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FLOOD03719
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FLOOD03719
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Last modified
1/25/2010 6:28:05 PM
Creation date
10/4/2006 11:57:45 PM
Metadata
Fields
Template:
Floodplain Documents
County
Adams
Community
Thornton
Stream Name
Big Dry Creek
Basin
South Platte
Title
Floodplain Information Report
Date
4/15/1988
Prepared For
Thornton
Prepared By
McLaughlin Water Engineers, Ltd.
Contract/PO #
&&
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />I <br />I <br />II <br />I <br />II <br />I <br />II <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />'I <br />I <br />I <br /> <br />As in previous studies, Big Dry Creek is modeled with the split flow option along <br />Washington Street. A separate HEC-2 run (see the Technical Appendix) was used to <br />determine water surface elevations just to the east of Washington Street. <br /> <br />Since no improvements downstream (north) of 136th Avenue will occur during golf course <br />construction, the U DFCD f100dway defined for this reach should remain as shown in <br />Muller's outfall study. <br /> <br />PROPOSED CONDITIONS DOWNSTREAM OF 136TH AVENUE <br /> <br />Muller's (Aug. 1987) outfall study defines improvements to a portion of the creek <br />channel where it crosses under North Washington Street. The improvements include a <br />low-flow channel, a drop structure, bridge replacement, relocation of the German Ditch <br />diversion, and raising Washington Street from 136th Avenue north for a distance of <br />4,150 feet (approximately 142nd Avenue). <br /> <br />These proposed improvements were modeled with HEC-2 analysis of Big Dry Creek. The <br />change in the tailwater elevation at 136th Avenue was negligible when compared to the <br />existing condition (including the recent road improvements) analysis. The proposed <br />improvements HEC-2 file, including improvements to the 136th Avenue bridge, are <br />provided in the Technical Appendix. The improvements are shown in plan view on <br />Drawing No.2. <br /> <br />The proposed storm water outfall system improvements were based on criteria requiring <br />that North Washington Street not be overtopped during the 100-year flood. The water <br />surface profile for the proposed bridge shows that the low chord would be submerged <br />during the 100-year flood. After consulting with City Public Works Department <br />engineering staff members and the UDFCD, the criteria was revised to provide 1.0 foot <br />of clearance at the low chord of the proposed bridge for this study. These revisions <br />and the associated channel and road improvements were modeled with HEC-2. This <br />analysis also proved to have negligible effects on the tail water conditions at 136th <br />Avenue. <br /> <br />During the course of the analysis of these proposed improvements, required changes to <br />the present outfall study became evident. The proposed bridge structure on North <br />Washington Street (approximately 138th Avenue) must be raised approximately 1.6 feet <br /> <br />1lI-3 <br />
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