Res 172-91 11/5/1991RESOLUTION NO. /791 -97
RESOLUTION TO APPROVE THE CRITERIA FOR STORM
WATER FACILITIES IN THE CITY OF WICHITA FALLS
WHEREAS, the City of Wichita Falls has previously adopted a
storm water retention ordinance:
WHEREAS, Section III of the storm water retention ordinance
requires the adoption by resolution of the criteria for thePY
design of storm water facilities; and,
WHEREAS, the Council has examined the proposed criteria for
storm water facilities and desires to adopt same.
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 5th day ovember, 199
M A Y O R
ATTEST:
i
CITY CLERK '
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
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 capacity
Detention/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 paved
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.
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 storm
event.
5.4 Design calculations shall be submitted of a sufficient
nature to define any required detention volume.
5.5 All required offsite facilities shall include an easement
dedicated to the City with the owner responsible for
maintenance.
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.
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RATIONAL METHOD RUNOFF COEFFICIENTS
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
ArrAGHME NT
I Aill i
Vic itaFalls
TEXAS
DEPARTMENT OF PUBLIC FORKS
may 10, 1988 SHIM'1 OF 3
EXAMPLE OF DETENTION
AND
RELEASE RATE REQUIREMENTS ik s
OF 1CITYOFWICHITAFALLSORDINANCE
NO. 41-88 "RETENTION ORDINANCE"
i
ROPOSED
EXAMPLE 1 T 1
o Ac.a
TOTAL DRAINAGE AREA: 65 Ac. IE\\1\1• P• •'
DRAINAGE AREA TO BE DEVELOPED:
30 Ac. Moderate to High Density Single Family Subdivision.
tc Undeveloped) = 35 min.
C Undeveloped) = 0.35 (per ordinance)
I100 Undeveloped) = 6.0 in/Hr
tc Developed = 20 min.
j
C Developed = 0.45
1100 Developed = 7.7 in/Hr
I
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
Q100= 104 CFS
100
DETERMINE DETENTION VOLUME : 80
Cu.Ft. 03 60
20 Min x 60 Sec./Min x (104 15
Cu.Ft. ) 13 40Sec. Sec.
106.800 ft
3
20 15 cFS
0
o 10 20 30 40 50
Time(Min)
yArrAcHME-Nr