FLOOD00925

• County: Statewide
• Community: State of Colorado
• Title: Colorado Flood Hydrology Manual - Section 22 Program
• Date: 9/1/1993
• Prepared For: CWCB
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Educational/Technical/Reference Information

assumption of uniform-flow conditions, the energy slope is considered equal to the
average channel bed slope; therefore, this approach should not be applied in
backwater areas.

Observed inflow and outflow hydrographs can be used to compute
channel storage by an inverse process of flood routing. When both inflow and outflow
are known, the change in storage can be computed, and from that a storage vs.
outflow function can be developed. Tributary inflow, if any, must also be accounted
for in this calculation. The total storage is computed from some base level storage at
the beginning or end of the routing sequence.

Inflow and outflow hydrographs can also be used to compute routing
criteria through a process of iteration in which an initial set of routing criteria is
assumed, the inflow hydrograph is routed, and the results are evaluated. The process
is repeated if necessary until a suitable fit of the routed and observed hydrograph is
obtained.

2.2 DETERMINING THE NUMBER OF ROUTING STEPS. In reservoir
routing, the Modified Puis method is applied with one routing step. This is under the
assumption that the travel time through the reservoir is smaller than the computation
interval At. In channel routing, the travel time through the river reach is often greater
than the computation interval. When this occurs the channel must be broken down
into smaller routing steps in order to simulate the flood-wave movement and changes
in hydrograph shape. The number of steps (or reach lengths) affects the attenuation
of the hydrograph and should be obtained by calibration. The maximum amount of
attenuation will occur when the channel routing computation is done in one step. As
the number of routing steps increases, the amount of attenuation decreases. An initial
estimate of the number of routing steps (NSTPS) can be obtained by dividing the total
travel time (K) for the reach by the computation interv;'ll At.

K=!.

(3)

Vw

NSTIPS = K
At

(4)

Where:

K

= total travel time through the reach
L = channel reach length
V w = velocity of the flood wave
NSTPS = number of routing steps

The time interval At is usually determined by ensuring that
there is a sufficient number of points on the rising side of the inflow hydrograph. A
general rule of thumb is that the computation interval should be less then 1/5 of the

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