FLOOD03181

• County: Weld
• Community: Greeley
• Stream Name: Cache La Poudre River
• Basin: South Platte
• Title: Master Floodway Plan
• Date: 8/1/1983
• Prepared For: Greeley, CWCB
• Prepared By: Simons Li & Associates Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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III. HYDRAULIC ANALYSIS

Floodplain Analysis

The original floodplain analysis performed by the U.S. Army Corps of

Engineers (COE) utilized an in-house computer model to calculate backwater

curves. AS part of this study, the results of the previous hydraulic analysis

were verified utilizing the U.S. Army Corps of Engineers' HEC-2 water surface

profile computer model (reference 4).

The first step of the verification process was to conduct field surveys

to measure existing structures and to select appropriate Manning's "n" rough-

ness coefficients. A separate nn" value report has been prepared and is on

file at the City of Greeley and the Colorado Water Conservation Board

(reference 5). The "n" values used in this study are from 0.035 to 0.040

for the main channel and from 0.035 to 0.055 for the overbanks. Digitized

cross sections from the previous COE study were obtained and located on the

topographic maps. The cross sections were checked for accuracy against the

topographic mapping and, in general, no major discrepancies were noted. The

digitized cross sections were then formatted into HEC-2 format. Initial

analysis was performed using the original COE parameters and comparisons of

the 100-year water surface elevation were made. The initial analysis showed

discrepancies in water surface elevation of about one foot at some locations.

The verification process was continued by modifying the loss coefficients

such as expansion and contraction losses, bridge and pier losses, and roughness

values until the water surface elevations determined by the previous study and

by this study were within 0.1 foot.

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Floodway Analysis

Once the 100-year water surface elevation was verified, an encroached

floodway section was established. The floodway is defined as that area that

is required to convey the 100-year flood allowing a one-foot rise in the

natural water surface elevation. A schematic of a floodway is shown in

Figure 6. The overbank area is encroached to a point where the 100-year water

surface is no more than one foot higher than the natural 100-year water sur-

face. This allows development of property located within the area between the

floodway boundary and floodplain boundary.

The HEC-2 water surface profile computer model has six methods for deter-

mination of the floodway. The method used in this study was to establish

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encroachments based on reduced conveyance of the overbank section. This

method encroaches the floodplain on either side of the channel proportional to

the natural capacity of the overbank. The floodway determined in this manner

is approximately equal to the floodway as given in the report entitled "Flood

Insurance Study, City of Greeley, COlorado, Weld COunty" (reference 3). The

floodway for the Greeley reach of the Cache la Poudre River is shown in

Figures 7, 8, and 9.

Bridge Analysis

A factor compounding flooding problems within the City of Greeley is the

presence of undersized bridges. The bridges at 8th Street, 5th Street and 6th

Avenue cannot pass the 100-year flood. These bridges are overtopped by floods

exceeding a 50-year return period. In addition, the limited hydraulic capa-

City of these three bridges causes backwater conditions resulting in increased

water surface elevations upstream of the bridges. This backwater condition

also results in deposition of sediment which reduces channel capacity.

The 11th Avenue, 8th Avenue, U.S. 85, and Union Pacific Railroad bridges

all have sufficient hydraulic capacity to pass the 100-year flood without

overtopping. A summary of bridge capacities is given in Table 2.

Flow Splits

The Greeley reach of the Cache la Poudre River experiences flow splits

during flooding. At some locations along the river (i.e., upstream of 6th

Avenue), the bank will be overtopped and floodwaters will flow along a path

parallel to the river, rejoining the main flow further downstream. In some

cases within the City, the existing flood control levee prevents these over-

bank flow splits from rejoining the channel, creating ponded areas within the

floodplain. This occured along the west bank between 5th Street and 8th

Street during the June 1983 flooding, The dikes upstream of 8th Avenue con-

tained the floodwaters in 1983, but if the dikes had failed, a large residen-

tial and commercial area would have been inundated, resulting in considerable

property damage.