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Not so fortunate were the community of Amherst, Colorado State Highway 23, and the Railroad line that parallels highway 23, The same storm that dumped harmless rain on Holyoke was much more intense over Amherst and the area near the Colorado..Nebraska state line, Basement flooding lnd street flooding were a result of the storm in Amherst, Portions of highway 23 between Amherst and Holyoke were completely under water and impassable for as much as 48 hours, while the railroad embankment, tracks, and several culverts near Amherst was severely washed-out. The railroad line was out of commission for several days while constmction crews replaced culverts and repaired the embankment and tracks, The damage was similar to what happened during the June 1997 flood event, but was not quite as extensive, eWCB staff did perform a rainfall bucket survey for this storm event and also collected field data to estimate peak flood flows for a number of streams in the Amherst/Holyoke area, Rainfall estimates were obtained from a "door to door" survey of homeowners, ranchers, and farmers residing in northeastern Phillips County, CWCB staff measured cross..sectional areas, channel slopes, and high water marks (HWM's) on several streams to estimate the July 23rd flood discharges, The measurements were performed using conventional field survey techniques. Good HWM's did exist at the subject sites, From the field data, CWCB staff computed indir'~ct flow measurements using normal depth calculations and Manning's equation, A rainfall isohyetal map for the July 23'd storm is included in the following figure, The maximum point rainfall amount for the storm total was approximately 6 inches, which fell in about 3 hours ;rom 5pm to 8pm, The center of the storm appeared to be located over the Colorado..Nebraska state line approximately 7 miles due east of Amherst. The estimated peak discharges for Phillips County are presented in the table below, Technical backup data for the discharge estimates is included in tbe appendix, Stream and Location Unnamed tributary to Wildhorse Creek Near inter. of CR 49 and CR34 u/s ofhwy 23 Unnamed tributary to Wildhorse Creek Downstream of CR 51 near hwy 23 Wildhorse Creek Just u/s ofhwy 23 Unnamed tributary to Frenchman Creek Upstream ofhwy 23, I mile NE of Holyoke Drainage Area (sq ll1i) 16 Peak Flow (cfs) t250 -- Estimated Frequency* 10 to 25-year 5-year ---- 10 to 25-year 10 to 25-year --~--- ] 8 - -----:480**- --TI''''''--'--n 50 - , 9 580 Notes: *Flow frequency estimated from the draft USGS regression equations (Vaill, 1996) **Peak flow was considerably less than the upstream flow due to floodwater storage behind the Highway 23 and railroad embankments, ***Total watershed area is greater than 100 square miles, but the stOlm only produced rain over the lower 15 square miles (est.) c:\seca\flood\1998\ 1998 NE COlD flood review,doc 3