FLOOD01787

• County: Boulder
• Stream Name: South Boulder Creek
• Title: South Boulder Creek/ Flatirons
• Date: 12/5/1997
• Prepared For: Boulder
• Prepared By: Consultants
• Floodplain - Doc Type: Community File

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Boulder Creek. Ground elevations in the valley are often lower west of Dry Creek
Ditch No.2, therefore, flood flows can be carried west of this Ditch.

From Highway 93 to Highway 36, another nine square miles are potentially
tributary to South Boulder Creek at Highway 36 (36.0 square miles below Gross),
ranging in elevation from 5350 MSL to 8461 MSL at Bear Peak. The character of
the watershed varies significantly and includes Bear Peak and steep mountain
slopes aligned with the Flatirons on the west; David's Draw which is also on the
west side and has some dense development; moderately steep to mildly sloping
agricultural, open space, riparian cottonwood groves, and gravel pit areas along the
valley; a large foothills tributary on the east which includes the Marshall Reservoir
and a downstream basin which has steeply sloped valley sides with a mixture of
brush, grass, and trees; and moderately sloping agricultural plains areas on the east
side of the valley which naturally drains to South Boulder Creek, but whose
drainage is interrupted by several irrigation ditches.

The wide floodplains and multiple flow paths of South Boulder Creek provide an
important balancing effect by storing, routing, and slowing the flashy peak runoff
rates from the steep slopes and tributary streams. From Highway 93 to Highway
36 the main channel conveys day to day flows, and whatever flood flows do not
spill upstream. As documented in the Phase I report, the majority of the 100 year
flood will be carried in the West Valley Overflow route. However, the West Valley
Overflow is forced to carry much of the water back to South Boulder Creek by the
Upper and Middle Flatirons Quarry Pit levees just upstream of Highway 36. Some
minor amount of the overflow will not return to South Boulder Creek, probably no
more than 300 cfs, which will be conveyed by Dry Creek Ditch No.2 and limited
overbank conveyance to join the South Boulder Creek spill just downstream at
Highway 36.

Downstream of Highway 36 and the main channel of South Boulder Creek there are
another 6.2 square miles of potential tributary area (42.2 square miles below Gross
Reservoir, 135.9 square miles total basin). This basin ranges in elevation from
5170 feet M.S.L. at the confluence with Boulder Creek to 6050 feet M.S.L. at the
top of the Viele watershed. The watershed is largely urbanized with notable
exceptions being the riparian corridor directly along the main channel, the highest
portion of the Viele watershed, the golf course, and some of the hills on the east
side. The valley bottom is a mile wide and is an alluvial fan, with varying cross
slope direction and, fortunately for drainage function, some moderate down valley
slope. The Viele watershed is on the southwest side of the basin, and begins on a
ridge just south of the confluence of Bear Canyon Creek and Fern Canyon. It has
four significant storage facilities and an outfall channel on the south side of the
valley that crosses Highway 36 at the Foothills Interchange and then joins South
Boulder Creek just north of South Boulder Road.

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