FLOOD01405

• County: Jefferson
• County: Arapahoe
• Community: Jefferson County, Arapahoe County
• Stream Name: Massey Draw, North Tributary SJCD
• Title: Major Drainageway Planning
• Date: 7/1/1979
• Prepared For: Jefferson County / Adams County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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III.

HYDROLOGIC ANALYSIS

SJCD (South) - Basin 6200

The drainageways were divided into thirteen separate sub-basins which are
shown on Sheet 1. Flood hydrographs were developed for each sub-basin for
the 2, 5, 10, 50 and 100 year floods for the future basin development con-
dition.and for the 100 year flood for the existing condition. Hydrologic
parameters including tributary area, basin length, length to centroid, and
basin slope were measured from U. S. G. S. quadrangle maps, and detailed topo-
graphic mapping provided by the UDFCb. Estimates of existing and projected
impervious areas were made from analysis of a~rial photographs, inspection of
other developed areas in the Denver Metropolitan area, and through data rela-
ting density of development to impervious area for Denver and other areas of
the country. All hydrologic information concerning the North Tributary of
SJCb (South) - Basin 6200 was taken from a major drainageway planning study
prepared by CH2M-Hill, Inc. for the Urban Drainage and Flood Control District,
Jefferson County and Arapahoe County in July 1976 (1).

At the Nevada Ditch, SJCD (South) - Basin 6200 drains approximately 4.0 square

miles. There are three branches including the North Tributary and Tributaries

1 and 2. The North Tributary will become tributary to SJCD (South) - Basin 6200

when drainage improvements are constructed in accordance with the North Tribu-

tary's Master Plan (1) (July 1976). This future condition was included in this

analysis. Below the Nevada Ditch, gravel is being mined and no definable drain-

age outlet to the South Platte River exists. The lower portion of the basin

is in Arapahoe County, while the major portion is in Jefferson County. The up-

stream limit of study extends to an elevation of 5770 feet. Main channel slopes

throughout the re~ch average 1.5%. SJCb (South) Tributary No. 1 joins the main

branch at a point below South Wadsworth Boulevard. It drains an area of 0.7

square miles, passes through the existing Fairview Heights subdivision and has

an average channel slope of 2.2%. Tributary No.2 joins SJCD (South) at a point

near Ken Caryl Road. It drains an area of 0.2 square miles with an average

channel slope of 1.9%.

For all sub-basins east of the Hogback Ridge, the Colorado Urban Hydrograph
Procedure (CUHP) was utilized. The procedure is described in the urban Storm
Drainaqe Criteria Manual (4) published in 1969. The UDFCD issued an interim pro-
cedure for determining the unit hydrograph coefficients c and c as functions
t p
of basin slope and impervious area. It is described in their June, 1977 issue
of "Flood Hazard News" (5), and was used to define the unit hyd:iograph coefficients
in this study. For the upper basin of Massey Draw (Sub-basin. F), another syn-
thetic hydrograph procedure called SYNHYD was utilized. It was developed
specifically by LRCWE for application to non-urban basins. It is based on
Snyder's synthetic hydrograph method and differs from the CUHP in the compu-
tation of the time to peak, inclusion of basin slope as a variable, definition
of c and c coefficients, and the Shape of the unit hydrograph. A descrip-
t p
tion of the program is included in the hydrologic backup data for this study
which is on file with the UDFCD. Basin parameters and summarized on Table III-I.

Basin Development

Existing development was determined through analysis of aerial photographs and

base maps provided by Jefferson County, and by field inspection. Future develop-

ment was projected with the aid of a map titled, "Southeast Jefferson County

Development" provided by the Jefferson County Planning Department, and through

discussions with Jefferson County Planning and Engineering personnel. In addition,

a number of development plats were obtained and reviewed.

Design rainfall was taken from the publication, "Rainfall/Runoff Information,

Magnitude/Frequency, Design Rainfall, Small Urban Basins" (6). It is sunnnarized in

Tables 111-2 through 111-4. For all sub-basins east of the Hogback Ridge infil-

tration losses recommended by the Urban Storm Drainaqe criteria Manual were

~ Leonard Rice Consulting Water Engineers, Inc.