Integrating Precipitation and Soil Moisture Measurements to Understand Landslide Movements along Alabama Highways

Landslides along Alabama highways are a relatively common occurrence in many regions of the state. These landslides can lead to damage to transportation infrastructure and significant traffic disruptions. The current practice identifies landslide locations primarily through maintenance personnel reports or motorist complaints. Once an unstable region is identified, the suspected slide area is commonly instrumented with inclinometers, which are then read at regular intervals to understand the slide plane location and identify changes in behavior. This inclinometer data has been collected at unstable sites across the state for many years and provides a unique dataset to understand how precipitation events influence landslide behavior along highways. Previously developed precipitation thresholds considering storm magnitude and duration were consistent with landslide events observed around the state, but there are many non-triggering events that fall above the thresholds (false positives). Approximately 70% of false positive storm events occurred during drier than average periods based on normalized soil moisture data from NASA’s SMAP instrument, while large movements occurred primarily during periods of average or above-average soil moisture. This suggests that adding soil moisture data to landslide threshold predictions may help to reduce false positive events and to assess the likelihood of large movements occurring. These findings are now being used to develop improved warning thresholds that can highlight when landslides are likely to occur, allowing inspections and preventative maintenance to be prioritized.