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

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ALTERNATIVES ANALYSIS The with -project model surface was then modified to evaluate alternatives for flood reduction at Bowers Field and mitigation of flood impacts to downstream properties. The following alternatives were identified through discussion between WSE, Century West, and Kittitas County, and the analysis and results for each evaluated alternative are discussed in the sections below: Increasing in -channel flow conveyance of Mercer Creek by modifying the channel banks and/or replacing the culvert under Hungry Junction Road • Construction or modification of drainage ditches to capture and route flow around the airport • Construction of flood storage (in combination with modifications to drainage ditches) INCREASED IN -CHANNEL CONVEYANCE OF MERCER CREEK -BANK MODIFICATION Flow escaping the Mercer Creek channel downstream of Hungry Junction Road contributes to overbank flooding of airport facilities. Actions considered to increase channel capacity and reduce overtopping included raising the channel bank at a channel bend on airport property downstream of Hungry Junction Road. WSE updated the with -project model geometry to represent multiple potential bank configurations and ran the model to simulate the 100 -year flood event. An example of the resulting change in 100 -year flood depth from raising the channel bank (compared to existing conditions) is shown in Figure 3. For any one of the configurations tested, raising the channel bank reduced peak flooding at the airport but retained more flow within the Mercer Creek channel, leading to increased flood depths along Mercer Creek downstream from the airport. No feasible measures were identified to mitigate these impacts. INCREASED IN -CHANNEL CONVEYANCE OF MERCER CREEK -HUNGRY JUNCTION CULVERT REPLACEMENT The Mercer Creek culvert under Hungry Junction Road is undersized and contributes to backwatering and road overtopping that floods the north end of Bowers Field and the airport runways. WSE updated the with -project model to simulate replacing the existing 4 -ft corrugated metal pipe (CMP) culvert with a larger structure and ran the model to simulate the 100 -year flood event. An example of the resulting change in 100 -year flood depth from a combination of the upsized culvert and raised channel bank (compared to existing conditions) is shown in Figure 4. Results indicate reduced road overtopping along Hungry Junction Road and reduced peak flooding at the airport. However, more flow within the Mercer Creek channel led to increased peak flood depths along Mercer Creek downstream from the airport. No feasible measures were identified to mitigate these impacts. DITCH MODIFICATION OR CONSTRUCTION The following alternative ditch locations and alignments were evaluated: Clean out the existing taxiway ditch along the east side of runway 11-29 to capture and route flooding from the airport runways downstream to Mercer Creek WATERSHED T- r p 3