DESIGN METHOD FOR STREAM RESTORATION FOR EROSION CONTROL AND SEDIMENT TMDLS BASED ON MANAGEMENT OF THE TIME INTEGRATED VALUE OF EROSIVE SHEAR STRESS (WORK DONE) ON STREAM SEGMENTS
Abstract:A design method based on the time integrated value of excess work done (W) has proved to be a effective in sizing and evaluating stream restoration designs in an incised channel. The technique relies on geomorphic measurements of channel reaches and cross-sections, continuous simulation hydrologic modeling, hydraulic depth and velocity calculations, followed by shear stress computation before and with the implementation of BMPs. It compares the work done on stable and preurban sections of the creek at the same location or adjacent to where creek is showing signs of excess erosion, using the following equation:
W = index of total effective work done over the length of flow record.
τc = critical shear stress that initiates bed mobility or shear erosion. Critical values were estimated from permissible velocity tables published in ASCE Manual No. 77, p 334 (1992).
τi = applied hydraulic shear stress, computed as ρgdS, where d = depth of water, S = longitudinal slope, g = gravity constant, ands ρ = density of water
Δt = duration of flows (in units of time)
n = number of bins in the flow-duration histogram, corresponding to length of flow record
The time increment (Δt) is determined by generating a histogram of flows from the continuous simulation model, which in our case are hourly data. For each flow range (a Bin in the histogram), the histogram provides the count or duration of time that flows are within the designated flow range. For the average flow within each Bin, the corresponding depth, velocity, and shear stress are computed for each of the cross sections. The equation is then solved for each flow Bin to compute the excess work done by that specific flow. When equation 8 is summed over the long-term flow record (histogram), the result (W) is a measure of the total excess work done on the stream channel boundary. This work moves the bed or bank material.
Through designs that use grade controls to change inter-grade slope with pool structures to dissipate excess energy gained over each grade control, channel widening and flood plain recreation at the bankfull elevation of the urban watershed, and bank protection, or a combination of measures, the excess work is reduced back down to that observed for currently stable or for stable preurban stable cross-section adjacent to or at the same location where excessive erosion is observed under current conditions (or future conditions). This allows the creek bed and bank to rehabilitated to an acceptable level of erosion and be rehabilitated to support natural vegetation and aquatic life.
Document Type: Research Article
Publication date: January 1, 2004
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