FLOOD07750

• Designation Number: 448
• County: Arapahoe
• Community: Unincorporated Arapahoe County
• Stream Name: Big Dry Creek
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Big Dry Creek & Tributaries
• Date: 11/1/1996
• Designation Date: 3/1/1997
• Floodplain - Doc Type: Floodplain Report/Masterplan

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III. HYDROLOGIC ANALYSIS

A. GENERAL

The hydrologic analysis for the study area was performed to define the storm runoff peak flows and

volumes for the various sub-watersheds of the Big Dry Creek watershed. The runoff information is

subsequently used to evaluate the existing drainage facilities, potential drainage problems, alternative drainage

improvements, and in the development of the Flood Hazard Area Delineation (FHAD) and Preliminary Design

(Pbase -B) studies and reports.

The study area was divided into a total of 162 sub-watersheds. Peak flows at each sub-watershed
outlet point were calculated for the 2-, 5-, 10-, 50-, and lOa-year recurrence intervals. Tbe study area and
sub-watershed areas are shown in Figures III -I A tbru I C.

B. DESIGN RAINFALL

The modified 2-hour design storm distributions recommended by the Urban Storm Drainage Criteria
Manual (USDCM)(Reference I) were used to produce runoff hydrographs for the flow routing required in the
UD&FCD Stormwater Management Model (SWMM)(Reference 2). The incremental rainfall depth option was
used in the Colorado Urban Hydrograph Procedure (CUHP)(Reference 3) runoff module. Zone I rainfall
depths for 5-minute increments were used for this study and are listed in Table III-I. The values listed in
Table III-I are from the 2-hour design storm distribution for watersheds with areas between 5 and 10 square
miles.

C. COLORADO URBAN HYDROGRAPH PROCEDURE PARAMETERS

The CUHP computer program was used to compute storm runoff hydrograpbs for the various sub-

watersheds. The version used for the study was CUHPEI/PC. The sub-watershed input parameters required

for CUHP are: sub-watershed identification number, drainage area, length and centroidallength, drainageway

slope, impervious percentage, detention and retention storage losses, and infIltration loss rates. The weighted

channel slopes were adjusted using Figure 4-1 of the Runoff Chapter of the USDCM for input into CUHP.

Since the 5 to 10 square mile 2-hour design storm was used for this study, the CUHP rational method option

for drainage sub-watersheds less than 90 acres in size was not used. The optional parameters to model

111-1