O-232-12-77CITY OF ALLEN, TEXAS, ORDINANCE N0. 232
AN ORDINANCE AMENDING ORDINANCE NO. 125 (THE SUB-
DIVISION ORDINANCE TO THE CITY OF ALLEN, TEXAS) BY THE
ADDITION THERETO OF CLAUSES REGARDING STORM DRAINAGE
DESIGN; PROVIDING FOR READOPTION AND PROVIDING FOR AN
EFFECTIVE DATE.
BE IT ORDAINED BY THE CITY COUNCIL OF THE CITY OF ALLEN,
TEXAS:
SECTION I: That from and after the effective date of
this Ordinance, Ordinance No. 125 (the subdivision ordinance
to the City of Allen) is hereby amended in the following
manner and by the addition of the following provisions,
to -wit:
a) STORM DRAINAGE DESIGN STANDARDS FOR THE CITY OF ALLEN,
TEXAS:
All storm drainage studies and storm structure design to
be submitted to the City of Allen shall be prepared by a
Registered Professional Engineer. The amount of storm flow
is to be calculated using the rational formula which is
Q = C*I%A, where
Q
= storm
water flow in
cubic
feet per
second;
C = coefficient
of
runoff;
I = intensity
of the
rainfall
during
the travel time in inches per hour; and, A = area of the
watershed in acres from which the runoff will occur.
b) COEFFICIENT OF RUNOFF:
The value of "C", the coefficient of runoff is to be taken
from the following table according to the type of use of the
watershed. The coefficient of runoff is to be according to
the future planned land uses as shown on the City's "Future
Land Use Plan" map. In the event the watershed includes
areas not shown on the future land use maps, generally such
areas are to be considered as residential land. If other
development is anticipated within the reasonable future in
H
land areas not included in the "Future Land Use Plan", the
designer is to make_a rough estimate of such uses and adjust
his calculated storm flow accordingly.
RUNOFF COEFFICIENTS
TYPE OF AREA RUNOFF COEFFICIENT INLET TIME IN
MINUTES %;
Business Areas 0.80 10
Industrial Areas 0.70 10
Residential Areas 0.50 15
Park Areas 0.30 20
This inlet time may be used in lieu of calculating inlet
time.
c) STORM FREQUENCY:
The maximum allowable storm flow for streets shall be
such that at least a 10 foot wide traveling lane is left in
the center of the street with a 5 -year frequency storm and so
that the water flowing in any street will not be higher than
the top of the lowest curb for a 25 -year frequency storm.
Storm sewers and inlets into catch basins for storm sewers
must be designed accordingly. In addition, culverts across
residential streets shall also be designed for 25 -year frequency
storm water flow.T Open channels, culverts across thoroughfares
and bridges must be designed toca.rry 50 -year frequency storms.
d) TRAVEL TIME:
Travel time is defined as the time required for runoff
from the most remote portion of the drainage -area to reach
the point at which the flow is to be calculated.
e) RAINFALL INTENSITY:
Rainfall intensity for different frequencies and time
of duration shall be calculated from data presented in
r
"Technical Paper No. 40" published by the U. S. Weather
Bureau.
f) STORM FLOW STRUCTURES:
Manning's formula shall be used for calculating the
flow in All drainage structures.
Bridges and channels under all bridges shall be so
designed that the maximum flood -water elevation from a
50 -year frequency storm is at least 12 inches below the
lowest part of the bridge deck.
SECTION II: Ordinance No. i�5
(the subdivision
ordinance of the City of Allen) as so amended is hereby
readopted by the City Council of the City of Allen, Texas.
DULY PASSED AND APPROVED by the City Council of the
City of Allen, Texas, on this 15th day of DECEMBER ,
1977 .
4vnIgg`eLr. yor of Allen
CORRECTLY RECORDED:
(71ty Secretary