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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SECTION IV
BASIS OF DESIGN

frequency storm with no embankment or spillway overflow, The facility

is within property designated as part of the Lakewood Parks and Open-

space system for passive recreation. Outlet works capacity selected was

a 36 inch reinforced concrete pipe with a 24 inch orifice. This arrange-

ment would allow the continual release of low flows in the outlet pipe

while detaining flows from the larger storm events. Because the soil

conditions at the proposed detention facility were not fully investigated,

a typical embankment section with an impervious core was used. The exact

material for the embankment should be selected during final design. Since

the detention facility will contain the 100-year frequency storm for rela-

tively short periods of time and because the area would be empty most

of the time, natural gasses were used for embankment protection. Rip rap

was provided for spillway protection if an emergency overflow occurred.

A concrete-lined spillway is also a feasible alternative.

GENERAL

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The recommended alternative plan is a combination of flood plain im-

provements and open space detention storage. The design consists of

closed conduits, open channels, street crossings and detention facil-

ities. Design criteria used in this report are described in the

following paragraphs.

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UNDERGROUND CONDUITS

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Since the Sloans Lake basin is almost completely developed, open channel

improvements were not practical in most instances~ Underground circular

conduits designed to carry a 5-year frequency storm were used for most

local drainage improvements, with box culverts used for most street

crossings. However, box culverts or eliptical conduit would also be a

feasible alternative to the circular conduit because of reduced conflict

with existing utilities, Box culverts were deSigned to carry a 25-year

frequency ftorm and to have free-flow conditions with the water surface

approximately 6 inches below the top to prevent surcharging the culvert.

Surcharging would decrease the box culverts' carrying capacity. Manning's

n value used for box culverts was 0.015 based on the assumption that they

would be cast-in-place, Wing walls were provided at box culvert entrances

and exist to ensure smooth transitions.

OPEN CHANNELS

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Although open channels should be utilized wherever possible for improve-

ments within the Sloans Lake basin, existing development in the basin has

limited their use to a few locations, Grass lined channels where chosen

because they provide a green belt and recreation area (adding social

benefits) and have a relatively low construction cost.

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DETENTION STORAGE

Channel sections were designed for a 25-year frequency storm with 1 foot

freeboard., A Manning's n value of 0.03 was chosen for an average rough-

ness coefficient for flow depths of 4 to 5 feet. A maximum design velocity

of 7 - 7-1/2 fps for the 25-year frequency storm was used. The maximum

side slope used was 4: 1 (4 horizontal to 1 vertical). Included in the

channel design is a low flow concrete lined ditch located along the thalweg

of the drainageway to provide for trickle flows from urban runoff caused by

intermittant rain showers, street cleaning, car washing and/or lawn sprink-

ling.

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Pipes were designed for less than flowing full condition. Manning's n

value of 00013 was used for precast concrete pipes with ordinary joint

alignments. Maximum allowable velocity was set at 20 fps.

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Storm detention facility proposed to be located in the vacant space

between Vance Street and Wadsworth Boulevard near 25th Street was designed

to contain the runoff volume from the tributary drainage area for a 100-year

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