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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

SECTION V

RECOMMENDATIONS

SUMMARY

The Phase I and II work effort has revealed significant problems. In order to
address these problems there are three basic recommended actions:

1. Refine South Boulder Creek and local tributarv hvdrology. The Corps'
hydrology is dated, has basic errors in physical parameters, particularly area,
and the rainfall data used does not adhere to NOAA or UDFCD practices.
While some hydrology has been conducted by McLaughlin Water Engineers
and WRC for local drainage in the overflow area, it is not adequate to
simulate the spills and hydraulic phenomena that will actually take place in a
100 year local or South Boulder Creek spill event. Recommended
refinements include:

. Determination of catchment areas below Gross Reservoir,
. Use of a current rainfall methodology,
. Correction of other catchment and stream element parameters.

2. Floodolain and hazard delineation. The public needs to be notified of the
problem, and the problem needs to be better defined. Thus a delineation of
the West Valley area needs to be prepared, both for direct local drainage and
the West Valley overflow. A significant portion of the South Boulder Creek
flow, depending on the size of the runoff event, will be spilled to the West
Valley Overflow at Highway 36 (approximately 42% of the 100 year event)
and routed to points downstream such as Baseline (approximately 40% of
the 100 year event) and Arapahoe (approximately 52% of the 100 year
event). Fortunately much of this West Valley flow will be shallow to
moderate depths in streets (1.5 to 3.5 feet) with exceptions at storages,
ditches, sumps, and points where the flow concentrates. Nevertheless the
public needs to be made aware and encouraged to purchase flood insurance
and evaluate measures that can mitigate or prevent damage such as
flood proofing.

3. Master Planning. The need for a collaborative planning effort has been
previously stressed. All parties will suffer with the present situation. There
is no apparent simple or quick solution. The ultimate plan will probably use a
number of hazard management techniques and physical improvements
including local drainage conveyance enlargement and measures to limit the
frequency and severity of South Boulder Creek spills to the West Valley.

V - 1