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<br />OCT-30-96 WED 14:03 <br /> <br />BRR BLD 53 DFC <br /> <br />FAX NO. 2365034 <br /> <br />P. 07 <br /> <br />. ~ <br /> <br />partially obatructed by debris caught on a metal cattle fence, thus, causing the reported ponded <br />water. The flow through the culvert probably wu ..timated to b. about 360 ft3ls. For a flow <br />depth of 18 inch.. oyer the road (unverified eyewltne'l 'Itimate), the corresponding width i. <br />about 400 feet Ther. Is evidence of some riU and gully erosion on the downstream side of the <br />highway embankment and flood debris on the downstream Jence averagIng about 0.25 foot depth <br />above the ground. Thl. supports that although some water appears to have flowed over the road, <br />depths and veloclti.. were not large enough to cause more erosion or debris to be collected ~Jicrr" .'?~ <br />higher on the fence. The estimated road overflow (using the road overflow equation Q . CLH~ <br />was about 1,200 to 1,300 Ws for a total of about 1,500 to 1,600 Ws. Because no data <br />(othar than eyewitnasa ntimatea) of flow depth over the road could be found to verify the depth <br />of flow over the road, this flood estimate II considered extramaly poor, , <br /> <br />Downstream from thia culvert and highway embankment to Cow Hollow Creek, good high-water <br />marks (HWMs) are preserved throughout the floodplain, which correspond to the flow depth at <br />the culvert outlet. The HWMs consist of organic debris and bottles lodged in the sage brush, fine- <br />grained, loose "dimenta deposited downstream from sage (obstructions that reduced velocity <br />causing deposition) on top of very hard soil. aplays 01 fIood-deposited gravel, wash lines (flood <br />waters removed fines from flood-plaln aurface leaving a surface having numarous pebbles), and <br />surfacII undisturbed by flood watera. Peak discharges were estimated for 31 sites (both visIts) <br />for tha Opalatonn in 11190. As with the Morgan lummary, only selactad flood estimates ara <br />provided to deJlne the maximum peak runoff and areal distribution of the 1990 storm runoff. <br />The peak discharge for Cow Hollow Creek downstream /rom Highway 30 (site 2) was about 1100 <br />ft3Is. A peak discharge of 110 ft3Ia was estimated for Cow Hollow Creek about 1.2 miles <br />upstream from Highway 30 (site 12). A peak discharge of about 800 ft3Is also was estimated <br />about 1 mile downstream near thl confluence wilh Hame Fork. The smaller discharge at the <br />mouth probably is dua to a combination of tha approximate nature oJ the discharge estimate, <br />lesser rainfall (substantially Jewar and amaller rills and overland flow), Elnd ,ttenualion of the <br />flood peak. Attenuation is likely because of the ralalively flat slope (0.0037 ftlrt) and wide, <br />densely vegetated flooclplain. The nel peak di.charge (site 2. site 12) is about 800 ft3/s for <br />about 3 mi2; tha corresponding unit discharge Is about 265 fl3Islmi2. Spatially, the 1990 <br />Opal storm runoff was limited to epproxlmately 10 mi2 to a maximum of 15 mi2 (figure 4). <br />The area of the 11190 storm runoff was estimated 'rom a delineation oJ aitas having minimal <br />streamflow runoff and rills and llully erosion from lhe intense runoff during the 1990 <br /> <br />6 <br />