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<br />Not so fortunate were the communily of Amherst, Colorado Stale Highway 23, and the Railroad <br />line lhat parallels highway 23. The same slorm lhat dumped harmless rain on Holyoke was much <br />more inlense over Amhersl and the area near the Colorado-Nebraska state line. Basement flooding <br />and streel flooding were a result of lhe storm in Amherst. Portions of highway 23 between Amherst <br />and Holyoke were completely under water and impassable for as much as 48 hours, while the <br />railroad embankment, tracks, and several culverts near Amherst was severely washed-out. The <br />railroad line was out of commission for several days while construction crews replaced culverts and <br />repaired the embankment and tracks. The damage was similar to whal happened during the June <br />1997 flood event, but was not quite as extensive. <br /> <br />CWCB staff did perform a rainfall bucket survey for this storm evenl and also collected field data to <br />estimate peak flood flows for a number of streams in the AmherstIHolyoke area. Rainfall estimales <br />were oblained from a "door 10 door" survey of homeowners, ranchers, and farmers residing in <br />northeastern Phillips County, CWCB slaff measured cross-sectional areas, channel slopes, and high <br />water marks (HWM's) on several streams to estimate the July 23rd flood discharges. The <br />measurements were performed using conventional field survey techniques. Good HWM's did exist <br />at the subject sites. From the field data, CWCB staff computed indirecl flow measurements using <br />normal depth calculations and Manning's equation, <br /> <br />A rainfall isohyetal map for the July 23rd storm is included in the following figure. The maximum <br />point rainfall amounl for the storm total was approximately 6 inches, which fell in about 3 hours <br />from 5pm to 8pm, The center ofthe storm appeared to be located over lhe Colorado-Nebraska state <br />line approximalely 7 miles due east of Amherst. <br /> <br />The eSlimated peak discharges for Phillips County are presented in the lable below, Technical <br />backup data for the discharge eslimates is included in the appendix. <br /> <br /> Drainage <br /> Area Peak Flow Estimated <br />Stream and Location (sq mi) (cfs) Frequency* <br />Unnamed tributary to Wildhorse Creek 16 1,250 10 to 25-year <br />Near inter. of CR 49 and CR34 u/s ofhwy 23 <br />Unnamed tributary to Wildhorse Creek 18 480** 5-year <br />Downstream of CR 51 near hwy 23 <br />Wildhorse Creek 15*** 1,150 10 to 25-year <br />Just u/s ofhwy 23 <br />Unnamed tributary to Frenchman Creek 9 580 10 to 25-year <br />Upstream ofhwy 23,1 mile NE of Holyoke <br /> <br />Notes: *Flow frequency estimated from the draft USGS regression equations (Vaill, 1996) <br /> <br />**Peak flow was considerably less than the upstream flow due 10 floodwaler storage <br />behind the Highway 23 and railroad embankmenls. <br /> <br />***Total watershed area is greater than 100 square miles, but the storm only <br />produced rain over the lower 15 square miles (est.) <br /> <br />c:\seca\flood\1998\1998 NE Colo flood review,doc <br /> <br />3 <br />