2019_KittitasCountyHMP_Volume 1_For Adoption

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CHAPTER 11. FLOOD 11-5 11.2.1 Geomorphology Geomorphology refers to the relationship between the shape and other physical characteristics of a river and the rocks and sediments of the valley in which it flows. The river creates its channel, which reflects the force of the flowing water and the material of which the bed and banks are made. Changes in watershed conditions can affect the amount of runoff and the amount and size of sediment that enters the river. Changes in runoff and sediment loading affect the river’s behavior, including flood characteristics. The Yakima River’s character changes in response to local geology as it flows downstream. Much of the river is braided, with interlaced channels and gravel bars and an active channel area; however, there are areas where basalt geology constricts the lateral movement of the river. All forks of the Teanaway R iver generally are constrained in their upper reaches. Moving downstream to the Teanaway River valley, the river is fairly channelized, but has free lateral movement. 11.2.2 Stream Flow During ordinary years, much of the precipitation in Kittitas County remains as snowpack for several months after it falls, providing for higher flows during the spring thaw; however, much of the runoff is stored in one of the three reservoirs for irrigation purposes later in the year. In high precipitation years, rain-on-snow events decrease the snowpack and increase stream flow to the point of flood events. This was most apparent during the 1990, 1995, and 1995 flood events. Cool spring temperatures increase peak stream flows, as snow remains in the mountains throughout the early spring, then melts and runs off more quickly when temperatures increase in later spring or early summer. When large amounts of water runoff at one time, high flows occur. Higher peak flows increase the possibility of flooding. Exchanges between surface water and groundwater also drive stream flow in the Yakima River basin, but the relationships between the two are complex. Permeable glacial sediments are thought to provide for a high degree of hydraulic continuity between surface water and groundwater in mos t parts of the basin. Where surface water and groundwater are in continuity, the condition of the river corridor will have strong impact on groundwater resources as well as on flooding. Riparian vegetation both slows flows and helps water percolate to the zone from which it can recharge the aquifer. Similarly, changes in land use that affect groundwater quantity and quality and aquifer recharge potential will be reflected in the river. 11.2.3 Principal Flooding Sources Riverine Flooding There are many flood problem areas in Kittitas County. Large-scale developments with urban densities adjacent to the Yakima and Teanaway Rivers—specifically, Elk Meadows, Elk Meadows Park, Pine Glen, Sun Island, Sun Country, Teanaway Acres, and the Teanaway Wagon Wheel—have experienced substantial flood damage. The county also has numerous streams with large and unpredictable floodplains and flood capacities. These include, but are not limited to, Cabin, Cole, Big, Little, Silver, Gold, Manastash, Taneum, Wilson and Reecer Creeks. Floods on the Yakima, Teanaway and Cle Elum Rivers occur as the result of snowmelt in spring and early summer and occur after heavy rains in November and December. Ice and debris can have an impact on flood stages when culverts and bridges are obstructed. The spring/summer snowmelt floods are characterized by slow rise and long duration of high flow; river stages may be increased by ice and debris jams. The fall/winter flood crests are reduced because flood storage is available after the irrigation season in Kachess, Keechelus, and Cle Elum Lakes. However, these reservoirs control only a small part of the