FLOOD05793

• County: Boulder
• Community: Longmont
• Stream Name: Dry Creek
• Basin: South Platte
• Title: Technical Addendum Floodplain Information and Flood Control and Drainage Plan
• Date: 4/1/1980
• Prepared For: Boulder County
• Prepared By: Water Resource Consultants
• Floodplain - Doc Type: Floodplain Report/Masterplan

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HYDRAULIC ANALYSIS

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A. General

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Water surface profiles have been computed for the study reach for

5-, 10-, 50-, 100- and 500-year frequency floods. These flows are listed

1n Table-4 already presented on page 32.

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The hydraulic characteristics of the study reach were documented based
on two separate field investigations. Culvert and bridge geometry, channel
and overbank roughness coefficeints, flow obstructions, and the overall
characteristics of the area were assessed during these investigations.
The Manning's roughness coefficients used in the hydraulic analysis were
documented in a separate report prepared as part of the technical addendum.
Photographs were included in the report to support the recommended values.
Recommended roughness coefficients ranged from 0.030 to 0.060 for the
channel and from 0.040 to 0.070 for the overbank areas.

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B. Procedures

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The water surface elevations for the 5-, 10-, 50-, 100- and 500-year

floods were computed using the Corps of Engineers HEC-2 Step Backwater

Computer Program. This program utilizes a solution to the one-di~ensional

energy equation to determine the shape of the profile between control

section where the water surface elevation is known or can be assumed. The

procedure for a steady flow profile calculation is called the "Standard

Step Method." In this method, the distance from a downstream or upstream

point, where the conditions are known, to the point where the backwater effects

are to be determined, is divided into reaches by cross sections at fixed

locations along the river. Starting from one control point, calculations

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