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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The model bases parameters on the City topography maps and hand level surveys
for selected hydraulic controls. Instrument surveys of sensitive features such as
extended weirs, levees, channels, and flow splits have not been undertaken.
However, the study does indicate where such efforts would be more important.

The model does not simulate a local storm, but the regional event assumed by the
Corps. Some of the West Valley flow paths particularly along Dry Creek Ditch
No.2 would probably be dominated more by an intense local storm than by spills
from South Boulder Creek.

Ultimately we believe that refined South Boulder Creek hydrology complying with
regional and NOAA practices will result in larger flows than those indicated herein.

SPECIFIC NOTES AND ASSUMPTIONS

The SWMM model elements cannot simulate flow in both directions, such as may
very well occur through the Viele culvert under the on ramp from Foothills to
Highway 36 eastbound. Due to this and the likely breach of the lower levee, spills
from South Boulder Creek are assumed to first discharge through the local 48-inch
culvert under the on ramp to its capacity, with the remaining flow traveling west to
the Viele channel and offline storage upstream of the ramp.

The Anderson Ditch culvert under the on ramp and freeway was assumed to be 25
percent plugged based on our observations.

The Anderson Ditch culvert may consist of an 8-foot by 4-foot concrete box culvert
at the upstream end and switch to either a double 54-inch RCP or double 48-inch
RCP at the downstream end. To simplify the calculations a single 8-foot by 4-foot
culvert was assumed for the entire length.

The exact outlet configurations of the Viele side channel storage and of the Table
Mesa storage were not known. The best information available was used in the
model. The outlets should probably be surveyed and the correct discharge storage
relationships used.

The size and route of the existing storm sewer running along Foothills Parkway has
not been verified. The initial model assumes a 2-foot diameter pipe discharging to
the west storage at Foothills and Baseline.

The Thunderbird Lake at Pawnee Drive and Mohawk Drive was ignored for flood
control purposes as directed by the City.

The area to the west of Foothills Parkway and Baseline Road along Inca Parkway,
Mohawk Drive, and the surrounding area have storm sewers that appear to

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