FLOOD00584

• County: Boulder
• County: Weld
• Community: Erie
• Stream Name: Coal Creek
• Title: Hydraulic Analysis and Floodplain Revision for Coal Creek Flood Control Project
• Date: 9/1/1993
• Prepared For: Erie
• Prepared By: WRC Engineering, Inc.
• Floodplain - Doc Type: Project

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The project includes construction of a storm sewer outfall at Evans Street (between Sections 18,5 and
18.6), The outfall was recommended in the Town of Erie, Master Drainage Study, prepared by ],R,
Engineering, August, 1989 (reference 7) and will provide drainage from the Town behind the levee, The
outfall will be constructed with a flap gate to prevent high flows from entering the Town, In addition to this
outfall, the aforementioned master plan recommended storm sewer outfalls at Wells Street and Moffatt Street
(between Sections 19,2 and 20,0), There is no levee along this reach and the proposed improvements do not
alter the existing grades in this reach. Therefore, they are not included as part of this project.

V. HYDROLOGY

The flood discharges used in this study were developed by the Soil Conservation Service (SCS) -

Reference No, I, An independent detailed hydrologic analysis was not included as part of the engineering

service for this study, After a review, it was determined that the SCS flood discharges adequately reflected

the potential year 2010 basin conditions and thus adjustments to the existing hydrologic analysis were not

justified,

The flood discharges used in this study are presented in Table-I. The discharges fluctuate slightly
. along the study reach due to the changes in floodplain storage and drainage area,

VI, IMPROVEMENT HYDRAULICS

A hydraulic analysis of the study area was completed to determine the water surface elevations and

t100dplain delineation for the 10-, 25-, 50-, 100- and 500-year storm events, The elevations were complied

using the Corps of Engineers' HEC-2 water surface profile computer program for personal computers, Cross-

sections were measured from left to right looking downstream utilizing aerial photography provided by the

Town and Colorado Water Conservation Board (CWCB),

The HEC-2 analysis for this study was started using the original Section 14.4 from the FIRM study,

The CWCB study was used for the starting water surface elevations of the FIRM study, Original Sections

27 and 28 of the Y1ichael Baker, Ir" Inc, study, terminate the HEC-2 analysis, A detailed section by section

discussion of the improvements as they affect the existing HEC-2 analysis is ~resented in Chapter VIII,

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