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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waters by irrigation systems, such as transbasin diversions and storm drainage
conveyance by ditches, is not considered in the sense of having a positive
mitigating or other effect on flood control or conveyance because the local
governments normally have no control of the ditch companies and because the
ditch companies are not usually responsible for, nor are they required to convey
runoff water which enters the ditches.

The irrigation diversions and ditches do have physical effects, both positive and
negative, on the drainage and flood control for this watershed in the following
ways:

1 . Aggradation of stream beds can occur at diversion structures which can
cause changes in flood flow distributions and levels. Examples include the
South Boulder Ditch, Dry Creek Ditch No.2, the Marshallville Ditch, the
Shearer Ditch, the Leggett Canal, the McGinn Ditch, the New Anderson
Ditch, the New Dry Creek Ditch, the Enterprise Ditch.

2. Redistribution and collection of flood waters, particularly in the South Boulder
Creek overflow, occurs because of the ditches and related conveyance
structures. Examples include South Boulder Ditch, Dry Creek No.2 Ditch,
Marshallville Ditch, the Anderson Ditch, the Wellman Ditch.

The East Boulder Ditch appears to have little potential effect.

Bear Canyon Creek can spill to the Anderson Basin (and Ditch) at Table Mesa and
Broadway (Highway 93). However, no spillage or transbasin diversion is assumed
herein according to the contract and accepted practice.

Downstream of Highway 36 on the west side of the basin there are three subbasins
which are drained by storm sewers to Bear Canyon Creek, but which may
contribute to the South Boulder Creek basin because of street drainage patterns and
the Wellman Ditch. The Wellman Ditch can also bring in significant runoff from
other watersheds, particularly Bear Canyon Creek. No transbasin flow is assumed.

There are three irrigation ditches which have been incorporated into the SWMM
model, on a limited basis, because their bank topography and overflow carrying
capacity significantly influence the redistribution of South Boulder Creek flood
waters. These include:

1 . The Wellman Ditch from upstream of Foothills to South Boulder Creek

2. The Anderson Ditch from Highway 36 to Viele and South Boulder Creek

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