Drainage Design Manual, Hydrology - Flood Control District of ...

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
5.6.4 Estimation of Lag
The application of an S-graph requires the estimation of the parameter, basin lag. A general relationship
for basin lag as a function of watershed characteristics is given by Equation (5.11):
Lag =
C LL ca -----------
S p
where:
(5.11)
Lag = basin lag, in hours,
L = length of the longest watercourse, in miles,
L ca
S
C
= length along the watercourse to a point opposite the centroid, in miles,
= watercourse slope, in feet per mile,
= coefficient, and
m and p = exponents.
The Corps of Engineers often uses C = 24K n , where K n is the estimated mean Manning’s n for all
the channels within an area, and m = 0.38. The USBR (1987) has recommended that C = 26K n
and m = 0.33. Both sets of values in Equation (5.11) will often result in similar estimates for Lag.
Traditionally the exponent, p, on the slope is equal to 0.5.
It should be noted that K n is a measure of the hydraulic efficiency of the watershed and it is not
necessarily a constant for a given watershed for all rainfall depths and rainfall intensities. As
rainfall depth and/or rainfall intensity increases the efficiency of runoff increases and K n
decreases. Therefore, some adjustment in K n should be made for use with rainfalls of different
magnitudes (frequencies). Generally, K n is the smallest for extreme floods such as PMFs and
increases as the frequency of event increases.
Selection of K n
The selection of a representative K n value for a particular watershed is an inherently subjective
process. However, some guidelines are given for the selection of K n in Maricopa County in conjunction
with the four recommended S-graphs. Table 5.6 contains a summary of these guidelines.
Additional guidance may be gleaned from the calculated K n values for numerous
watersheds provided in Appendix D.1. Care should be taken to keep in mind the limitations discussed
above when selecting K n for any given watershed.
Several graphical relations are available for estimating basin lag. One such relation (U.S. Army
Corps of Engineers, 1982a) is shown in Appendix D.1. Several other relations that should be
5-28 August 15, 2013