Impact of Streambank Stabilization on Sediment Deposition and Erosion in Central Nebraska Streams

Stabilization projects are increasingly used to mitigate the effects of anthropogenic streambank erosion, yet the effectiveness of these practices has been insufficiently monitored and assessed to date. Sound monitoring practices promote engineered effectiveness, in addition to allowing adjustments in implementation and maintenance to improve practices over time. However, current methods to quickly and efficiently quantify deposition and erosion within a stream continue to be costly and inefficient. Therefore, the objectives of this project were to 1) Measure streambank migration of three reaches at Cedar River in Nebraska, from 1993 to 2006 (pre-stabilization) and from 2006 to 2018 (post-stabilization) using aerial imagery and 2) Quantify sediment deposition around jetties from 2006 to 2018 and in 2019 following a large flood using survey equipment. Results from objective 1 showed that erosion rates decreased significantly where stabilization practices were installed, and in some instances, increased deposition in the reach. Results from objective 2 reinforce findings from objective 1, showing increases of up to 406% in sediment deposition from 2018 to 2019. The surveys were completed seven months following the 2019 flood, demonstrating that the significant increase in deposition was a long-term impact, influenced by the jetties in the reach. To expand on our findings, we broadened our scope and assessed the impacts of stabilization structures on upstream and downstream sections of the river. To do this, we: 1) Measured the amount of riverbank loss/gain 1.5 wavelengths upstream and downstream of each stabilized reach and on the opposite bank from 1993 to 2006 (pre-bank stabilization), and 2006 to 2018 (post-bank stabilization) on Cedar River, in North-Central Nebraska using ArcGIS and historical aerial imagery. Unexpectedly, the differences in erosion from pre- to post-stabilization showed little to no statistical significance and deposition was significantly greater pre-stabilization in some reaches, supporting bank stabilization at Cedar River may be effective at the location of installation, but have little to no impact on decreasing erosion rates upstream or downstream. The methodology proposed in this project to quantifying sediment deposition in the stream system, along with the stream migration information collected for adjacent segments of the stream, serve to reinforce the need for additional investigations to be completed to improve streambank stabilization projects, as well as the importance of subsequent stream monitoring programs. Advisor: Aaron R. Mittelste