REACH SCALE EFFECTS OF BANK STABILIZATION ON POINT BAR ACCRETION AND PHOSPHORUS STORAGE

Bank stabilization structures are widely used in streams and rivers to protect critical infrastructure. As evidence accumulates indicating that bank erosion is an important nutrient input process, bank stabilization structures are increasingly being proposed as nutrient reduction practices. Prior work suggests that bank stabilization structures are effective at locally reducing cutbank erosion, but their net impact on bend- and reach-scale transfer of sediments and associated nutrients between eroding and depositing banks and surface waters remains unclear. To examine the influence of bank stabilization on point bar sediment and Phosphorous (P) deposition, we studied six meander bends on the West Nishnabotna River in southwest Iowa. Three stabilized and three unstabilized bends were selected for a detailed study consisting of topographic surveys, point bar sediment sampling and geospatial analysis using aerial LiDAR point clouds.

Point cloud differencing between 2008 and 2020 LiDAR point clouds reveals cutbank erosion and point bar deposition occurs across all meander bends but is larger in volume, area, and extent on unstabilized meander bends. The total volume of sediment deposited on the point bars of unstabilized meander bends is approximately 30 percent greater than on the stabilized meander bends. Point bars in stabilized bends are truncated and have steeper transverse slopes (29% on average, versus 14% for unstabilized bends) resulting in a narrower bankfull channel with a smaller cross-sectional area. At river stages 0 – 2 meters above baseflow, deposited sediment packages in the unstabilzed meander bends are more extensive, contain finer sediment and have more total P than their stabilized counterparts. Bank stabilization structures evidently suppress the creation of lateral sediment accommodation space, limiting deposition to mostly vertical accretion at higher stages. Our results suggest that suppressing natural meander migration with bank stabilization structures not only reduces bank erosion but also reduces some of the compensating sediment deposition. Thus, stabilization efforts aimed at reducing nutrient inputs should consider both erosion and deposition in quantifying nutrient reduction.