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2009-05-07_REVISION - M2000158
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2009-05-07_REVISION - M2000158
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Last modified
6/16/2021 6:23:32 PM
Creation date
5/8/2009 11:15:15 AM
Metadata
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Template:
DRMS Permit Index
Permit No
M2000158
IBM Index Class Name
REVISION
Doc Date
5/7/2009
Doc Name
TR Submittal
From
The Hayes Law Firm LLC
To
DRMS
Type & Sequence
TR5
Media Type
D
Archive
No
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CIVIL RES-M)URCES,LLC <br />ENGINEERS & PLANNERS <br />MEMORANDUM <br />TO: United Water and Sanitation District, Project File <br />FROM: Civil Resources, LLC <br />DATE: February 13, 2009 <br />RE: Gilcrest Resource - Floodplain Analysis <br />This memo summarizes assumptions and results of modeling efforts to estimate the floodway limits of the South Platte <br />River adjacent to the Gilcrest Resource property. This data was used to evaluate mining options for the property. <br />Location <br />The site is an approximately 2,150 acre property and is located two and a half miles west of the intersection of Weld <br />County Road 38 and Highway 85 just south of Gilcrest, Colorado, This property is located east of the South Platte River <br />and the St. Wain River confluence in Section 35, Township 4 North, Range 67 West and Section 2, Township 3 North, <br />Range 67 West. See Figure 1. <br />Existing dafa <br />Aerial topography was generated for the site and provided to Civil Resources on February 7, 2009. The aerial <br />topography was assumed to significantly represent the existing disturbed site conditions and was generally confirmed by <br />visual inspection. Autodesk Civil3D was used to create surfaces and cut cross-sections for use in the USACE hydraulic <br />modeling software HEC RAS. Cross-sections used in the model are attached to this memorandum. Per FEMA there <br />have been no detailed floodplain studies on the South Platte near the subject property. <br />HEC-RAS Model <br />Civil Resources utilized the River Analysis System developed by the Hydrologic Engineering Center (HEC-RAS) Version <br />3.1.3 as the hydraulic method to analyze the hydraulics, floodplain and the floodway of the South Platte River adjacent to <br />the site. Below is a list of inputs that were used in the model: <br />• Cross-Sections: Cross-sections of the existing grade were taken from 2-foot photogrammetric mapping; <br />• Mannings (n) Roughness Coefficient - Roughness coefficient (n) values of 0.055 and 0.04 were applied for the <br />for the river channel and adjacent area (approximately 600 feet wide river bottom) and the overbanks, <br />respectively. <br />The modeling incorporated three separate river reaches reflecting: 1) the St. Vrain River ; 2) the South Platte River <br />upstream of the confluence; and 3) the South Platte River downstream of the confluence respectively. The 100-year <br />flow in the South Platte was estimated to be 33,600 cfs based on a previous study by Tetra Tech RMC accepted by <br />Weld County for a reach located upstream of Gilcrest Resource site near State Highway 7. The flow in the St. Vrain <br />River was estimated to be 17,290 cfs for the 100-year storm event, however, only fifty percent (8,645 cfs) of this peak <br />flow was assumed to be coincident with and therefore additive to the South Platte River Basin peak considering the low <br />probability that the two basins would observe 100-year precipitation events at the same time and that the peaks would <br />closely coincide due to differences in basin characteristics and associated response times. <br />323 FIFTH STREET . FREDERICK, COLORADO 80530 • PHONE: 303.833.1416 • FAX: 303.833.2850
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