Erosion control is an important consideration on any construction project. Seldom, though, is control of erosion the overriding consideration on a project. Such was the case on the Vitale Fly Ash Consolidation and Habitat Restoration Project in Beverly and Wenham, MA. Control of erosion and dust was a driving force during design and construction of the project, which was located adjacent to, and within, the community’s drinking-water supply. Exceptional cooperation between the public, regulators, and various parties involved in the construction resulted in a highly successful project that benefits both the environment and the local community.
The Vitale project involves the removal of fly ash from a drinking-water reservoir and an active stream course and associated wetlands, and the restoration of the stream course and wetlands. Fly ash is a byproduct of coal-burning power plants. During combustion, fly ash particles are carried in the flue gases and are removed using electrostatic precipitators or mechanical collection devices. Fly ash particles are generally silt-size, spherical in shape, and non-plastic. Because of their lack of cohesion and small size, fly ash particles are highly susceptible to erosion by water and wind. In the field, fly ash has the appearance of gray silt. When dry, the material has the consistency and behavior of talcum powder. Fly ash typically contains elevated levels of heavy metals associated with the parent coal material.
Vitale Site History
During the 1940s and 1950s, approximately 300,000 cubic yards of sand and gravel were mined from the Vitale quarry in Beverly. From the 1950s to the 1970s, the mined-out pit was filled by the former property owner with fly ash from a nearby power plant.
Over the years, Airport Brook, which ran through the site, eroded approximately 40,000 cubic yards of fly ash from the landfill. The eroded fly ash was carried downstream and deposited within the streambanks and adjacent wetlands, as well as a portion of Wenham Lake, located a half-mile downstream of the Vitale site. Wenham Lake is the drinking-water supply for the neighboring communities.
The erosion of fly ash from the Vitale site buried the downstream wetlands under 1 to 3 feet of ash. The fly ash reduced the health and diversity of the wetlands as invasive species such as common reed (Phragmites australis) and purple loosestrife (Lythrum salicaria) invaded large portions of the wetlands. The dominance of invasive species also reduced the wetlands’ ability to support a diversity of wildlife species. In addition, the deposition of the fly ash adversely impacted the hydraulics of Airport Brook, resulting in periodic flooding of nearby Route 97.
Erosion at the Vitale site also resulted in areas of dangerous instability. Over the years, Airport Brook eroded a 30-foot-deep channel through the fly ash at the site. This area (referred to as the “canyon” by the project team) was highly unstable. Numerous scarps along the canyon slopes were evidence of past landslides where huge quantities of fly ash tumbled into the brook. Other slopes around the fly ash landfill were also unstable and showed evidence of widespread erosion.
Addressing the Problem
Although the Vitale site was abandoned by the former owners and is now owned by the City of Beverly, New England Power Co. (NEP, a National Grid company) agreed to do the cleanup. NEP was the owner of the power plant at the time the fly ash was generated and placed at the Vitale site.
NEP hired Haley & Aldrich Inc. of Boston, MA, to evaluate the environmental impact of the fly ash. Over the next two years, Haley & Aldrich completed a comprehensive exploration and testing program that delineated the horizontal and vertical limits of the fly ash; determined the environmental impact of the ash on the groundwater, surface water, and wetlands in the area; and assessed the risk of the fly ash to wildlife and humans in the vicinity of the site.
An extensive public input process was initiated by NEP to keep the local community informed on the progress of the Haley & Aldrich study, and on NEP’s plans to address the fly ash. In addition, a technical advisory group (TAG) was formed to provide input to NEP and its consultants during the environmental assessment and subsequent remedial design. The TAG included members of local public interest groups, environmental consultants, lawyers, the water supply board, state environmental regulators, the mayor and city council of Beverly, the board of selectmen of Wenham, and other interested parties.
Design to Restore Wetlands and Stabilize Fly Ash Landfill
The final design included the following major tasks:
- Re-routing a 1,200-foot length of Airport Brook at the Vitale site to its historic alignment and infilling the canyon
- Flattening and stabilizing the slopes of the existing 15-acre fly ash landfill
- Removing fly ash from the banks of Airport Brook, the adjacent wetlands, and the exposed reservoir bottom during seasonal low lake levels. Excavated fly ash was hauled to the Vitale site where it was spread and compacted.
- Restoring 16 acres of wetlands where fly ash was removed. Restoration included installation of extensive plantings and wildlife habitat features such as coir rolls, root wads, basking logs, brush piles, and boulder piles.
- Placing geotextile and soil cover materials over the stabilized fly ash landfill and hydroseeding the surface
Erosion and Dust Control During Design
To protect the reservoir and public health, it was critical for the project to control erosion and dust. Also, because virtually all of the fly ash removal was taking place in wetland resource areas, it was essential to protect wetlands and open water within and outside the active work areas.
These goals were an integral part of the design from the start. The contract drawings and specifications contained rigorous requirements to minimize the risk of erosion and airborne fly ash, including the following:
- Clear language in the contract documents advising the contractor that fly ash is highly susceptible to water and wind erosion, and stressing the critical need to control erosion and dust during construction
- Covering all wetland areas where fly ash was removed with biodegradable erosion control blankets. Various grades of erosion control blanket were used in the design, depending on anticipated flow conditions and the desired degradation rate for the blanket.
- Limiting the area of active fly ash removal such that no more than a half-acre of fly ash or soil was exposed at any time prior to covering with erosion control blankets
- After cutting and removing aboveground portions of trees, requiring that stumps and other belowground portions remain in place to stabilize soils and prevent erosion until the contractor was ready to begin excavation of fly ash in the area
- Requiring an “upstream-to-downstream” construction sequence to minimize the risk of re-depositing fly ash in areas that had been restored
- Including in the contract drawings a construction sequence plan that showed a technically feasible approach for performing the work under the contract, and then requiring the contractor to submit a plan showing his actual proposed sequence.
- Showing on the contract drawings the location and type of basic erosion control measures. Since a comprehensive erosion control system must be adapted to actual field conditions and construction methods, the contractor was required to submit an Erosion and Sedimentation Control Plan detailing contractor-designed supplemental erosion control measures tailored to his planned construction methods and sequence.
- Incorporating into the design the temporary diversion of Airport Brook around the active work areas. Temporary diversion channels were lined with erosion control blankets and crushed stone.
- Establishing during design the location of an off-road construction access roadway, and requiring that all fly ash be hauled to the Vitale site on the designated access roadway. This required negotiating an access agreement with the owner of the abutting property, but eliminated the need to haul fly ash on public roads.
- Requiring the contractor to prepare and submit a Dust Control Plan and Perimeter Air Monitoring Plan
- Establishing allowable dust levels and requiring full-time perimeter dust monitoring during construction
- Submitting to the engineer and local water supply board manufacturer’s literature and Material Safety Data Sheets on the contractor’s proposed dust suppression agents. Because of the proximity to the reservoir, only polymer-based agents were used.
- Installing a perimeter fence equipped with wind screen to further reduce the amount of dust migrating offsite
- Installing wheel wash stations and stabilized construction entrance pads
Erosion and Dust Controls During Construction
NEP selected T Ford Co. Inc. (TFCI) of Georgetown, MA, to construct the project. TFCI has a history of proven performance on environmentally sensitive projects for both NEP and Haley & Aldrich. TFCI subcontracted Great Meadows LLC of Rowley, MA, to install the erosion control blankets and the 52,000 plants included in the wetlands restoration design.
In the field, a variety of erosion control measures were employed. Hay bales, silt fence, and a combination of the two were used to separate the area of fly ash removal/wetland restoration from areas outside the project that were unimpacted by fly ash.
Downstream of active work areas, a series of check dams and filtration berms was installed. These were constructed from various materials, including hay bales, silt fence, crushed stone and filter fabric, and straw wattles, depending on the field conditions.
Homemade Straw Wattles
The straw wattles became an integral part of TFCI’s erosion control system. The rolls proved to be easily deployed, highly versatile, and cost-effective. Because of the need to isolate the active work area from the surrounding wetlands, the erosion control measures changed on a daily basis. As Jack Enos, TFCI principal, describes it, “Our biggest challenge was to maintain the aggressive production schedule while continuously keeping our eyes on potential erosion issues as the work progressed. The owner and engineer expected full attention to the ever-changing erosion control situations.”
It was often necessary to install erosion controls in sensitive wetlands accessible only by foot. Hay bales were cumbersome in these conditions, especially after they became wet-a saturated hay bale can weigh more than 100 pounds. The foot traffic needed to install and remove hay bales itself can be a significant source of turbidity downstream of the work area.
Although commonly used in the western US, manufactured straw wattles are difficult to obtain in Massachusetts, so TFCI developed an innovative way to fabricate these onsite. TFCI purchased several rolls of Conwed Plastics’ Vexar plastic tubular mesh netting (now available from NSW). The tubular netting was slipped over the end of an 8-inch-diameter flexible plastic hose connected to the outlet of a straw mulcher. When a hay bale is fed into the mulcher, straw is blown into the tubular netting. As the netting fills, it is gradually retracted to produce a uniform 10-inch-diameter wattle. Each hay bale produces approximately 12 linear feet of wattle.Because of their ease of installation and low cost, several rows of wattles can be installed in succession downstream of the work area. Each row of wattles provides an incremental reduction in sediment load. The cumulative effect of multiple rows of wattles can significantly reduce turbidity.
Erosion Control Blankets
Following removal of fly ash and placement of wetland organic soil, the work area was covered with erosion control blankets. To limit the amount of exposed fly ash and soil, the specifications limited the size of the active work area to a half-acre prior to covering with erosion control blankets.
Several grades of North American Green erosion control blankets were used. Representatives from North American Green visited the site regularly to observe installation of the blankets and provide technical input. The blankets were staked down using 6-inch- or 12-inch-long wooden Eco-STAKEs.
Monitoring of Erosion Controls
As required by the specifications, TFCI regularly inspected the erosion controls and documented its observations in a daily log maintained at the site.
Haley & Aldrich, as part of full-time construction monitoring, measured turbidity several times per week in Airport Brook and Wenham Lake. At least once per month, and after rainstorms of a half-inch or more, water samples were obtained at established points in the brook and lake, and submitted for chemical analysis.
The project’s wetland scientist regularly walked the site with representatives of the local conservation commissions. The head of the water supply board visited the site at will and often attended weekly site meetings. NEP also provided three public tours of the construction site, conducted on Saturdays, to show local residents and other interested parties the progress of construction.
There was ongoing dialogue between the contractor, engineer, owner, and construction manager regarding the erosion controls in place, and what additional measures might be needed to address upcoming construction and weather conditions. The goal was to provide “proactive and redundant” erosion control measures to protect the site from whatever Mother Nature might send.
Moment of Truth
After a summer of nearly ideal construction weather, the site was pummeled in the fall with three back-to-back 3- to 4-inch rainstorms brought on by the effects of Hurricanes Katrina and Wilma. The rainstorms transformed the site, which had seen relatively little flow throughout the summer, into a raging torrent.
When the skies cleared and the water receded, the site emerged nearly unscathed. The erosion control blankets provided exceptional protection. There was virtually no erosion within the 20 acres where erosion control blankets had been installed. The performance was impressive, especially considering that none of the planted vegetation had yet become established.
The only damage to the erosion control blankets was a 30-foot by 50-foot area that ripped. This was caused by concentrated flows that exceeded the shear strength of the blanket (reduced after several months of biodegradation) at that particular location. North American Green observed the area, and recommended the torn section be repaired with a stronger grade of blanket.
The other erosion control measures functioned as intended.
The Third Storm
By the time the third big storm hit, the fly ash had been completely excavated from the wetlands and exposed lake bottom, and the project was nearing completion. The lake was at historic low levels and aided in maximizing the area of fly ash accessible to TFCI.
While Great Meadows was completing the wetland restoration work near the lake, flow from Airport Brook was still being directed to a temporary diversion channel. During the third storm, the massive flow breached the diversion channel at the lake and water rushed across the exposed lake bottom. Fortunately, the lake bottom had been cleaned of fly ash. However, the flow cut a channel across the lake bottom, exposing sand and gravel deposits below.
The last construction planned for this area was excavation of the final section of stream channel connecting the restored Airport Brook to the lake. After observing the channel eroded by the breach, and seeing the functional and attractive sand and gravel deposit in the bottom, the team decided to grade the eroded channel and make it the final connection to the lake. After all, why fight Mother Nature?
Long-Term Benefits for the Environment and Community
The Vitale project has resulted in the restoration of 16 acres of valuable wetland habitat and increased protection of the community water supply. In addition, the Vitale site, now stabilized and securely covered, will be used by the City of Beverly for athletic fields.
Erosion control was an integral part of the design from the earliest stages. The contract drawings and specifications contained rigorous requirements to minimize the risk of erosion and airborne fly ash. These requirements, along with the efforts of a conscientious contractor, resulted in minimal erosion and dust levels well below the allowable limits.
Several types of erosion controls were used to address the variety of field conditions occurring during construction. Straw wattles, fabricated onsite by the contractor, proved to be versatile, easily deployed, and cost-effective. And, when Mother Nature sent major rainstorms, the erosion control blankets worked exceptionally well in protecting the area.
Through the public input process, interested parties were kept informed throughout the project. The community is now benefiting from the corporate/public partnership that resulted in the restoration of a natural habitat and beneficial use of the Vitale site. Joe Kwasnik, vice president environmental with National Grid, summed up the project by stating, “We recognized early on that a key to the success of this project depended upon the erosion control systems that would be used in a very dynamic environment such as we faced at the Vitale site. It was the solid engineering, expert execution, and extensive monitoring that allowed for an extremely successful project. We are very proud of what was accomplished here.”
