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

Drainage Design Manual for Maricopa County
Hydrology: Unit Hydrograph Procedures
As an alternative to the above procedures, the DDMSW program will compute the rainfall
excess directly and perform the necessary iterations to compute the T c and R parameters.
5.7.2 S-Graph
1. From an appropriate map of the watershed, measure drainage area (A), L, L ca and S.
LL ca
2. Calculate the basin factor
0.5
.
S
3. Using the data in Appendix D.1 or the tables in the Design of Small Dams or the
USBR Flood Hydrology Manual, attempt to identify watersheds of the same physiographic
type and similar drainage area and basin factor. Make a list of the watersheds
with similar drainage areas and basin factors and tabulate the estimated value
of K n for those watersheds and the measured lag.
4. Estimate K n for the watershed by inspection of the tabulation from Step 3.
5. Calculate the coefficient (C) and select the value of the exponent (m) corresponding to
the source (Corps of Engineers or USBR) that was used to estimate K n . If the source
of K n is unknown, then use the Corps of Engineers version of Equation (5.11).
6. Using Equation (5.11), calculate the basin lag. Compare this value to the measured
lags of watersheds from Step 3.
7. Select an appropriate computational time interval (NMIN) and compute Q ult using
Equation (5.10).
8. Select an appropriate S-Graph and tabulate the percent Q ult , percent lag and the
accumulated time.
9. Transform the S-Graph into an X-duration (NMIN) unit hydrograph using linear interpolation
with Δt = NMIN.
10. Adjust the “tail” region of the S-Graph by lagging that portion by Δt and subtracting the
ordinates.
As an alternative to the above procedure, the DDMSW will transform the S-Graph to a
unit graph automatically.
August 15, 2013 5-31