FLOOD06925

• County: Larimer
• Community: Greeley
• Stream Name: Cache La Poudre River
• Basin: South Platte
• Title: Master Floodway Planning
• Date: 3/11/1983
• Prepared For: Greeley
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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8

eight short reaches and known starting water surface elvation

because the river is so flat that the flooding is mostly caused

by backwater. The adjustment of "n" values and channel expan-

sion and contraction coefficients can be applied to the short

reaches for better results. Modification of the channel bed

elevation will not be used unless it is absolutely necessary.

The calibration procedure involves a trial and error process.

Based on our experience, the COE study results can be repro-

duced to an acceptable range utilizing known starting water

surface elevations at short reaches and the adjustment of "n"

values and channel expansion and contraction coefficients.

After the HEC-2 output is properly calibrated, a floodway will be

established. The criteria for delineating the floodway will be the FEMA cri-

teria of allowing a one-foot rise in water surface elevation over the existing

lOO-year water surface elevation in the floodplain. The HEC-2 water surface

profile computer model has six different methods of determining floodways.

These methods are as follows:

el) Encroachments are established by program user.

(2) Encroachments are established by a width specified by the
program user.

(3) Encroachments are determined by a specified reduction in area;
both overbanks are reduced by equal areas.

(4) Encroachments are determined by a specified rise in water sur-
face elevation. Encroachments are established so conveyance is
equally reduced in overbanks.

(5) Encroachment stations are established by optimizaton techniques
based on a target change in water surface elevation.

(6) Encroachments are established by the same technique as Method 5
except that instead of a change in water surface elevation, a
change in energy grade line is specified.

In addition to these six methOds, the State's la-inch water depth and velocity

criteria will also be investigated.

The initial floodway run will utilize Method 4 using a specified rise in

water surface and equal encroachments. Subsequent floodway calculations will

be made to fully utilize the areas designated by the City as desired floodway