FLOOD01644

• Designation Number: 46
• County: Denver
• Community: Denver
• Title: Flood Hazard Boundary Map - Supplementary Report, Sanderson Gulch & Weir Gulch, Denver, Colorado
• Date: 3/1/1975
• Designation Date: 7/1/1975
• Floodplain - Doc Type: Floodplain Report/Masterplan

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All computer work was completed on the company IBM 1130 Computer.
The computer has an 8K storage capacity which can be increased
to 12K with the use of disc storage units. This added capacity
made operation of the total model quite simple. The program
offers an optional amount of computer output. All hydrographs
were available in storage at all times and could be recalled for
any operation.

All storm hydrographs were computed on a two minute time increment.
This had a number of advantages. First of all, it provided for
a more accurate determination of the peak flow rate and time to
peak, as well as the computation of the total hydro graph volume.
Also, it provided for greater accuracy in the routing program
where the determination of required storage is quite important.
Thirdly, the two minute increment provided the ability to lag
hydrographs to the nearest two minutes of flow time. In urban
drainage, where normal storm hydrographs characteristically
have steep narrow peaks, the flow time increment becomes very
important. For most conditions, the flow time varied from 0
to 16 minutes.

Lagging and adding the total hydrograph in the downstream direc-
tion provides a fairly realistic representation of the actual
flow condition. Ideally, the hydro graph should be attenuated
by recognizing channel storage. In the case of Sanderson and
Weir Gulches, it was felt that the existing reservoirs provide
storage detention to a much larger degree than channel storage
and the effect of channel storage wou~d be negligible in
comparison. The lagging effect, however, keeps the time frame
of the hydrograph in the proper perspective.

Overall, utilization of the computer model as an analytical
tool permitted the calculation of nearly 1,300 hydrographs,
as required by the contract, in a comparatively short period
of time. Also, the computer model offered a much greater
degree of flexibility.

HYDRO DESCRIPTION

As mentioned earlier, the HYDRO model directly follows the
Colorado Urban Hydrograph (CUHP) Procedure as outlined in Part
4 of the Runoff Section of the Urban Storm Drainage Criteria
Manual. A quick review of the CUHP indicates that this pro-
cedure is based on the derivation of a synthetic unit hydro-
graph and statistically determined rainfall information.
The unit hydrograph approach to storm runoff determination,
when historical recorded information is not available, was
originally developed by Sherman in 1932 and expanded upon by
Snyder in 1938. The unit hydrograph reflects the integrated
effects of basin characteristics such as area, shape, slope

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