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upper threshold discharge assuming supercritical flow with an "n" value of 0.045. The staffs best estimate of a peak flow value is approximately 300 cfs (see Appendix). An estimate of the "bulking factor" of the flow due to sediment loading was not scientifically detennined for this analysis; therefore the estimated discharge represents the total water and sediment flow. However, if one assumes that 20% of the total discharge was due to sediment and debris, then the estimated clearwater discharge would be 240 cfs. Study Findings The CWCB staff finds that the July 30, 1998 flood event on the unnamed gulch was slightly greater than the 100-year flood event computed by Love & Associates. Although stonn nmoff from the gulch has historically flowed down Backus Street and onto Main Street, dumping of construction ruble from nearby projects in the upper portion of the basin has contributed to a heavier sediment load, and increased the damage potential from intense thunderstonns. The large paved parking lot near the headwater area of the basin has also exacerbated runoff and flooding problems. In addition, the CWCB staff believes that the 100-year design discharge of 190 cfs for the unnamed gulch may be too low. This conclusion is anecdotally based on the findings that an approximate 100-year stonn event (2 to 2.5" of rain in an hour) caused a peak nmoff (clearwater) of about 240 cfs. The current design discharge would seem to be more realistic if a pennanent detention facility were constructed below the large parking lot. Recommended Flood Mitigation Measures . Outfall System Plans are currently under review to build a parking structure in the area surrounding the mouth of the gulch, which would effectively obliterate the lowest reach of the gulch itself. The design engineers, Futura Engineering, have indicated that they plan to install a culvert under the parking structure to redirect the flows into North Clear Creek. Upstream of the culvert inlet, a series of debris and sediment basins have been designed to prevent the culvert from clogging. It is our understanding that the proposed culvert would handle a portion of the 100-year discharge, while the remaining overflow would be handled by the downstream street section as it did historically. . Review of the Basin Hydrology and Design Plans Consideration should be given to revlsltmg the design hydrology. Consequently, the current design capacity of the proposed culvert near the Golden Gates Casino may be inadequate unless detention storage is provided upstream. It is also recommended that an engineered floodwall be constructed on the back side of the Golden Gates Casino to protect that structure from future flood damage in case of a project failure. . Detention Pond and Slope Stabilization In addition, slope stabilization techniques should be implemented on the large fill embankment just below the large parking lot at the top of Miner's Mesa. The easterly sector of the parking facility acts as a detention pond. However, the embankment needs bank stabilization measures to resist any future overtopping and failure. This action should take place in addition to the previously mentioned mitigation measures. Slope stabilization would help prevent massive quantities of rock and sediment from being transported to the lower part of the basin. 1:\wctrans\seca\I998FloodsIBJackHawk :;