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FLOOD00385
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Last modified
11/23/2009 1:21:52 PM
Creation date
10/4/2006 9:15:03 PM
Metadata
Fields
Template:
Floodplain Documents
County
Boulder
Community
Boulder
Stream Name
Boulder Creek
Title
South Boulder Creek Interim Hydrology Study
Date
7/1/2000
Prepared For
Boulder County
Prepared By
Taggart Engineering Associates, Inc.
Floodplain - Doc Type
Floodplain Report/Masterplan
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<br />1-2 <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 />I <br /> <br />Another historic facility that has substantial effects on actual hydrology is the rail <br />line to Moffatt Tunnel. When the rail grade was constructed, the material <br />excavated from ridges and tunnels was placed in valleys. <br /> <br />As the culverts placed are generally small, substantial storage exists. At one <br />location, South Draw, the crossing provides on the order of 80 to 100 acre feet of <br />storage, and practically reduces the 100-year peak to less than half. The next <br />tributary to the west has a much smaller storage with no apparent culvert, and the <br />runoff seeps through the embankment. We have not visited the other railroad <br />storage sites. We believe there are 20 to 30 such storages, and total storage could <br />amount to 500 to 1000 acre feet. There are roughly 5.1 square miles of tributary <br />area to the rail line below Gross (or about 12 percent of the watershed below <br />Gross) and 1.2 square miles of tributary area to the rail line above Gross. <br /> <br />The hydrology herein does not and cannot assume these rail line storages to be in <br />place under current regulations until adequate institutional arrangements and <br />appropriate improvements are made. It is also important to recognize these railroad <br />storage areas pose a potential washout hazard, which could result in higher floods <br />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 <br />flashy stream. Some segments with a shallow main stream overflow to <br />moderately wide meadows. These segments would attenuate flows. In addition, <br />there are other main stream segments which have deeper and steeper channels <br />which would increase peaks. <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 the West <br />Valley Overflow. As shown on Drawings 4 through 11, South Boulder Creek and <br />the West Valley Overflow repeatedly diverge and converge. The West Valley route <br />often has a lower elevation than the corresponding elevation of South Boulder <br />Creek across the floodplain. Dry Creek Ditch No.2 diverts irrigation water in the <br />midst of the upper split (see Drawing 4, element 1423). Generally Dry Creek Ditch <br />No.2 follows a shallow ridge between the two flow paths. However, at several <br />locations the Dry Creek Ditch No.2 is crossed by or carries flood waters. <br /> <br />The Flatirons Gravel Pit and Levee pushed the West Overflow toward South Boulder <br />Creek (see Drawings 5 and 6). Without the levee, most of the West Valley <br />Overflow would flow directly to the west of the Foothills and Highway 36 <br />Interchange. Regardless of the levee, significant portions of flows in South Boulder <br />
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