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[Portal ID #1937]

Grays Harbor County - Wishkah Road Comprehensive Flood Study Project

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Update from Rob Wilson     10/17/2016

From Larry Karpack (Watershed Science & Engineering):

"WSE completed development of a combined topographic surface for the Wishkah River and floodplain. This effort combined channel bathymetric and bank data provided by PGS with new LiDAR collected by GeoTerra defining the remaining floodplain. The new surface was then merged with a topographic surface previously developed for the lower Wishkah River within Aberdeen from the Northshore levee project.  Upstream flow and downstream tidal data were developed both for design events as well as for historical events including the recent flood events in January and December 2015. A 2D hydraulic model was then developed using these data and HEC-RAS 5.0. The comprehensive flood study model was joined to the lower Wishkah hybrid 1D/2D model from the North Shore levee project. The USGS National Land Cover Database was used to generally define areas of differing surface roughness. Roughness n-values were initially assigned and then further refined through calibration to the January 2015 flood with comparison to the measured high water mark data. Additional simulations were then made for the December 2015 event as well as three other “design” events that could represent 100-year flood conditions: 100-year inflow with 2-year tide, 10-year inflow and 10-year tide, 2-year inflow with 100-year tide. These model runs provide the information needed to characterize baseline flood conditions. Additional runs of the model will be made to look at the effects of lesser (i.e. 2-year) and greater (i.e. 500-year) flood conditions. This data will be (was) shared with stakeholders during a public meeting in Aberdeen on October 13, 2016."

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