FLOOD00591

• County: Weld
• Community: Greeley
• Stream Name: Cache La Poudre River
• Title: Floodplain and Floodway Analysis
• Date: 2/20/1994
• Prepared For: CWCB
• Prepared By: Brown & Caldwell
• Floodplain - Doc Type: Floodplain Report/Masterplan

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

4

4. Construction within the floodway and floodplain requires that the first floor
elevation be higher than the IOO-year water surface elevation.

5. If development is done within the floodplain or floodway to address a health
and/or public safety issue, the FEMA regulations are sllperseded by public safety.
Also, EPA regulations are more restrictive and take precedence over FEMA.

Floodplain Modifications

The current regulatory floodplain for the Poudre River was adapted prior to the
construction of new bridges which cross the Poudre River at Ash Avenue and at 8th Street. The
original Ash Avenue bridge was undersized causing tailwater flooding throllghout the WPCF.
The 8th Street bridge was also undersized, causing upstream tailwater. This 8th Street tailwater
flowed across U.S. Highway 85, north of the Highway 85 Poudre River bridges, creating a split
flow condition where the northern flow path became hydraulically disconnected from the river
channel. The northern flow path entered the WPCF property at the northwest comer where it
combined with the tailwater from the old Ash Avenue bridge. The resultant floodplain was
wide, encompassing the entire WPCF.

The new bridge construction, replacing Ash Avenue and 8th Street bridges with larger
structures, has reduced the floodplain at the WPCF. The present floodplain is not impacted by
tailwater from Ash Avenue since the new bridge passes the major stonn flow. Split flow still
enters the WPCF at the northwest comer; however, the flow volume has been greatly reduced
by the increased bridge size at 8th Street. The flooding at the WPCF is now hydraulically
connected to the Poudre River channel and is shallow overbank flooding.

Areas of naturally high ground on the WPCF, namely the primary clarifiers, aeration
basins, headworks, chlorine contact facilities, blower building, and the sludge digester complex,
are now islands raised above the IOO-year water surface elevation. Floodplain model results are
included as Appendix I.

Floodway Modifications

The current regulatory floodway at the WPCF reflects the backwater condition caused
by the old Ash Avenue bridge. The resultant floodway was large, encompassing the entire
WPCF plant site. BC modeled the floodway under present conditions for the lOO-year stonn
event using HEC-2 program. Floodway model results are included as Appendix J. As the
model shows, the replacement of the Ash Avenue bridge, and the subsequent reduction of the
upstream floodplain, reduces the floodway. This removes large areas of the WPCF from the
redefmed floodway.

'--:-"::::;J''!!'.:'':;

.,:;-,,:"

21/7387