FLOOD07509

• Designation Number: 150
• County: Adams
• Community: Thornton
• Stream Name: Direct Flow Area 0054
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Direct Flow Area 0054 - Including Major Drainageway Planning DFA 0054 Phase A Report 11/79
• Date: 10/1/1979
• Designation Date: 1/1/1980
• Floodplain - Doc Type: Floodplain Report/Masterplan

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FLOOD HAZARD AREAS

The 100-year flood has been selected by the Federal Government, the State. of
Colorado, the Urban Drainage and Flood Control District, and local governments as the
flood event to be used for floodplain delineation and regulation. Thus, the Flood Hazard
Area maps depict the profile and floodplain limits of the lOO-year flood based on future
developed conditions in the basin as determined from current master planning maps.
Floodplains for the 2-, 5-, 10-, and 50-year events determined as part of the concurrent
major drainageway planning study indicate a number of areas of DFA 0054 exposed to
severe flood hazards even in lesser events. In addition to the 100-year flood, the 10-year
flood profile is shown on the drawings.

Floodplain and floodway reference data including potential 100-year flood
discharge, thalweg elevation, flood elevations and widths, are listed by cross-section in
Tables 4, 5, and 6. Floodplain reference data corresponds to the 100-year floodplain as
shown on the Flood Hazard Area Delineation Maps. Valley cross-sections at specified
locations are presented on the drawings to indicate ground level across the valley and the
overlying flood depths.

The floodway is defined as the minimum channel required to pass the 100-year
flood without increasing flood depths or velocities more than a specified amount over the
natural floodplain conditions. The floodway data for DF A 0054 listed in the reference
tables was developed using the more restrictive of either the Urban Drainage District or
local criteria. The following floodway criteria have been used for the respective
jurisdictions:

Adams County: Velocities of 2 feet per second or depths greater than 18", and/or
1/2 ft. rise in the water surface profile based on equal encroachment.

Thornton: The UD&FCD criteria of 1/2 ft. rise in the water surface based on equal
encroachment.

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Flooded Areas

The 100-Year floodplain limits and the 100-year and 10-year water surface profiles
for Direct Flow Area 0054 are shown on the Flood Hazard Area Delineation Maps. The
sheets are ordered to follow Direct Flow Area 0054-1, 0054-2, and 0054-3, all beginning at
the upstream end. The Plan and Profile Sheets are supplemented with Plan View Sheets
where areas subjected to flooding become very wide.

Direct Flow Area 0054-1

The upstream region of Direct Flow Area 0054-1 begins west of Colorado Boulevard
in a residential area that is near total development. The majority of the basin in this area
drains into a smail detention pond located just south of East 115th Avenue and west of
Jackson Street. The pond has a capacity of just under 2 acre-feet capacity, which when
considered in conjunction with the 12 inch diameter RCP outlet, is adequate only to retain
storms of magnitude equal to or less than the 2-year event. All storms of greater
magnitude would overtop the pond, continue down East 115th Avenue and cross over
Colorado Boulevard.

Runoff which crosses over Colorado Boulevard is assumed to continue on East 115th
Avenue to where thi<; study begins above Ash Circle, Sheet 1. From here, flow continues
on 115th, before being intercepted by Birch Drive. To this point, the 100-year flow
remains confined to the streets and front yards of residences. The storm sewer system
which also follows the route of the surface flow is adequate to intercept and convey
runoff from events of the 2-year magnitude and less.

Beyond Birch Drive, runoff again changes direction as it turns down Clermont Way.
As flow continues, approximately 180 cfs of the 100-year storm is diverted down Clermont
Drive. Three residences on the southeast side of Clermont Drive are subjected to near
flooding as the flOOdplain just approaches the front of the homes.

At the intersection of Clermont Way and East 113th Avenue, the flow divides with
approximately 60% of the flow continuing down Clermont Way onto 112th Place, while the
remaining flow follows East 113th Avenue, Sheet 2. Flow from the storm sewer system
empties into a small earthen channel that parallels Cherry Drive, where the outflow joins