FLOOD06982

• County: Weld
• Stream Name: Cache La Poudre River
• Basin: South Platte
• Title: Flood Plain Information Report Cache La Poudre River
• Date: 4/1/1996
• Prepared For: Orr Land Company
• Prepared By: Messner Engineering Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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HYDRAULIC ANALYSIS

Analysis of the hydraulic characteristics ofthe study reach of the Cache La Poudre River was
carried out to provide estimates of the water surface elevations of floods at the selected
recurrence intervals. The water surface elevations were computed through use of a hydrologic
and hydraulic software program, which is compatible with the Corps of Engineers HEC-2 step-
backwater computer program. Cross sections for the analysis of the Poudre River were obtained
from digitized cross sections of a topographic map for the area and field survey.
Channel roughness factors (Manning's "n") for these computations were assigned on the basis
of field inspection of the floodplain areas and check for consistency with values used in other
previous studies. Manning's values range from 0.03 to 0.05.
Average velocities in the floodplain range from 9,8 to 13.1 feet per second, while average
velocities for the floodway range from 9.8 to 13.2 feet per second, Scour holes do occur under
both of the bridges. This is a common occurrence and in some cases scour holes can be several
feet deep. Cross section data from the topographic mapping was adjusted for actual stream bed
elevations under the bridges where scour had occurred.
Scour holes under and near bridge structures in most cases continue under the entire bridge
and increase the hydraulic capacity of the structure slightly; therefore these depressions were
considered in the hydraulic analysis. These scour holes are a result of flood events that have
occurred before in this area, but do not appear to be filling with sediment during low-flow
periods.
Details of the hydraulic analysis is included in the Technical Addendum,

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