Influence of bank stabilization on floodplain sediment and phosphorus storage
Eroding river banks are frequently stabilized with rock, concrete or wood when infrastructure or property is threatened. There is also growing interest in using bank stabilization as a way to reduce nutrient loading from bank erosion. However, little is known about the impacts of bank stabilization on the sediment deposition process on opposing point bars and the adjacent floodplain, whose dynamics are closely coupled with bank erosion. If arresting bank erosion also reduces sediment and phosphorus deposition and storage elsewhere, the nutrient reduction benefits of streambank stabilization may be less than expected.
Researchers plan to compare the sediment and P concentrations of eroded and deposited sediment in five stabilized and five unstabilized river bends in the Nishnabotna River watershed to determine whether bank stabilization has any systematic impact on reach-scale sediment and P storage.
Aerial imagery and LiDAR elevation data will be used to estimate the total volumes of sediment eroded and deposited from each stabilized and unstabilized bend. Researchers will then map and sample different sediment types on each point bar and adjacent floodplain to characterize both the sediment eroded and deposited and its P concentration. The results will be compared to determine whether sediment and P storage on point bars and adjacent floodplains is reduced in bends with bank stabilization structures.
Note: Project reports published on the INRC website are often revised from researchers' original reports to increase consistency.
A crew including graduate student Tanner Lambert and three undergraduate research assistants completed a full season of fieldwork at selected sites along the West Nishnabotna River in southwest Iowa. They laid out transects on six point bar and floodplain features, surveyed cross-sections, described vegetation and sampled sediment for texture and total P analyses. Those lab analyses are now underway at a partner lab (USDA Agricultural Research Service National Laboratory for Agriculture and the Environment). Meanwhile, the first stage of LiDAR point cloud comparison is complete, and researchers are analyzing a DoD (DEM of Difference) computed from the geomorphic changes identified between 2010 and 2020.
Other activities included working with INRC to develop a video describing our project, which is now available on the INRC website.
The graduate student helping lead the project has assembled a field crew (four undergraduate field and laboratory assistants) and developed standard field procedures for collecting field data during the summer 2022 field season, currently underway. A reconnaissance crew paddled canoes down approximately 30 miles of the West Nishnabotna River, which is the focus of this study. Initial preliminary point cloud differencing has been done to estimate geomorphic change in the study reach and to select candidate sites for field study. Ongoing field data collection and sample processing will continue through the remainder of the summer field season, July-August 2022.
Other activities included one presentation.
Since project initiation in summer 2021, a graduate student (ISU Geology, M.S. student 2021-2024) was recruited, who began work on the project in July and who will complete much of the work detailed in the proposal in the coming two years.
Thus far, activities have been limited to developing standard field procedures for accessing, describing and sampling point bar and floodplain sediments and soils. In addition, the final days of 2021 saw the first availability of point clouds from Iowa's 2019-2020 LiDAR flights, though only for a limited number of counties. Even so, we have begun to develop a workflow for geomorphic change detection using the 2010 and 2020 LiDAR point clouds using the software package CloudCompare. This has been tested in and around a short reach of the Thompson River, near Davis City, Iowa. Once point clouds are available for the study area (primarily the middle sections of the Nishnabotna River, expected to be available Jan. 2022), the analysis workflow will be scaled up to estimate sediment erosion and accretion at selected migrating bends of the river. From these data, target areas will be identified for sampling in the summer 2022 field campaign.