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

Drainage Design Manual for Maricopa County
Hydrology: Channel Routing
channel is uniformly distributed. This is a proper assumption for a case when overland flow runs
directly into a gutter. It is also a reasonable approximation of the flow as it passes through a
storm drain system from a catch basin and within the collector pipes.
7.3.2 Main Channel
The main channel element can be used to route inflow from an upstream subbasin or a combination
of inflows from collector channels along a subbasin. The flow is assumed to be uniformly
distributed, which appears to be a reasonable assumption when the flow is received from collector
channels at several locations.
7.3.3 Parameter Selection
The data requirements for Kinematic Wave channel routing include surface drainage area, channel
length and slope, channel shape and geometry, Manning’s n, and the inflow hydrograph. The
designer is referred to the HEC-1 manual for the proper selection of these parameters.
When working with the Kinematic Wave method, it is important to be familiar with the computational
procedures inherent in the model. In order to solve the governing equations, which theoretically
describe the Kinematic Wave method, proper selection of time step and reach length are
required. The designer will specify a channel reach length and a computational time step for the
inflow hydrograph. This time step could very well be different from the one selected by the computer
for computational purposes. Furthermore, the computer will use this information to select
distance intervals based on the given reach length.
The computational process could unrealistically attenuate the outflow peak. It appears that a longer
reach length results in more attenuation. To overcome this problem, more recent versions of
HEC-1 will calculate the outflow peak by applying both the time step selected by the designer as
well as the one selected by the program. If the resulting peaks are not reasonably close, the
designer can modify the selected time step or the reach length to improve the calculations. It
should be noted that the program will compare peak flow values for the main channel and not the
collector channels.
7.4 MUSKINGUM ROUTING
Flow routing through natural channels can be accomplished by applying the Muskingum Routing
technique. The main characteristic of natural channels with respect to routing is that the outflow
peak can be drastically attenuated through storage loss, a process which is simulated by Muskingum
routing.
August 15, 2013 7-3