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An explanation of why downstream flood risks will not increase following removal of Potter Hill Dam was provided during the June 10 public information meeting.
In summary, the Potter Hill Dam is a “run-of-the-river” dam that provides negligible flood storage as the river’s normal water surface elevation upstream of the dam is at or slightly greater than the elevation of the dam’s spillway crest (depending on the time of year). Unlike “flood control” dams that have storage capacity to hold additional water during a flood event, for “run-of-the-river” dams, the additional water from precipitation is not held back and continues to spill over the dam (and adjacent land areas depending on the height of water during the flood).
As a result, peak flood flows discharged downstream of the dam subsequent to its removal will be equivalent to or less than those currently (with the dam in place) discharged to downstream areas. In fact, lowering normal flow elevations upstream of the dam will increase available temporary storage within the river channel and restored floodplain upstream of the dam that is no longer impounded during normal flows. As a result, there will not be any increased flood risks to downstream properties, and an incremental decrease in flood risk.
The above findings have been confirmed by detailed hydraulic (HEC-RAS) modeling for this project (and other dam removal projects on the Pawcatuck River, including at White Rock Dam). These analyses use methods that have been approved, and are required by, the Federal Emergency Management Agency (FEMA), the federal agency responsible for developing and issuing flood maps for all regions of the United States. Following removal of the White Rock Dam, HEC-RAS modeling files and as-built survey information were provided to the FEMA team (which was led by USGS) that was at the time completing a study to update Flood Insurance Rate Maps (FIRMs) for southern Rhode Island.
In addition to the above consideration of downstream flood risks during storm events, removal of the dam avoids any scenario where a breach would release a flood wave from the impoundment downstream, causing inundation of downstream properties and infrastructure.
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Dams can negatively impact the ecology of rivers and surrounding ecosystems by reducing habitat quality, altering ecological processes, and contributing to species population fragmentation. Removal of the Potter Hill Dam will both restore this section of the Pawcatuck River to its natural free-flowing state and mitigate flood hazard risks and flood damages to both upstream and downstream residents and structures along the river.
The Potter Hill Dam was first evaluated by the state as being structurally compromised in 1993, and its condition has only gotten worse since that time. Removing the dam will eliminate the risk of the dam and/or millrace channel headgates from breaching in the future. If this were to happen, the entire upstream length of the river impoundment (about 6 miles) could be suddenly released in an uncontrolled manner, causing damage to downstream properties and fish and wildlife habitats, and potential loss of water supply to residents in Hopkinton with shallow groundwater wells that are immediately adjacent to the river. Both the dam and debris from the mill are imminent hazards to boaters and downstream properties, including bridges and other public infrastructure. The Town of Westerly seeks to eliminate this hazard and potential liability by removing the dam expeditiously. Increased frequency of high rainfall events attributed to climate change are more and more commonly occurring, and increasing flood hazards and risks will continue until the dam is removed and the river is restored.
The dam is the last barrier on the main stem of the Pawcatuck River to upstream fish migration as the existing fish ladder is outdated and ineffective due to the poor location of the fishway entrance and the limited functioning of the fishway within an excessively small range of river flows (which will only worsen with climate change). Currently, migrating fish are delayed below the dam as they attempt to navigate the downstream river channel in search of the fish ladder entrance. With this delay, the fish become easy prey to cormorants, gulls, osprey and other predators. While this is a natural process, the loss of high numbers of spawning fish limits recovery of the river populations of herring and shad. Diadromous fish from the Pawcatuck River are a food source for important marine recreational and commercial species downstream. Improving fish passage will increase the use of nursery habitat in the Pawcatuck, which will in turn provide Little Narragansett Bay, Rhode Island Sound, and the Western North Atlantic with a greater number and diversity of fish. The local fishing economy will benefit as a result by having greater numbers of species such as striped bass, bluefish, and black sea bass.
In 2019, the Wood-Pawcatuck River watershed was designated as a Wild and Scenic River under the federal Wild and Scenic Rivers Act. Of the country’s 3.6 million miles of streams, less than 1% have received this designation. Removing the dam will improve the river’s ecological integrity, quality, and resilience, and will enhance recreational and aesthetic values of the river, thus improving the health and value of the Wood-Pawcatuck River watershed as a whole.
Removing the dam will provide significant flood risk reduction benefits to upstream properties and roadway infrastructure while reducing downstream flood risks due to dam failure. The project is consistent with the Town’s 2020-2040 Comprehensive Plan, the Town’s Municipal Resiliency Plan and the 2017 Wood Pawcatuck Watershed Association Flood Resiliency Plan.
There is no owner willing to assume responsibility of the costs of repairing the dam, and the Town of Westerly does not have funds available for repairing the dam, nor does the town want to be responsible for the liability of a failing structure that is a public safety hazard. Federal funds secured by the town are only eligible for removing this barrier to migratory fish passage. The project partners are focused on dam removal as the preferred alternative to address the purpose of the project to restore unimpeded fish passage, reduce flood risk, and eliminate a public safety hazard.
Repairing the dam would also not adequately address any of the ecological concerns previously mentioned. Continual maintenance of the dam would be needed, and the town does not have the financial resources or staff capacity for on-going dam operation, maintenance and repairs.
Partial removal of the dam would require other in-river structures to provide hydraulics needed for efficient fish passage, would require future maintenance, and would also require a design to minimize public safety hazards associated with recreational boating. Substantial regulatory challenges, particularly filling of river channel and floodplain, would be associated with a partial dam removal with in-river nature-like fishway (NLF) alternative, as well.
With the preferred approach, no future operation, maintenance, or repairs will be required, and funds are available for dam removal.
Not all dams are the same. The unique hydrologic features of (and infrastructure in or over) each river requires thorough engineering assessments to evaluate options to address defunct and failing dams. In the case of the Potter Hill Dam, several options were evaluated before it was decided that removing the dam and restoring the river to its natural state is the best option in terms of improving the ecology of the river and reducing flood risks due to flooding to Westerly, Hopkinton, and Pawcatuck, CT.
The Potter Hill Dam is characterized by a narrow-length spillway with high spill and leaking gate structure. Immediately downstream of the dam, the river widens, and multiple millrace channels within the crumbling mill structure are present that release flows back into the river. The construction of a NLF would require a set of large stone weirs, for which a design would need to take into account this wide downstream section of the river.
To design and construct a full-height series of NLF weirs that will not only provide effective fish passage but also maintain current headpond levels, such as the system used for the Bradford project, would require a larger impact footprint, greater wetland disturbance, and the need to import a significant amount of fill material into the river to construct the step-pool system. Regulatory permitting agencies including the Rhode Island Department of Environment Management’s Freshwater Wetlands Division and the U.S. Army Corps of Engineers require the “least environmentally damaging project alternative” (LEDPA) that meets the project purpose(s) be implemented. The Town of Westerly and its project partners consider dam removal as the LEDPA for the Potter Hill dam, based on detailed engineering analyses completed to date.
In addition, the cost of this type of fishway would be greater to construct than the other alternatives considered and would require long-term maintenance and repair. The Town of Westerly will not be responsible for the operations and maintenance of such structures at the Potter Hill site.
In addition to the alternative approaches addressed in the previous questions (repair dam and replace fish ladder, remove dam and construct a NLF like at Bradford) other alternatives have been evaluated, which were presented during the March 18 Public Information Meeting. Wetland permitting agencies (Rhode Island Department of Environmental Management and US Army Corps of Engineers) require an evaluation of alternatives that demonstrates that the selected alternative be the least impactful approach that meets the project purpose(s). The permit application materials that will be submitted to these agencies this fall will describe the project team’s evaluation of alternatives, which is briefly summarized below.
● “Do Nothing” Approach○ Dam, millrace gates and crumbling mill buildings near the dam would remain a significant public safety hazard○ Mill building debris would remain in the river○ No improvements to recreational boating/portage safety○ No improvement to other recreational opportunities or public access○ No upstream flood risk reductions○ Continued risk of downstream flooding and loss of water to residents along river with shallow wells due to breach of dam remains○ No water quality improvements○ No ecological wetland/wildlife habitat restoration or enhancements○ Fish ladder currently performs poorly due its layout - entranceway lies in the turbulent area below the dam’s spillway, and the fishway operates effectively only during a limited range of river flows○ No ownership, no funding to repair or to maintain dam in future
● Repair/Replace Dam and Replace Fish Ladder○ No funding is available to repair dams without safety improvement benefits○ No upstream flood risk reduction for properties and roads○ No improvement to recreational boating opportunities or risks○ Same limitations in fish ladder capacity vs. full channel width for fish passage○ Requires ongoing, long-term operation, maintenance and repairs○ On-going public safety hazard in the vicinity of the dam
● Remove Dam and Construct Full-Height Nature-Like Fishway (maintain impoundment like at Bradford NLF)○ No flood risk reduction for upstream properties and roads○ Would increase flood risk to Maxson Street and adjacent residences○ No water quality improvements○ Limited increase in wildlife benefits○ Reduced fish passage capacity versus full channel width for fish passage○ Unlikely to be permitted due to greater direct wetland disturbance and significant amount of fill material required in river○ No current funding for the increased cost to construct due to large size structures and fill in the river○ Requires ongoing, long-term operation, maintenance and potential repairs● Remove Dam Only (No Additional Channel Modifications)○ Would not assure efficient fish passage or boating safety○ Continued erosion of riverbanks and risk of undermining Maxson Street requiring future maintenance and repairs
● Remove Dam and Construct Nature-Like Roughened Channel orRiffle-Pool Channel○ Roughened Channel■ Simplest approach to restore channel to pre-dam condition – lower construction cost, shorter construction duration■ Enhances recreational boating opportunities and public safety■ Reduces upstream flood risks■ Improves upriver water quality and river ecology (lower water temperatures, higher dissolved oxygen levels)■ No future operation, maintenance or repairs required■ Resilient to potential climate change effects ○ Riffle-Pool Channel■ Similar benefits as the roughened channel design but could potentially provide enhanced fish passage efficiency and recreational boating opportunities
While evaluations of the preferred alternative are currently ongoing, based on the engineering analyses completed to date, the option to fully remove the dam and construct a nature-like riffle-pool channel is most advantageous due to the more significant benefits it would provide to fish passage, flood risk reduction and recreational boating opportunities.
Remnant mills and low-head dams in general pose serious risk to people who recreate near them. Potter Hill Mill is a set of building structures first constructed in the mid-1800s, and currently they are in serious disrepair. Trespassers on the mill site may access these dangerous structures, and serious injury may result. Additionally, the mill buildings are collapsing into the river, representing a significant safety hazard to recreational boaters passing downriver and other water recreation users.
The Potter Hill Dam has a dangerous hydraulic pattern at the base of the structure during high flows (known as “keepers” or “roller waves'') where recirculating currents occur. For the Potter Hill Dam site, this dangerous hydraulic pattern exists even with lower flows, and presents an increased risk due to the dam’s narrow spillway width. The hydraulic effects and currents are very difficult to get out of and account for multiple injuries and drownings nationally each year.
The videos below provides detailed explanations of the risks of low head dams.
Removing the dam will not change (neither raise nor lower) water levels downstream of the Potter Hill Mill site during normal or storm/flood flows.
Upstream of the dam’s current location, the extent that water levels will be lowered depends on how far a location is upstream; water levels will be lowered more closer to the Potter Hill dam site, and the magnitude of reductions will diminish at locations further upstream of the dam.
Approximate water level reductions in respective segments of the river are summarized below, for different (seasonal and flood) flow conditions.
● Immediately Upstream of Potter Hill Road Bridge○ Average Seasonal Low Flow (August – October): 6.5 feet○ Average Spring Flow (March – June): 6.3 feet○ 100-Year Flood Flow: 2.5 feet
● Immediately Downstream of Route 3 Bridge○ Average Seasonal Low Flow (August – October): 4.25 feet○ Average Spring Flow (March – June): 3.9 feet○ 100-Year Flood Flow: 1.7 feet
● Immediately Upstream of Route 3 Bridge○ Average Seasonal Low Flow (August – October): 4.1 feet○ Average Spring Flow (March – June): 3.7 feet○ 100-Year Flood Flow: 1.3 feet
● Pawcatuck River at McGowan Brook (Aguntaug Swamp) Outlet○ Average Seasonal Low Flow (August – October): 3.5 feet○ Average Spring Flow (March – June): 2.4 feet○ 100-Year Flood Flow: 0.6 feet
● Bradford Fishway○ Average Seasonal Low Flow (August – October): 1.4 feet○ Average Spring Flow (March – June) 0.9 feet○ 100-Year Flood Flow: 0.1 feet
The town and its consultant seek more specific information to address this concern. A certified engineer would be able to conduct an evaluation of your land, and nearby subsurface soils and hydrologic features to better determine whether or not your well may l be impacted. An engineer will be able to guide you on how to best proceed if you begin to experience issues with your well. Properties that have deep wells and those that are not close to the river will not be affected.
Groundwater supply at a property is influenced in a number of ways including its proximity to rivers or other surface waterbodies, soil permeability (how easily or with difficulty water moves through soil), elevation difference between a waterbody and the property’s well, and rate of water withdrawal for each home residence. Properties that are close to, and near the same elevation as the river, may more likely experience changes in groundwater supply following dam removal. This is especially true where permeable soils are present, unless an upland waterbody that supports underlying groundwater is nearby.
If you have a concern about your well, please contact the Town of Westerly for our project team’s consulting engineer to arrange a site visit and to collect needed information.
Groundwater supplies in the Pawcatuck River watershed are naturally governed by a number of factors, including the local geology, the availability of water supply recharge areas (where infiltration of rainwater occurs), the depth/condition of bedrock and in some cases, river conditions. Based on the project team’s preliminary assessment, the project team has determined that approximately 125 properties warrant further evaluation. Earlier this year, the project team sent a survey to potentially affected property owners; slightly fewer than 50 responses have been received by the Town of Westerly and its consulting engineer to date. Of those that have responded to date, a number of have indicated that their wells have previously experienced lack of water, or draw water from shallow sand formations saturated by river water.
In the coming months, the project team will continue to work with the towns of Westerly and Hopkinton to refine its assessment of private wells, based on the following criteria:
● depths below ground (i.e., deep and bedrock wells will not be affected)
● location with respect to the river (the higher the elevation and the greater the distance from the river, the lower the potential for effects)
● proximity to other tributaries and surface waterbodies and subsoil/bedrock conditions (these other sources reduce reliance on the Pawcatuck River for groundwater supply and more permeable/porous soils and fractured bedrock allow water to move underground to private wells more readily)
● type of well (certain types, like hand-dug wells, are more likely to be affected)
● record of performance (whether they have experienced lack of water during previous dry or drought conditions)
Even if no action is taken, in future years, wells that are affected by changes in river flows will have limited capacity to provide a reliable source of drinking water, as leakage rates through the Potter Hill dam’s failing millrace headgates increase river levels between Potter Hill Dam and Bradford will continue to drop as a result.
Further, scientists predict that as a result of climate change, Rhode Island will experience longer and more frequent periods of drought, that will put more stress on shallow, vulnerable wells, both in terms of water quantity and quality. When the headgates ultimately fail and the river breaches the dam through the millrace channels, these substandard wells will likely experience a sudden failure, with no plan or resources in place to replace the failed well in a timely manner.
Based on the findings of previous studies of the aquifers in this section of the Pawcatuck River, Westerly’s municipal water wells will not be affected. Previous studies by the U.S. Geological Survey determined that the Pawcatuck River between Potter Hill Dam and Bradford is receiving water from upland areas in the contributing watershed and is not feeding water into the deep groundwater aquifer from which public drinking water wells in this stretch of the river (Crandall and Bradford wells) draw their water. A review of sub-surface mapping published in USGS’s studies of this area indicates that the nearest well (the Crandall well, within Aguntaug Swamp) is supplied by the water entering the aquifer from the south (ocean-side of the well) and the surrounding swamp, and is not being recharged by the river.
The river would recharge the aquifer in this area only if water withdrawals from these municipal and other private wells increased significantly beyond today’s level (to or above the aquifers’ full yield capacity) causing the existing water table along the river to be significantly lowered. During drought conditions, only the Noyes Avenue and White Rock wellhead protection areas (which are downstream of the dam by more than two miles and thus not affected by removal of the Potter Hill Dam) supplies water to the aquifer due to the extensive paved areas limiting water infiltration along this stretch of the river.
Shallow private drinking wells that draw water from the river via shallow sandy soil formations are our priority and the primary focus of our assessment.
Flood risks will decrease upstream of the dam, especially for properties bordering the river near and downstream from the Route 3 bridge. As noted in the March 18 public information meeting, 100-year flood elevations in this portion of the river will be lowered between 1.3 and 2.5 feet, depending on the location of the property. This will reduce the potential for flood damage during 100-year floods for residences on Lima Drive, Forest Drive, Saratoga Avenue, Hiscox Road, Boy Scout Drive and Fieldstone Way in Westerly, and along Laurel Street, Maxson Street, and River Road in Hopkinton.
Elevations for all river flow events (normal, 100-year flood and larger floods) downstream of the dam will not increase following removal of the dam. The magnitude of pre- and post- dam removal water surface elevations decreases as the magnitude of flows associated with any particular seasonal or storm flow event increases. As shown by modeling analysis results presented during the March 18 public information meeting, there are more significant changes (reductions) in water surface elevations at respective locations upstream of Potter Hill Dam following its removal during lower flow conditions (e.g., seasonal low flow) in comparison to higher flow conditions (normal spring flows, 100-year flood flows, and higher flow events).
As a result, there will still be reductions in upstream flood elevations during a 500-year flood, but they will be less significant than 100-year event reductions. In terms of changes to downstream flood elevations, there will be no changes to downstream flood risks for the 500-year flood event or any other flood event following removal of the dam.
Removing the dam will improve the long-term ecological health and resiliency of the river and enhance recreational opportunities such as kayaking and fishing. The river is used for kayaking, paddling, canoeing, paddle boarding and fishing. By prioritizing the health of the river as a federally designated Wild and Scenic River, the project partners expect recreational opportunities will continue to be available to the public and improve water-based recreational values. Most of the restored river reach is expected to remain as “flat water”, although water quality improvements (lower seasonal water temperatures and higher dissolved oxygen levels) and unimpeded passage by migratory and resident fish with a removed dam will improve water quality and recreational fishing.
Removal of the dam will also improve the safety of water recreation in the vicinity of the dam, and provide eco-tourism opportunities to both Hopkinton and Westerly. Canoeists and kayakers will be able to float down the lower river, and depending on the final design that is implemented for the dam removal site, some may readily pass the restoration area and continue downriver to downtown Westerly and tidal waters.
Removing the dam will substantially lessen public safety hazards at the dam site where accidents have occurred, and restored conditions should lessen canoe portages across Potter Hill Road and public safety issues to bypass the dam. Optimally, the mill site may become a future public access site.
Decades of data support that dams have negative impacts on river systems and that removing them most often improves fish and wildlife habitat quality. Dams increase water temperatures at least seasonally, alter natural water flows and sediment transport, inundate and transform upstream floodplains, and disrupt river continuity for migratory fish and other aquatic animals to pass upstream. Restoring the natural flow of a river promotes improved habitat quality and as a result, can increase the overall biodiversity of both the river and the surrounding wetlands and associated plants and animals linked to rivers.
In the case of the Pawcatuck River, removing the Potter Hill dam will improve fish passage for spring-migrating river herring, American shad and American eel, which are important species in the food chain for birds and other animals along the river and marine life in Rhode Island’s coastal waters and beyond. . Spotted and wood turtle populations, which are listed as endangered by the International Union for Conservation and Nature, will be able to move up and down the river freely.
Other dam removal projects such as the White Rock and Lower Shannock Falls Dams, have led to an increased passage by river herring and other fish species, as well as the return of osprey, great blue heron and other wildlife that rely on fish as a primary food source. Overall, the project partners seek to restore the natural free-flowing river conditions so the Pawcatuck River is more resilient to the effects of climate change.
The wetland system of the Lower Pawcatuck River watershed encompasses approximately 2,000 acres of forested, shrub and emergent wetlands based on information available in National Wetland Inventory (NWI) maps and state (RIGIS) land cover maps. Of these wetlands, some are located directly along the impounded Pawcatuck River upstream of Potter Hill Dam. Others are situated in the landscape where surface water and groundwater may hydrologically connect the wetlands to the river.
The project partners engaged the University of Rhode Island, Department of Natural Resources Science (NRS) to conduct a wetland assessment of existing conditions, and how wetlands may change by removing the dam. This work is ongoing and is expected to be presented and summarized in fall 2021. To date, NRS professors and students have improved on the accuracy of the wetland type mapping, and are thoroughly assessing soils and hydrology along the lower river. This information is needed to predict changes to wetlands such as one wetland type converting to another wetland type (e.g., emergent marsh to shrub wetland) with dam removal. Once the NRS study is completed, the data results are expected to be used in the regulatory permit applications for the project.
Removing the Potter Hill Dam will restore the ability of the upriver wetlands to temporarily store flood flows and trap sediments that are suspended in the river during high flow events. These will result as the impoundment that inundates former vegetated wetlands is lowered. Currently, the dam is artificially holding water in areas that would otherwise provide temporary flood storage capacity. Based on outcomes from past dam removal projects in New England and elsewhere, it is expected that up-gradient vegetated wetlands will adjust to the change in water depth and return to their natural composition, relatively rapidly (within 1-2 growing seasons/years).
The project engineers have also modeled the river by updating a “HEC RAS” model of the Pawcatuck prepared by the Federal Emergency Management Agency. The model includes use of bathymetric data (the river bottom elevations collected by professional land surveyors and other experts) and long-term river flow data from government flow gauges on the river to predict how water surface elevations will seasonally change with dam removal. The study includes a number of cross-sections of the lower river to help predict water surface elevations with the lowering of the impoundment and changes in the river channel.
Results of the HEC RAS study were presented during the initial two public informational meetings, and will be included as part of the regulatory permit applications for the project. The project engineers continue to work collaboratively with NRS faculty and students to review environmental data collected for the Lower Pawcatuck River, and to thoroughly analyze what changes are expected to occur.
Yes. As our region experiences more frequent and intense weather events such as heavy precipitation or droughts as a result of climate change, it is expected that this will increase stress on rivers and wetlands bordering rivers. Naturally flowing, unfragmented rivers with vegetated wetlands and floodplains are more capable of responding to severe environmental stressors than rivers that are dammed. Increased water temperatures and variable water levels as a result of climate change will negatively impact the ecology of the river over time, but removing the dam will make the Pawcatuck River more resilient to these changes.
Floodplains serve to temporarily store flood flows, while run-of-the river dams like Potter Hill Dam have upstream impoundments with no floodwater storage. Vegetated riparian wetlands provide a root network to strengthen riverbanks and floodplains, and help to trap sediments transported during high river flows. These vegetated riparian plants also provide habitat structure for wildlife that overall is higher quality habitat than an open water impoundment. With climate change effects, a diversity of riparian plants will provide conditions more favorable to supporting wildlife associated with the Pawcatuck River watershed.
● Town of Westerly
● Southern Rhode Island Conservation District
● Wood-Pawcatuck Watershed Association
● The Nature Conservancy
● National Oceanic and Atmospheric Administration
● U.S. Fish and Wildlife Service
● U.S. Geological Survey
● Rhode Island Department of Environmental Management
Project team members have completed hundreds of dam removal and ecological restoration projects across the United States, and dozens of projects within Rhode Island. Please visit http://www.westerlyri.gov/PotterHill for more information about the qualifications and experience of the organizations leading this project.
● Year 1 (Ending September 2021): Field Investigations & Alternatives Assessment Being Completed ○ Document dam, gates, millrace channels ○ Field investigation of river levels, soils, groundwater, and wetlands
● Year 2 (July 2021- September 2022): Supplemental Investigations, Design, Permitting, and Start of Construction ○ Shallow groundwater well assessments ○ Third-party dam condition assessment ○ Historical/cultural resources assessment (document pre- and post-colonial conditions)
● Year 3 (July 2022- Winter 2022): Complete Construction and Begin Post-Construction Wetland and Biological Monitoring
Please visit http://www.westerlyri.gov/PotterHill for more information, or to submit your questions and comments. Recordings of the public meetings that were held in March and June 2021, as well as the presentation slides, can be found on this website.