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FLOOD05699
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FLOOD05699
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Entry Properties
Last modified
1/25/2010 6:49:57 PM
Creation date
10/5/2006 1:43:08 AM
Metadata
Fields
Template:
Floodplain Documents
County
Jefferson
Community
Idledale, Morrison, Tinytown
Stream Name
Sawmill Gulch, Bear Creek, Mt. Vernon Creek
Basin
South Platte
Title
Flood Hazard Area Delineation
Date
12/1/1981
Prepared For
Jefferson County
Prepared By
UDFCD
Contract/PO #
&&
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />- Bear Creek at Idledale. (Sheets 2 and 3) <br /> <br />T!1e lOa-year discharge is quite large and flood depths on the <br />developed overbank are expected to range from 4 to 6 feet. Therefore, <br />the flooding hazards are high. Because of depths and steep slopes at <br />the edge of the floodplain, no floodway was delineated for this reach. <br /> <br />- Sawmill Gulch. (Sheets 12 t!1rough 14) <br /> <br />- Bear Creek at Morrison. (Sheets 4 through 6) <br /> <br /> <br />Data from a previous study made by the Army Corps of Engineer <br /> <br /> <br />(Reference 4) was used for the floodplain delineation. Within Morri- <br /> <br />son corporate limits, the floodway delineated for the Morrison Flood <br /> <br /> <br />Insurance Study (Reference 5) in accordance with the FEMA criteria was <br /> <br /> <br />adopted. Outside the corporate limits, the criteria adopted by <br /> <br /> <br />Jefferson County permits very little encroachment on the floodplain <br /> <br />because flow depths are over 2 feet even in areas where the floodplain <br /> <br /> <br />is wide. Also, development with~n the floodplain of this reach would <br /> <br /> <br />be the cause of greater flooding in the community. <br /> <br />The very steep slope of the streambed and the deeply incised <br /> <br /> <br />valley results in a narrow floodplain and, therefore, no floodway was <br /> <br /> <br />delineated. Some limited flooding might occur at the Grapevine Road <br /> <br /> <br />crossing at the upstream end of the reach where the culvert is under- <br /> <br /> <br />sized. A portion of the discharge at this location might cause <br /> <br />sheetflow down Grapevine Road. <br /> <br />- Turkey Creek at Tiny Town. (Sheets 15 through 17) <br /> <br />- Mount Vernon Creek. (Sheets 7 through 11) <br /> <br /> <br />Data from a study made by the Army Corps of Engineers (Reference <br /> <br />4) was used to delineate the floodplain for the lower reach of the <br /> <br /> <br />study. Within Morrison corporate limits the floodway delineated for <br /> <br /> <br />the Morrison Flood Insurance Study (Reference 5) in accordance with <br /> <br />FEMA criteria was adopted. At the downstream end of t!1e reach, Mount <br /> <br /> <br />Vernon Creek is not contained within its banks and flooding will occur <br /> <br /> <br />on the highly developed overbanks. Upstream, the valley is deeply <br /> <br /> <br />incised and the lOa-year discharge will be contained within a rela- <br /> <br /> <br />tively narrow area. No development has encroached onto the floodplain <br /> <br /> <br />in this area and the two major crossings (Red Rocks Access Roads) can <br /> <br /> <br />handle the flow without overtopping. No floodway has been delineated <br /> <br /> <br />for this reach because the adopted criteria does not allow for a <br /> <br /> <br />substantial encroachment. <br /> <br />Upstream of its confluence with Parmalee Gulch, Turkey Creek has a <br /> <br />relatively mild slope and a wide floodplain. The only major existing <br /> <br /> <br />development in the floodplain is the tourist attraction of "Tiny Town" <br /> <br />whic!1 includes small stvuctures and a model railroad with a few <br /> <br /> <br />bridges whose sizes would not significantly affect the floodplain. <br /> <br /> <br />Some encroachment is possible in a few areas east of Highway 8 and on <br /> <br /> <br />a portion of the overbank area where depths are relatively shallow. <br /> <br />At the downstream end of the study where Highway 8 crosses the stream, <br /> <br /> <br />overtopping of the structure will occur and some shallow flooding is <br /> <br /> <br />expected down Highway 285 until the topography or a. local drainage <br /> <br /> <br />structure allows this portion of the discharge to go back into the <br /> <br /> <br />channel. <br /> <br />- Turkey Creek U/S Bear Creek Reservoir. (Sheets 18 through 21) <br /> <br /> <br />This portion of Turkey Creek has a steep slope and the topography <br /> <br />indicates that the main channel has meandered in the past. No signi- <br /> <br /> <br />ficant development has yet taken place in the floodplain. Very little <br /> <br />encroachment is possible on most of the reach. At the downstream end <br /> <br /> <br />of the study, a 2000 feet long 8'x8' reinforced concrete box culvert <br /> <br />carries a portion of the flow under the Highway 285 and Soda Lakes <br /> <br /> <br />Road interchange. The culvert was designed for a 50-year discharge of <br /> <br />1100 cfs. Hand calculations show that the culvert would carry appro~ <br /> <br /> <br />imately 1300 cfs during a laO-year flood and that the remainder of the <br /> <br /> <br />discharge would flow overland along Highway 285, under the Soda Lakes <br /> <br /> <br />Road overpass, and eventually back into the original channel near Bear <br /> <br />Creek Reservoir. This overland flow was modeled accordingly. <br /> <br />-8- <br />
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