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Last modified
1/29/2010 10:15:26 AM
Creation date
10/5/2006 5:01:13 AM
Metadata
Fields
Template:
Floodplain Documents
Designation Number
148
County
Adams
Arapahoe
Douglas
Community
Denver Metro Region
Stream Name
Harvard Gulch, West Harvard Gulch & Dry Gulch
Basin
South Platte
Title
Flood Hazard Area Delineation - Harvard Gulch, West Harvard Gulch and Dry Gulch
Date
12/1/1979
Designation Date
1/30/1980
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 <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />West Harvard Gulch <br /> <br />A floodway represents a part of the floodplain which is required <br /> <br />to pass a lOa-year flood event without raising the flood profile <br /> <br />more than an acceptable amount and also represents that part of <br /> <br />the floodplain most hazardous to personal safety and welfare. In <br /> <br />this study, the floodway defines a flow area required to convey the <br /> <br />lOa-year event without increasing the lOa-year water surface by more <br /> <br />than 0.5 foot and having a depth greater than 1.5 feet. Also, flood- <br /> <br />plain areas where average velocities exceeded 2 to 3 feet per second <br />were considered to be within the floodway. Floodway widths, which <br />represent the maximum limits of encroachment into the floodplain, <br />are tabulated in Tables 3, 4 and 5. <br /> <br />At the confluence of the South Platte River, the main channel of <br />West Harvard Gulch flows through a 7-foot diamater concrete pipe <br />under the Platte River Drive. This pipe is inadequate to carry <br />all the laO-year flood flow. Consequently, approximately 1,200 <br />cfs will flow overland causing shallow flooding in the area of the <br />Arapahoe Power Plant. Depths will be approximately 1 to 2 feet <br />with very low v~locities (Sheet 12 and 13). <br /> <br />Between Platte River Drive and the Colorado and Southern Railroad <br />crossing, the laO-year flood flow is contained in the existing <br />natural channel (Sheet 12) . <br /> <br />Flood Velocities <br /> <br />The channel between the Colorado and Southern Railroad and South <br />Zuni Street has been designed as a recreation area with a low <br />flow pipe buried underneath. During the laO-year flood event, <br />200 cfs will be carried in the low flow conduit and approximately <br />1,400 cfs will flow above ground. The floodplain is confined to <br />the channel; however, the Tejon and Raritan crossings will be inun- <br />dated to depths of 2 to 3 feet (Sheets 12 and 14). <br /> <br />Average flow velocities, characteristic of a lOa-year flood event <br />on Harvard, west Harvard and Dry Gulches are shown on the profile <br />portion of the Flood Hazard Area Delineation drawings. The flow <br />velocity in the channel would range generally between 4 and 10 <br />feet per second during a lOa-year event in the study area. Over- <br />bank velocities would range between 1 and 4 feet per second. <br /> <br />Water flowing at a rate greater than 6 to 8 feet per second may <br />cause severe erosion of stream banks and is capable of transporting <br />large amounts of debris. Velocities in the range of 6 to 8 feet <br />per second could erode fill around bridge abutments. <br /> <br />The reach from Zuni Street to South Federal Boulevard consists of <br />the natural channel with few improvements. The lOa-year flood <br />event is confined to the channel except for some ponding and <br />shallow overtopping at the Zuni, Clay and Decatur Street crossings <br />(Sheets 14 and 15). <br /> <br />- SUMMARY - <br /> <br />With the exception of the shallow flooding area around the Arapahoe <br />Power Plant, there are no structures in the West Harvard Gulch <br />floodplain. <br /> <br />This report has identified the probable flooding limits of a po- <br />tential lOa-year flood event in the Harvard and West Harvard Gulch <br />drainage basins. <br /> <br />Floodplain and Floodway Data <br /> <br />The limits of future flooding for Harvard, West ~arvard and Dry <br />Gulches are graphically outlined on the Flood Hazard Area Delinea- <br />tion Haps and tabulated in Tables 3, 4 and 5, Floodplain and <br />Floodway Reference Data. The tables list the potential flood ele- <br />vations, floodplain and floodway widths, floodway location, flood- <br />way surcharge elevation and thalweg elevation at each cross section. <br /> <br />The floodplain limits are based on (1) planned, future development <br />in the basin, (2) existing and mapped floodplain conditions, and <br />(3) no blockage at the existing culvert and bridge structures. <br />Even with these assumptions, floods of greater magnitude can and <br />will occur in which the limits of flooding will exceed those <br />shown in this report. <br /> <br />-7- <br />
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