Dam engineering on a deadline: When Leominster, Mass. was inundated by an epic rainfall event

Engineers who design, inspect, and plan repairs and upgrades of the more than 80,000 dams across the United States usually are thinking in timeframes of years—how long it will take to design and permit and complete projects? And of the decades or even centuries dams need to be counted on to last.

Every once in a while, however, dam engineers have to think and react in timeframes of just days and even hours.

Such was the case in September 2023 in the North-Central Massachusetts community of Leominster, Mass., when the city was inundated with a virtually unheard-of 9.5 inches of rain in less than six hours. The “rain bomb” and resulting sudden flooding, an increasingly common occurrence in communities all across the United States, strained every dam in the region, with particularly dire impacts to the Department of Public Works’ Distributing Reservoir Dam and the Parks and Recreation Department’s Barrett Park Dam.

After being overtopped by flood waters for a number of hours, both dams experienced erosion of the embankment slopes and faced a threat of structural collapse unless the city and its contractors moved within a matter of hours to implement emergency repairs.

Fortunately, Leominster was well-positioned to manage this crisis. Under the leadership of its veteran mayor, Dean Mazzarella, the city administration has for years maintained clear visibility into the conditions of and maintenance challenges facing these dams. My firm, GZA GeoEnvironmental Inc., and I personally have had a long and positive relationship providing consulting engineering services to the city in support of its dam inspection and maintenance plan.

So when the floodwaters came, GZA was able to get into the field in a matter of hours and begin determining what needed to be done, immediately, to prevent this enormous challenge from turning into a crisis.

After assessing the damage to the two embankments, one need that was immediately clear was the installation of a filter and buttress on the downstream side of these two earthen dams in order to ensure that excess hydrostatic pressure acting on the eroded dams would not result in an internal erosion failure. The intent is somewhat like using a cheesecloth while preparing a kitchen recipe that calls for draining out excess moisture: By trapping fine particles of soil while allowing excess water to pass through the earthen dam, the filter prevents a self-accelerating cycle of water washing out soil from the dam that, in turn, lets more water pass through and carry out more and more fine soil until the dam ultimately is breached or even collapses.

Leominster already had an on-call arrangement for emergency infrastructure repairs with Blue Diamond Construction, based in Foxborough, Mass., and there was an on-going stream restoration project being performed in the city by George R. Cairnes and Sons out of Windham, N.H. Through these two intiatives, the city was able to get materials and begin construction of emergency repairs within hours of the overtopping and erosion of the dams.

At the Barrett Park Dam, the city and its consulting engineers had been pursuing local Conservation Commission approval to clean out the main spillway, which had become partially clogged with sediment and invasive vegetation, in order to allow unrestricted flow into and through the spillway.  Moving quickly to prevent potential issues, the Conservation Commission issued an emergency order that enabled Cairnes to move immediately to remove the vegetation and install three precast box culverts, with rock ramps on either side to allow construction equipment and vehicles to move through the site, while allowing for safe discharge of the flood waters.

What might normally have been a 12- or 18-month process of preparing design drawings and environmental analyses and submitting them for regulatory review and approval became, of necessity, a project completed on the fly, in less than 72 hours, in order to stave off a potential dam breach and flooding for many homes and businesses downstream.

With the worst of the weather event past, engineers had the chance to go back and do everything possible to make these repairs sustainable for the long term and document the work completed.

What we learned is that, as global climate change drives more frequent and more damaging “rain bomb” precipitation like what Leominster experienced, we’re going to need to re-think the standards to which we design new and upgraded dams. The Regulatory Design Storm for new dams of intermediate size and significant hazard classification such as the Barrett Park Dam is a 500-year flood event. More analysis remains to be conducted of the precipitation that fell on September 11, 2023, but it does appear tentatively that what Leominster experienced was something more akin to a 1,000-year (or less likely) flood–and that going forward, we need to re-evaluate dam safety standards to meet the climate challenges of today and tomorrow.

Thanks to a well-prepared city government with strong, established relationships with its consulting engineers and contractors, Leominster was able to move fast and avert disaster when epic flooding hit. Many communities, however, aren’t and won’t be as well-positioned. Taking the time to make and implement thoughtful plans before the next “rain bomb” hits will always be easier, and vastly less nerve-wracking, for municipal leaders and their professional partners than designing emergency repairs in the middle of the night while the rain’s still pouring down. What Leominster learned, the hard way, is the climate crisis shows no signs of slowing down.

Anders Bjarngard, a specialist in geotechnical engineering, dam engineering, and recently waterfront engineering, is a senior principal at GZA GeoEnvironmental Inc., company-wide government client sector lead, and manager of GZA’s office in Amesbury, Mass. He is a Massachusetts Licensed Professional Civil Engineer and a member of the Society of American Military Engineers, the American Society of Civil Engineers, the Boston Society of Civil Engineers, and the Association of State Dam Officials.