Impacts of Stream Bed Adjustment on Local Brisge Morphology at Highway 281

Nebraska sand-bed rivers have long been a part of sediment transport studies and have been used in the development of numerous well-known empirical bedload equations. Some of these studies have shown how adjustments in streambed elevation led to changes in local stream morphology. The sand-bed rivers of Nebraska are highly susceptible to such changes due to their wide, shallow cross-sections and erodibility. The objective of our study is to synthesize a hydrodynamic model with high-resolution field data and improved methods of representing sediment transport to better predict both short and long-term channel adjustments for an aggraded sand-bed river. The field component of our study targets the reaches of the Niobrara River in northern Nebraska near two bridges, the Highway 11 bridge and the Highway 281 bridge. These reaches are of particular interest because they are situated upstream and downstream of Spencer Dam, which failed in 2019, releasing large quantities of sand into the downstream reach. We began with collecting Unmanned Aerial System (UAS) data that provides us with point clouds and aerial photos (for roughness estimation) of the sites and hand-collected PPK GPS data for streambed topography. The point clouds were aligned with USGS data and then created into Digital Elevation Models (DEMs). The point clouds were also used to calculate local sediment erosion. QGIS was used to create polygons to represent areas of available sediment in the reservoir and areas of erosion. A script was used to calculate volume loss at the local area of erosion. HEC-RAS was used to create a simple one-dimensional model of the Niobrara River at the local Highway 281 bridge. August 2023 collected data was incorporated in the HEC-RAS one-dimensional model to assess model performance and to adjust the model to recreate the observed data. The results found that the slope of the longitudinal profile of the river decreased between the USGS data and the collected 2022 September data, and the increase in water surface elevation at the bridge is 0.45 meters. The 2023 August data follow the same slope as the USGS data, but the water surface elevation increased another 0.06 meters at the bridge. The sediment volume loss calculation between 2022 September and 2024 February was approximately 85,690 cubic meters. The standing waves shown in the aerial imagery and the results of the one-dimensional model, the flow of the river is critical through the entire reach. Based on these findings, additional data should be collected to create a quasi-unsteady sediment transport model or two-dimensional model to accurately model the short- and long-term channel morphological changes of the Niobrara River. Advisors: David Admiraal and Tirthankar Roy