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

Drainage Design Manual for Maricopa County
Hydrology: Application
iv. Use the following ratios (Cudworth, 1989) to estimate V m , the velocity
of the floodwave:
Channel Geometry
V m
V
Wide rectangular 1.67
Wide parabolic 1.44
Triangular 1.33
The value of K is then estimated by dividing the reach length by V m .
c. X: For wide, shallow channels with low to moderate slopes and significant
overbank flow during the design flood being modeled, choose X = 0.15 to
0.25. For steep to very steep, narrow, deep channels with little overbank flow,
choose X = 0.25 to 0.40.
9.5.4 Application of Muskingum-Cunge Routing
1. For constructed channels and some natural channels, this routing option can be used
by providing all input on the RD record only. This requires selection of a predetermined
channel shape (see the HEC-1 User’s Manual). Complex channel geometry
and/or variable channel roughness (channel and overbank) can be modeled with the
additional use of RC, RX and RY records. An eight-point cross section is input on the
RX and RY records to describe the representative channel geometry.
2. Execution of the HEC-1 program may terminate with a math error message if the
inflow to the routing reach is zero (no runoff generated from the upstream watershed).
This may occur in situations that have either very low rainfall depth (intensities) or
exceptionally high rainfall losses, or zero diversion (most often).
9.6 INDIRECT METHODS
9.6.1 Procedures
The following instructions should be followed for verifying peak discharges that are derived by
analytic methods (Rational Method or rainfall-runoff modeling).
A. Verification with Unit Peak Discharge Curves:
1. For a given watershed of drainage area (A), in square miles, divide the 100-year primary
peak discharge estimate by A.
August 15, 2013 9-65