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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A tyoical input data sheet for the reservoir routing program
is shown in Figure 111-2. The objective of this input sheet
is to provide a description of the reservoir and the outlet
control section as well as the methods used in determining
the stage-volume-discharge curves which are characteristic
of the reservoir and spillway. For the most part, volumes
for the reservoirs were determined from the topographic maps
which were available. In nearly every case the normal water
surface of the reservoir was assumed to be at the spillway
elevation. Spillway overflow and berm overtopping were
computed utilizing the basic broad crested weir formula.

No input data sheets were made for the lagging and adding
program. Once the input data was established for all phases
of the HYDRO model, the information was transferred to
computer coding sheets from which the input cards were
punched and run. A sample computer output from the HYDRO
model is shown in Figure 111-3.

DESIGN RAINFALL

The design rainfall for the study area was taken from isohyetal
maps in the Rainfall Section of the Urban Storm Drainaqe Criteria
Manual. These maps provide the rainfall depths for the 2, 10,
and 100 year frequencies and 1, 6 and 24 hour storm durations.
Since the gulches extended approximately nine miles from the
South Platte River, the storm rainfa~l typical to each sub-
basin was used, rather than assuming the same design
storm over the total basin.

From the storm rainfall input, the effective precipitation was
computed following the procedure presented in the Runoff
Section of the Manual. The effective precipitation is a
function of the degree of impervious and pervious area, sur-
face detention and depression storage, infiltration, vegeta-
tion interception and an impervious area loss percentage.

The determination of these abstraction coefficients was based
on an analysis of the recorded rainfall and runoff hydrograph
on Harvard Gulch at Logan Street for the storm of June 8, 1969.
In order to reproduce the storm hydrograph, the rainfall ab-
stractions were manipulated to give the required excess pre-
cipitation. A review of the data indicated an infiltration
rate for this event which varied from 1-1/2 inches per hour
in the earlier part of the storm to less than 1/2 inch per hour
toward the end of the storm. The pervious detention-depression
storage for that storm was about 0.80 inches while the impervious
storage was 0.30 inches with a 15% loss factor. These values
were comparatively high yet relatively realistic. Comparing

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