_REVISION - M1981302 ‎(117)‎

• Permit No: M1981302
• IBM Index Class Name: Revision
• Doc Name: FLOOD HAZARD AREA DELINEATION SOUTH BOULDER CREEK JULY 1986
• Type & Sequence: AM2
• Media Type: D
• Archive: No

rGeneral descriptions of flooding conditions along South Boulder Creek are shallow flooding.
provided in the following paragraphs.
Baseline Road to State Highway 93 (South Broadway)
Colorado and Southern Railroad to Baseline Road
rBetween Baseline Road and State Highway 93 (South Broadway) , a distance of about
This 1 .4 mile study reach is characterized by moderate development in and 3.4 miles, South Boulder Creek flows in a northeasterly direction through a
adjacent to the floodplain. Sheets 1 through 6 show limits of the 100-year broad, relatively flat floodplain. With the exception of a few scattered dwel-
floodplain. Water surface profiles are indicated on Sheets 22 through 23 and lings, existing land is used for pasture and open space. Sheets 7 through 14
29A• present the 100-year floodplain limits. Water surface profiles are shown on
Sheets 23 through 26.
Vegetation along the creek channel consists of moderately heavy brush and trees,
while channel overbank areas are fairly flat and open. Crossings of South Crossings and other structural features in this reach include bridges at: South
Boulder Creek in this study reach include Arapahoe Road, McSorley Lane, and Boulder Road, U.S. Highway 36 (Denver-Boulder Turnpike) , State Highway 170
Baseline Road. Diversion headworks for Valmont Reservoir are also contained in (Marshall Road) , and State Highway 93, as well as irrigation diversion structures
this reach as is the juncture of the Wellman Canal Ditch with South Boulder for McGinn Ditch, South Boulder Canyon Ditch, Schearer Ditch, and Goodhue Ditch.
Creek. The average 100-year floodplain widths vary from 1800 to 3300 feet.
Average 100-year floodplain widths vary from 600 to 2400 feet. Improvements are
Between the downstream study limit (C&S Railroad) and Arapahoe Road a special planned for the South Boulder Road crossing which consist of a new bridge and two
split flow analysis was performed to determine the divided floodway and distri- additional lanes for eastbound traffic. These improvements were incorporated in
bution of flood flows between South Boulder Creek and the adjacent Dry Creek the hydraulic modeling. It should be noted that flow depths indicate that during
No. 2 Ditch drainageway. North of Arapahoe Road , commercial -industrial a 100-year event, some flow may overtop North Cherryvale Road to the east just
structures are situated in the floodplain. Between the C&S Railroad and Arapahoe downstream of South Boulder Road. This flow would leave the South Boulder Creek
Road, the 100-year discharge tributary to the South Boulder Creek channel was basin and pass into the Dry Creek basin, flowing towards Baseline Reservoir.
estimated at 2600 cfs while the remaining 4030 cfs of the total 6630 cfs However, hydraulic modeling for this study was completed assuming that all flow
(100-year) is conveyed by the Dry Creek No. 2 Ditch channel . These results are remained in the South Boulder Creek basin.
consistent with the previous analysis completed by Simons, Li and Associates,
Inc. (Reference 6) . At U.S. Highway 36 approximately 1660 cfs of the total 6160 cfs (100-year) will
overtop the highway near its intersection with State Highway 157. The remaining
Upstream of Arapahoe Road , residential dwellings located adjacent to South 4500 cfs passes through the bridge opening. A split flow analysis was used to
Boulder Creek along Old Tale Road to the west and Gaptor Road to the east, are evaluate this distribution of flow. The split flow returns just downstream of
affected by flooding. The dividing line between the Dry Creek No. 2 Ditch and U.S. Highway 36 near South Boulder Road.
the South Boulder Creek drainageways generally parallels Old Tale Road. During a
100-year flood, flow in South Boulder Creek overtops this divide and spills into Gravel mining activity presently occurs upstream of U.S. Highway 36 on the west
' the Dry Creek No. 2 Ditch drainageway. The cumulative overflow to the Dry Creek side of the drainageway. Areas landward of the embankment were assumed
No. 2 Ditch, which occurs all along Old Tale Road, equals 2300 cfs. Much of the ineffective for flow conveyance.
' municipal golf course situated west of the creek channel will be inundated by
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