FLOOD04928

• County: Denver
• County: Jefferson
• Community: Denver, Edgewater, Lakewood, Wheatridge
• Stream Name: Sloans Lake Basin
• Basin: South Platte
• Title: Major Drainageway Planning
• Date: 12/1/1977
• Prepared For: Denver / Lakewood
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Per cent of imperviousness was determined by measuring impervious

surface areas of eight sample street blocks throughout the basin.

The selected blocks were judged to be typical of fully developed

representative areas for the various sub-basins. Field visits veri-

fied the development characteristics of the sample blocks.

After evaluation of the soil characteristics, infiltration rate and

pervious runoff, the following hydrological factors were selected and

used in the computation of storm hydrographs:

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Soil Infiltration

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The coefficients used in the computation of the unit hydrographs, Ct

(coefficient reflecting time to peak) and C (coefficient related to
p

peak rate of runoff) were determined and adjusted in accordance with

the procedure contained in the USDCM.

First 1/2 Hour

5-year Return Period 1 inch/hour
10-year to 100-year Return 0.5 inch/hour
Impervious Depression and Dentntion Storage:
Pervious Depression and Detention Storage:

Remainder of Storm

0.5 inch/hour
0.5 inch/hour
0.1 inches
0.3 inches

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HYDROGRAPHS

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Table 1 lists the information developed for the various parameters

associated with the Sloans Lake basin hydrologic investigation, based

on a totally developed study area.

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DESIGN RAINFALL

Design storm hydrographs were computed at each of the design points

(except for Point A) for the four storm frequencies (5-, 10-, 25- and

100-year) under consideration for the projected total development con-

dition. These hydrographs, developed in accordance with the Colorado

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That portion (about 60%) of the Sloans Lake Drainage Basin lying

north of Colfax Avenue and west of Sheridan Boulevard is located in

Township 3 South, Range 69 West (Jefferson County). That portion

(about 35 per cent) of the Basin lying north of Colfax and east of

Sheridan is in Township 3 South, Range 68 West (Denver County). A

very small portion (about 5 %) lies south of Colfax Avenue and is in

Township 4 South, Ranges 68 and 69 West. The design rainfall presented

in the Rainfall/Runoff Information Report published under Project REUSE

for these townships and ranges varies insignificantly. The values for

Jefferson County are generally slightly higher and were the values

used for runoff computations in the Sloans Lake Basin hydrologic in-

vestigation. These data are presented in Tables B-1 and B-2 of

Appendix B. The depths and durations for the four design rainfalls

(5-, 10-, 25- and lOO-year) are presented in Tables B-3 through B-8

of Appendix B.

Urban Hydrogr<1ph procedure were used to determine peak des iqn flows

for each sub-basin and design point within the study area. Graphical

illustrations of these hydrographs are presented in Figures B-1 through

B-15 in Appendix B.

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Hydrographs at the Colfax Avenue/Federal Boulevard interchange (Sub-

basin A) were not developed due to the lack of any drainageway point-

of-concentration for runoff flows. Peak flows in Sub-basin A were

approximated by summing the Sub-basin Al flow and that portion of flow

from Sub-basins A2b and A3 which is diverted to Sub-basin A. Two

assumptions inherent to this procedure included:

l. The peak flows from the contributory sub-basins (AI, A2b
and A3) occur at approximately the same time;

2. The local rainfall in the immediate vicinity of the inter-
change is small in comparison to the tributary inflow.

A summary of the peak flows at each of the design points for each fre-

quency is presented in Table 2.

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