FLOOD02174

• County: Denver
• Community: Lakewood
• Stream Name: Dry Gulch
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 7/1/1979
• Prepared For: Denver
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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6

III HYDROLOGIC ~NALYSIS

to develop the flood flows in the Dry Gulch basin, using the
CUHP.

General

The Urban Drainage and Flood Control District directed that four

specific flood flows were to be evaluated during the development

of the master plan for Dry and North Dry Gulches. These were

the floods which could be expected to occur, on the average, once

every five, ten, fifty, and one-hundred years. These floods are

referred to as the five-year, ten-year, fifty-year, and one hun-

dred-year floods; they have likelihoods of occurring or being

exceeded in anyone year of 20%, lO%, 2%, and l% 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 Dry Gulch basin. Additionally, the flood flows which

were to be evaluated were those which would occur under the

simulated condition of "full development" in the basin. For

these reasons, a less direct but more flexible method was employed

to estimate the flood flows.

Delineation of Basins

The initial step in the analysis was to define the routes which

flood flows would take in the basin. While this is a very simple

procedure in many drainage basins, it became very complicated in

the basins of this analysis. The flows frequently leave their

apparent "natural" channels and travel along street or other un-

natural routes. The flows sometimes diverge into two routes and

then converge some distance downstream. These flow patterns will

be addressed in more detail in a subsequent section. Key points,

designated "points of hydrologic computation," were selected

along the channels of Dry and North Dry Gulches as points at

which calculations would be made of flow rates. The basins and

sub-basins tributary to each point were defined by locating the

dividing ridges between them. Sheet 2 of l8 in Section VII

illustrates the main channel routes, the "points of hydrologic

computation," and the basin boundaries.

The Urban Drainage Criteria Manual (Reference 3), presents a

method for predicting flood flows based upon a statistical analysis

of rainfall and runoff characteristics. The method is known

as the Colorado Urban Hydrograph Procedure (CUHP) and makes

possible the development of a flood hydrograph from an analysis

of rainfall. Presented in this section are the steps employed

Rainfall

Rainfall data for storms of various return periods were obtained

from Reference 3. Synthetic rainstorms of 200 minute lengths

were then developed for the five-year, ten-year, fifty-year and

one hundred-year events. The total rainfall amounts are presented

in Table III-l.