2019-03-25-minutes-public-works-study-session

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Additional locations for flood storage were considered, including areas northeast, east, and southeast of the airport runways. These options were eliminated due to limitations caused by terrain features, slope, and available area; therefore, no hydraulic analysis of these options was completed. PREFERRED ALTERNATIVE WSE presented results of alternative model simulations to the County and Century West during online meetings on 1/15/19 and 2/4/19. At the 2/4/19 meeting, the team selected a preferred alternative consisting of the following combination of measures to reduce flooding to airport facilities and mitigate downstream impacts (see Figure 6 diagram): • Extend/cleanout the taxiway ditch to the north end of runway 11-29 • Construct an east -west ditch along abandoned taxiway at north end of runway 11-29 to capture and route overland flow away from Bowers Field and towards Whiskey Creek to prevent flooding across runways • Construct storage ponds at west end of Bowers Field to detain flow from the new east -west ditch and allow controlled release to Whiskey Creek. Size culverts at the pond outlet to restrict outflow to Whiskey Creek and prevent any increase to peak flooding downstream. • Construct storage ponds to the west of Whiskey Creek on Kittitas County property upstream of Hungry Junction Road. Size ponds and outlet structures to attenuate peak flow on Whiskey Creek and to work in conjunction with storage ponds west of Bowers Field to avoid downstream flood impacts on Whiskey Creek. The taxiway ditch cleanout extends approximately 1700 additional feet parallel to the runway from the existing cleanout, and the constructed east -west ditch extends approximately 2800 feet from the access road at the north end of Bowers Field to the abandoned taxiways at the west end of the airport property. Constructed storage ponds consist of berms ranging from a maximum of 4 to 7 feet and 600 to 1000 feet long with aft x 3ft to 8ft x Sft rectangular outlets. The five storage ponds on Bowers Field provide a peak storage volume of approximately 21 acre-feet during the 100 -year flood event. The four storage ponds upstream of the airport provide a total peak storage volume of approximately 12 acre- feet during the 100 -year flood event. Resulting 100 -year flood depths and change in depth from the preferred alternative are shown in Figures 7 and 8, respectively. In general, the preferred alternative routes flow around the airport through a combination of ditches and storage, eliminating flooding across the airport runways and downstream properties between Whiskey and Mercer Creeks. The storage ponds fill to maximum capacity during the flood peak, then drain completely within 12 hours. Figure 8 indicates that there are some changes in flood depth outside of the constructed ditches and storage ponds, although all of these impacts are very minor (less than 0.1 ft change in depth) and are generally contained on County property. WATERSHED P a k> P 15