HomeMy WebLinkAboutOrd 64 Estahblishing a Forumla for determining Flood DischargesORDINANCE NO. 64
AN ORDINANCE ESTABLISHING A FORMULA FOR DETERMIN-
ING FLOOD DISCHARGES IN THE TOWN OF WdESTLAKE, ADOPTING
A RUN --OFF COEFFICIENT; PROVIDING FREQUENCY CURVES
FOR DETERMINING RAINFALL INTENSITY: DEFINING "SPREAD
OF WATER"; PROVIDING A STANDARD FOR DESIGN OF STORM
SEWERS; PROVIDING A STANDARD FOR DESIGN OF OPEN DRAIN-
AGE CHANNELS;, AND PROVIDING A STANDARD FOR DESIGN OF
ROADWAY CULVERTS
BE IT ORDAINED BY THE CITY COUNCIL OF THE TOWN OF WESTLAKE, TEXAS, that:
I.
The terms and provisions of Exhibit "A", attached hereto, incorporated herein and
made a part hereof, are hereby approved and adopted as the Storm Water Drainage
Design Standards of the Town of Westlake, Texas.
II.
The terms of Exhibit "A" shall be binding upon all developers, land planners and
all other persons, firms and corporations who shall hereafter develop housing
projects, shopping centers, business establishments or industrial areas.
III.
All construction designed to cope with Storm Water Drainage in the Town of Westlake
shall conform to the terms of said Exhibit "A".
this da
PASSED AND APPROVED y of f c, L<.c. , 197 A.
ATTEST:.
City Secretary '
APPROVED:
Mayor
I
TOWN OF WESTLAKE, TEXAS
EXHIBIT "A"
STORM DRAINAGE CRITERIA
A. General - The design of storm drainage improvements in the Town of Westlake shall
be based on flood discharges determined from the Rational Formula. The formula
for calculating storm flows in this manner is:
Q = CIA, where
Q is the storm flow at a given point
C is a coefficient equal to the ratio
that the peak rate of runoff bears
to the average rate of rainfall
I is the average intensity of rainfall
for a period equal to the time of
flow from the farthest point of the
drainage area to the first inlet
point on the storm sewer
A is the area tributary to the design
point
B. Runoff Coefficient - The runoff coefficient which considers the slope of the
terrain, the character of the land use, the length of overlsnd flow, and the
imperviousness of the drainage area shall be determined from the ultimate land
development as shown on the Future Land Use Map, Town of Westlake Master Plan.. The
runoff coefficients for the appropriate land uses shall be as follows:
Business Areas 0.90
Industrial Areas 0.70
Residential Areas 0.40
Park Areas 0.30
C. Rainfall Intensity - Frequency - The rainfall intensity - frequency curves
iwhich are shown on Plate 1 are plotted from data compiled by the U. S. Depart-
ment of Commerce We::ther Bureau in Technical Paper No. 40 (modified). The
intensity, I, in the formula Q = CIA is determined f)-om these curves by arriv-
ing at a time of concentration and adapting a storm frequency upon which to
base the drainage improvements.
1. Time of Concentration - The time of concentration, which is the time of
-low from the farthest point of the drainage area to the first inlet in
the system, consists of the time required to flow overland plus the time
required to flow in the gutter to the inlet. A minimum time of concen-
tration of ten (10) minutes shall be used for all areas except business
areas and a minimum time of concentration of five (5) minutes shall be
used in business areas. A nomograph, shown on Plate 2, is attached for
estimating the time of concentration.
- 2 -
Ordinance No. 64
2. Storm Frequency
improvements in
Type of
Facility
- Recommended design
the Town of Westlake
DESIGN STORM
Description
of Area
to be
Drained
storm frequencies for
are shown in a table
FREQUENCY
Maximum
Time of
Concentration
(Minutes)
the storm drainage
as follows: ...
Recommended
Design
Frequency
-(Years)
*Storm
Residential,
30
5
Sewers
Commercial and
Manuf8cturing
*Culverts,
Any type of area
30
5
Bridges,
less than 100
Channels
acres
and Creeks
"Culverts,
Any type of area
45
10
Bridges,
greater than 100
Channels
acres but less than
and Creeks
1,000 acres
***Culverts,
Any type of area
60
25
Bridges,
greater than
Channels
1,000 acres
and Creeks
Vfi en the maximum time of concentration or area to be drained is exceeded, the
design shall be based on a ten (10) year frequency.
**When the maximum time of concentration or area to be drained is exceeded, the
design shall be based on a twenty-five (25) year frequency.
***Whenever, in a storm sewer system, an inlet is located at a low point so that
flow in excess of the storm sewer capacity would be directed onto private property,
the design frequency shall be increased beyond five (5) years. If the inlet loca-
tion is such that overflow could cause damage or serious inconvenience, it may be
desirable to increase the design frequency to as much as twenty-five (25) years.
D. Area - The area used in determining flows by the "Rational Formula" shall be
calculated by subdividing a map into drainage areas within the basin contri-
buting storm water runoff to the system.
E. Spread of Water - During the design storm, the Quantity of storm water that is
allowed to collect in the streets before being intercepted by a storm drainage
system is referred to as the 'spread of water". In determining the limitations
for carrying storm water in the street, the ultimate development of the street
shall be considered. The use of the street for carrying storm water shall be
limited to the following:
- 3 -
ORDINANCE NO. 64
SLREAD OF WATER
Expressways - Eight feet (8') from face of curb.
Major thoroughfares (divided) - One traffic lane
on each side to remain clear.
Thoroughfares (not divided) - Two traffic lanes
to remain clear.
Secondary streets -- One traffic lane to remain
clear.
Residential streets - Six inch (6") depth of
flow at curb or no lanes completely clear.
Curves are provided on Plate 3 for determining the spread of water for certain
gutter slopes, gutter discharge and pavement crown.
F. Storm Sewer Design - Storm water in excess of that allowed to collect in the
streets shall be intercepted in inlets and carried a+,ay in a storm sewer system.
Storm sewer capacity shall be calculated by Mannings Formula
Q = 1.486 AR 2/3 S 1/2 were
n
Q is
the
discharge in cubic feet per second
A is
the
cross-sectional area of flow in square feet
R is
the
hydraulic radius in feet
S is
the
slope of the hydraulic gradient in feet
per
foot
n is
the
coefficient of roughness (n= .013 for new
concrete pipe)
Pipe Flow Charts for determining required pipe sizes based on mannings formula and
computation sheets 1, 2 and 3 are provided for inlet and storm sewer design cal-
culations.
In the design of the storm sewer system, the elevation of the hydraulic.gradient
of the storm sewer shall be a minimum of 1.5 feet below the elevation of the adja-
cent street ,gutter.
Storm seiner pipe sizes shall be so selected that the average velocity in the pipe
will not exceed 15 feet per second.
�Open Channel Design - Stor.. water runoff in excess of that allowed to collect
in the streets in developed areas and runoff in undeveloped area may be carried
in open channels (not in the street rights of way). Open channel capacity shall
be calculated by Mannings Formula and roughness coefficients shall be as follows:
Type_ of Lining
Earth (Bermuda Grass)
Earth (Non Vegetated)
Concrete Lines
Rou hness Coefficient"n"
0.035
0.030
0.015
MW
Maximum Permissab le
Mean Velocity
8ft. per sec
5 ft. per sec
15 ft. per sec
ZiI"Cinance i,qo, ce,
Open channels may be constructed with various cross-sectional shapes but are usually
V-shaped or trapezoidal. The trapezoidal ditch which has a more natural shape and
has greater capacity shall have side slopes no steeper than 2:1 in earth and 1:1 to
1.5:1 when lined with concrete.
H. Culvert Design - At locations of creels cro8sings with proposed roadway improve-
ments, it is sometimes necessary to receive and transport storm water under the
roadway in culverts. The quantity of flow shall be determined by the Rational
Formula and the capacity of the culvert shall be calculated by Manning's Foxmula.
Design of culverts shall include the determination of upstream bacicivater condi-
tions as well as downstream velocities and flooding conditions. Consideration
shall be given to the discharge velocity from culverts and the following limita-
tions are allowed:
CULVERT DISCHARGE - VELOCITY LIMITATIONS
Culvert Discharging
On To
Earth
Sod earth
Paved or riprap apron
Shale
Rock
Maximum Allowable
Velocity (f.p,s.)
6
3
15
10
15
Generally, all culverts shall be designed with a free outfall and the following
head losses shall govern the design of the culvert:
1. Frictional Head Loss
hf = sfL
when e
sf = Slope of frictional gradient in
feet_ per foot
L = Length of culvert in feet
2. Head Loss clue to change in Velocity
2
by = v2 - v12 where
2g 2g
v2 = Velocity in culvert
vi - Velocity in channel above culvert
g = Acceleration clue to gravity.
3. Head Luss at Upstream Entrance to Culvert Due to Entrance and
Change in Section
lie = V22 where vl is equal to or less than
2g six ft. (6') per sec.
he = v2 2 - 0.5 vi 2 where vl is greater than six ft.
2g 2g (6') per sec.
5 -
TOWN OF WESTLARE
CURB AND CUTTER REQUIREMENTS
I N D E X
ORDINANCE NUMBER 65
PACE
SECTION l...DEQ'INITIONS.......................................... I
SECTION 2...COUNCIL TO DETERMINE NATURE & EXTENT OF IMPROVEMENT.. 1
SECTION 3...PERMI.T.............................................. 1
SECTION 4...PERMIT FEES .......................................... 1 & 2
SECTION 5 -BOND REQUIRED ........................................ 2
SECTION 6 ... HOMEOWNERS EXCLUDED-- ............................ 2 & 3
SECTION 7...SPECIFICATTONS....................................... 3
SECTION 8...SUPERVISION & APPROVAL OF WORK ....................... 3
SECTION 9...WARNING DEVICES REQUIRED FOR PROTECTION OF TRAFFIC... 3
SECTION 10..EMERGENCY CLAUSE ..................................... 3
EXHIBIT A. - SPECIFICATIONS FOR CONSTRUCTION OR REPAIR OF CURB AND CUTTER
I...........DEFINITION........................................... 4
II.......... PLANS ................................................ 4
III ......... MATERIALS ............................................ 4
IV .......... STANDARD DETAILS ..................................... 4
V ........... EXCAVATION ........................................... 5
.FORMS . 5
VII ......... REINFORCEMENT ........................................ 5
VIII........ CONCRETING & INSPECTION ..............................6
IX .......... CONSTRUCTION ......................................... 6
X........... FINISH ............................................... 7
XI.......... CURING ............................................... 7 & 8
XII......... TESTING ..............................................8
XIII ........ BACKFILL, .................. .............................8
XIV......... FINAL ACCEPTANCE ..................................... 8