FLOOD10337 ‎(2)‎

• County: Arapahoe
• Community: Greenwood Village
• Stream Name: Upper Goldsmith Gulch
• Title: Flood Hazard Area Delineation, Upper Goldsmith Gulch
• Date: 4/1/2005
• Prepared For: Urban Drainage and Flood Control District, Arapahoe County and Greenwood Village
• Prepared By: Moser and Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTION 4 FLOOD HAZARD AREA DELINEATION
4.1 GENERAL 4.3 COMPARISON TO CURRENT EFFECTIVE FLOOD INSURANCE STUDY
The FHAD identifies those land areas, buildings, structures, and roadways which are expected to be The streams studied as part of this FHAD and the study limits coincide with the effective FIS study. The
inundated during a 100-year flood event. In addition, floodway areas within the toO-year floodplain must 100-year floodplain from the applicable flood insurance rate maps was scanned and overlaid onto the
be kept free from obstructions to ensure that the flood can be conveyed without adversely increasing the project mapping to make a comparison with the FHAD 100-year floodplain. In general, the FHAD
water surface elevation and the energy grade line. The floodway is the zone whose boundary is defined by floodplain and the FEMA effective condition floodplain are consistent.
the effects of encroachment raising the water surface or energy grade line to a certain limit. For this study,
the floodway was established for an increased energy grade line of up to 0.5 feet and 1.0 feet.
4.2 FLOODPLAIN AND FLOODWAY
The toO-year floodplain and water surface profile are illustrated on Sheets 1 through 9 in Appendix E. Also
included on the profile is the 10-year event. Other hydraulic information included on the drawings is
channel cross section locations, drainage structure locations, drainage structure waterway opening and
elevations, and the channel thalweg. Each of the sheets also includes a cross section of the channel
depicting the toO-year discharge and average flow velocity.
The floodplain information in Appendix A describes the results for the hydraulic modeling for the 10-, 50-,
100-, and 500-year flood events. It also includes the floodway modeling results for both the 0.5 feet and 1.0
foot increase in the energy grade line.
The water surface profiles were determined using the HEC-RAS program. The floodway was established
using encroachment Method 5 in the HEC-RAS program. The floodway was further refined using
Method 1. Encroachments were made based on equal conveyance reduction until approximately the target
level of energy grade line increase (either 0.5 foot or 1.0 foot) was realized.
There are several locations within the study limits where implementation of the regulatory floodway would
result in only a small distance of encroachment. There are also locations where existing residential
development is directly adjacent to the floodplain that lies within established green-belt areas. In these
locations, there was no encroachment allowed and the floodway was set equal to the floodplain boundary.
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