FLOOD04702

• Designation Number: 347
• County: Jefferson
• Community: Unincorporated Jefferson County
• Stream Name: Upper Lena Gulch
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Upper Lena Gulch
• Date: 1/1/1993
• Designation Date: 9/1/1993
• Floodplain - Doc Type: Floodplain Report/Masterplan

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III. HYDROLOGY

The study area was divided into seventy-seven sub-basins ranging in size from 30 acres to 145

acres, The average sub-basin size is 81 acres, Figure 3 presents the sub-basin delineation,

Sub-basin characteristics such as area, length, and slope were measured from USGS 7,5

minute quadrangle maps of Colorado for "Ralston Buttes", "Golden", "Arvada", 'Evergreen",

"Morrison", and "Fort Logan",

General

Hydrologic analyses for Upper Lena Gulch above Maple Grove Reservoir for future conditions

were conducted in accordance with the procedures outlined in the Urban Drainage and Flood

Control District's (UDFCD) "Urban Storm Drainage Criteria Manual" (USDCM), These analyses

update previous studies performed as part of the Lena Gulch Drainaae Master Plan. June 1975

prepared for the UDFCD, the Cities of Golden, Lakewood and Wheat Ridge, and Jefferson

County,

Runoff hydrographs from each sub.basin for the 10, 50, and 100-year recurrence interval
storms were generated using CUHP, The computer program CUHPE/PC was used for actual
runoff computations, One-hour rainfall depths were input to CUHPE/PC as well as basin
characteristics such as length, length to centroid, percent impervious area, basin slope, and
parameters to define infiltration in the basin, CUHPE/PC used the one-hour rainfall to develop
a two-hour design storm, computed excess precipitation, and developed runoff hydrographs for
subsequent combination and routing, A detailed description of the computational procedures
used in CUHPE/PC may be found in the USDCM,

Modelina Methodoloav

The methodology used to analyze and quantify storm runoff has changed significantly over the
past 20 years, The hydrologic analyses for the 1975 Master Plan were performed using the
Massachusetts Institute of Technology Catchment Model computer model, also known as
MITCAT, and used a rainfall distribution developed from information and procedures contained
in the UDFCD "Urban Storm Drainage Criteria Manual" (USDCM), The currently accepted
approach for computing urban runoff in the Denver area is the Colorado Urban Hydrograph
Procedure (CUHP) which is outlined in the current version of the USDCM, The methodology
presented in the USDCM is based on extensive research and practical experience in modeling
urban runoff in the Denver area and includes updated rainfall data derived from studies of
gaged precipitation records in the Denver area,

The basin runoff hydrographs were combined and routed to develop discharge hydrographs
from each basin using the UDFCD computer program UDSWM2/PC, A complete description
of the UDSWM2/PC program's capabilities and limitations may be found in the program's user
manual available from UDFCD, The UDSWM2/PC program was selected for use in routing the
storm hydrographs because of its ability to handle a variety of types of conveyance elements as
well as detention storage facilities, and because of its ease of interfacing with the CUHPE/PC
using runoff hydrographs as direct input Figure 4 presents a schematic diagram of the
hydrologic modeling structure,

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