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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.osle.y Wash Drainag~ lv!aster Plan Phase 2: Alternative Drainage Facility Evaluation A large variety of combinations exist, each of which offers some flood control benefit. To develop a manageable number of alternatives, a number of objectives were developed. The first of these was that the alternative should control flooding during a specific flood event. This helped identify a target storm threshold below which no damages are anticipated. For example, a lO-year system would protect against damages for floods up to, and including, the lO-year flood. Another objective was to control the damage associated with flooding in areas away from the main stem of Bosley Wash. In other words, it was intended to minimize the overflows that caused water to flow out of the drainageway. Finally, it was deemed desirable to minimize the cost of improvements necessary to meet the other objectives. Two primary alternatives have been identified during this study: the lOa-year altErnative and the lO-year alternative. In each case, elements were assembled to meet the desired level of protection. Because of the nature of runoff within the Bosley Wash Basin, detention is an important component of each. This helped to reduce the cost of downstream facilities and allowed the sponsors to formalize the exiting inadvertent detention that has proven so successful in limiting flood damages. 4.2 lOO-year System The 100-year system of improvements identified for Bosley Wash will eliminate the flood hazard along the main stem of the drainageways within the Watershed. The improvements consist of strategically placed stormwater detention facilities that will reduce the amount of water flowing in the channels, and channel and culvert improvements along the Stub Ditch and the Bosley Wash mainstem. Another important element of this system is the recognition that routine and active maintenance of the channels and culverts must be undertaken. In combination, these improvements will protect the citizens of Bosley Wash from flooding during the lOa-year event. Important elements of the plan include construction of detention at the Reservoir A location. This project will include the acquisition of property above 1-70 so that the large detention basin (75.5 AF) can be constructed. This reservoir limits releases to 50 cfs and significantly reduces the flow of water from the upper Watershed. A reservoir located at L.~I5~:1~fI~,~~-~:'I:&V;;~:IL~~:~;-~:I:;~ -':..L!.s::~_^ ~:;- i":(~:li-'i',: j',~) ":"'-{7:.~"..uWi-!o:i.," Page 17 /"----...,. ~~ @'1.