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nhc <br />The dominance of down -valley meander migration Illustrates how channel migration is expected to <br />mostly occur In areas relatively close to the existing active channel. In particular, the meander beit— <br />defined as the area within an envelope connecting the outside of meander bends—is the most <br />vulnerable to future erosion (Olson at al., 2014). Furthermore, the dominance of down valley meander <br />migration, as opposed to cross -valley meander amplification, results in a fairly narrow channel migration <br />zone (CMZ) (Illustrated in Appendix A) compared to that which would be extrapolated from the average <br />bank erosion rates computed for the river if most migration occurred In the cross -valley direction. <br />BEDLOAD TRANSPORT RATING CURVE <br />As described above, a bedload transport rating curve (Figure 5) was developed based on estimated <br />channel hydraulics and bed material composition. This provides a conservative estimate of the maximum <br />expected impact to channel migration caused by the Increased flow In the reach between the current <br />and proposed lick Creek confluence. The Wilcock and Crowe (2003) bedload transport function, as <br />implementaed in BAGS (Pitlick at al., 2009b, 2009a) sediment transport program, was utilized to do this. <br />The channel cross-section was cut from Bathymetric, Green Band L OAR data (PSRC and Quantum <br />Spatial, 2015) provided by the County, and a floodplaln roughness of Manning's n-0.05 was assumed, <br />with values of 0.025 and 0.08 used to test sensldvity to the assumption. Channel roughness was <br />estimated from the bed material grainsize distribution as a part of the BAGS hydraulic computations (as <br />described by Pitlick et al., 2009a). The reach average long profile slope (0.008) was used as the input <br />slope (rather than a hydraulic gradient) because a hydraulic model of the reach was outside the scope of <br />this investigation. This bed slope is similar to the modeled 1"ar water surface slope (0.01) in the <br />existing Flood Insurance Study (FEMA 2018) as it suggests that the use of the bed slope to characterize <br />shear stress at typical channel4brming flows Is appropriate. The slope was determined as the average <br />riffle -crest to riffle -crest reach slope. This was measured between the first riffle upstream of the cross- <br />section location, utilized for sediment transport calculations, to a riffle crest near the existing <br />confluence. <br />Figure 5 shows variability (shown as dashed lines) in the estimated bedload transport rating curve due to <br />changes in the floodplain roughness—which Impacts flow partitioning between the channel and <br />floodplaln and flow depth in the channel. The calculations are also extremely sensitive to the assumed <br />channel slope because the bedload transport rate is an exponential function of shear stress (which is a <br />direct function of slope). however, the general shape of the bedload transport rating curve Is relatively <br />Insensitive to these uncertainties, and so the proportional change in sediment transport, due to a <br />change in flow, will be similar regardless of the specific input values chosen. <br />Potential impact of Lick Creek relocation on the NF Teanaway River Morphodynamics 13 <br />