Res 158-92 10/6/1992RESOLUTION NO. /5y--u
RESOLUTION MODIFYING THE CRITERIA FOR STORM
WATER FACILITIES IN THE CITY OF WICHITA FALLS
WHEREAS, the City of Wichita Falls has previously adopted
the criteria for storm water facilities; and,
WHEREAS, it is the desire of the City Council to relieve the
owner of maintenance responsibilities for sole purpose dedicated
storm water retention basins; and,
WHEREAS, the Council has examined the proposed criteria for
storm water facilities and desires to adopt the enclosed
modifications.
NOW, THEREFORE, BE IT RESOLVED BY THE CITY COUNCIL OF THE
CITY OF WICHITA FALLS, TEXAS, THAT:
The enclosed criteria for storm water facilities are hereby
adopted.
PASSED AND APPROVED this the 6th ctober,
M A Y O R
ATTEST:
CITY CLER
CITY COUNCIL AGENDA
October 6, 1992
ITEM/SUBJECT: Resolution approving modified criteria for storm
water facilities in the City of Wichita Falls
INITIATING DEPT: Public Works
Commentary:During the regular City Council meeting of
September 15 , 1992 , the City Council discussed the report
received from the Storm Water Detention Task Force. As part of
these discussions, the City Council requested an appropriate
action to remove the requirement for owner maintenance of
dedicated storm water facilities.
This resolution will modify the "Criteria for Storm Water
Facilities" as required in the storm water detention ordinance to
relieve the owner of this responsibility.
APPROVED) DIRECTOR)
CITY MANAGER'S REMARKS: This resolution would change the present
practice of requiring developers to place storm water detention facilities
where required and to maintain those facilities. Under this p •:- ed lutir-n
the City would be responsible for maintaining detention requ' erne i in _sidential
areas. The City staff would be'able to work with the develo:•-rs o •- tify the
most practical methods of retention facility for each locat • .'
CI Y R. )
ASSOCIATED INFORMATION: Resolution
BUDGET CERTIFICATION: (Sources/Account Nos. )
T. OFFICER)
LEGAL CERTIFICATION: 4
cI A4Rk
CODIFICATION: Yes No\k
Page 1 of 9 Pages
Agenda Item No. 7.b.
Revision A
CRITERIA FOR STORM WATER FACILITIES
I. PURPOSE
1. 1 It is the purpose of this document to state the criteria to
be used in designing storm water facilities which are to be
constructed in the City of Wichita Falls, Texas. It is the
intent of this document to state acceptable criteria while
encouraging innovation and creativity on the part of the
technically qualified designer. Designers are encouraged to
coordinate their projects with the Engineering Division of
the City of Wichita Falls early in the design process.
1. 2 This document accepts the premise that preventive measures
addressing storm water are ultimately less expensive to the
tax payers than implementation of corrective measures. It
is recognized that the cost of storm water facilities
ultimately is borne, and is inevitably passed on, to the • •
user of the product via a higher cost of goods, housing, or
services.
1. 3 These criteria are intended to supplement the storm water
detention/retention ordinance (No. 41-88) .
II . DESIGN FLOWS
2 . 1 It is recognized.-that urban hydrology is both an art and a
science. It is the intent of this document to use only a
level of precision suitable for the situation under
consideration. The level of precision shall be based on
sound engineering judgement by technically qualified
personnel .
2 . 2 Storm water flows from watersheds of less than 150 acres
will be developed using the "rational formula" Q = CIA
where:
Q = Peak rate of runoff in cubic feet per second
C = Coefficient of runoff (see Attachment 2)
I = Rain intensity in inches per hour (see Attachment 1)
A = Basin area in acres
Page 1 of 9 pages
Agenda Item No. 7.b.
Revision r,
2 . 3 For storm water flows from watersheds in excess of 150
acres, the engineering staff for the City of Wichita Falls
should be consulted for guidance in methodology to be used.
2 . 4 Determination of required storage capacityDetention/Retention Ordinance 41-88) may be determined by
either:
a) Modified rational hydrographic method as set forth in
Urban Storm Water Management Special Report No. 49) .
b) Wichita Falls rational method as set forth in
Attachment 3 . It is recognized that the method as set
forth in Attachment 3 is generally easier to apply and
will normally result in lesser storage volume
requirements than (a) above.
2 . 5 All storm channels and conduits will be designed to give a
mean velocity, when flowing full , of not less than 2 . 0 feet
per second.
2 . 6 Open storm channels are acceptable and may be either pa."ed • •
or unpaved. It should be recognized that an unpaved channel
has a much larger easement requirement as compared to the
paved channel . Unpaved channels shall be grass lined for
erosion control . An additional consideration which should
be born in mind by the designer is the potential displeasing
aesthetics involved in an unpaved channel during periods of
low flow.
2 . 7 The storm channel or conduit, shall be designed for a 10-
year reoccurring storm event without the channel or conduit
becoming surcharged. .
2 . 8 The storm facility shall be designed to prevent storm water
from entering any structure during a 100-year recurring
storm event.
III . INLET AND CONDUIT DESIGN
3 . 1 Inlets on streets and conduits shall be designed the meet
the following conditions:
a) The water depth in the public street shall not exceed
top of curb during a 10-year storm event.
b) The minimum storm sewer pipe diameter shall be 12
inches.
c) New storm water conduits shall have required clearances
with potable water and sanitary sewer lines as set
forth in the applicable state regulations.
Page j of 9 pages
Agenda Item No. 7.b.
Revision A
IV. CONSTRUCTION
4 . 1 Where the storm conduit crosses an existing or proposed
sidewalk, driveway or street, the trench shall be backfilled
with a suitable material compacted to 90% of modified
proctor density, or portland cement stabilized sand
containing no less than 1 . 5 sacks per ton.
4 . 2 Manholes or inlets with rings and covers shall be installed
on enclosed storm sewer systems as follows:
a) At the end of each line.
b) At all changes in grade, size, or vertical and
horizontal alignment.
c) At distances no greater than 600 feet for sewers of 30
inches or less in diameter.
V. SUBMITTALS
5. 1 Design calculations shall be submitted with all storm water • •facility designs including runoff coefficients, time of
concentration, area, rainfall intensities and flow rates .
5. 2 The drainage calculation submitted shall include a contour
map delineating the appropriate drainage area.
5 . 3 The hydraulic grade line shall be shown for a 10 year stor.:n
event.
5 . 4 Design calculations shall be submitted of a sufficient
nature to define any .required detention volume.
5 . 5 All-33^ed-- -15-ai- e--fae s-sha44--i rE.qtrea-e--ett-easement
dedicated--te--the--eity--Wth--the--owner--responsible--for
mantenanee. All required detention facilities shallincludeaneasementdedicatedtotheCity. Such easement
shall provide for:
a) owner maintenance for detention facilities serving
all other than single family residential land use.
b) owner maintenance for multi-purpose detention
facilities serving single family residential land
use.
c) City maintenance for sole purpose detention
facilities serving single family residential land
use.Such detention facilities shall include
7 provisions for adequate access by City forces and
shall be designed to minimize maintenance
requirements.
Page of 9 pages
Agenda Item No. 7.b.
r •
Revision A
5 . 6 Commercial development requiring detention shall include a
site grading and drainage plan showing all storm water
facilities.
5 . 7 A master drainage plan shall be submitted for a parcel under
active consideration for development. Construction plans
will only be required at the time of final platting.
Page_of 9 pubAgendaItemNo. 7.5
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Agenda Item No. 7.b.
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Nor
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RA"'TONAL METHOD RUNOFF COEFF1CT.NIS
Description of area Runoff coefficients
Businesses:
Downtown areas 0.80 to 0.95
Shopping malls 0.85 to 0.95
Neighborhood areas 0.50 to 0.70
Residential:
Single-family areas 0.35 to 0.50
Multi-units, detached 0.40 to 0.60
Multi-units, attached 0.60 to 0.75
Residential
half acre lots or more) 0.35 to 0.45
Industrial 0.60 to 0.90
Parks, Cemeteries 0.20 to 0.35
Playgrounds 0. 20 to 0.35
Unimproved areas 0.20 to 0.30
Page 7 of 9 ges
Agenda Item No. 7.b.
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TEXAS
Ye 10, 1999 DEPARTMENT .OF PUBLIC NORX5
filT ! Of1
EXAMPLE OF DETENTIONI
AND
RELEASE RATE REQUIREMENTS,
I ;
I OF II
CITY OF WICHITA FALLS ORD t. ;INANCE
NU. 41-88 'RETENTION ORDINANCE' I
ROPOSE``
EXAMPLE 1 b 6-
TOTAL DRAINAGE AREA: 65 Ac. n.
DRAINAGE AREA TO BE DEVELOPED:
30 Ac. Moderate to High Density Single Family Subdivision.
tc Undeveloped = 35 min.
C Undeveloped = 0.35 (per ordinance)
ltoo Undeveloped = 6.0 in/Hr
tc Developed = 20 min.
C Developed = 0.45
I100 Developed = 7.7 In/Hr
RELEASE RATE :
Limit I Q Developed = 0.45 x 7.7 in/Hr x 30 Ac. = 103.95 cfs
Q Undeveloped = 0.35 x 6.0 in/Hr x 30 Ac. = 63.00 cfs
40.95 cfs
Limit II Q Allowable = 30 Ac. x 0.5 cfs/Ac. 15.0 cfs
Use lessor of two limiting factors : 15.0 cfs i
0100- 104CFs
100
DETERMINE DETENTION VOLUME : 80
Cu.F t 60
20 Min x 60 Sec./Min x (104 Cu Ft. :2-15
Sec. Sec. )
1 106.800 ft
3
20 ANA\ —15 cfs
0
Page 8 of 9 pages o 10 20 Btg4Qcof __-pages
Agenda Item No. Timef; t da Item No