FLOOD03450

• County: Jefferson
• Community: Lakewood
• Basin: South Platte
• Title: Storm Drainage Criteria City of Lakewood
• Date: 8/9/1982
• Prepared For: Lakewood
• Floodplain - Doc Type: Educational/Technical/Reference Information

Page 12

The rational formula is: Q=CIA where: Q= discharge in cfs
C= runoff coefficient
1= rainfall intensity
in in/hr
A= area in acres

1) Runoff Coefficients: The runoff coefficients used with
the Rational Method shall be based on land use as given in
Table III. This table includes the adjustment factor (Cf)
and no adjustment is to be made to the runoff coefficients
from Table III.

2) Time of Concentration: The time of concentration is
necessary for use of the Rainfall Intensity - Duration
Curve (Fig. 2) . A separate time of concentration is
necessary for the overall basin and each sub-basin or
design point. The time of concentration is also known as
the "inlet time" and represents the time for a unit of
water to travel from the most remote portion of the basin
to the design point. The time of concentration (T ) is
composed of the overland flow time (t ) and channgl or
conduit flow time (t). The time of con~entration formula
shown below, with at 5 year coefficient factor, shall be
used to obtain the rainfall intensity for all frequencies
of storms. The minimum time of concentration-shall be as-
sumed to be five minutes.

T = t + t
cot
Whe re T
c

= time of concentration, minutes
(five minutes minimum)
= overland flow time
channel or conduit flow time

t
o
t =
t

From USDCM, the overland flow time (t ) may be taken as:
o
\
to = 1.8 (l.l-CS)L

(8)1/3

Where: t = time of overland flow, minutes
LO = distance of overland flow, ft (not to
exceed 400')
S = slope of basin, in percent
Cs = Runoff coefficient for five year storm

or, for developed watersheds only,

t
o

= L .,. 10 (min)
180

whichever is lesser.