FLOOD04241

• County: Broomfield
• Community: Broomfield
• Stream Name: Nissen Res., City Park, Gay Res., West Lake Basins
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 6/1/1979
• Prepared For: UDFCD
• Prepared By: Wright-McLaughlin Engineers
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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FLOODPLAIN DESCRIPTIONS

The 100-year floodwater surface has been delineated on plan and profile

drawings for each drainage basin, as indicated on the sheet index (Figure

IV-I), Descriptions of flooding problems on a sheet by sheet basis are

presented in the following text,

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CITY PARK BASIN

Sheets 1 and 2 indicate the extent of flooding upstream and west of U,S, 287,

In this area, the channel travels in the form of a swale through a sparsely

developed but rapidly growing industrial area between the Community Ditch

and U,S, 287, The capacity of the culvert at U,S, 287 is severely reStricted

by a smaller culvert and access road just downstream which acts as a dam,

thus causing most of the flow to inundate the highway, From this point to

Midway Boulevard (Sheet 3), 100-year flows are confined to the North Midway

Park area, Flow overtops the road at Midway Boulevard and continues through

South Midway Park where additional tributary flow enters just upstream of

Emerald Street (Sheet 4),

The flow approaches 120th Avenue in a wide floodplain (Sheet 6), which

results in a splitting of the flow at the box culvert located there.

Approximately 550 cfs of the total 1,550 cfs does not reach the culvert and

flows eastward across the open field toward Sheridan Boulevard, The re-

maining 1,000 cfs flows through the culvert, joi ns with an additional tribu-

tary flow of 490 cfs from the west, and continues eastward in a channel

between the frontage road and the eastbound lanes of 120th Avenue,

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The Sheridan Boulevard culvert (Sheet 7) does not have the capacity to handle

1,490 cfs and, as a result, 150 cfs overtops the frontage road to the south,

while 160 cfs flows across Sheridan Boulevard and back into the Channel to

combine with 1,180 cfs carried by the culvert, However, the channel immed-

iately downstream of Sheridan Boulevard has the capacity to carry approxi-

mately 1,000 cfs, thus the excess flow (340 cfs) overtops the frontage

road to the south, combines with the previously lost 150 cfs, and is lost

from the basin, Approximately 1,000 cfs continues eastward in the channel.

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Sheets 9 and 10 delineate the 100-year floodplain for the South Tributary,

where flows in excess of the existing underground drainage system travel

along Nickel Street, West 3rd Avenue, and Garnet Street before combining

with the main channel flow upstream of Emerald Street, It appears that

only residences in the vicinity of West 3rd Avenue and Garnet Street are

affected by 100-year flooding, however, field surveys would be required to

determine house elevations and the extent to which each is subject to flood-

ing,

The 550 cfs which enters the Sheridan Boulevard - 120th Avenue intersection

from the northwest flows eastward along l20th Avenue to the vicinity of

the existing waste ponds to combine with overbank flow from the Nissen

Reservoir Basin,

Sheet 8, in conjunction with Sheet i8, shows the confluence of the City Park

Basin and the Nissen Reservoir Basin, The culvert at 120th Avenue can carry

the 1,000 cfs in the channel, but flows to the north of 120th Avenue not

carried in the Nissen channel inundate much of the overbank,

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Flow overtops the roadway at Emerald Street and most of the flow continues

downstream through Emerald Park, but some flow travels down Agate Way

to the low point, where a swale conveys it back to the main channel,

100-year flows in excess of the culvert capacity at Main Street cross the

roadway and spread out to inundate a wide area in Broomfield Community

Park (Sheet 5),

NISSEN RESERVOIR BASIN

Sheets 11 and 12 show that the upper basin area is dominated by single-family

residential development, with the exception of the high school adjacent to

Main Street and south of Miramonte Boulevard. Development upstream from

Miramonte Boulevard has been well planned with respect to drainage and

represents a fine example of multipurpose use in the floodplain area,

The subdivision open space is located along the major drainageway and includes

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