FLOOD00959

• Designation Number: 143
• County: Denver
• Community: Denver
• Stream Name: Marston Lake North
• Title: Flood Hazard Area Delineation - Marston Lake North
• Date: 2/1/1979
• Designation Date: 3/18/1979
• Prepared For: Denver
• Prepared By: UDFCD
• Floodplain - Doc Type: Floodplain Report/Masterplan

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IV. HYDRAULIC SUMMARY.

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Hydraulics

Hydraulic analyses of the study area were completed to determine the

water surface elevations for the 10- and 100-year storm events. The

elevations were compiled using the Corps of Engineers' HEC-2 Water

Surface Profiles computer program. Valley cross-sections were measured

from left to right looking downstream on the one-inch ~ 100-feet scale,

two-foot contour topographic mapping provided by the District. A field

investigation was conducted to determine the channel and floodplain

conditions, vegetative cover, and roughness coefficients.

HEC-2 modeling purposes. In areas where flood flows move overland, the

roughness coefficient was held constant along a cross-section. It should

be noted that the roughness coefficients listed in Table II are the rough-

ness values used in the SWMM for computing channel and overland flow for

flood routing.

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Channel and Floodplain
The natural channel draining the Marston Lake North basin is characterized
by mildly sloping channels covered with medium-dense vegetation on the
channel and overbanks. Because the channel is filled with water only
during storm events, vegetation so~etimes overgrows drainage culverts,
leaving them with negligible capacity, with the exception of the 84-inch
culvert under Colorado State Highway 121 and the twin arch culverts
under Sheridan Boulevard where the capacities are not reduced signifi-
cantly. In the reaches of the drainageway through the Pinehurst golf
course, little or no channel is evident, with the flow moving overland
on dense turf. Little or no channel is defined upstream or downstream
of Lakes Lake, forcing higher frequency flows overland into the sump
area behind the Colorado State Hignway 121 highway embankment. From the
Marston Sump, a well-defined channel continues along the north bank of
Marston Lake, to the Marston Water Treatment Plant, where the flow is
forced overland to the Pinehurst golf course because the channel capacity
is limited by a 30-inch outlet. Tne channel downstream of Incinerator
Lake is well defined, with the steepest slopes occurring near the con-
fluence with Bear Creek.

Water Surface Profile Computations

During the water surface profile computations, the channel was broken

down into three reaches: Reach 1 from Bear Creek to upstream of the

Marston Water Treatment Plant; Reach 2 from the treatment plant to

Colorado State Highway 121; and Reach 3 from Colorado State Highway 121

to the end of the study area. No bridges or culverts were modeled in

any of the reaches; however, various assumptions were made in this

respect. All culverts, except the 84-inch culvert at Colorado State

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Highway 121,
flood flows.

were considered to carry a negligible percentage of higher

The 84-inch culvert backs up the flow sufficiently to

force flow from storms greater than the 10-year over-the-road.

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~oughness Coefficients

Channel and overbank roughness coefficients were estimated by field inspec-

tion h d . M, annl.'ng's "n" values for channels varied between
along t e ral.nageway.
k h varied between 0.03 and 0.035 for
0.02 and 0.03 and the overban roug nesses

The natural channel bordering Marston Lake between cross-sections 128

and 138, has a limited capacity due to flow obstructions (such as high

spots in the channel), and adverse slopes which occur throughout this

reach. From the water surface profile computations, it was determined

that this reach has a capacity of approximately 650 cubic feet per

second. Above this flow rate, the Marston Lake embankment is overtopped

and water will enter the lake. If it is assumed that the flow obstructions

are eliminated during a flooding event by washing out, the capacity of the

channel between cross-sections 128 and 138 is estimated at 1,100 cfs and a

decrease in floodplain widths results within this reach of drainageway, as

compared to the floodplain when the flow obstructions are assumed and flows

enter Marston Lake. Therefore, to delineate the floodplain in this reach

of drainageway and model the potentiallY worst flooding conditions, the

flow obstructions are assumed to remaill in place and therefore a flow

reduction is assumed due to the floodwaters which would be lost to

stormwater moving over the Marston Lake embankment within chis reach. The

majority of the flows which enter Marston Lake over the embankment occurs

in the vicinities of stations 105+00, 117+00 and 134+00 along the l1arston

Lake North drainageway. Table IV shows the flow reduction in this reach

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