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Merrimack Village Dam: Results of Removing a Dam in New Hampshire

Post-dam removal monitoring is ongoing to identify the impacts of removing Merrimack Village Dam in New Hampshire. The dam was removed to restore diadromous fish habitat, improve water quality, restore natural sediment transport and avoid high maintenance costs associated with the aged structure.

By Mark Wamser

In recent years many New England states, including New Hampshire, have been removing low-head dams that no longer serve a useful purpose. To support these efforts, competitive grant funding is available from federal, state and private entities that can be used to partially defray costs associated with feasibility studies and demolition. Most dams eligible for this funding are those along the coast where removal would restore diadromous fish (Atlantic salmon, American shad, river herring and American eel) to former spawning grounds and restore habitat connectivity.

Merrimack Village Dam - the lowermost dam on the Souhegan River in Merrimack, N.H. - was purchased by Pennichuck Water Works in 1964 for use as a supplemental water supply source. These plans never materialized, and in 2004 PWW received a letter from the New Hampshire Department of Environmental Services regarding numerous dam deficiencies. PWW opted to retain Gomez and Sullivan Engineers P.C. of Henniker, N.H., to conduct a feasibility study regarding removal of the dam. Removal achieved many goals: diadromous fish restoration, habitat connectivity, restoration of natural sediment transport, elimination of operation and maintenance costs and removal of a potential safety hazard.

From the onset, this project was a cooperative effort among PWW, state and federal agencies, and many non-profit organizations that invested technical experience and grant funds for the successful removal and restoration of the Souhegan River. In 2008, this dam was removed, thereby allowing diadromous fish to once again ascend the Souhegan River.

Project history and setting

The first documented dam construction on this site at Merrimack Village occurred in 1744 for use as a grist mill. Over the centuries, ownership exchanged hands and modifications were made to the dam and power canal. The river at this site was harnessed to power various saw mills, grist mills, shoe industries and a chemical plant. The most recent changes to the dam occurred circa 1934 when the stone masonry dam was overlain with concrete. The dam was about 20 feet high and 145 feet wide along the arched spillway.

As the local population grew, the dam came to be located along a heavily traveled road in a downtown area of Merrimack. A stone's throw below the dam is a vintage stone masonry bridge, called Chamberlain Bridge. From this bridge, drivers and pedestrians could easily view the dam. Although the dam was highly visible, the bulk of the impoundment was relatively hidden from view and no development around the impoundment exists. The impoundment had artificially widened as a result of sediment buildup, resulting in the creation of two small islands. Beneath Chamberlain Bridge the Souhegan River cascades over bedrock before slowing into a shallow, sandy pool. From here, the sandy bottom river slowly winds 1,500 feet to the Merrimack River. From just below Chamberlain Bridge to the confluence with the Merrimack River, the Souhegan River is backwatered by the Merrimack River.

Dam deficiencies and funding opportunities

In January 2004, NHDES' Division of Dam Safety issued PWW a letter of deficiency, stating the dam did not meet current dam safety criterion. Deficiencies included inadequate spillway capacity, deteriorating concrete, inoperable gate structures and excessive leakage due to loose interior stonework on a training wall. The NHDES also indicated in the letter that PWW may opt to remove the dam with assistance from the NHDES' Dam Removal and River Restoration Program.

A combination of the letter of deficiency, the fact that the dam was not used for water supply, the long-term liability and operations and maintenance costs prompted PWW to consider dam removal. After discussions with NHDES about the Dam Removal and Restoration Program, PWW pursued a study to evaluate the feasibility and costs associated with removing the dam. NHDES also assisted PWW with securing federal, state and private grants to help defray the costs of a feasibility study and potential removal.

Initial discussions between PWW and project partners resulted in the de-cision to conduct a feasibility study to determine if dam repair or removal was the best alternative for the site. Major project partners that provided technical and financial support for removal of the Merrimack Village Dam included NHDES, PWW, U.S. Fish and Wildlife Service, New Hampshire Fish and Game, Conservation Law Foundation, American Rivers, National Oceanic and Atmospheric Administration, Gulf of Maine Council on the Marine Environment, Coastal Conservation Association and Restore America's Estuaries. PWW contracted with Gomez and Sullivan in 2004 to conduct the feasibility study.

This aerial view of the Souhegan River shows what it looks like after the Merrimack Village Dam was removed. Monitoring is ongoing to better understand the geomorphic response to dam removal.
This aerial view of the Souhegan River shows what it looked like before removal of the Merrimack Village Dam. Monitoring is ongoing to better understand the geomorphic response to dam removal.

Feasibility study issues

When evaluating dam removal projects, several resource impacts are considered and can vary depending on site specific conditions. Major issues evaluated by Gomez and Sullivan as part of the feasibility study included:

- Quantity, quality, transport and management of impounded sediment;

- Impacts of dam removal on wetlands and cultural/historical resources; and

- Access to private lands in order to remove the dam.

Given that a dam had been present at this location for centuries, the impoundment was essentially filled with sediment and was artificially widened. The Souhegan River is very sinuous, carving through easily eroded soils that are then transported to the lower Souhegan River. The river is well-known for transporting heavy sediment loads during storm events. The sediment volume in the impoundment was quantified by driving steel rods to the point of refusal every 5 to 10 feet across several transects within the approximate 1,800 foot-long impoundment. The sediment consisted primarily of uniform sand/silt both vertically and horizontally within the impoundment; the gross volume of sediment was about 81,000 cubic yards.

Sediment cores were obtained and tested for the presence of metals, polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls, volatile organic compounds, semivolatile organic compounds, pesticides and total organic carbon. Cores also were evaluated for physical characteristics, specifically grain-size distribution. Samples were obtained above the influence of the impoundment, below the dam and within the impoundment. Only elevated levels of PAHs were detected, and these came from two samples within the impoundment. As a result, a 10-day toxicity test was conducted to determine if there was an impact on organism growth and survival. Based on the sediment chemical analysis and toxicity testing, it was concluded that the contaminants were not readily bio-available and posed no risk to downstream ecosystems.

Before its removal, Merrimack Village Dam was a 20-foot-high and 145-foot-wide stone masonry dam overlain with concrete. The dam blocked the migration of diadromous fish and had outlived its usefulness as a water supply structure.
Before its removal, Merrimack Village Dam was a 20-foot-high and 145-foot-wide stone masonry dam overlain with concrete. The dam blocked the migration of diadromous fish and had outlived its usefulness as a water supply structure.

Using the hydraulic model of the river system, along with the grain-size distribution data, a sediment transport analysis was conducted and revealed that the bulk of impounded sediment would be transported downstream. Because of the large volume of sediment which made it impractical to cost-effectively dredge; the low-risk to aquatic life based on the chemical and toxicity testing; and the poor-quality habitat in the river reach below the dam, it was proposed by PWW to allow the sediment to transport naturally downstream. In the end, this method of sediment management was chosen after considerable discussions with state and federal regulators.

Another affected resource was a 4-acre open water wetland that was hydraulically connected to the impoundment and located behind the town's fire department. A wetland delineation was conducted according to the U.S. Army Corps of Engineers' Wetland Delineation Manual. Much of the wetland was classified as palustrine open water with some palustrine scrub-shrub and palustrine forested areas along the edges. Based on hydraulic modeling, dam removal would result in converting the open water wetland into more scrub-shrub and/or forested wetland. All parties understood that if not for the presence of the dam, no open water wetland would have existed. Thus, no off-site wetland mitigation was required.

Dams more than 50 years old and needing a federal permit for removal or receiving federal funding must undergo cultural studies with the New Hampshire Division of Historical Resources in accordance with the National Historic Preservation Act. This coordination is needed to determine whether any properties that could be affected by removal of the dam (including the dam itself) are eligible for listing on the National Register of Historic Places. Two assessments were conducted: a phase I assessment of historic, architectural and engineering resources and a phase IA archeological assessment. The phase I assessment required a qualified historian to complete a project area form and an individual dam inventory form detailing the historic context and evolution of the dam, as well as provide photographic documentation, and a description of the possible effects on the historic appearance of the area.

The phase IA archeological investigation included an evaluation of the potential impact to archeological resources in the area and along the impoundment shoreline. Based on the phase IA findings, a limited phase IB study (test pits) was required, although no artifacts of significance were found. In the end, the dam was eligible for the registry and thus a memorandum of agreement was reached with NHDHR dictating specific mitigation measures.

After evaluating all options and costs, PWW made the decision to remove the dam. With this decision, Gomez and Sullivan prepared engineering design drawings, permits and bid documents for contractor bids. In preparing the design drawings, one complication arose involving access to the demolition site: Getting there from the east was not viable due to existing infrastructure (a power canal), extreme slope, and the close proximity of the fire department's ingress/egress routes. The only viable access was from the west on private property.

After discussions with the private land owner, an agreement was reached whereby the owner stipulated that no erosion of the riverbank could occur and the disturbed area would be reseeded and planted accordingly. Due to the sandy bank, there was concern that excessive erosion would result in the loss of property and potential for eroding the bank to the point of encroaching on the nearby roadway. In the end, the contractor was allowed to use the property to stage its equipment and demolish the dam. Because of the sharp left turn of the river, just prior to the former dam, the river bank was dressed with stone and numerous plants were placed between the stones to create a stable bank.

A ground-level view of the Souhegan River after the Merrimack Village Dam and its apron were removed shows the river in its more natural state as well as some of the work done to fortify the shoreline.
A ground-level view of the Souhegan River after the Merrimack Village Dam and its apron were removed shows the river in its more natural state as well as some of the work done to fortify the shoreline.

Prior to the demolition work, vibration monitors were installed at the Chamberlain Bridge abutments due to its close proximity to the dam. The monitoring equipment was set so that if vibration levels reached a pre-determined level, work would cease; however, this never occurred. The dam removal work was conducted from below the dam atop a large concrete apron (part of the dam), which provided a level working surface for equipment. From here, an excavator equipped with a hammer breached the far side of the dam to allow the impoundment to slowly drain. After the initial breach and draining of the impoundment, two excavators were used to demolish the structure. Working from one side of the dam to the other, the dam and concrete apron were removed. As expected based on the historic review, beneath the dam's concrete overlay was an earlier vintage stone masonry dam. Much of the original stone was reused on-site for bank stabilization. A few of the larger granite blocks were provided to the town for use at a nearby park.

In terms of the timeline for removal, clearing and grubbing of land and access road construction occurred in July 2008. However, because of unexpected heavy rains, in-water work did not begin in earnest until Aug. 6, 2008, when the dam was initially breached and all in-water work was completed later that month. Additional work included stabilizing the bank and completing the plantings plan.

Monitoring the project

Data is needed on the impacts of dam removal on river geomorphology, plants and fisheries. Generally, funding for pre- and post-monitoring studies is limited and, in most instances, there are no monitoring requirements. For this project, pre- and post-monitoring studies were conducted and generally entail documenting fish composition above and below the dam; identifying the plant composition along transects; and conducting geomorphic investigations above and below the dam.

The majority of these efforts have been placed on understanding the geomorphic response to dam removal, which included surveying the same transects above, below and within the impoundment before and after dam removal. Boston College was contracted to conduct the bulk of the geomorphic work. Adjustments have included scouring of the channel bed in the upper reach of the former impoundment, which has exposed a long riffle covered with small boulders. Vertical and lateral adjustments are continuing today as the river is exposed to a range of flow conditions.

Conclusion and thoughts

Removal of the dam was only possible with technical and financial assistance from various project partners. As with any successful project, considerable effort was placed on communicating and educating all stakeholders involved with the process. There was considerable public outreach via televised public meetings, question-and-answer sessions and individual meetings. In Gomez and Sullivan's experience, educating decision-makers and stakeholders is critical. Many members of the public do not understand how dams operate or how river systems function. Commonly heard misconceptions about removing low-head dams have included statements that it will "dry up the river," there will be a loss of flood protection, or unsightly mudflats will be present along the exposed shoreline for years. Gomez and Sullivan has found that a rendering of the project area with and without the dam is a good visual tool.

Overall, removal of Merrimack Village Dam opened 14 miles of river habitat, allowing diadromous fish to access former spawning grounds and restoring natural habitat connectivity. The next barriers on the Souhegan River are the McLane and Goldman dams in downtown Milford, N.H. In 2010 Gomez and Sullivan was awarded a contract to evaluate the feasibility of removing these two dams. A feasibility report is expected to be released in 2012.

Mark Wamser, P.E., is a senior water resources engineer with Gomez and Sullivan Engineers P.C.

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