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<br />GLENWOOD SPRINGS FIS <br /> <br />The base flood is the I OO-year flood and the designated height for this study is one foot. <br /> <br />The unique nature of the Roaring Fork and Colorado River presented some <br />difficult challenges in determining the floodway while eliminating negative surcharges. <br />Both rivers in the study area are confined to entrenched channels. The 100'year <br />floodplains are therefore largely confined within the river channel except for a few <br />locations where the floodwaters exceed the main channel capacity. Initial HEC,RAS runs <br />resulted in numerous large negative surcharges in both channels when the floodplain was <br />encroached upon. At the boundary condition, a one-foot surcharge is added to the base <br />condition, because much of the Colorado and Roaring Fork Rivers are entrenched, the <br />water surface will return to the base condition before an area is reached where the flow <br />can be encroached upon by the floodway. When this condition occurs (section can be <br />encroached upon, while downstream section water surface is at the base condition) and <br />the flow is encroached upon a negative surcharge results. The negative surcharges occur <br />because the 100-year floodplain is mostly confined within the river channel. When the <br />encroachments are added on the floodplain the wetted perimeter is drastically decreased <br />while the cross,sectional area is only slightly decreased. This greatly increases the <br />hydraulic radius (cross-sectional area divided by wetted perimeter), which increases the <br />velocity, so the water surface drops. Removing the encroachments and allowing the <br />floodway to be the floodplain could eliminate all the negative surcharges. As soon as the <br />encroachments on the floodplain are inserted in the modeling runs, small negative <br />surcharges began to appear and they tend to propagate to upstream sections. The final or <br />"optimal" run was selected which minimized the number of negative surcharges for the <br />entire study reach but which allowed for the maximum encroachment on the floodplain <br />up to the one foot limit allowed by FEMA guidelines. <br /> <br />Table 4.1 shows the one location where a negative surcharge in the HEC-RAS <br />model that was unable to be removed. River station 154+07.5 on reach 2 of the Colorado <br />River, upstream of the junction with the Roaring Fork River, it is at a bridge section, and <br />the negative surcharge was unable to be removed because the downstream flow is <br />contained inside the stream banks (no backwater effects were able to be generated). It is <br />uncertain as to why the negative surcharge was generated at the above station because <br />there are no encroachments on the flow below or above the mentioned section. <br /> <br />Table 4.1. <br /> <br />River <br />Colorado <br />Colorado <br /> <br />Location of negative surcharge in Glenwood Sprin s, HEC-RAS model. <br />roTOeffiil <br />Reach River Sta Profile W,S. Elev WS I <br />2 15407.5 BR DBase 100yr 5730.12 I <br />2 15407.5 BR D 100yr + 1 ft 5730.1 .0.02 <br /> <br /> <br /> <br />5 <br />