When it Rains

Most Vermonters would say it started raining in mid-June and never really stopped. By early July, western Vermont began to see the first of the rising waters.

Heavy rains began statewide on July 9th, and steadily increased as the storm system stalled. Over the next 36 hours, the peaks of the Green Mountains were to receive 6 to 9 inches of rainfall, surging into rivers already saturated by the unanticipatedly wet June. Rivers began to crest, vibrant downtowns became submerged, and hundreds of roads and bridges began to buckle, having been impacted by an immeasurable number of mudslides, road washouts, plugged culverts and undermined bridge abutments.

Recognizant of Tropical Storm Irene, water levels in portions of Vermont rose to heights surpassed only by the flood of 1927, the greatest natural disaster ever recorded in the state. Following the primary flood event, a federal disaster declaration was issued for all fourteen counties.

The Call to Action

GEODesign was rapidly engaged by both the Vermont Agency of Transportation (VTrans) as well as the Vermont Dam Safety Program (VTDSP) to assist with the flood recovery efforts.

In the days immediately following the flooding event, VTrans brought GEODesign onboard to be a part of a multiple-disciplinary task force that jointly assessed over 100 damage sites in southeastern Vermont. This included sites that had already undergone emergency repairs, were in the process of being repaired, as well as those that had yet to be repaired. 

The initial response effort was focused on triaging sites into a) those needing significant immediate action, b) those that could be temporarily stabilized in the near term but would require further engineering evaluation for permanent repairs, and c) those where repair recommendations could be provided based on the initial site observations. Where emergency repairs had already been made or were currently underway, GEODesign provided input on the suitability of the emergency repairs with an eye towards resiliency for future flooding.

Next, GEODesign provided geotechnical consultation and subsurface explorations for select sites that were identified as requiring further engineering evaluation, as well as those where construction oversight assistance was needed to reopen closed roads due to large scale slope failures.

Simultaneously to our work with VTrans, GEODesign also assisted the VTDSP with performing secondary inspections on over two dozen dams identified as potentially sustaining some level of damage or overtopping during the flooding event.

This included report preparation, providing dam owners with the results of inspection findings, and recommendations to correct deficiencies where deemed prudent.

Key Takeaways

  1. The Best Solutions Are Multi-Disciplinary. Slope failures along rivers following flooding events require multiple disciplines to appropriately evaluate failure mechanisms and identify the best solutions for a long-lasting repair. These situations generally require coordinated input from geotechnical, hydraulics, hydrology, and infrastructure design experts to appropriately develop a repair solution designed to withstand future flooding events.
  2. When it comes to rock fill, size matters. This event provided us an opportunity to evaluate areas that had received flooding during both T.S. Irene as well as the July 2023 flood. Typical repair details generated in the immediate aftermath of the T.S. Irene repairs called for the use of very large stone sizes (Type IV) to resist the high shear forces that can develop along stone fill slopes during a flood event. Those repairs constructed with appropriately sized stone held up well. Those constructed with undersize rock fill did not.
  3. Follow the recommendations of dam safety professionals. Following routine inspections, it is common for VT Dam Safety to provide suggestions to dam owners meant to keep their dams in proper working order. In general, dams that were maintained in accordance with common dam operation and maintenance safety criteria performed well during the flooding event. Those that had been allowed to fall into disrepair or neglected to follow through on maintenance recommendations generally did not perform as well.
  4. Keep Low-Level Outlets Operational. Often, one of the first items to go into disrepair on a dam is the low-level outlet. In general, low-level outlets should be exercised at least annually to ensure their functionality and prevent seizing. These safety features come in a variety of configurations, and are designed to allow additional water to bypass the dam in a controlled manner. These features are beneficial during a high-water event by helping maintain a lower pool level behind the dam, thus decreasing the stress put on the dam structure. Many of the dams we observed where damage was sustained during the flood event either did not have a low-level outlet, or the low-level outlet had become non-functional after years of neglect.

As we put the final touches on this latest GEONote, we again are facing the aftermath of another damaging flood event following unseasonably heavy rain combined with snowmelt this past Sunday into Monday. Vermont Governor Phil Scott held a press conference calling this storm a “gut-punch” and reminding everyone to be safe.  

The saying, at least here in Vermont, is true: when it rains, it pours. While the damage from this latest storm will not become apparent until the floodwaters fully recede, GEODesign will again be ready to assist with the fallout. Like our resilient neighbors involved in all aspects of the cleanup, we are built for the task.

Featured photo credit: Air Force Senior Master Sgt. Michael Davis

Author: Jacob Wimett, P.E., Geotechnical Engineer, Senior Associate


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