Cedar River Hydrographic Survey (WA)

WEST Consultants, Inc. was contracted by Seattle Public Utilities to develop steady and unsteady flow hydraulic models for a 12-mile reach of the Cedar River near North Bend, Washington. This effort was made in support of a plan for long-term management of large woody debris (LWD) within the Cedar River watershed by Seattle Public Utilities (SPU). The intended use of the hydraulic model is the estimation of hydraulic parameters for evaluating the stability of LWD between the Cedar Falls Power Plant and the diversion structure at Landsburg. The model will also be useful in the prediction of a wide range of hydraulic parameters for the modeled reach. It will allow for the estimation of the travel time of flood events through the reach and for better prediction of flow-dependent water quality conditions as Landsburg. The model can also be used to estimate zones of high shear stress that could lead to bank erosion and wood recruitment.

A major component of the work was a hydrographic survey of the channel to collect topographic data for the Cedar River necessary for development of the hydraulic models. The survey was conducted using a combination of Global Positioning System (GPS) and Total Station control work. Due to dense vegetation and the rugged physical geography of the study area, the use of GPS through the study area was limited. At the completion of the surveying task, more than 20 GPS control points had been measured, over 240 traverse points were established, and approximately 2,200 side shots were measured to define 85 river cross sections and four bridges that comprise the Cedar River hydraulic model geometry. Digital photographs were taken at each cross section location for documentation. A description of the bed material (cobbles and boulders, gravel, etc.) was also recorded for later use in the development of the hydraulic model.