When it comes to dams operated by the federal government, most of us tend to think of the big ones named after former presidents. But the Agriculture Department plays a little-recognized role in managing the nation’s dams too: Tens of thousands of them. Most of them are relatively small, and they’re showing their age. But USDA’s Agricultural Research Service has found innovative ways to extend their lives and prevent potentially catastrophic floods. Sherry Hunt is a supervisory research engineer who’s led ARS’ work on this topic – and she’s a finalist in this year’s Service to America medals program. She talked about her research – and USDA’s role in dam management – with Jared Serbu on Federal Drive with Tom Temin.
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Sherry Hunt: USDA is the engineer of record of approximately 1/3 of the 90,000 dams in the national inventory of dams. And nearly 12,000 of these dams were constructed under the authority of the USDA small watershed program. Now the vast majority of these dams provide flood protection, but they also provide other benefits, including things such as irrigation water, for your crop production, provides role municipal water supplies, so huge benefit of economic development for rural communities, increase wildlife habitat, among others. So that is a little bit of the role we play, getting the water to the agricultural land for crop production, and that flood protection.
Jared Serbu: And then as far as the subset of dams that actually are vulnerable to failure, what did you find makes them vulnerable? What are some of their characteristics? Age is probably one.
Sherry Hunt: Of course, those 12,000 dams, the vulnerability is age, just like any other infrastructure, like buildings, roads, bridges, dams can also show signs of age. And they’re a large percentage of those 12,000 dams that I mentioned, are designed with planned service lives of 50 years. With construction dating back as far back as the 1940s. These dams have experienced a lot of changes, like deterioration of structural components, sedimentation of the uncounted reservoirs behind the dams, and changes in the landscape surrounding them. Take for instance, it may have been constructed to protect agricultural land, but now you may have millions of people downstream. Now, does this mean that these dams are in imminent danger of failure? No. It just means that we need to pay closer attention to them, just like any other infrastructure, and determine when it’s time that they need to be upgraded.
Jared Serbu: Yeah. And upgrade is, as I understand it, really the only option on some of these, right? Because at least in some cases, there’s been some urban or suburban encroachment, that just makes it not really feasible to do a full on replacement. Is that about right?
Sherry Hunt: That would be correct. Some are now landlocked, and so you have to come up with creative ways to upgrade these dams. In some cases, if the benefits are no longer there, it may mean decommission the dam. So take the dam out if they are no longer providing a benefit to the community.
Jared Serbu: Tell us about some of the interesting ways that you found to upgrade these in lieu of decommissioning them or replacing them.
Sherry Hunt: Well, as a scientist, I work with a team of engineering support personnel, and we develop standardized design guidance to rehabilitate or upgrade these aging dams. An example of this design guidance is for roller compacted concrete steps, doorways, these are spillways that have many advantages over other alternatives. But, for instance, they can be placed over the existing them that may be landlocked by residential or commercial properties. The construction materials are more feasible as they don’t require reinforced fill that many conventional concrete designs require. And they can provide shorter construction schedules because of the nature of the construction practices in which they are placed. Another example of the work that we are doing is working with collaborators at the USDA Natural Resources Conservation Service and Kansas State University, where we are developing technologies to analyze the potential of earthen dam erosion processes and failure as a result of water either spilling over the top of an existing dam or water coming through an internal hole within the dam. So we’ve developed software known as wind dam, which stands for Windows dam analysis modules, that can provide information on such things as the timing of the failure of the vegetation or rock that may be on the downstream slope of the dam, the erosion rates of earthen materials, the breach width and the time I mean that what’s the reservoir impounded by the dam is drained.
Jared Serbu: I’m trying to visualize, for example, that the roller compacted concrete example that you talked about a minute ago, what does that actually do in terms of providing additional safety? How does it interact with with the water behind the dam or potentially spilling over the dam?
Sherry Hunt: So the roller compacted concrete is actually a dry mix of concrete. When it is light, it comes up in a stair step fashion, it’s placed over the existing dam. So if there is a extreme storm event that comes through, the water rises, and spills over the top of the dam, which then goes down this step spillway, and so it allows the water to be slowed, and move on downstream to lessen the impact of the erosion in the downstream channels.
Jared Serbu: And what are some of the things that you and your team actually did to figure out these potential future causes of failure so that you could design these interventions?
Sherry Hunt: Some of the things we do is take a look at case studies from historical documentation, look at construction as built, we are starting to get into real time data monitoring and inspection. But also just the real time, dam breach, incidents and failures, and going out and doing forensic investigations, can all provide clues and sue the causes of failures? We then take that information along with the increased knowledge that we obtained from our research to develop additional research plans to better understand those mechanisms that trigger a dam failure. For instance, how well is the vegetation maintained on a dam? If these are constructed in rural environments? Has there been cattle left to graze on these dams that may have trampled down the vegetation and therefore causing initiation point for erosion to occur? We also have learned how the soil materials are placed with regards to soil type for more sphere content, soil compaction energy, and what role that plays and how fast or how slow erosion processes may occur during earthen dam failure.
Jared Serbu: And so I guess the last thing I wonder is based on everything that you and your team have learned throughout this process, is there information that we have now that would tell USDA how to build a dam in the year 2021 that we just didn’t know in the 1940s?
Sherry Hunt: Yeah, there’s been lots of lessons learned. Technology transferred from our research as it relates to the performance of embankment dams and related structures, that provides practicing engineers with knowledge and making adjustments to their construction practices of new earthen dams. There was quite a bit of workloads and a lot of technical experience, even back in the 1940s when these dams were first constructed. But an example of something we’ve learned since then, is just a slight change in the moisture content of the soil placed in the in the embankment can have very different differing results, and the timing of of a breach. Therefore practicing engineers can take this information and knowledge today to make stronger dams. In addition, our research provides engineers with the tools to identify how to prioritize existing dams for rehabilitation and upgrade. So the research provides them with more information and how well the dam will perform. So a lot of information is garnered from the research over the years to make improvements along the way to lessen the vulnerability of them they have.
Jared Serbu: And can you think of a specific example of a dam that actually has failed, that wouldn’t have failed if one of these techniques that you that you talked about had actually been employed when it was built?
Sherry Hunt: So in 2019, I was invited to travel to Wisconsin, southwest Wisconsin, to take a look at five dam failures that occurred during a storm event of extreme rainfall over a short period of time. And that is just one example of water had spilled over the dams and there were vulnerabilities at which the soils were placed and how it was tied into the natural landscape. Had we had that information back then. It may have caused the design engineers to rethink how that dam may have been placed.