Description

Thomas/Wright, Inc. (T/WI) prepared multiple restoration designs for the Thornton Creek watershed. The Thornton Creek drainage basin is a highly urbanized watershed that is experiencing various problems including man-made obstructions to fish passage, a volatile flow regime and associated bank erosion and down-cutting, loss or degradation of fish habitat, diminished stream side vegetation and other buffer encroachments, and the introduction of various nonpoint source pollutants. All of these problems have a detrimental effect on salmonid fish species in the stream which include coho salmon and cutthroat trout.

As part of a Department of Ecology Centennial Clean Water funded grant, Seattle Public Utilities (SPU) conducted reconnaissance surveys of the main stem, north and south branches of Thornton Creek to compile an inventory of potential in-stream improvement projects. In June of 1998, T/WI and The Watershed Company (TWC) were retained by SPU to jointly develop criteria to evaluate and prioritize 28 sites from the inventory for design and construction in 1999. As part of this effort, field investigations were conducted along many reaches of the creek to evaluate existing conditions, prioritize projects for design and construction, and develop preliminary concept level designs.

T/WI prepared design plans and specifications for each of the 19 stream reaches and project sites. The design phase included an analyses of channel hydraulics, scour at proposed channel grade control structures, susceptibility of stream bed and bank to erosive shear, stable channel cross-section geometries, in-stream and riparian corridor habitat enhancement, active vs. historic fluvial processes, and easement and access requirements. The designs included: culvert repair and replacement, fish ladder repair, channel grade control structures, biotechnical and structural stream bank stabilization, and in-stream and riparian habitat enhancement.