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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HYDROLOGIC ANALYSIS

A. Flood Characteristics

Flooding along Dry Creek No.1 is most likely to Occur during the

May through August period. Major flooding will result from intense thunder-

storms which produce heavy rainfall over relatively small areas. A 100-year

flood is expected to rise to its peak. flow within 2 to 3 hours from the most

intense part of the storm and recede within 6 to 8 hours after the initial rise.

The hydrologic analysis required extensive hydraulic computations

to determine peak flows at locations where the flood water was divided.

Diversion of flows at road crossings, particularly at State Highway 119,

had a significant effect on theoretical peak flows downstream. The

methods used in the hydraulic analysis are explained in the following

chapter.

B. Method of Analysis

The hydrologic analysis established the 5-, 10-, 50-, 100-, and

SOD-year frequency flood discharges along the Dry Creek No.1 study reach.

These discharges were developed based on existing basin development con-

ditions. The projected flood discharges for the year 2000 were also

predicted for the stream reach downstream from S.H. 119. The future

discharges were based on anticipated densities of development at that

time.

Discharge - frequency relationships were developed using the Environ-

mental Protection Agency's "Storm Water Management Model," (SWMM) computer

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