FLOOD04065

• County: Weld
• Stream Name: St. Vrain Creek
• Basin: South Platte
• Title: St. Vrain Floodplain Update
• Date: 4/2/1998
• Prepared For: Weld County Unincorporated
• Prepared By: consultants
• Floodplain - Doc Type: Community File

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51. Vrain Creek Floodplain Update

March 5, 1998

3.0 WORK PLAN

The timing of the floodplain update is linked to the construction of the McConnell Drive
Bridge by the developers of the Lyons Valley Park subdivision. At present the design
for the McConnell Drive Bridge shows an encroachment of the floodway coupled with a
compensatory overflow channel to maintain floodplain conveyance without raising base
flood (1 ~O-year) elevations by more than 1 ft. Based upon discussions with Mr. Fred
Metzler of FEMA Region VIII, a Letter of Map Revision will be required by FEMA if
such an encroachment occurs. Alternatively, if the bridge were constructed so that the
piers and abutments are outside the floodway (fully spanning the floodway of about 175
feet) no Letter of Map Revision submittal and review would be required. Because of the
sensitivity of floodway modeling and importance of the Town's compliance with NFIP
regulations, BRW recommends that the submittal by the developer of Lyons Valley Park
be referred to FEMA even if a span of a floodway is provided such that a technical peer
review is made by John Liou of the Denver Region VIII office.

To create a work plan for this assignment, BRW consulted with Mr. Brian Hyde of the
CWCB and Curt Parker of Boulder County to review project goals and objectives, gain
better understanding of available models and field data, and agree upon a process by
which the town can submit this information for inclusion in an updated map to be
processed by Boulder County and the CWCB for submittal to FEMA. The work plan
performed by BRW consisted of the following:

1. Research available information including previous floodplain studies and
records of flooding in local newspapers and perform a cursory evaluation of
previous hydraulic models prepared by other consultants.

2. Conduct a limited field survey to resolve benchmark issues in the Lyons area
and obtain localized topographic information around the McConnell Bridge
area. Additionally, BRW personnel walked the reached of the St. Vrain Creek through
the Town of Lyons to conduct a visual field reconnaissance for verification of the
reasonableness of the existing models.

3. Update the HEC-2 models previously prepared by Engineering Professionals
Inc. (EPI) and Love Associates (LA). This included encoding additional sections
at Station 152.9 to reflect recent construction of a pedestrian bridge by the Town
of Lyons, updating cross sectional data at several sections, and making
modifications and corrections to the previous modeling efforts.

4. Review model results, make modifications for calibration, and assess the overall
reasonableness of the floodplain delineation.

5. Submit preliminary draft to the CWCB for technical review.

6. Revise per comments and submit to Boulder County for inclusion in an updated
St. Vrain Creek Floodplain Delineation Study.

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