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

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nhc The banks are characteristically composed of coarse alluvial material with a simllar texture to that observed in the channel (Photo 6), with little overbank fines. Roots of Cottonwood and Ponderosa Pine were observed In some locations; these provide some limited bank cohesion which increases the bank's strength slightly and provides some resistance to channel widening and lateral channel migration. Systematic exposure of sandstone bedrock along the channel suggests that the depth of alluvial fill In the valley bottom Is similar to, or less than, typical channel scour depths. Bedrock formed grade -controlling sills are present in the bottom of scour features and wherever the channel abuts the valley wall. These bed and bank conditions suggest limited lateral channel mobility and that most bedload transport is conveyed down the channel without exchange of material between the channel and floodplain. A preliminary review of aerial photos, described in the following section, supports this Interpretation as the channel bar and scour patterns are highly dynamic, while the lateral position of the active channel has been quite stable. 3.2 Historic Channel Migration Channel positions were delineated from historic aerial photos from 1963,1984,1998, 2006, and 2017 to evaluate a baseline channel migration rate for the subject reach (Figure 4). individual time period, reach average bank erosion rates are very slow, ranging from 0.8 to 2.4 faet/year, with a'whole period average of 1.6 feetfyear. Individual actively migrating meanders moved somewhat faster, with a maximum observed long-term migration rate of about 5 feet/year. The above reported values are reach average channel migration rates, but there is substantial variability depending on local conditions along the channel. Bedrock exposures largely act as barriers to lateral channel migratlon, as can anthropogenic features or—at least temporarily— bank reinforcement from the roots of large trees or accumulations of large woody debris. Local lateral migration rates are shown for three of the most active meanders in the project area in Figure 4 and Include rates up to 5 feet year. The most upstream meander (Meander 1 In Figure 4) has slowly amplified and translated down -valley with a fairly consistent bank erosion rate of about 3 feet/year. A side channel is present on the left bank/south west valley wall opposite this meander. It first became apparent in the 1984 aerial photo, but It is not dear whether it formed between 1963 and 1984, or It possibly was not visible in the relatively poor quality 1963 aerial image. The middle meander (Meander 2) translated rapidly down - valley between 1963 and 2006 but stalled out around 2006. it appears to have amplified to the point that flow cuts across the Inside orf the meander bend in a chute cutoff, leaving the outside of the bend relatively Inactive for the time being. Detailed local investigation of bank conditions and river hydraulics would be required to better understand the likelihood of the outside of the meander reactivating. The downstream most meander (Meander 3) has been moving at a rate of about 1 foot(year. In some areas between active meanders—particularly where the channel is entrained against bedrock on the left valley wail—no measurable channel migration has occurred since 1963. Potential impact of lick Creek relocation on the NF Teanaway River Morphodynarnics 11