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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valley, and that Dry Creek Ditch No.2 at first follows the left or west side of the
valley until it crosses the present location of Highway 36, where it goes down the
middle of the valley. To the left or west side of the ditch, the contours are lower
than the ditch location. Thus flood flows could make it to the west side of the
valley once they were higher than Dry Creek Ditch No.2, which is a likely
occurrence in times of flood.

There are two geologic zones of alluvium, based upon USGS geology maps and
report. The large center zone, denoted as Piney Creek Alluvium, is primarily
associated with alluvium coming from the mountain area drained by South Boulder
Creek and is related directly to "modern stream floodplains." The adjacent
Broadway and deeper Louvers Alluvium have surface grades that are generally
higher than the modern floodplains, but can have beds below the modern
floodplain. They illustrate a broad potential floodplain. The Piney Creek Alluvium
coupled with the historical topography illustrates a much wider potential floodplain
than the FHAD or FIRM maps and the tendency for the streams and floodplains to
migrate.

There is a significant tendency for the main channel to spill to the West Valley
above Highway 93 and Highway 36. If a major event were to transport sediment
from the mountains, we believe this tendency would increase. The east to west
valley tendency occurs downstream of Marshall Road and in most of the lower
South Boulder Creek floodplain.

Sediment transport potential for long term movement of sediment and bed change
is slowed by Gross Reservoir, which traps sediment from a majority of the
watershed. However, peak flood flow rates at the mouth of the canyon are unlikely
to be significantly reduced by Gross Reservoir because of the steep, narrow, and
large area below the reservoir. Therefore the stream at flood stage has nearly the
same capacity to transport large sizes of material as in pre-Gross conditions, but
the total volume of sediment would be reduced.

Further, there are numerous local effects which could lead to deposition and could
lead to migration, such as at bridges, diversions, and low capacity channel sections.
Also there are reaches where migration is potentially more likely, such as in the
washing pits and at the bank upstream of Highway 36 where the spill presently
occurs. These tendencies can affect the flow splits assumed herein, and thus
affect the hydrology of South Boulder Creek and West Valley Overflow flows and
duration of flooding.

The hazards associated with breaching or overtopping of the upper or middle
segment of the Flatirons Levee need to be carefully analyzed for the aspect of major
stream migration to the West Valley and for related worsening of the flood hazard
for extreme events.

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