FLOOD04995

• County: Washington
• Community: Otis
• Basin: Republican
• Title: Town of Otis Master Drainage and Flood Study
• Date: 5/1/1986
• Prepared For: Otis
• Prepared By: Engineering Professionals, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

runoff volumes were determined for existing basin
conditions without ponding behind roadway and railroad
embankments. However, hydraulic analysis did account
for the impact of Dade Street, Highway 34 and 63, as
well as the the railroad with respect to the southern
basin. Table 1 contains a summary of 10- and 100-year
discharges and tributary areas at various points of
interest throughout the basins. Following is a summary
of the techniques and assumptions used to develop the
hydrology. Further information, calculations, and HEC-l
computer output may be found in the technical addendum.

Rainfall depths for the 10- and 100-year return
periods were obtained from the "Precipitation Frequency
Atlas of the Wester United States, Atlas 2, Volume II,
Colorado" (reference 1). The Colorado Urban Hydrograph
Procedure (reference 5) was used as a guide in
distributing the incremental rainfall values to obtain
10- and 100-year design storms. The six-hour design
storms used in the analysis are listed in Table 2.

The unit hydrograph option in the United States
Army Corps of Engineers' HEC-l Flood Hydrograph Package
(reference 3) was used to estimate peak flows and
runoff volumes. A unit hydrogrpah is defined as the
hydrograph resulting from a unit storm, that is, one
inch of direct runoff from the tributary area.
Implicit in unit hydrograph theory are the assumptions
that the unit hydrograph represents the integrated
effects of basin size, shape, channel routing effects,
and basin slopes. Normally, unit hydrographs are
developed on a basin-by-basin basis by analysis of
historic rainfall-runoff records. Where such records
are not available, a synthetic unit hydrograph may be
developed for the basin. For this study, the Snyder
unit hydrograph as modified by the Denver Regional
Council of Governments' Urban Drainage and Flood
Control District (UDFCD) was used (reference 5). The
Snyder unit hydrograph technique as modified by UDFCD
assumes that the unit hydrograph shape may be
determined by two coefficients, the peaking coefficient
Qp, and the time to peak coefficient tp. In turn,
these coefficients are assumed to be functions of basin
slope, area, and imperviOUS area.

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