2005-07-13_REVISION - M2000016 (28)

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amec~ suejea LaFarge H&H made W ACR Joe 4271-716054 Ft. Lupton Drainage cneacedty ~ft-~. J" ~ Date 10/11/04 Perimeter Channel Design Approved by sneer 1 of 2 OBJECTIVE: Assess the existing drainage along the east side of the Rittenhouse property and design a channel to route intermittent flows azound the property to the South Flatte River. Provide a typical cross-section and lining requvements for the channel. METHOU.i Estimate the•peak flow for the contnbuting drainage basin using the Rational Method. Calculate rainfall intensity in incfies/hour according to the methodology described in NOAH, 1973. Calculate time of concentration based on sheet flow,. shallow concentrated flow and channel flow. Design the channel design using a spreadsheet based on Manning's formula. ASSUMPTIONS: • The design will be based on existing conditions. • The County will not require floodplain and floodway analysis of the South Platte River and/or the rerouted tributary. • A wetland evaluation will be completed by others. • .Easements and rights-of--ways will be available on Lafarge controlled property to complete this project and any legal recording or survey of a new easement orright-of--way will be completed by Lafarge or under a separate agreement with AMEC. • Lafarge will supply adequate survey information for the proposed alignment. • AMEC will only define the contributing subbasin and the related peak flow. • Lafarge will provide AMEC with the channel slope. • AMEC will not provide a plan & profile, specifications, anerosion control plan or construction details (except maybe the lining). • AMEC will not meet with the County, and will not obtain permits for this project. • Rational Method Runoff C-Coefficients for light industrial = 0.8 • Rational Method Runoff C-Coefficient for pasture = 0.3 • Manning's n for grassed channels = 0.030 for stability calculations, and = 0.035 for capacity calculations. • Allowable velocity forgrass-lined channel in erosive soils during the 100-year storm event = 5.0 fps (Urban Drainage Table MD-2). • Apply the 100-yr, 1-hr rainfall intensity for subbasins with times of concentration (Tc) longer than 60 minutes. CALCULATIONS: • Basin Areas (see Figure 1 for basin locations): 2000 ft 2000 ft lacre Area measured w/ planimeter = 0.8680in2 * * * = 79.7ac ? 80ac in in 43560ft2 2000 ft 2000 ft lacre Hardpack Area measured w/ planimeter = 0.2480ittZ * * * = 22.8ac =_ 23ac in in 43560 ft: Remainder of Area is pasture, characterized by short, scrub grass and alfalfa = 80-23=57 ac