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Construct a new ditch from east to west along an abandoned taxiway between runway 11-29 and <br />Hungry Junction Road to capture water overtopping Hungry Junction Road and route it west <br />toward Whiskey Creek <br />• Extend the constructed east -west ditch (described above) south along the west side of the airport <br />to activate flood storage within an existing depression on airport property <br />WSE updated the with -project model geometry to represent each of these alternatives, including <br />variations on ditch length, size, and outlet location. Results indicate a reduction in flooding at airport <br />infrastructure but significantly increased flood depths at and downstream of each of the ditch outlets. <br />Ditch options were therefore considered in combination with flood storage (see next section) to reduce <br />peak outflow and eliminate downstream impacts. <br />FLOOD STORAGE CONSTRUCTION <br />Multiple flood storage configurations were evaluated in conjunction with drainage ditch construction to <br />reduce peak flows and mitigate downstream impacts from routing flooding away from airport facilities. <br />Storage alternatives were simulated by adding multiple low -height berms in series to maximize storage <br />within the steeply sloping terrain and to reduce the required height of each berm. Each storage area was <br />modeled with a low-level culvert outlet to meter outflow and allow stored water to drain during the flood <br />recession. Three storage configurations were examined: <br />1. Storage ponds at the northwest corner of Bowers Field, fed by a constructed east -west ditch <br />(described above). Water would be released Whiskey Creek through a culvert outlet. <br />Storage ponds at the southwest corner of Bowers Field (an alternative to the first configuration) <br />covering a natural depression in this area that would provide additional storage volume. Water <br />would be released to Whiskey Creek through a culvert outlet. <br />3. Storage ponds to the west of Whiskey Creek and north of Hungry Junction Road. These ponds <br />would be located away from the airport on property owned by Kittitas County. <br />Berm layouts for each storage configuration were determined through an iterative process to achieve <br />adequate flood storage. Berms were simulated by creating vertical walls on top of the existing model <br />terrain without any changes to the surrounding terrain surface. Equivalent storage volume could also be <br />achieved through a combination of shorter berms paired with excavation of the area upstream. Pond <br />outlets (culverts) were sized to maximize storage volume during the flood peak while allowing for <br />adequate drainage during the flood recession. <br />Final configuration results showed that storage alternatives 1 and 3 were effective at reducing 100 -year <br />peak flows and mitigating downstream impacts of flow routed through the ditches. However, storage <br />alternative 2 resulted in increased downstream flood depths that could not be mitigated due to the <br />location of the drainage outlet. Figure 5 shows the change in 100 -year flood depth (compared to existing <br />conditions) resulting from storage alternative 2 in conjunction with the Mercer Creek bank -raising <br />measure described above, indicating a significant increase in downstream flood depths downstream of <br />the storage outlet. <br />�=WATERSHED F [; e a <br />