FLOOD00734

• Designation Number: 213
• County: Adams
• Community: Thornton
• Stream Name: Brantner Gulch and Tributaries
• Title: Flood Hazard Area Delineation - Brantner Gulch and Tributaries
• Date: 1/1/1983
• Designation Date: 5/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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7

v. HYDROLOGIC ANALYSIS

makes possible the development of a flood hydrograph based on

analysis of a predetermined rainfall and watershed character-

istics. Presented in this section is a summary of the steps

employed to develop flood flows in the study basins, using the

CUHP.

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General

The Urban Drainage and Flood Control District directed

that five specific flood flows were to be evaluated during the

development of the hydrology for Brantner Gulch and the Regional

Park Tributaries. These were the floods which could be expected

to occur, on the average, once every two, five, ten, fifty, and

one hundred years. These floods are referred to as the two-

year, five-year, ten-year, fifty-year, and one hundred-year

floods; they have likelihoods of occurring or being exceeded in

anyone year of 50%, 20%, 10%, 2%, and 1%, respectively. Whenever

possible in a hydrologic analysis, estimates of peak flow rates

would be determined through a statistical analysis of recorded

floods. However, no usable records exist of floods in the study

basins. Additionally, the flood flows which were to be evaluated

were those which would occur under the condition of

"full development" in the basin. For these reasons, a less

direct but more flexible method was employed to estimate the

flood flows.

The Urban Storm Drainage Criteria Manual (Reference 1),
presents a method for predicting flood flows based upon a statis-
tical analysis of rainfall and runoff characteristics. The method
is known as the Colorado Urban Hydrograph Procedure (CUHP) and

The first step in the hydrologic analysis was to define

the routes which flood flows would follow in the basin. Limits

of Brantner Gulch watershed and its sub-basins were delineated

by locating the dividing ridges tributary to each channel. In

very flat areas such as the land adjacent to the South platte

River, defining the low-flow channel was more difficult and

required additional efforts. Key points, designated "points of

hydrologic computation," were selected along the Brantner Gulch

channel, its tributaries, and the Regional Park Tributary channels

as points at which calculations of flow rates could be made.

Figures 16 and 17 illustrate the main channel thalweg, the "points

of hydrologic computation," and the basin and sub-basin boundaries.

Rainfall data for storms of various return periods were

obtained from the Urban Drainage and Flood Control District.

The District provided charts for the determination of C C
t' p

and rainfall distribution. Other parameter values were taken

from the Urban Storm Drainage Criteria Manual. Synthetic rain-

storms of 120 minute lengths were developed for the two-year,

five-year, ten-year, fifty-year and one hundred-year events (see

Table 1).

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