39-2 Using Remote Sensing and Field Observations to Assess the Stability of a Restored Cobble-Bed River

Session: Functions of River Corridors, Floodplains and Wetlands

Presenting Author:

Authors:

Abstract:

Flooding is one of the most dangerous natural disasters in the United States, posing serious risks to human lives, damaging infrastructure, and necessitating costly repairs. Human modifications to rivers have led to channel incision and disconnection from floodplains, increasing fluvial erosive power and driving channel instability. The incised cobble-bedded Zealand River, located in northern New Hampshire (44.2461° N, 71.4968 W°), was recently reconnected to its historic floodplain in hopes of restoring form and function. Specifically, in Summer 2024, the legacy channel was blocked by a berm, and flow was diverted into multiple newly (re)constructed channels on the former floodplain. During Fall 2024 and Spring 2025, uncrewed aerial systems (UASs) were flown to collect high-resolution Light Detection and Ranging (LiDAR) topography data on river bars, banks, and floodplain areas throughout the reach before and after two overbank floods. To evaluate the geomorphic response to the floods, successive LiDAR digital elevation models (DEMs) were compared to characterize reach-scale erosion and deposition. In addition, LiDAR point clouds were used to estimate grain size distributions on bar and bank areas. We considered point cloud metrics including standard deviation, entropy, and roughness height to predict the median grain size of measured pebble counts. Using the best performing model, we created the first continuous map of grain size change on bar and bank areas throughout a 2-km-long river reach before and after flooding events, thus advancing remote sensing techniques for river monitoring in cobble-bed rivers. Maps confirmed minimal channel and grain size change in cobble-bedded areas, indicating that the restoration appears to be stable. Finally, floodplain DEMs, surveyed channel bathymetry, surveyed high water marks, and grain size maps were used to develop a two-dimensional hydraulic model, giving estimates of shear stress throughout the reach during high flows. Correspondence between expected and observed locations of erosion and deposition can improve understanding of hydraulic drivers of channel change during flood events, increasing confidence in similar modelling techniques and informing planning at other sites requiring restoration, thus mitigating flood impacts and enabling infrastructure protection.

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