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I I I I I I I I I I I I I I I I I I CENTRAL CITY MASTER DRAINAGE STUDY 16 SECTION 4. EUREKA GULCH DRAINAGE 4.1 General There are two efforts to reduce drainage and sediment damage, and maintenance requirements on Eureka Gulch. The first is major drainage improvements to convey storm waters and sediment collected in the watershed above the City. The second is to improve local drainage for frequent events such as snowmelt and normal summer rain showers. 4.1.1 Major Drainage Eureka Gulch flows through several drainage structures along its path from the west city limit to its confluence with Nevada Gulch. A field review indicated that critical locations include Prosser Loop pipes (Figure 4.1.1 and 4.1.2), MacFarlane Foundry flume (Figure 4.1.2) and the entrance to the flume at the west edge of the Opera Association parking lot (referred to herein as the the Opera House flume, Figure 4.1.2). .overtopping floods occur at the Opera House flume for the 2.6- year flood using information in CWCB (1987). Flood flows that exceed the capacity of the flume flow east onto the parking lot, down to the Opera House where they pond until they spill to the north onto Eureka Street and to the south behind the Opera House. Local experience would dispute that the frequency of flume overtopping and street flooding is every other year. CWCB hydrology is compatible with the Black Hawk study. Flume hydraul ics are, perhaps, too conservative. However, with the abundance of debris along the channel and sediment sources, it is possible that a two-year flood could be responsible for overtopping the flume. T,herefore, the object of the conceptual design for Eureka Gulch is to make the occurrence of street flooding less frequent. Street flooding, once it occurs, greatly increases the flooded a~ea and the property at risk since water does not return to the channel until it flows through the downtown area. From an analytical standpoint, damage due to overtopping the flume and subsequent street flooding and damage has a 38 percent chance of occurring each year. If this can be reduced to the 13- year event, or a 7.5 percent chance of damage, then the risk associated with street flooding, on an annualized basis, will be greatly reduced (with risk measured in dollars of damage). Other elements of risk assessment such as loss of life and loss of business revenue will also be favorably affected. Two opportunities are available, for decreasing the overtopping frequency, they are detention stdrage and increased Opera House flume capacity. The City owns a detention storage location at Dorothy Lee placer site (Figure 1.1.1). A 9.9 foot high, non- jurisdictional dam at approximately the location of the existing ~ HydroDynamics Incorporated