2019-10-28-minutes-public-works-study-session

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nhc 7 CONCLUSION This assessment suggests that moving the confluence of Lick Creek upstream may have an impact on channel migration rates In the reach that will receive additional flow between the current and proposed confluence locations. The increase Is minor compared to the baseline conditions and would most likely Influence areas within the existing CMZ (Appendix A) where substantial exposure to channel migration risks are already present. The analysis Is based on a vary conservative estimate of the potential Increase In lateral channel migration rates of 17% over what appears to be a fairly stable baseline condition (average of 1.5 feet/year), based on the modeled change in bedload transport capacity for the 2 -year flow (Figure 5, Table 2). The maximum expected increase In reach average lateral migration rate would be about 0.3 feet/year. Over a 50 year period, this equates to an additional 14 feet of lateral channel movement compared to the baseline condition which would result in SD feet of channel migration over the same 5U -year period. Due to a dominance of down -valley meander translation, this additional migration will mostly influence areas already exposed to channel migration hazards that are near the channel or within the meander belt. The potential for the added flow to increase the rate of channel erosion Into terraces surrounding the floodplain cannot be quantified with available data but is most likely to be a very small fraction of the Increase in predicted lateral channel migration rates. In contrast, the channel cross section width is not expected to substantially change. Rational Regime model assessment suggests the expected widening Is on the order of 2 feet, or about a 3% Increase In the width. The magnitude of this impact Is dwarfed by along -channel variability In width, with areas as narrow as 60 feet and as wide as 15a feet, that results from localized variability in the principal controlling factors. While contributions of Lick Creek further upstream from the current discharge point is relatively minor, the existing baseline risk for the Teanaway River channel movement within the CMZ is currently substantial and has the potential to migrate within the CMZ. 4 Potential impact of Lick Creek relocation on the NF Teanaway River Morphodynarnics 19