Bank Stability Assessment Tool for Bridge and Abutment Infrastructure in Iowa - RB10-014

(2021) Bank Stability Assessment Tool for Bridge and Abutment Infrastructure in Iowa - RB10-014. Transportation, Department of

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Abstract

In Iowa, bank erosion is a persistent and severe problem. Yet, the lack of field data and remote sensing imagery inhibits characterization of the spatial and temporal variability of soil strength and erodibility, which influences bank erosion mechanisms. There is a critical need for a geomorphic-geotechnical-hydraulic approach that utilizes currently available tools (e.g., LiDAR; GIS; PEEPs; recirculating flumes; numerical models) to quantify the extent of bank erosion near bridges at a suitable spatial scale and over time. The Iowa Department of Natural Resources, Iowa Geological Survey, and the University of Tennessee- Knoxville provide a multi-dimensional approach to identify and map currently eroding banks in 3rd - 6th order streams that intersect with bridge structures in Iowa. This approach includes geotechnical and hydraulic data that capture the spatial and temporal variability of the bank soil strength under changing climate, moisture and land-use conditions to provide the likelihood and severity of bank erosion. This study substantiates spatial patterns and temporal cycles of bank erosion in the Major Land Resource Areas (MLRAs) of the state. MLRAs 107A, 107B, 108C, and 108D exhibit similar patterns due to the loess soils covering western and southern Iowa. These soils have higher average critical shear stresses than the coarser, till-derived soils in MLRAs 103 and 104 of north central and northeast Iowa. However, MLRAs 107A and 107B have Factors of Safety less than one for at least 95% of their observed flows suggesting that fluvial erosion is highly likely and thus, they have the highest density of eroding banks. Regression models using existing databases and new GIS coverages developed during this project were established using eleven parameters including bank height, stream sinuosity, stream slope, available water capacity, clay content, and bulk density, among other parameters. Stream length to bridge length ratios were used to identify the potential threat for bridges in the near future. In addition, streambank polygons were developed and intersected with right-of-way features to identify roads that may be impacted by lateral channel migration. Bridges with a high ratio and a stream having a high potential for migration were flagged. The greatest number of bridges threatened by stream migration were found in MLRAs 107B and 108D, followed closely by MLRAs 103 and 107A. The greatest risk to roadways were in MLRAs 107B and 103. The project identified 1,515 bridges and 281 road right-of-ways in Iowa that were considered to be at high or moderate risk of future erosion by channel migration. The study provides a means to prioritize those bridge structures across the state that need further protection from pending bank failures.