FLOOD00385

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Title: South Boulder Creek Interim Hydrology Study
• Date: 7/1/2000
• Prepared For: Boulder County
• Prepared By: Taggart Engineering Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Flow arriving at the area of Highway 36 Bridge over the main channel of South
Boulder Creek can potentially spill to the northwest. There are three major and
several minor routes which the flow can take:

1. Main Channel. The main channel route conveys flow in a largely riparian
cottonwood grove floodplain which has a mixture of open space, agriculture,
residential, and commercial uses. As the main channel passes by the
Wellman Ditch additional spills to the West Valley occur through a residential
area and the golf course (element 237, Drawing 7). Below Arapahoe, the
Leggett Canal to the Valmont reservoirs (Hillcrest, Valmont, Leggett) can also
carry flood flows (236, Drawing 8). Also, there is another potential spill to
the west to Dry Creek Ditch No.2 near the railroad tracks, which has not
been evaluated herein due to complex backwater analysis and perceived
likely low magnitude.

2. Direct return to South Boulder Creek: Drv Creek No.2. Viele. and the
Anderson Ditch. The South Boulder Creek spillage upstream of Highway 36
that flows to the northwest would be joined by flows from Dry Creek Ditch
No.2.

There are three modest conveyance systems which will carry an initial
portion of the flow across Highway 36 and, for the most part, back to South
Boulder Creek just below South Boulder Road. The first 220 cfs would be
conveyed in the Dry Creek Ditch No.2 box culvert under Highway 36 (263
and 376, Drawing 10\, and via an irrigation ditch to a spill structure (375\
near South Boulder Road which would shed most of the flow to the New
Anderson Ditch, which subsequently could spill to Viele (378). Greater
spillage flow from South Boulder Creek, on the order of 500 to 800 cfs, will
be intercepted by the Viele drainage system (265 and 1267) adjacent to the
Foothills Parkway Interchange and conveyed under Highway 36, and then
combine with the spillage from the lower Anderson Ditch and the Dry Creek
Ditch No.2, and flow under South Boulder Road to South Boulder Creek.

Greater South Boulder Creek spillage would overtop the Viele system and
flow onto the freeway (292). Some portion of this spillage and local
drainage flows can be conveyed by the Anderson Ditch to the east to South
Boulder Creek, subject to some spillage to the north at several points, the
first and most significant of which would occur near the Anderson culvert
under the Highway 36 eastbound on ramp (383).

3. West Vallev Overflow. Greater South Boulder Creek spillage (from 700 to
1000 cfs) would flow onto Highway 36 (360) and pass by the Anderson
Ditch, and then largely overflow the Highway 36 barrier (352 and 1812) and