Manastash Creek Post Flood Investigation & Flood Relief Alternatives

In May, 2011 a major flood on Manastash Creek near Ellensburg, WA caused significant damage to private property and public infrastructure.  To stop the flooding, excavators removed 2,200 cu yds of sediment from the stream and built flood protection berms upstream and downstream from the Cove Road Bridge, but because the berms were constructed as an emergency, no floodplain development permits were obtained from Kittitas County.  The berms cut off a major overflow path across the Manastash Alluvial Fan, but unfortunately this will increase flood discharges along the main channel downstream, which likely will cause adverse impacts to downstream properties .

Kittitas County desired to work with local property owners to determine the best course of action.   To aid in this effort, Kittitas County asked WATERSHED Science & Engineering (WSE) to provide specialist river engineering services to determine the cause of the flooding and to identify actions that could both resolve the permit violation and provide meaningful flood relief.

Role / Services: WSE conducted a detailed post flood investigation, which included discussions with property owners and County personnel. Analyses pertained to hydrology, hydraulics, and geomorphology, including an examination of alluvial fan and sediment transport/deposition characteristics.  WSE identified and examined multiple flood relief actions and provided recommendations.  In January, Jeff made a presentation of results to the Kittitas County Board of County Commissioners.

Outcome:  Stakeholders gained a solid understanding of the cause of the problems based upon sound scientific information.  They also gained an appreciation for the complexity of the stream system and an understanding that flood reduction needs to be addressed on a reach scale rather than site by site.