WIDESPREAD SLOPE FAILURES IN VERMONT DUE TO HEAVY RAINS ON JULY 9 TO 11, 2023

Heavy rains that fell on July 9 to 11, 2023 totaled in excess of 125 mm (5 inches) throughout most of eastern Vermont and large areas east of the Green Mountains received in excess of (175 mm (7 inches). Peak rainfall at some sites east of mountains was in excess of 225 mm (9 inches). As there had been abundant precipitation in the preceding weeks, soils were already saturated and runoff quickly brought rivers to extreme flood levels that resulted in devastating flash flooding, gully erosion, alluvial fan activity, stream bank erosion, and numerous landslides.

The Vermont Geological Survey and partners have been evaluating landslides and other slope failures (SF) for over 20 years. Based on our experience with previous floods (especially flooding from Tropical Storm Irene in 2011), we expected to receive reports from public agencies and the public of slope failures (SF) that were driven by some combination of high pore pressure and extensive toe erosion of slopes by streams. In the days following the July flood, we did indeed receive many reports of landslides caused at least partly by stream erosion, and as in previous floods, many of these were on slopes that had previously failed. However, we also received many reports of SF at sites where there was little or no connection to recent stream erosion. Some of these were at sites of earlier SF while others were at sites that had apparently not been unstable previously.

In the weeks following the flood we worked with partners to respond to SF at approximately 80 sites that ranged widely in size and in the human impacts, from no damage to extensive damage to roads and other public infrastructure and the destruction of homes. Although fortunately no lives were lost due to the SF, many homes were made unsafe to occupy, further stressing the already tight housing market in Vermont. Several private homes have been recommended for FEMA buyouts and others are under consideration for buyouts. Whole neighborhoods are now performing detailed geotechnical evaluations to better understand risks associated with their landscape positions. The financial costs due to SF are thus quite high and ongoing.

One of the lessons that we are taking from these events is that we need to develop landscape-scale slope stability models that encompass more than just stream-side landslides. This work is ongoing.