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FLOOD10164
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Last modified
1/26/2010 10:12:11 AM
Creation date
10/5/2006 4:56:16 AM
Metadata
Fields
Template:
Floodplain Documents
County
Arapahoe
Community
Littleton
Stream Name
Slaughterhouse Gulch
Basin
South Platte
Title
Major Drainageway Planning
Date
4/1/1974
Prepared For
Littleton, DRCG, DHUD
Prepared By
Frasier & Gingery, Inc.
Contract/PO #
&&
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />i I <br />I <br />i I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />The coefficients used in the computation of the unit hydrograph, <br />Ct and Cp, have been determined both from information published <br />in the Urban Storm Drainage Criteria Manual, and more recent <br />informat1on received from the Corps of Engineers on studies of <br />recorded runoff events in the Cherry Creek basin. Our analysis <br />of these coefficients for each of the five basins gave an <br />average value for Ct of .32 and an average value of Cp of .49 <br />for the total basin. These compared to a Ct of .36 and Cp of <br />.47 computed for the basin under the Project REUSE study. The <br />differences can be attributed to the increased development in <br />the basin and the existence of more storm sewers and paved <br />streets to speed the runoff process, as well as, perhaps a more <br />detailed investigation. <br /> <br />C. DESIGN RAINFALL <br /> <br />The design rainfall for the study areas was taken from iso- <br />hyetal maps in the Rainfall Section of the Urban Storm Drainage <br />Criteria Manual. These maps provide the ra1nfall depths for <br />the 2, 10, and 100-year storm frequencies and 1 and 6 hour <br />storm durations. A 2-hour storm duration is generally used as <br />the critical storm duration in this area. The rainfall data is <br />tabulated in Table III. Since the basin is relatively small, <br />a storm rainfall typical to the basin was used rather than <br />assuming a different design rainfall over each sub-basin. <br /> <br />TABLE III <br />DESIGN RAINFALL <br /> <br />Storm Frequency <br />2 -year <br />10-year <br />100-year <br /> <br />Basin <br />1 hour <br />(1n. ) <br /> <br />.95 <br /> <br />2.10 <br /> <br />2.55 <br /> <br />1-5 <br />6 hour <br />(in. ) <br /> <br />1. 55 <br /> <br />3.30 <br /> <br />4.25 <br /> <br />From the storm rainfall input, the effective precipitation was <br />computed following the procedure outlined in the Runoff Section <br />of the USDC Manual. The effective precipitation is a function <br />of the degree of impervious and pervious area, surface deten- <br />tion, depression storage, infiltration, vegetation interception <br />and an impervious area loss percentage. After evaluation of the <br />soils characteristic of the Slaughterhouse Gulch basin, an <br />infiltration rate of 1/2 in./hr. was selected as being typical of <br />the basin and used in the computation of the storm hydro graphs <br />are: <br /> <br />Impervious depression/detention storage: 0.10" <br />Pervious depression/detention storage: .30" - .40" <br />Impervious loss: 5% <br /> <br />IH-3 <br />
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