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I I I I I I I I I I I I I I I I I I I to accommodate the IOO-year storm runoff below the spillway. The dam is less than 10.0 foot high, and the surface area is less than 20 acres, so the dam is not cons i dered under the juri sdi ct i on of the State Engineers Office. The facil ity reduces 100-year event peak runoff flows from 241 cfs to 40 cfs. Eureka Gulch crosses "H" Street and Prosser Road with a 4' diameter culvert at each location. Both culverts are undersized for major storm runoff. However, overflow in 100-year events remains in streets and eventually returns to the channel. Approaching the downtown area, Eureka Gulch encounters a culvert inlet upstream from the historic MacFarlane foundry. Having limited capacity, excess flow is directed to and conveyed by Eureka Street and eventually encounters inlets and storm drainage collection pipes which are sized for the 100-year storm event. Intercepted flow is taken to the Opera House flume. The Opera House flume has already been discussed as a key component. The flume is in fair condition. Previous investigation of the flume revealed only one section presenting a significant hydraulic restric- tion, which was at the downstream side of Pine Street. Several large boulders had entered the flume before a trash rack was installed, and were deposited near a low flume ceiling. This problem, however, has been corrected. Also part of Eureka Gulch watershed is a steep hillside north of Eureka Street. A storm drain system exists on County road which conveys flow to the Opera House flume, and also on James Street, which conveys flow through Register Alley to the Opera House flume. B. Nevada Gulch Watershed Nevada Gulch drains to Nevada streets to Gregory Gulch. condition. flume which goes under parking lots and The flume is in excellent hydraulic C. Spring Gulch Spring Gulch flows may exceed culvert capacity under the HooperjRoworth Street intersection. Overflow rem a ins withi n Roworth and Spri ng Street below top of curbs. IV. HYDROLOGY A. General Adequate streamflow data does not exist on the Eureka, Nevada, and Gregory Gulches which would allow direct determination of design storm runoffs. Runoff rates must therefore be estimated for the areas of interest. In earlier studies, hydrological analyses consisted of estimated storm runoff from the entire Eureka Gulch and Nevada Gulch, or dividing 3