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Last modified
1/25/2010 6:26:44 PM
Creation date
10/4/2006 11:34:39 PM
Metadata
Fields
Template:
Floodplain Documents
County
Boulder
Community
Boulder
Stream Name
South Boulder Creek
Basin
South Platte
Title
South Boulder Creek Draft Hydrology Study Phase III
Date
2/10/2000
Prepared For
UDFCD
Prepared By
Taggart Engineering Associates, Inc.
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />. <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 />average of 7.7% of the reservoir volume (3219 acre feet) available in June when <br />the maximum volume of water is stored. The reservoir operating practices could <br />change these statistics. <br /> <br />Another historic facility that we believe has substantial effects on actual hydrology <br />is the rail line to Moffatt Tunnel. When the rail grade was constructed, the material <br />cut from ridges and tunnels was placed in valleys. As the culverts placed are <br />generally small, substantial storage exists. At one location, South Draw, the <br />crossing provides on the order of 80 to 100 acre feet of storage, and practically <br />reduces the 100-year peak to less than half. The next tributary to the west, has a <br />much smaller storage with no apparent culvert, and the runoff seeps through the <br />embankment. We have not visited the other storages, as under current regulations, <br />the effects of such storages cannot be assumed until engineering adequacy is <br />documented and institution arrangements are secured for their maintec.ance. We <br />believe there are 20 to 30 such stor:s, ~nd total storage could aFl'leu~,lLbn5.o0 to <br />nI ,,~o v.J<..w''''<<1 ,~: <br />1 ictJaiCre feet. There is roughly 5.1 ~ri utary to the rail line below Grossl'{or <br />about 12 percent of the watershed below Gross) and 1.2 sm above Gross. <br /> <br /> <br />Again, the hydrology herein does not and cannot assume these rail line storages to <br />be in place under current regulations and without institution arrangements made. It <br />is also important to recognize these railroad storage areas pose a potential washout <br />hazard, which could result in higher floods than predicted by the model herein. <br /> <br />The main waterway of South Boulder Creek has substantial effects on the <br />hydrology because of its flood routing characteristics. In the mountains the <br />sections vary widely. The steeper, narrower tributaries and higher elevation main <br />streams can accelerate flows from upstream and cascade, or in lay terms, cause a L <br />flashy stream. Then there are some segments with a shallow main stream .wllie'" ...;. ,,-1 <br />overflows to a moderately wide meadow. These segments would attenuate flows. <br />Then there are other main stream segme,nts which have deeper and steeper <br />channels ~ would increase peak(, -fl""" ~, <br /> <br />As the flows leave the mountains, the waterways change dramatically to a wide <br />alluvial valley. When depths exceed 2.5 to 4 feet, the water overflows into wide <br />floodplains. Just above Highway 93, South Boulder Creek overflows to what we <br />have referred to as the West Valley Overflow. As shown on Drawings 4 through <br />11, these two routes repeatedly diverge and converge. The West Valley route <br />often has lower elevation than the corresponding elevation of South Boulder Creek <br />across the floodplain. Dry Creek Ditch No.2 diverts irrigation water in the midst of <br />the upper split (see Drawing 4, element 1423 begins the split). Generally Dry <br />Creek Ditch No.2 follows a shallow ridge down between the two flow paths. <br />However, at several locations the Dry Creek Ditch No.2 is crossed by or carries <br />flood waters. <br /> <br />1-2 <br /> <br />
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