FLOOD00734

• Designation Number: 213
• County: Adams
• Community: Thornton
• Stream Name: Brantner Gulch and Tributaries
• Title: Flood Hazard Area Delineation - Brantner Gulch and Tributaries
• Date: 1/1/1983
• Designation Date: 5/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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5

The drainage basin consists of Vlm loams at the upper end

of the basin and channel, Samsil-Shingle complex soils in the

middle channel and basin, and Nunn Clay loams and loamy alluvial

land near the river. These soils are described in the Soil

Survey of Adams County, Colorado, published by the Soil Conser-

vation Service, as having moderate to very low permeability.

The static water table in the loamy alluvial land near the river

is seasonally about three feet below the surface.

The Regional Park Tributary Basin watershed consists of

8.75 square miles, and is about as long as it is wide. The two

independent tributaries flow eastward and combine just before

their confluence with the South Platte River in Adams County.

The lOa-year floodplains presented in this report, however,

are based upon future or "fully developed" land use conditions.

Anticipated conditions show a development pattern of residential

housing in Adams County at a density of four dwelling units per

acre. Future development of Brantner Gulch Basin within

Thornton City Limits is assumed to be the same as that illustrated

in Thorntan's 1979 Future Land Use Map.

III. ADDITIONAL STUDIES AND PERTINENT DATA

General

The land use in the drainage basins has been herein des-

cribed as residential for the upper Brantner Gulch Basin,

agricultural or undeveloped for the middle Brantner Gulch Basin

and commercial for the lower Brantner Gulch Basin. Land use in

the Regional Park Tributary (RPT) Basin is primarily undeveloped

or agricultural property. The existing land use in the lower

RPT basin is predominantly commercial.

Two prior flood studies have been performed for the City of

Thornton. In 1980, Merrick and Company prepared a Master Drainage

Plan for the City. This study and report were very comprehensive

but did not address the need for a Probable Maximum Precipitation

(PMP) flood analysis in Eastlake Darn No. 2 and No. 3 as it relates

to flood control in the Brantner Gulch Basin. Some of the recom-

mendations in the Merrick report were considered in this FHAD study.

In 1981, Rocky Mountain Consultants, Inc. prepared a report

on the condition of Eastlake Dam and Reservo~r No.3. The report

addressed the need for PMP flood routing considerations and

requ~red ~mprovements to the dam. The PMP storm used in the 1981

report appeared to be of smaller magnitude than the PMP storm

determined by the method shown in "Design of Small Dams" by the

U.S. Bureau of Reclamation (recommended by UDFCD) and used in

this study and report.

The County Fairgrounds area is located in the level plain

adjacent to the Platte River. Flows from Brantner Gulch and the

South Regional Park Tributary VI combine for the lOa-year flood

and cause a majority of the County Regional Park area to be

flooded for the laO-year event.