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

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<strong>Drainage</strong> <strong>Design</strong> <strong>Manual</strong> for Maricopa County<br />

<strong>Hydrology</strong>: Application<br />

9.5 CHANNEL ROUTING<br />

9.5.1 Application <strong>of</strong> Normal-Depth Routing<br />

1. Routing reaches should have relatively constant characteristics along the entire reach<br />

(i.e. geometry, slope, roughness, etc). If not, then consider subdividing the reach.<br />

2. Too short <strong>of</strong> a routing reach may cause numeric instabilities and/or increase the peak<br />

discharge. The model output should be checked for unstable warning messages. If<br />

unstable warning messages are reported, then check the discharge range <strong>of</strong> instability<br />

in comparison to the peak discharge and plot the hydrograph for inspection.<br />

3. If several short routing reaches occur in succession and attenuation is anticipated,<br />

then the channel routing operation can be replaced by a hydrograph lag operation.<br />

4. Channel geometry must have sufficient capacity to convey the peak discharge.<br />

5. The number <strong>of</strong> computational subreaches (NSTPS), should correspond to the lag<br />

time computed by HEC-1 for the routing reach. Example:<br />

An inflow hydrograph with a time to peak <strong>of</strong> 4.5 hrs is routed down a 5000 ft natural<br />

channel. The estimated NSTPS is 2 and NMIN is set to 5 min. The resulting time to<br />

peak <strong>of</strong> the routing operation is 4.92 hours, a lag <strong>of</strong> 25 minutes. The actual NSTPS<br />

should be (lag/NMIN)=5. This is an interactive process that should be repeated until<br />

NSTPS*NMIN approximates the lag.<br />

9.5.2 Application <strong>of</strong> Kinematic Wave Routing<br />

1. Kinematic Wave routing is most appropriately used where peak attenuation and channel<br />

transmission losses are not expected to be significant. The usual applications are<br />

for defined urban channels and short, steep natural channels, with minimal overbank<br />

flow.<br />

2. When working with Kinematic Wave routing, channel capacity must be checked to<br />

assure proper conveyance <strong>of</strong> flow prior to the HEC-1 run. Otherwise, if the channel is<br />

undersized, the program will automatically extend channel boundaries to contain the<br />

flow.<br />

3. The guidance, comments, and warnings in the HEC-1 User’s <strong>Manual</strong> should be studied<br />

and carefully observed in applying the Kinematic Wave method.<br />

August 15, 2013 9-63

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