph Method 3.2 Downstream Assessment 3.3 Hydraulics 3.3.1 Streets 3.3.2 Closed Conduits 3.3.3 Storage Design 3.3.4 Open Channels 3.3.5 Culverts 3.3.6 Bridges 3.3.7 Energy Dissipation 4.0 APPENDIX Detention Facilities Agreement 1.0 INTRODUCTION: 1.1 Purpose and Scope The development of this design manual is authorized by the City of Wichita Falls' Stormwater Management Ordinance (the "Stormwater Ordinance ") to protect and provide for the safety and welfare of the general public and to mitigate flood damage to private and public property within the City and its extraterritorial jurisdiction. This manual establishes standard principles and practices, design guidance, and a framework for incorporating effective and environmentally sustainable stormwater management into the development and construction processes within the jurisdiction of the City and is intended to encourage a greater uniformity in developing plans for stormwater management systems. The design methodology, procedures, factors, formulae, and graphs described in the following pages are intended to serve as guidelines for the analysis of drainage matters for local government review and approval purposes; however, the responsibility for the adequacy and effectiveness of the actual design remains with the design engineer and sound engineering judgment must always be applied. Users of this manual should be knowledgeable and experienced in the theory and application of drainage engineering principles. Any deviation from the requirements of this manual must be approved by the Director of Public Works. 2.0 DEVELOPMENT PROCESS: 2.1 Predevelopment Conference Prior to beginning any design and concept layout for a development project within the City, the developer and his /her design engineer(s) are strongly advised to schedule, in advance, a Predevelopment Conference with the City Engineering Division. Appointments can be made by calling the City Engineer at (940) 761 -7477. General information as to the project location, site size, intended use, and other relevant factors should be given when scheduling. The developer, developer's engineer and City Engineering staff can then meet and discuss specific drainage and infrastructure issues for the project site prior to beginning the design process. Available City electronic information such as topographic maps and existing hydrologic and hydraulic studies can be obtained. Additionally the meeting will serve to review the City's expectations for studies and plans to be prepared and submitted for City review and approval in keeping with this manual during the design and approval process. 2.2 Concept /Preliminary Study Once concept plans for a project are prepared, the design engineer shall provide to the City Engineering Division a written report that describes and documents the background, intent and methodology intended to be used along with preliminary plans and sufficient analyses to indicate that the requirements of the Stormwater Ordinance can be addressed. For a subdivision, this submission would be commonly referred to as a master drainage plan. This plan and study shall be prepared by a Professional Engineer licensed in the State of Texas with demonstrated knowledge of the study of drainage issues and proficiency with drainage analysis and modeling tools and should include, but is not limited to: • a preliminary plat and /or overall concept development plan for phased projects; • background topographic data for the site including off -site drainage area(s) (existing City topographic data may be used for this submission); • depiction of existing FEMA floodplain and floodway lines on the site and drainage area; • any proposed revisions to the FEMA floodplain; • initial hydrologic analyses to assess the stormwater impact of the proposed development; • approximate structure, pond, and conveyance sizes and proposed flow line grades; • indications that appropriate easements can be provided, dedicated or obtained; and • other information that will assist City staff in determining if the project can reasonably comply with the Stormwater Ordinance. If the design engineer intends to claim that there will be no downstream impact from the development, then a complete downstream assessment as later described including all necessary documentation, analysis and background materials shall be provided for review and approval by the City at this stage. After appropriate review, the City Engineering Division will notify the design engineer of any comments, suggested revisions and its conditional consent to proceed with final plans. This does not constitute tacit approval of the project plans but simply indicates that the initial proposal appears to be conceptually viable. Department of Public Works approval of a preliminary plat is contingent upon this conditional consent. 2.3 Final Study /Construction Plans After conditional consent for a project is obtained, the design engineer shall provide to the City Engineering Division final construction plans and analyses that indicate and document the specific improvements that will fully address the requirements of the Stormwater Ordinance. This plan and study data shall be prepared by a Professional Engineer licensed in the State of Texas with demonstrated knowledge of the study of drainage issues and proficiency with drainage analysis and modeling tools and shall include, but are not limited to: • project specific on -site and off -site grading, drainage and /or detention plans with details of proposed improvements intended to provide compliance with the Stormwater Ordinance including the 100 year hydraulic grade line surfaces being specifically noted on the profile sheets; • surveyed topographic data for the site and pertinent off -site features and verified topographic data for related drainage area(s); • depiction of existing FEMA floodplain and floodway lines on the site and drainage area; • final hydrologic and hydraulic analyses and calculations to document the stormwater impact of the proposed development; • final downstream assessment as described in Section 3.2 hereof; • specific structures, pond, and conveyance sizes along with profiles and representative cross sections of drainage channels; • copies of required permits obtained including all application data as submitted to the approving agency; • specific identification of required phased improvements within the proposed development sequence; • signed easement documents with metes and bounds descriptions and /or a copy of the final plat with dedications noted; and • other information that will assist City staff in determining that the project complies with the Stormwater Ordinance. The City's approval of the final study and construction plans is a precondition of obtaining approvals from the Department of Public Works for final plats or for building permits. Approval will be evidenced by the signature of the City Engineer or his /her designee on the cover page of the plan set and by a separate letter detailing any other matters required to comply with the Ordinance, this Stormwater Manual, or for approval and acceptance of the drainage improvements. 2.4 Construction Review and Engineer Certification 2.4.1 Public Drainage Improvements The City Engineering Division shall provide on -going inspection of construction activities and all work related to drainage improvements that are being constructed for City acceptance and maintenance. The City inspector will keep a log of construction site visits and the project contractor will be required to maintain a red -lined set of project drawings indicating any variations noted between plans and actual construction. Upon completion of the drainage improvements, the design engineer shall, as deemed appropriate, field verify: flow line grades of structures and conveyances, measurements of structures, and actual volumes of storage facilities. The engineer shall then produce an as -built set of project drawings using the red -lined plan set from the Contractor, notes kept by the City Inspector, appropriate verified grades and volumes, and other information known to the engineer. Two hard copy sets of the as -built drawings and an electronic file of the drawings in AutoCAD format shall be provided to the City. The as- built plans and any required maintenance bond shall be provided to the City in satisfactory format prior to the City's acceptance of the improvements. 2.4.2 Privately Maintained Drainage Improvements Upon completion of project site improvements that will be privately maintained, such as a commercial site detention pond, the design engineer shall field verify the as -built flow line grades of structures and conveyances; obtain actual measurements of structures, and verify actual volumes of storage facilities and shall provide a detention facilities agreement in form and substance as provided by the City, a copy of which is included in the Appendix to this Manual, which includes: • the notarized signature of the fee simple owner of the subject property; • an Exhibit "A" containing a reduced copy of the as -built plan of the drainage facility with a sealed and signed certification by the engineer as to the volume and release rate of the facilities; and • an Exhibit "B" containing the metes and bounds description of the area comprising the detention facility. The City Engineering Division's approval of the project for a certificate of occupancy will be contingent upon receipt and acceptance of the signed and sealed detention facilities agreement. 3.0 DESIGN CRITERIA: 3.1 Hydrology 3.1.1 Hydrologic Methods The following methods are approved to support hydrologic site analysis for the design methods and procedures included in this Manual: • Rational Method • Modified Rational Method • SCS Unit Hydrograph Method • Snyder's Unit Hydrograph Method Table 3.1 lists the hydrologic methods and the circumstances for their use in various analysis and design applications. Table 3.2 provides some limitations on the use of several methods. In general the Rational Method is recommended for small highly impervious drainage areas such as parking lots and roadways draining into inlets and gutters. Table 3.1 Applications of the Recommended Hydrologic Methods Method Size Limitations' Ration SCS Modifie Snyder's Method Related al Metho d Unit Section Med tho d Ration Hydrograph Method can be used for Modified Rational al estimating runoff volumes Gutter Flow Section for storage design. Method can be used for and Inlets 3.3.1 estimating peak flows and hydrographs for all design Closed Section applications. Method can be used for Conduits 3.3.2 estimating peak flows and (Snyder' S)3 hydrographs for all design Storage Section applications. Facilities 3.3.3 Open Section Channels 3.3.4 Culverts Section 3.3.5 Bridges Section 3.3.6 Energy Section Dissipation 3.3.7 Table 3.2 Constraints on Using Recommended Hydrologic Methods Method Size Limitations' Comments Method can be used for estimating peak flows and Rational 0 — 200 acres the design of small site or subdivision storm sewer systems. Method can be used for Modified Rational 0 — 200 acres estimating runoff volumes for storage design. Method can be used for Unit Hydrograph (SCS)2 0-2000 acres estimating peak flows and hydrographs for all design applications. Method can be used for Unit Hydrograph 1 sq. mile and larger estimating peak flows and (Snyder' S)3 hydrographs for all design applications. Table 3.2 Constraints on Using Recommended Hydrologic Methods Size limitation refers to the drainage basin for the stormwater management facility (e.g., culvert, inlet). 2 This refers to SCS methodology included in many readily available programs (such as HEC -HMS or HEC -1) that utilize this methodology. 3 This refers to the Snyder's methodology included in many readily available programs (such as HEC -HMS or HEC -1) that utilize this methodology. If local stream gage data are available, these data can be used to develop peak discharges and hydrographs. The user is referred to standard hydrology textbooks for statistical procedures that can be used to estimate design flood events from stream gage data. Note. It must be realized that any hydrologic analysis is only an approximation. The relationship between the amount of precipitation on a drainage basin and the amount of runoff from the basin is complex and too little data are available on the factors influencing the rainfall - runoff relationship to expect exact solutions. 3.1.2 Rainfall Estimation Rainfall intensities for the City of Wichita Falls have historically been provided in that Intensity Duration Frequency Curve prepared by Forrest and Cotton Engineers in January 1965 which may be used for hydrologic analysis within the City. Values may also be calculated using the following formula: i = b /(t, + d)e (3.1) where i is inches per hour and t, is the rainfall duration in minutes. The parameters b, d and e for storm frequencies of 2 year through 100 year events for Wichita County, Texas are shown in Table 3.3: Table 3.3 Parameters for Formula 3.1 — Wichita County, Texas Event e b d 2 yr. 0.803 51 9.4 5 yr. 0.784 62 8.7 10 yr. 0.795 76 8.7 25 yr. 0.792 88 8.7 50 yr. 0.797 104 8.7 100 yr. 0.792 114 9.4 3.1.3 RATIONAL METHOD 3.1.3.1 Introduction When using the Rational Method some precautions should be considered: • In determining the C value (runoff coefficient based on land use) for the drainage area, hydrologic analysis should take into account any future changes in land use that might occur during the service life of the proposed facility. • Since the Rational Method uses a composite C and a single t, value for the entire drainage area, if the distribution of land uses within the drainage basin will affect the results of hydrologic analysis (e.g., if the impervious areas are segregated from the pervious areas), then the basin should be divided into sub - drainage basins. • The formulae and tables included in this section are given to assist the engineer in applying the Rational Method. The engineer should use sound engineering judgment in applying these design aids and should make appropriate adjustments when specific site characteristics dictate adjustments are appropriate. • The Rational Method should not be used for calculating peak flows downstream of bridges, culverts, or storm sewers that may act as restrictions causing storage that impacts the peak rate of discharge. 3.1.3.2 Equations The Rational Formula is expressed as follows: 1011. Q = CIA (3.2) where: Q = maximum rate of runoff (cfs) C = runoff coefficient representing a ratio of runoff to rainfall per Table I = average rainfall intensity for a duration equal to the t, (in /hr) A = drainage area contributing to the design location (acres) 3.1.3.3 Time of Concentration Use of the Rational Formula requires the time of concentration (t,) for each design point within the drainage basin. The duration of rainfall is then set equal to the time of concentration and is used to estimate the design average rainfall intensity (1). The time of concentration consists of an overland flow time to the point where the runoff is concentrated or enters a defined drainage feature (e.g., open channel) plus the time of flow in a closed conduit or open channel to the design point. For each drainage area, the distance is determined from the inlet to the most remote point in the tributary area. From a topographic map, the average slope is determined for the same distance. In urban areas, the length of overland flow distance should realistically be no more than 50 — 100 feet. Table 3.5 gives recommended minimum and maximum times of concentration based on land use categories. The minimum time of concentration should be used for the most upstream inlet (minimum inlet time). Computed downstream travel times will be added to determine times of concentration through the system. For anticipated future upstream development, the time of concentration should be no greater than the maximum. Table 3.5 Times of Concentration Land Use Minimum Maximum (minutes)' (minutes) Residential Development 15 30 Commercial and Industrial 10 25 Central Business District 10 15 3.1.3.4 Runoff Coefficient (C) Table 3.6 provides certain runoff coefficients for the Rational and Modified Rational Methods. The design engineer may also calculate and submit a site specific C value by using the actual site areas and percentages of different land uses within the site being considered rather than arbitrarily using the values from Table 3.6. Clear documentation of the C value determination shall be submitted for review and approval by the City Engineering staff. If it is important to locate a specific land use within the drainage area, then another hydrologic method should be used where hydrographs for each individual drainage area can be generated and routed through the drainage system. It may be that using only the impervious area from a highly impervious site (and the corresponding high C factor and shorter time of concentration) will yield a higher peak runoff value than by using the whole site. This should be checked particularly in areas where the overland portion is grassy (yielding a long t,) to avoid underestimating peak runoff. Table 3.6 Runoff Coefficient Values Description of Area Runoff Coefficients (C) Lawns: Sandy soil, flat, <2% Sandy soil, average, 2 - 0.10 7% 0.15 Sandy soil, steep, > 7% 0.20 Clay soil, flat, <2% 0.17 Clay soil, average, 2 - 0.22 7% 0.35 Clay soil, steep, > 7% Agricultural (cultivated) 0.30 Mesquite Pasture 0.25 Streams, Lakes, Water 1.00 Surfaces Business /Commercial /Industrial: calculate Residential: Single Family (6 lots /ac) 0.55 Single Family (4 lots /ac) 0.50 Single Family (3 lots /ac) 0.45 Single Family (2 lots /ac) 0.40 Single Family (1+ acre calculate lots) calculate Multi - Family Projects Parks, cemeteries 0.25 Playgrounds 0.35 Railroad yard areas 0.40 Streets: Asphalt and Concrete 0.95 Brick 0.85 Drives, walks, and roofs 0.95 Gravel areas 0.50 Graded or no plant cover: Sandy soil, flat, 0 - 3% 0.20 Sandy soil, flat, >3% 0.25 Clayey soil, flat, 0 - 3% 0.25 Clayey soil, average, 0.35 >3% 3.1.4 MODIFIED RATIONAL METHOD 3.1.4.1 Introduction For drainage areas of less than 200 acres, a modification of the Rational Method can be used for the estimation of storage volumes for detention calculations. The Modified Rational Method is a procedure based on the Rational Method by which hydrographs are developed rather than only a peak flow. The hydrographs determined by the Modified Rational Method are based on the assumption that runoff begins and increases linearly to the peak volume of runoff. The time in which the peak is reached is the time of concentration (t,). The peak is maintained for the storm duration and then linearly decreases to zero. The duration (horizontal axis) for both the rising and falling limbs of the inflow hydrograph equal t, and the peak flow of the hydrograph is maintained for the storm duration. A triangular hydrograph results when the storm duration (td) is equal to t, and represents the same peak flow as calculated by the Rational Method. When td is increased beyond t,, the hydrograph takes a trapezoidal shape as shown in Figure 3.1 below. As td is lengthened, the peak flow decreases, but the volume of runoff, the area under trapezoid, increases. An allowable release rate is set (Qa) based on pre - development conditions. The allowable release rate increases linearly until it reaches the receding limb of the inflow hydrograph. The td is varied incrementally until the storage volume (shaded gray area) is maximized. This method is normally an iterative process which can be done by hand or on a spreadsheet. Readily available software programs such as Bentley (Haestad) Pond Pack© Modified Rational Method "I" use this same methodology. Downstream analysis is not possible with this method as only approximate graphical routing takes place. tc td Time Figure 3.1 Modified Rational Definitions 3.1.4.2 Design Equations The allowable release rate can be determined from: Qa =CaiA (3.3) where: Qa = allowable release rate (cfs) Ca = predevelopment Rational Method runoff coefficient i = rainfall intensity for the corresponding time of concentration (in /hr) A = area (acres) The Modified Rational Method should be used for basins with fairly homogeneous land use and flow paths. Consideration should also be given to increasing the C factor for higher intensity ( >25 year) storms because infiltration and other abstraction losses have a proportionally smaller effect on runoff during such events. Care should be exercised in the calculation of the C factor and time of concentration used to determine the Qa to avoid oversizing the outlet device and thus reducing available storage. 3.1.5 SCS METHOD 3.1.5.1 Application The SCS method can be used for both the estimation of stormwater runoff peak rates and the generation of hydrographs for the routing of stormwater flows, thus it can be used for most design applications. It is assumed that most users of the SCS methodology will use a computer program such as HEC -HMS therefore this manual does not attempt to include the equations and concepts utilized as the methodology is adequately described in the HEC -HMS User's Manual and Technical Reference Manual. 3.1.5.2 Runoff Factor (CN) The SCS method uses a combination of soil conditions and land uses (ground cover) to assign a runoff factor to each area. Soils data can be obtained from a site specific geotechnical report or from the County Soils Survey information available on -line at http: / /soi Is. usda. gov /. Average antecedent soil moisture conditions (AMC II) are recommended for most hydrologic analysis. Table 3.8 gives standard curve number values for a range of land uses. When a drainage area has more than one land use, a composite curve number can be calculated based upon percentages of land uses within a basin Table 3.8 Runoff Curve Numbers' Curve numbers for Cover Description' hydrologic soil groups Cover type and hydrologic condition Cultivated Land: Without conservation treatment With conservation treatment Pasture or range land: Poor condition Good condition Meadow: Good condition Wood or forest land: Thin stand, poor cover Good cover Open space (lawns, parks, golf courses, cemeteries, etc. )3 Poor condition (grass cover < 50 %) Fair condition (grass cover 50% to 75 %) Good condition (grass cover > 75 %) Impervious areas: Paved; curbs and storm drains (excluding right -of -way) Paved; open ditches (including right -of- way) Gravel (including right -of -way) Dirt (including right -of -way) Urban districts: Commercial and business Industrial Residential districts by average lot size: 1/8 acre or less (town house) 1/4 acre 1/3 acre 1/2 acre 1 acre 2 acres Developing urban areas and newly graded areas (previous areas only, no vegetation) 1 Average runoff condition, and la = 0.2S Average percent A B C D impervious area 72 81 88 91 62 71 78 81 68 79 86 89 39 61 74 80 30 58 71 78 45 66 77 83 25 55 70 77 68 79 86 89 49 69 79 84 39 61 74 80 98 98 98 98 83 89 92 93 76 85 89 91 72 82 87 89 85% 89 92 94 95 72% 81 88 91 93 65% 77 85 90 92 38% 61 75 83 87 30% 57 72 81 86 25% 54 70 80 85 20% 51 68 79 84 12% 46 65 77 82 77 86 91 94 2 The average percent impervious area shown was used to develop the composite CNs. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. If the impervious area is not connected, the SCS method has an adjustment to reduce the effect. 3 CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations of open space cover type. 3.1.5.3 Urban Modification of the SCS Method Connected Impervious Areas The CNs for various land cover types were developed for typical land use relationships based on specific assumed percentages of impervious area. These CN values were developed on the assumptions that: 1. Pervious urban areas are equivalent to pasture in good hydrologic condition, and 2. Impervious areas have a CN of 98 and are directly connected to the drainage system. If all of the impervious area is directly connected to the drainage system, but the impervious area percentages or the pervious land use assumptions do not appear applicable, refer to the graphical chart provided by SCS to compute a composite CN. Unconnected Impervious Areas Runoff from these areas is spread over a pervious area as sheet flow. To determine CN when all or part of the impervious area is not directly connected to the drainage system, refer to the graphical charts provided by SCS to compute a composite CN. 3.1.5.4 Travel Time Estimation Travel time (Tt) is the time it takes water to travel from one location to another within a watershed, through the various components of the drainage system. Time of concentration (t,) is computed by summing all the travel times for consecutive components of the drainage conveyance system from the hydraulically most distant point of the watershed to the point of interest within the watershed. Travel Time Water moves through a watershed as sheet flow, shallow concentrated flow, open channel flow, or some combination of these. The type of flow that occurs is a function of the conveyance system and is best determined by field inspection. Travel time is the ratio of flow length to flow velocity: Tt = L /3600V (3.4) where: Tt = travel time (hr) L = flow length (ft) V = average velocity (ft/s) 3600 = conversion factor from seconds to hours Sheet Flow Sheet flow can be calculated using the following formula: Tt = 0.42 nL °,$ = 0.007 nL °.$ (3.5) 60 (P2) ' (S) ' (Pi)U.6(S)U.4 where: Tt = travel time (hr) n = Manning roughness coefficient L = flow length (ft), P2 =2 -year, 24 -hour rainfall S = land slope (ft /ft) Shallow Concentrated Flow After 50 to 100 feet, sheet flow usually becomes shallow concentrated flow. The average velocity for this type of flow can be graphically determined from information provided in the SCS manual or can be computed from the following equations. Unpaved V = 16.13(S) °'S (3.6) Paved V = 20.33(S) °'S (3.7) where: V = average velocity (ft/s) S = slope of hydraulic grade line (watercourse slope, ft /ft) After determining average velocity, use Equation 3.6 to estimate travel time for the shallow concentrated flow segment. Open Channels • Open channels are assumed to begin where surveyed cross sections have been obtained, where visible on aerial photographs, where identified by the local municipality, or where stream designations appear on USGS quadrangle sheets. • Manning's Equation or water surface profile information can be used to estimate average flow velocity. • Average flow velocity for travel time calculations is usually determined for bank - full elevation assuming low vegetation winter conditions. Manning's Equation is V = (1.49/n) (R) 2/3 (S) 1/2 where: V = average velocity (ft/s) R = hydraulic radius (ft) and is equal to A/PW A = cross sectional flow area (ft) PW = wetted perimeter (ft) S = slope of the hydraulic grade line (ft /ft) n = Manning's roughness coefficient for open channel flow (3.8) After average velocity is computed using Equation 3.10, Tt for the channel segment can be estimated using Equation 3.6. Limitations • Equations in this section should not be used for sheet flow longer than 50 feet for impervious surfaces. • In watersheds with storm sewers, carefully identify the appropriate hydraulic flow path to estimate t, • A culvert or bridge can act as detention structure if there is significant storage behind it. Detailed storage routing procedures should be used to determine the outflow through the culvert or bridge. 3.1.5.5 Hydrologic Stream Routing The routing methods selected for use in Wichita Falls are the Modified Puls and the Muskingum -Cunge 8 point section methods. 3.1.6 SNYDER'S UNIT HYDROGRAPH METHOD The Snyder method estimates a peak discharge and a time to the peak of the unit hydrograph. It also estimates shape parameters. Rainfall runoff models, such as HEC- 1, will typically complete the unit hydrograph based on assumed parameters and relationships. Typically, two parameters are needed to develop the Snyder Unit Hydrograph: • TL - lag time and • Cp - shape factor, also expressed as Cp640. The following equation to compute lag time should be used: TL = CT (L * LcA /S 0 ' 5 ) 0.38 (3.9) TL = Lag Time (hr) CT = coefficient L = hydraulic length of the watershed along the longest flow path (mi) LcA = hydraulic length along the longest watercourse from the point under consideration to a point opposite the centroid of the drainage basin (mi) S = weighted slope of the basin (ft /mi), measured from the 85% to the 10% points along the longest stream path in the basin. The value CT is a dimensionless parameter that is typically assumed to be consistent for various areas of the state. It can be estimated from neighboring areas or calibrated for the whole or portions of the basin, and then applied to multiple subbasins within the watershed. Note that there are multiple forms of the Snyder equation for TL. Some use ft /ft for the slope and some do not include the slope at all. If a regional CT value is used, verify that the same equation was used in the study within which it was developed. Values generally range from about 0.7 up to about 3.0 though values outside that range have been calibrated. The shape factor Cp reflects the sharpness of the hydrograph. High values, up to about 500, reflect a rapidly responding basin with a sharp peaked hydrograph. Low values, such as 250, generally reflect a flatter, slow responding basin with a longer, flatter hydrograph. These values are generally divided by 640 and entered into HEC -HMS as the Cp value ranging from about 0.4 to 0.8. 3.2. Downstream Assessment 3.2.1 Introduction The assessment should extend from the outfall of a proposed development to a point downstream where the discharge from a proposed development no longer has a significant impact on the receiving stream or storm drainage system. The assessment should be a part of the preliminary and final plans, and should include the following properties: • Hydrologic analysis of the pre- and post - development on -site conditions • Drainage path which defines extent of the analysis. • Capacity analysis of all existing constraint points along the drainage path, such as existing floodplain developments, underground storm drainage systems culverts, bridges, tributary confluences, or channels • Offsite undeveloped areas are considered as "full build -out" for both the pre- and post - development analyses • Evaluation of peak discharges and velocities for three (3) 24 -hour storm events • 2 -year storm • 10 -year storm; and ;eparate 100 -year storm • analysis for each major outfall from the proposed development Once the analysis is complete, the designer should ask the following three questions at each determined junction downstream: • Are the post - development discharges greater than the pre - development discharges? • Are the post - development velocities greater than the pre - development velocities? • Are the post - development velocities greater than the velocities allowed for the receiving system? These questions should be answered for each of the three storm events. The answers to these questions will determine the necessity, type, and size of non - structural and structural controls to be placed on -site or downstream of the proposed development. 3.2.2 Downstream Hydrologic Assessment Common practice requires the designer to control peak flow at the outlet of a site such that post - development peak discharge is equal to or less than pre - development peak discharge. It has been shown that in certain cases this does not always provide effective water quantity control downstream from the site and may actually exacerbate flooding problems downstream. The reasons for this have to do with (1) the timing of the flow peaks, and (2) the total increase in volume of runoff. Due to a site's location within a watershed, there may be very little reason for requiring flood control from a particular site. In certain circumstances where detention is in place or a master drainage plan has been adopted, a development may receive or plan to receive less that ultimate developed flow conditions from upstream. This might be considered in the detention needed and its influence on the downstream assessment. Any consideration in such an event would be with the prior approval of the City Public Works Director. This section outlines a suggested procedure for determining the impacts of post - development stormwater peak flows and volumes that are required as part of a developer's stormwater management site plan. 3.2.3 Methods for Downstream Evaluation The downstream assessment is a tool by which the impacts of development on stormwater peak flows and velocities are evaluated downstream. The assessment should consider the zone of influence of the proposed development and shall extend from the outfall of the development to a point downstream where the discharge no longer has a significant impact upon the receiving stream or storm drainage system Typical steps in a downstream assessment include: 1. Determine the outfall location of the site and the pre- and post - development site conditions. 2. Collect data for the stormwater facilities within the zone of influence, such as reviewing other studies and obtaining as -built plans. Based on this information, document whether or not the downstream facilities were designed for build out conditions for all property upstream and whether there are any know problems downstream such as road overtoppings, historical structure flooding, etc. If there are no such downstream problems and if the downstream systems are documented as being designed for build out conditions upstream, then this information shall be presented for the approval and consent of the City Public Works Director. If these criteria are not satisfied, then the assessment must continue. 3. Using a topographic map determine a preliminary lower limit of the zone of influence (approximately 10% point). 4. Using a hydrologic model determine the pre - development peak flows and velocities at each junction beginning at the development outfall and ending at the next junction beyond the "10% zone of influence" point. The 10% zone of influence can be considered to be the point where the drainage area controlled by the detention or storage facility comprises 10% of the total drainage area. For example, if a structural control drains 10 acres, the zone of influence ends at the point where the total drainage area is 100 acres or greater. Undeveloped off -site areas are modeled as "full build -out" for both the pre- and post - development analyses. The discharges and velocities are evaluated for the three design storms. 5. Change the land use on the site to post - development conditions and rerun the model. 6. Compare the pre- and post - development peak discharges and velocities at the downstream end of the model. If the post - developed flows are higher than the pre - developed flows for the same frequency event, or the post - developed velocities are higher than the allowable velocity of the downstream receiving system, extend the model downstream. Repeat steps 3 and 4 until the post - development flows are less than the pre - developed flows, and the post - developed velocities are below the allowable velocity. Allowable velocities are given in Table 3.13 in Section 3.3.4, Open Channels. 7. If shown that no peak flow increases occur downstream, and post - developed velocities are allowable, then the control of the flood protection volume can be waived by the City Director of Public Works. 8. If peak discharges are increased due to development, or if downstream velocities are erosive, one of the following options are required. • Provide an acceptable design to reduce the flow elevation and /or velocity through channel or flow conveyance structure improvements downstream; or • Design an on -site structural control facility such that the post - development flows do not increase the peak flows, and the velocities are not erosive, at the outlet and the determined junction locations. 3.3. HYDRAULICS 3.3.1 Streets Gutter Flow The City has chosen to calculate gutter depth using a straight crown cross section. Design guidance on gutter flow hydraulics may be obtained from the Federal Highway Administration's Urban Drainage Design Manual, HEC -22. Formula The following form of Manning's Equation should be used to evaluate gutter flow hydraulics: Q = [0.56/n] S.6 /3 S1/2 T8/3 (3.10) where: Q = gutter flow rate, cfs SX = pavement cross slope, ft /ft n = Manning's roughness coefficient S = longitudinal slope, ft /ft T = width of flow or spread, ft Manning's n Table Table 3.9 Manning's n Values for Street and Pavement Gutters Type of 'Gutter or Pavement Manning's n Concrete gutter, troweled finish 0.014 Concrete gutter with Smooth adjoining pavement Rough (broom finish) adjoining pavement 0.015 0.018 For gutters with small slopes, where sediment may accumulate, increase above values of n by 0.002 Stormwater Inlets Inlets used for the drainage of pavement surfaces can be divided into three major classes: • Grate Inlets — These inlets include grate inlets consisting of an opening in the gutter covered by one or more grates, and slotted inlets consisting of a pipe cut along the longitudinal axis with a grate or spacer bars to form slot openings. • Curb - Opening Inlets — These inlets are vertical openings in the curb covered by a top slab. • Combination Inlets — These inlets usually consist of both a curb - opening inlet and a grate inlet placed in a side -by -side configuration, but the curb opening may be located in part upstream of the grate. The City of Wichita Falls requires the use of curb type inlets. However, grate inlets may be allowed in certain design situations with the prior approval of the City Public Works Director. The City of Wichita Falls requires that any design using grate inlets must reduce that calculated capacity of the grate by 50% due to the probability of debris clogging the inlet. Inlets may be classified as being on a continuous grade or in a sump. Overflow provisions shall be provided in sump locations to handle excess stormwater flows that may exceed curb height in the event of a storm exceeding the design conditions or if the inlet were to clog. These overflow provisions shall not adversely affect adjoining private property. Design guidance for all inlet types of inlet hydraulics may be obtained from the Federal Highway Administration's Urban Drainage Design Manual, HEC -22, and from AASHTO's Model Drainage Manual. 3.3.2 Storm Sewer (Closed Conduit) Systems Closed conduit systems may be composed of different lengths and sizes of conduits (system segments) connected by appointment structures (system nodes). Segments are most often circular pipe, but can be a box or other enclosed conduit. The following requirements shall be applied to the design of storm sewers: • The minimum acceptable pipe size is 18" inside diameter • Manholes or junction boxes shall be provided at all changes in horizontal direction or slope, changes in pipe diameters, or pipe intersections with a maximum spacing on long pipe runs of 1000 feet. • Preformed wyes may be used only for single leads from an inlet to the main line. • Approved piping materials for conduits are: (1) Reinforced Concrete Pipe ( "RCP ") with pipe class determined by depth of cover and loading conditions installed per City Details, (2) High Density Polyethylene ( "HDPE ") with a smooth interior may be used only when a minimum of 24" of cover from finish grade to the top of pipe is provided and the pipe shall be properly embedded in strict accordance with manufacturers' specifications with graded gravel. Any HDPE used within five feet (5) of a street pavement edge or under street sections shall be encased with flowable fill on all sides per City details. (3) Poly Vinyl Chloride ( "PVC ") pipe with a smooth interior surface is allowable. Pipe stiffness shall be a minimum of 46 with actual pipe class determined by depth of cover calculations with pipe not to exceed 5% deflection. Capacity Calculations A closed conduit may be under pressure or at other times the conduit may flow partially full; however, the usual design assumption is that the conduit is flowing full but not under pressure. Under this assumption the rate of head loss is the same as the slope of the pipe (Sf =S), in ft /ft. The hydraulic capacity of storm drain pipes for gravity and pressure flows shall be determined by the following equation: V = (1.486/n) R2/3 S1/2 (3.11) where: V = mean velocity of flow, ft /s R = the hydraulic radius, ft - defined as the area of flow divided by the wetted flow surface or wetted perimeter (A/WP) S = the slope of hydraulic grade line, ft /ft n = Manning's roughness coefficient In terms of discharge, the above formula becomes: Q = (1.486/n) A R 213 S112 (3.12) where: Q = rate of flow, cfs A = cross sectional area of flow, ft2 For pipes flowing full, the area is (rr /4)D2 and the hydraulic radius is D /4, so, the above equations become: V = [0.590 D213S112]/n (3.13) Q = [0.463 D8/3S1/2]/n (3.14) where: D = diameter of pipe, ft S = slope of the pipe = Sf hydraulic grade line, ft /ft The Manning's Equation can be written to determine friction losses for storm drain pipes as: Hf = [0.453 n2V2L] /[R4 /3] Hf = [(2.87 n2V2L] /[Dais] Hf = [(185n2(V2/2g)L] /[D4i3] where: Hf = total head loss due to friction, ft (Sf x L) n = Manning's roughness coefficient D = diameter of pipe, ft L = length of pipe, ft V = mean velocity, ft /s R = hydraulic radius, ft g = acceleration of gravity = 32.2 ft /sect (3.15) (3.16) (3.17) Table 3.10 Manning's Coefficients for Storm Drain Conduits (H EC 22, 2001) Type of Culvert Roughness or Manning's 'n Corrugation Concrete Pipe Smooth 0.013 Concrete Boxes Smooth 0.013 HDPE Smooth 0.010 Polyvinyl chloride Smooth 0.010 (PVC) *NOTE: The Manning's n values indicated in this table were obtained in the laboratory and are supported by the provided reference. These numbers should be considered as the best possible for the pipe type. Actual field values for culverts may vary depending on the effect of abrasion, corrosion, deflection, and joint conditions. Minimum Grades and Desirable Velocities The minimum allowable velocity for closed conduits flowing full is 2.0 fps. The minimum slopes are therefore calculated by the modified Manning's formula: S = [(nV)2]/[2.208R4/3] (3.18) where: S = the slope of the hydraulic grade line, ft /ft n = Manning's roughness coefficient V = mean velocity of flow, ft /s R = hydraulic radius, ft (area divided by wetted perimeter) For circular conduits flowing full but not under pressure, R =D /4, and the hydraulic grade line is equal to the slope of the pipe. For these conditions Equation 3.20 may be expressed as: S = 2.87(nV)2/D413 (3.19) For a minimum velocity of 2.0 fps, the minimum slope equation becomes: S = 11.48(n2/D4/3) where: D = diameter, ft Maximum Velocities (3.20) Maximum velocities in storm drains should not exceed 15 fps. However, the outfall velocity shall not exceed the velocity of the receiving channel for the same storm event. Hydraulic Grade Line All drainage plans prepared for review by the City shall include hydraulic grade lines indicated on the profile views for the system. The energy grade line (EGL) represents the total energy along a channel or conduit carrying water. Total energy includes elevation head, velocity head and pressure head. The 10 -year storm hydraulic grade line for a closed conduit system shall be contained within the closed conduit system. E = V2 /2g + p/v + z (3.21) where: E = Total energy, ft V2 /2g = Velocity head, ft (kinetic energy) p = Pressure, Ibs /ft2 v = Unit weight of water, 62.4 Ibs /ft3 p/v = Pressure head, ft (potential energy) z = Elevation head, ft (potential energy) Bernoulli's Law expressed between points one (1) and two (2) in a closed conduit accounts for all energy forms and energy losses. The general form of the law may be written as: V12/2g + pl /v + z, = V22/2g + p2 /v + z2 - Hf - Y_Hm (3.22) where: Hf = Pipe friction loss, ft Y_Hm = Sum of minor or form losses, ft An in -depth presentation of the EGL and HGL calculations for a closed conduit system is provided in the Federal Highway Administration's Urban Drainage Design Manual, HEC -22 to which reference is herein made. Storm Drain Outfalls All storm drains have an outlet where flow from the storm drainage system is discharged. The discharge point can be a natural river or stream, an existing storm drainage system, or a channel which is either existing or proposed for the purpose of conveying the stormwater. The procedure for calculating the hydraulic grade line through a storm drainage system begins at the outfall. Therefore, consideration of outfall conditions is an important part of storm drain design. Storm drain outfalls shall include a headwall structure and a minimum 10 foot -long concrete apron with turned down footing at a transition into an earthen channel. However, the maximum velocity exiting the outfall cannot exceed the allowable velocity for the receiving channel. Refer to Table 3.12 for allowable velocities. Several aspects of outfall design must be given serious consideration. These include the flowline or invert (inside bottom) elevation of the proposed storm drain outlet, tailwater elevations, the need for energy dissipation, and the orientation of the outlet structure. The flowline or invert elevation of the proposed outlet should be equal to or higher than the flowline of the outfall. If this is not the case, there may be a need to pump or otherwise lift the water to the elevation of the outfall. Energy dissipation may be required to protect the storm drain outlet. A minimum 10 foot -long concrete apron shall be installed at the storm drain outlet into another conveyance. The outfall velocity shall not exceed that of the receiving stream or the maximum velocities provided in Table 3.12. Protection may be required at the outlet to prevent erosion of the outfall bed and banks. Riprap aprons or energy dissipators should be provided if high velocities are expected. The orientation of the outfall is another important design consideration. Where practical, the outlet of the storm drain should be positioned in the outfall channel so that it is pointed in a downstream direction. This will reduce turbulence and the potential for excessive erosion. If the outfall structure cannot be oriented in a downstream direction, the potential for outlet scour must be considered. For example, where a storm drain outfall discharges perpendicular to the direction of flow of the receiving channel, care must be taken to avoid erosion on the opposite channel bank. If erosion potential exists, a channel bank lining of riprap or other suitable material should be installed on the bank. Alternatively, an energy dissipator structure could be used at the storm drain outlet. The tailwater depth or elevation in the storm drain outfall must be considered carefully. Evaluation of the hydraulic grade line for a storm drainage system begins at the system outfall with the tailwater elevation. The starting point for the hydraulic grade line determination should be either the design tailwater elevation or the average of critical depth and the height of the storm drain conduit, (d, + D) /2, whichever is greater. Coincidental Occurrence If the outfall channel is a river or stream, it may be necessary to consider the joint or coincidental probability of two hydrologic events occurring at the same time to adequately determine the elevation of the tailwater in the receiving stream. The relative independence of the discharge from the storm drainage system can be qualitatively evaluated by a comparison of the drainage area of the receiving stream to the area of the storm drainage system. For example, if the storm drainage system has a drainage area much smaller than that of the receiving stream, the peak discharge from the storm drainage system may be out of phase with the peak discharge from the receiving watershed. Table 3.11 provides a comparison of discharge frequencies for coincidental occurrence for the 2 -, 5 -, 10 -, 25 -, 50 -, and 100 -year design storms. This table can be used to establish an appropriate design tailwater elevation for a storm drainage system based on the expected coincident storm frequency on the outfall channel. For example, if the receiving stream has a drainage area of 200 acres and the storm drainage system has a drainage area of 2 acres, the ratio of receiving area to storm drainage area is 100 to 1. From Table 3.11 and considering a 10 -year design storm occurring over both areas, the flow rate in the main stream will be equal to that of a five year storm when the drainage system flow rate reaches its 10 -year peak flow at the outfall. Conversely, when the flow rate in the main channel reaches its 10 -year peak flow rate, the flow rate from the storm drainage system will have fallen to the 5- year peak flow rate discharge. This is because the drainage areas are different sizes, and the time to peak for each drainage area is different. Table 3.11 Frequencies for Coincidental Occurrences (TxDOT, 2002) Area ratio* 2 -year design 5 -year design Main Stream Tributary Main Stream Tributary 10,000:1 1 2 1 5 2 1 5 1 1,000:1 1 2 2 5 2 1 5 2 100:1 2 2 2 5 2 2 5 5 10:1 2 2 5 5 2 2 5 5 1:1 2 2 5 5 2 2 5 5 Area ratio* 10 -year design 25 -year design Main Stream Tributary Main Stream Tributary 10,000:1 1 10 2 25 10 1 25 2 1,000:1 2 10 5 25 10 2 25 5 100:1 5 10 10 25 10 5 25 10 10:1 10 10 10 25 10 10 25 10 1:1 10 10 25 25 10 10 25 25 Area ratio* 50 -year design 100 -year design Main Stream Tributary Main Stream Tributary 10,000:1 2 50 2 100 50 2 100 2 1,000:1 5 50 10 100 50 5 100 10 100:1 10 50 25 100 50 10 100 25 10:1 25 50 50 100 50 25 100 50 1:1 50 50 100 100 50 50 100 100 *The Area ratio is the ratio of the overall the drainage area of the facility being evaluated. drainage area of the receiving stream to There may be instances in which an excessive tailwater causes flow to back up the storm drainage system and out of inlets and manholes, creating unexpected and perhaps hazardous flooding conditions. The potential for this should be considered. Flap gates placed at the outlet can sometimes alleviate this condition; otherwise, it may be necessary to isolate the storm drain from the outfall by use of a pump station. 3.3.3 Storage Design General Storage Concepts Storage of stormwater runoff within a stormwater management system is essential to providing the extended detention of flows for water quality protection and downstream streambank protection, as well as for peak flow attenuation of larger flows for flood protection. Runoff storage can be provided within an on -site system through the use of structural stormwater controls and /or nonstructural features and landscaped areas. Flood Level Dry Basin Flood Level Permanent Pool Stormwater Pond or Wetland Rooftop Storage ❑❑❑❑ Parking Lot Storage Landscaped ❑ ❑ ❑ ❑ Area Underground Vault Underground Pipe Storage Figure 2.1 illustrates various storage facilities that can be considered for a development site. Figure 3.3.2 Examples of Typical Stormwater Storage Facilities General Design Criteria • Outlet rates and design storms are defined in the City's Stormwater Ordinance; • HEC -HMS shall be used for large project designs (ponds with a drainage area of 200 acres or more); • A primary outlet device and corresponding storage volumes must be designed for discharge of 2, 10 and 100 year storms; • Stage- storage curve or table for the proposed storage facility shall be provided for all detention designs; • Stage- discharge curve or table for all outlet control structures shall be provided for all detention designs; • Ponds may not be located in existing drainage ways; • A secondary outlet device (emergency spillway) shall be provided at all facilities and designed to pass the 100 -year storm. A minimum of 6" of freeboard is required at all earthen dams or where erosion may occur from overtopping. • Maximum design WSEL shall be at least12" below the finish floor elevations of nearby structures • All storage facilities must be able to drain by gravity; • Earthen ponds shall have a minimum 0.5% slope across the flow line of the pond bottom and have minimum side slopes of 4:1 or flatter; • Permanent vegetation shall be established in all earthen ponds. In those ponds to be accepted for City maintenance, the vegetation shall be a drought tolerant blend containing Bermuda grass which is actively growing; covering the pond floor and side slopes with a 15 foot -wide belt around the top of berm; and having no bare spots greater that one square foot in size. • Retention ponds shall have a minimum, normal conservation pool depth of two feet unless the pond is intended to be stocked in which case at least 25% of the conservation pool depth shall be five feet deep or more. Measures shall be provided to insure that aerobic pond conditions are maintained. Only that volume existing above the normal conservation pool elevation shall be considered as storage. Discharge /outlet devices shall be designed as with a detention pond. • The maximum depth of ponding in parking areas shall be 18 "; however, the developer must clearly identify the area of potential ponding over 12" deep with signage and assumes all liability for vehicle damage; • Underground systems must have be designed with adequate manway access for cleaning and must be able to drain by gravity; • Rooftop system designs must also include the signed and sealed certification by a currently licensed Texas professional engineer that the entire structure (primary and secondary framing) has been properly designed to accommodate the additional stormwater loadings and that the building envelope has been designed to properly protect the interior from water intrusion as a result of the rooftop detention. General Storage Design Procedures The design procedures for all structural control storage facilities are the same whether or not they include a permanent pool of water. In the latter case, the permanent pool or spillway elevation is taken as the "bottom" of storage and is treated as if it were a solid basin bottom for routing purposes. It should be noted that the location of structural stormwater controls is very important as it relates to the effectiveness of these facilities to control downstream impacts. In addition, multiple storage facilities located in the same drainage basin will affect the timing of the runoff through the conveyance system, which could decrease or increase flood peaks in different downstream locations. Refer to Section 3.2 for the requirements of a Downstream Assessment. 3.3.4 Open Channels Open Channel Types The three main classifications of open channel types according to channel linings are vegetated, flexible, and rigid. Vegetated linings include grass with mulch, sod and lapped sod, and wetland channels. Stone riprap and some forms of flexible man -made linings or gabions are examples of flexible linings, while rigid linings are generally concrete or rigid block. Vegetative Linings — Vegetation, where practical, is the most desirable lining for an artificial channel. It stabilizes the channel body, consolidates the soil mass of the bed, checks erosion on the channel surface, provides habitat, and provides water quality benefits. Conditions under which vegetative cover only may not be acceptable include but are not limited to: • High velocities • Standing or continuously flowing water • Lack of regular maintenance necessary to prevent growth of taller or woody vegetation • Lack of nutrients and inadequate topsoil • Excessive shade Proper soil preparation, seeding, mulching, watering and any other work necessary to the establishment of healthy vegetation shall be provided. Channels shall not be accepted for City maintenance until vegetation is fully established on a minimum of 90% of the channel bottom and side slopes. Pilot Channels - Man -made earthen channels with longitudinal slopes of less than 0.5% or that serve an area where consistent low flows are or may become prevalent shall be provided with a pilot channel per City Engineering standards. Flexible Linings — Rock riprap, including rubble and gabion baskets, is the most common type of flexible lining for channels. It presents a rough surface that can dissipate energy and mitigate increases in erosive velocity. These linings are usually less expensive than rigid linings and have self - healing qualities that reduce maintenance. However, they may require the use of a filter fabric depending on the underlying soils, and the growth of grass, weeds, and trees may present maintenance problems. Rigid Linings — Rigid linings are generally constructed of concrete and used where high flow capacity is required. Higher velocities, however, create the potential for scour at channel lining transitions and channel headcutting. Manning's n Values The choice of Manning's n value can significantly affect discharge, depth, and velocity estimates. Since Manning's n values depend on many different physical characteristics of natural and man -made channels, care and good engineering judgment must be exercised in the selection process. Recommended Manning's n values for man -made channels with unlined, rigid, gabion and riprap linings are given in the following Table 3.12For natural channels, Manning's n values should be estimated using experienced judgment and information presented in publications such as the Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains, FHWA -TS -84 -204, 1984, FHWA HEC -15, 1988, or Chow, 1959. Table 3.12 Manning's Roughness Coefficients for Design of Open Channels Lining Type Manning' s Comments Max. Velocity n Grass Lined 0.035 6 Concrete 0.015 15 Lined Gabions 0.030 10 n = 0.0395d50 where d5o is the 10 Rock Riprap 0.040 stone size of which 50% of the sample is smaller Rip pd 0.028 FWHA 10 Uniform Flow Calculations Design Aids This manual does not attempt to provide an exhaustive review of open channel design. Following is a discussion of the equations that can be used for the design and analysis of open channel flow. The Federal Highway Administration Hydraulic Design Standard manuals have numerous design charts or nomographs to aid in the design of rectangular, trapezoidal, and triangular open channel cross sections. In addition, design charts for grass -lined channels have been developed. Numerous software programs are available for calculating open channel flows. All submissions of design data to the City must clearly define which programs were used for analysis. Manning's Equation Manning's Equation, presented in three forms below, is recommended for evaluating uniform flow conditions in open channels. Most packaged drainage software utilizes these basic formulae: V = (1.49/n) R 213 S112 Q = (1.49/n) A R 213 S112 S = [Qn/(1.49 A R213)]2 where: v = average channel velocity (ft/s) Q = discharge rate for design conditions (cfs) n = Manning's roughness coefficient A = cross - sectional area (ft2) R = hydraulic radius A/P (ft) P = wetted perimeter (ft) S = slope of the energy grade line (ft /ft) (3.23) (3.24) (3.25) For prismatic channels, in the absence of backwater conditions, the slope of the energy grade line, water surface and channel bottom are assumed to be equal. For a more comprehensive discussion of open channel theory and design, see the reference USACE, 1991/1994. 3.3.5 Culvert Design Overview A culvert is a short, closed (covered) conduit that conveys stormwater runoff under an embankment or away from the street right -of -way. The primary purpose of a culvert is to convey surface water, but properly designed it may also be used to restrict flow and reduce downstream peak flows. The hydraulic and structural designs of a culvert must be such that minimal risks to traffic, property damage, and failure from floods prove the results of good engineering practice and economics. For economy and hydraulic efficiency, engineers should design culverts to operate with the inlet submerged during flood flows, if conditions permit. Design considerations include site and roadway data, design parameters (including shape, material, and orientation), hydrology (flood magnitude versus frequency relation), and channel analysis (stage versus discharge relation). Design Criteria The design of a culvert should take into account many different engineering and technical aspects at the culvert site and adjacent areas. The following list of design recommendations should be considered for all culvert designs as applicable. • Storm Frequency o Refer to the Stormwater Ordinance. Culverts must pass a minimum of a 10 year event but are also subject to depth of water restrictions over the roadway. • Velocity Limitations o Culverts are limited to velocities of 15 fps; however, the maximum allowable velocity of the downstream conveyance shall not be exceeded. • Debris Control o In designing debris control structures, it is recommended that the Hydraulic Engineering Circular No. 9 entitled Debris Control Structures be consulted. • Headwater Limitations o Governed by depth over roadway limitation of the Stormwater Ordinance. • Tailwater Considerations o Flows must be kept in dedicated easements and at least 12" below downstream structures. • Culvert Inlets o Hydraulic efficiency and cost can be significantly affected by inlet conditions. The inlet coefficient Ke, is a measure of the hydraulic efficiency of the inlet, with lower values indicating greater efficiency. Recommended inlet coefficients are given in Table 3.13. • Inlets with Headwalls o Headwalls may be used for a variety of reasons, including increasing the efficiency of the inlet, providing embankment stability, providing embankment protection against erosion, providing protection from buoyancy, and shortening the length of the required structure. Headwalls are required for all culverts and where buoyancy protection is necessary. If high headwater depths are to be encountered, or the approach velocity in the channel will cause scour, a short channel apron should be provided at the toe of the headwall. o This apron should extend at least one pipe diameter upstream from the entrance, and the top of the apron should not protrude above the normal streambed elevation. • Wingwalls and Aprons o Wingwalls are used where the side slopes of the channel adjacent to the entrance are unstable or where the culvert is skewed to the normal channel flow. • Improved Inlets o Where inlet conditions control the amount of flow that can pass through the culvert, improved inlets can greatly increase the hydraulic performance of the culvert. • Material Selection o Reinforced concrete pipe (RCP), pre -cast and cast in place concrete boxes are recommended for use (1) under a roadway, (2) when pipe slopes are less than 1 %, or (3) for all flowing streams. High- density polyethylene (HDPE) pipe may also be used if encased in flowable fill as specified by City details. Table 3.14 gives recommended Manning's n values for different materials. • Culvert Skews o Culvert skews shall not exceed 45 degrees as measured from a line perpendicular to the roadway centerline without approval. • Weep Holes o Weep holes are sometimes used to relieve uplift pressure on headwalls and concrete rip -rap. Filter materials should be used in conjunction with the weep holes in order to intercept the flow and prevent the formation of piping channels through the fill embankment. The filter materials should be designed as an underdrain filter so as not to become clogged and so that piping cannot occur through the pervious material and the weep hole. • Outlet Protection o Culvert discharges shall be treated as a storm drain outfall. • Environmental Considerations o Where compatible with good hydraulic engineering, a site should be selected that will permit the culvert to be constructed to cause the least impact on the stream or wetlands. This selection must consider the entire site, including any necessary lead channels. • Safety Considerations o Roadside safety should be considered for culverts crossing under roadways. Guardrails or safety end treatments may be needed to enhance safety at culvert crossings. The AASHTO roadside design guide should be consulted for culvert designs under and adjacent to roadways. Table 3.13 Inlet Coefficients Type of Structure and Design of Entrance Coefficien t Ke Pipe, Concrete Projecting from fill, socket end (grove -end) 0.2 Projecting from fill, square cut end 0.5 Headwall or headwall and wingwalls Socket end of pipe (groove -end) 0.2 Square -edge 0.5 Rounded [radius = 1/12(D)] 0.2 Mitered to conform to fill slope 0.7 *End- Section conforming to fill slope 0.5 Beveled edges, 33.70 or 450 bevels 0.2 Side- or slope- tapered inlet 0.2 Box, Reinforced Concrete Headwall parallel to embankment (no wingwalls) Square -edged on 3 edges 0.5 Rounded on 3 edges to radius of [1/12(D)] or [1/12(8)] or beveled 0.2 edges on 3 sides Wingwalls at 300 to 750 to barrel Square -edged at crown 0.4 Crown edge rounded to radius of [1/12(D)] or beveled top edge 0.2 Wingwalls at 100 or 250 to barrel Square -edged at crown 0.5 Wingwalls parallel (extension of sides) Square -edged at crown 0.7 Side- or slope- tapered inlet 0.2 Although laboratory tests have not been completed on Ke values for High- Density Polyethylene (HDPE) pipes, the Ke values for corrugated metal pipes are recommended for HDPE pipes. * Note: "End Section conforming to fill slope ", made of either metal or concrete, are the sections commonly available from manufacturers. From limited hydraulic tests they are equivalent in operation to a headwall in both inlet and outlet control. Some end sections incorporating a closed taper in their design have a superior hydraulic performance. These latter sections can be designed using the information given for the beveled inlet. Source: HDS No. 5, 2001 Table 3.14 Manning's n Values Type of Conduit Wall & J 'oint Description' Manning's n Concrete Pipe Good joints, smooth walls 0.012 Good joints, rough walls 0.016 Poor joints, rough walls 0.017 Concrete Box Good joints, smooth finished walls 0.012 Poor joints, rough, unfinished walls 0.018 High Density Smooth Liner 0.011 Polyethylene (HDPE) Corrugated 0.020 Polyvinyl Chloride 0.011 (PVC) Source: HDS No. 5, 2001 Note: For further information concerning Manning n values for selected conduits consult Hydraulic Design of Highway Culverts, Federal Highway Administration, 2001, HDS No. 5, pages 201 -208. Comprehensive Design Guidance Comprehensive design discussions and guidance may be found in the Federal Highway Administration, National Design Series No. 5, document entitled Hydraulic Design of Highway Culverts, Second Edition, published in 2001. This document is available from the National Technical Information Service as Item Number PB2003102411 *DL. (http: / /www.ntis.gov /search.htm) Search for this document using the Item Number. 3.3.6 Bridge Design The following subsections present considerations related to the hydraulics of bridges. It is generally excerpted from Chapter 9 of the Texas Department of Transportation (TxDOT) Hydraulics Design Manual dated March 2009. Design Recommendations The design of a bridge should take into account many different engineering and technical aspects at the bridge site and adjacent areas. Bridges shall be designed to pass a 100 -year event with 12" of freeboard between the calculated 100 -year water surface elevation and the lowest structural member. Loss Coefficients The contraction and expansion of water through the bridge opening creates hydraulic losses. These losses are accounted for through the use of loss coefficients. Table 3.15 gives recommended values for the Contraction (K,) and Expansion (Ke) Coefficients. Table 3.15 Recommended Loss Coefficients for Bridges Contraction Expansion Transition Type & Ke No losses computed 0.0 0.0 Gradual transition 0.1 0.3 Typical bridge 0.3 0.5 Severe transition 0.6 0.8 3.3.7 ENERGY DISSIPATION General Criteria Erosion problems at culvert, pipe and engineered channel outlets are common. Determination of the flow conditions, scour potential, and channel erosion resistance shall be standard procedure for all designs. Energy dissipators shall be employed whenever the velocity of flows leaving a stormwater management facility exceeds the erosion velocity of the downstream area channel system. Energy dissipator designs will vary based on discharge specifics and tailwater conditions. Outlet structures should provide uniform redistribution or spreading of the flow without excessive separation and turbulence. Recommended Energy Dissipators For many designs, the following outlet protection devices and energy dissipators provide sufficient protection at a reasonable cost: Riprap apron Riprap outlet basins Baffled outlets Grade Control Structures Refer to the Federal Highway Administration Hydraulic Engineering Circular No. 1, Hydraulic Design of Energy Dissipators for Culverts and Channels, for the design procedures of energy dissipators. APPENDIX DETENTION FACILITY AGREEMENT STATE OF TEXAS § COUNTY OF WICHITA § KNOW ALL MEN BY THESE PRESENTS This agreement made this the day of 20_ by and between the City of Wichita Falls, Texas, hereinafter "City ", acting by and through its City Manager, and <<Current Property Owner >>, hereinafter "Owner ". The term owner shall include the above named owner, its successors and assigns. WITNESSETH: WHEREAS, <<Property Owner >>, is the owner of certain real property located in the corporate limits of the City, more fully described as <<Subdivision Legal Description>> and incorporated herein by reference ( the "Owner Tract "); and, WHEREAS, the Owner and the City desire that the development of the Owner Tract be in accordance with applicable storm water runoff regulations of the City, designed to promote the health, safety and general welfare of the citizens of the City. NOW, THEREFORE, in consideration of the covenants contained herein and other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the City and Owner hereby agree as follows: ARTICLE ONE In consideration of the City plat and site approval of the property described as <<Subdivision Legal Description >>, Owner hereby agrees to construct, maintain and repair a certain Detention Facility to be constructed on a portion of the Owner Tract, identified in Exhibits "A & B ", attached hereto and made a part hereof for all purposes (the "Detention Facility "). ARTICLE TWO Owner shall construct, maintain and repair the Detention Facility in a condition sufficient to provide storm water detention in accordance with the regulations of the City in effect on the date of this agreement. The Detention Facility and site grading shall be completed in accordance with Site and Grading Plans submitted by Owner and approved by City and shall be completed prior to City's issuance of a Certificate of Occupancy for any building constructed on the Owner Tract. The owner shall not allow any structure nor allow any modification within the limits of the detention facility, which will adversely affect the performance of the facility. In the event the Owner shall subdivide the Owner Tract into two (2) or more parcels which use the Detention Facility the owner of each resulting tract shall have the right to perform the maintenance necessary to retain the functionality of the detention facility. The maintenance obligation shall be a covenant running with the Owner Tract; provided, however, that in the event any Owner conveys its interest in the Owner Tract, such conveying Owner shall be released from any and all obligations under this agreement arising after the date of such conveyance. City shall have the right to inspect the Detention Facility at all reasonable times to insure compliance with this agreement and Owner hereby grants City access to and across the Owner Tract for this purpose. In the event Owner fails to fully perform its obligations under this agreement to maintain the Detention Facility, and such failure continues for thirty (30) days after written notice from the City to Owner, City shall have the right to perform the necessary maintenance and receive full reimbursement from the Owner for the reasonable expenses incurred by City in connection therewith. Any notice, request, demand or other communication to be given to the Owner hereunder shall be in writing and shall be deemed to be delivered; (1) when received if hand delivered, or (2) if sent by mail, three (3) days following deposit in a U.S. Postal Service receptacle, postage prepaid, as registered or certified mail, return receipt requested. ARTICLE THREE Owner agrees to indemnify and hold harmless the City, its officers, agents and employees from all suits, actions or claims, and from all liability and damages for any and all injuries or damages arising from or as a result of Owner's negligence in the performance or failure to perform its obligations under this agreement. ARTICLE FOUR Approval of this agreement by the City shall not create any financial obligation of the City, nor does such approval indicate approval of the appropriateness, adequacy or engineering of the Detention Facility. IN TESTIMONY WHEREOF, the parties have caused this instrument to be executed on the date shown above. Attest: City Clerk LIM City of Wichita Falls, Texas Darron Leiker City Manager Title: ATTEST: Secretary STATE OF TEXAS § COUNTY OF WICHITA § Owner This instrument was executed before me on this day of 20 , by Notary Public, State of TEXAS EXHIBIT "A" IT 1 N. 00'0000° E. XX.XX' /�JEIENIIUIV FA;ILI`UUINUHY SCALE 1 "=100' o TT R AS CONSTRUCTED / I MAX. STORAGE VOLUME VAAAAAA XXX.XX cult ' \P o MAX. RELEASE RATE 7770 BPm RIP -RAP / J IT 1 SCALE 1 "=100' o TT / I VAAAAAA o ' \P o RIP -RAP / J VVVVV VV \VY« VAAAAAVA VAAA� / a� 5' O - FLU \ \ \ \\ \ \�� // �55 WIDE CONCH FLUME @ 59 _ AAA VAAAAAA o / 1/ - /%0� A / �N CONC. CUTLET - T STRUCTURE / o o L -£ r��� /�� aAAAV���� VAEA IF POINT OF BEG INNINC S. 00'00'00" W. XX.XX' IT, \ �� BY PLACING MY PROFESSIONAL SEAL AND SIGNATURE ON THIS PAPER, I CERTIFY THAT THE DETENTION POND FOR THE ABOVE INDICATED PROJECT HAS BEEN CONSTRUCTED IN GFIVFHAI CUIVF OHMANCF WIIH SIUHMWAIFH UFSIGIV CHI IFRIP OF IHF CIIY OF WICHITA FALLS, AND THE OUTFLOW FROM THE POND IS EQUAL TO OR LESS THAN THE MAXIMUM OUTFLOW AS APPROVED BY THE CITY ENGINEER. ._. .. ..... I........DOC ...,, %.. o.. ......' .XXXXX e �hSTONAL FN, EXHIBIT "B" DETENTION FACILITY - FIELD NOTES A TRACT OF LAND MORE COMMONLY KNOWN AS THE DETENTION FACILITY IS A PORTION OF <<LOT, BLOCK, SUBDIVISION NAME >>, AN ADDITION TO THE CITY OF WICHITA FALL TEXAS, AS RECORDED IN VOLUME XX, PAGE X, WICHITA COUNTY PLAT RECORDS. THE BOUNDARY E DETENTION FACILITY IS MORE SPECIFICALLY DESCRIBED BY METES AND BOUNDS AS FOLD THENCE SOUTH 00'00'00" EAST ACRE INNING AND CONTAINING XX.XX