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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and ground cover, and defines the shape of the runoff hydrograph
for one inch of direct runoff over the area. For storms where
the effective precipitation depth is different from the unit
depth, the actual storm hydrograph is determined by multiplying
the ordinates of the unit hydrograph by the effective precipita-
tion depth and adding to obtain the final storm runoff hydrograph.

To facilitate accuracy and standardization in developing input
for the effective precipitation, unit hydrograph and storm hydro-
graph operations of the model, a HYDRO data sheet was developed,
as shown in Figure III-I. The input sheet provides the basin
name and number, which is used primarily for computer identifi-
cation, and also the design point of interest. The section,
township and range of the particular design point was also in-
cluded to assist in the determination of the storm rainfall
depths. Basin parameters were determined from the mapping
available. These parameters included the area, channel length,
channel length closest to the basin centroid and the average
channel slope through the basin. In addition, flow times along
the channel through the basin were computed using a predetermined
design velocity. The degree and type of development within the
basin was noted. When the type of development was quite irregular,
the areas of different development were noted to assist in deter-
mination of various basin characteristics. Basin or hydrological
characteristics included the percentage of pervious and impervious
areas, a constant infiltration rate in inches per hour, pervious
and impervious retention depths, the impervious loss percentage,
and hydrograph time and peak coefficients reflecting the type
and degree of development. At the bottom of the input sheet
the storm rainfall depths were recorded for the 2, 10 and 100
year frequencies and the 1 and 6 hour durations. These condi-
tions encompassed the range of frequencies and duration of
interest in the study. Additional comments concerning the basin
were noted at the bottom of the sheet.

The storm hydrograph coefficients, Cp and Ct' which reflect the basin

slope and degree of perviousness or imperviousness, and degree of
sewered area, are based on the values recommended in the Urban
Storm Drainage Criteria Manual.

An additional variable in the computation of the unit hydrograph
within the HYDRO model is the unit hydrograph width relationships
at the 50% and 75% peak flow points. For the Sanderson and Weir
studies the most recent width relationship developed by the Urban
Drainage and Flood Control District were used which reflect
characteristic widths of unit hydrographs in developed urban
areas.

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