Water Environment Federation (WEF)

Methods of Design and Financing River Restoration Projects Constructed to Address Erosion and Sediment TMDL Problems Resulting from Development in Watersheds

This paper presents the results of research and experience with design and implementation of creek or river rehabilitation in urban watersheds. The paper discusses a five-step method for designing instream projects for reaches of creek subject to erosion. It recommends a post project audit of design and construction practices to qualify a project as a stream rehabilitation project.

The general approach developed for creek or river rehabilitation is to identify stable reference reaches in the same watershed that has similar conditions and duplicate those in the eroding reaches. This requires matching velocities, shear stress and stream power for the same slopes and soil conditions. The research identified that design methodology based on reference reaches in rural watersheds needs to be adapted for the change in the frequency of bankfull flows experienced with urbanization. The frequency of bankfull events, measured at each reach of a creek, increases with the percent of the watershed area that is impervious at the reach. The design methodology has to be refined to evaluate channel stability in those sections of the creek that may have entrenchment ratios of 2 or less because of limited corridor width, when the stable channel design techniques may require entrenchment ratios of 2.5 to 4. Channel stability
analysis includes computations of excess shear stress at specific flows and the excess work based on a continuous flow record. The values of excess shear and excess work have to be matched to the stable reference reaches by incorporating bed protection techniques such as grade control structures with pools and bank protection elements such as use of rock toe protection, vegetated rock layer on the bank and vegetated banks. These techniques increase the critical shear stress of the bed and bank and reduce the excess shear and excess work.

The paper discusses seven constraints that the financing mechanism for instream programs has to address. These include the length of time that creeks take to respond to urbanization (20 years or longer), the extended period of time over which instream projects may have to be built in response to the disturbance, and the need to address the impacts on all affected reaches. A twostep mechanism is recommended. First, the developer fees are determined and assigned to specific project(s) completed at the time of issuing the permit to the developer. Second, an actual allocated cost is determined after 20 years and reconciled against the fees. Establishing a stormwater utility and charging fees based on a combination of initial and annual fees can be one method to administer the process.

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