Imagine a stormwater management system that provides sedimentation, variable denitrification and oxygen saturation, evapotranspiration, volatilization, and infiltration. Is this the newest high-tech engineered stormwater treatment system? No, it’s the original green infrastructure: the stream channel. In suburban and rural communities in New England and many other parts of the country, the typical small stream channel plays a critical role in drainage, flood control, groundwater recharge, pollutant transport, and pollutant sequestration.

In many stormwater-regulated communities, these streams are considered “waters of the United States” and are therefore viewed only as receiving waters at the end of the line for permitted stormwater dischargers. But regardless of legal definitions of a “water,” nature’s conveyance systems are an important part of a community’s green infrastructure, and enhanced management of these “natural” systems is another tool in the stormwater manager’s toolbox.

The town of Lexington, MA, just over 10 miles northwest of Boston, sits at the headwaters of three major watersheds: the Charles, Mystic, and Shawsheen rivers. A town with a rich history and significant involvement in the American Revolution, Lexington was first settled in 1642 by farmers, who were largely attracted to the area by its agricultural opportunities. The town continued to be a farming community until a railroad line from Boston was built in the 1840s, allowing for convenient travel back and forth from the city, turning Lexington into the suburb of Boston it’s largely considered to be today. The town has maintained much of its rural character with extensive open space and agricultural charm around a historic New England village.

As is typical in many suburban communities with extensive open space and conservation lands, the continued management of “natural” drainage systems that rely on open channel conveyance presents a challenge to effectively balancing conveyance and flood storage. This was the case in Lexington, where stormwater had been controlled by such measures as isolated piped systems, typical agricultural drainage systems like wetland tile drains, small headwater streams, channel straightening, and channelization. But as these agricultural drainage systems became less functional in the transition to suburban and conservation lands, it was difficult to maintain adequate drainage. At this point, it became clear that active management of streams and wetlands would need to be incorporated into the town’s long-term approach to managing its drainage system as effectively as possible.

Infrastructure and Water-Quality Challenges
The streams and waterways that receive Lexington’s drainage discharges provide valuable wildlife habitats, enhance the surrounding environment, and serve as a link between many open spaces and recreational areas—all of which makes them essential to the community. Just as importantly, these streams and open drains can and should provide floodwater protection and drainagefor neighborhoods and developed areas. Unfortunately, in 2010, issues were beginning to arise that signaled to the Department of Public Works (DPW) and Conservation Division that ­stormwater drainage enhancements were necessary.

Lexington streams tend to have small watersheds with very low gradients; these factors make drainage systems more susceptible to sedimentation and hydraulic restriction problems. In addition, aging drainage infrastructure and the accumulation of sediment in open channels and wetland areas were reducing drainage capacity, which was leading to localized flooding, increasing the risk of infrastructure failure, and potentially impacting stream health. When these issues began coming to light, the town, which has always prioritized the health of its residents and environment, knew something needed to be done, but stalemates between the methods preferred by the Conservation Division and drainage enhancement techniques advocated for by the DPW persisted in being difficult to overcome.

When faced with drainage issues, the approach used by public works can often be on the opposite end of the spectrum from the approach favored by conservation departments and wetland advocates. Dredging, straightening, and draining has been the long-standing approach to addressing flooding and drainage problems, but with the town’s focus on conservation ethics and a desire for restored wetlands, the ability to do this work had been very limited in recent years, which was now resulting in more numerous drainage problems. The potential for conflict at this phase of the project is one that many communities can relate to, as it is often the case that one department’s needs or priorities may appear to differ substantially from another’s.

However, instead of choosing an all-or-nothing approach from the perspective of Lexington’s DPW or Conservation Division, the two thought their efforts could be more impactful and long lasting if they combined them. The resulting strategy would integrate an assessment of built and natural systems, identify key drainage impediments and structural deterioration, assess stream channel geomorphology and watershed characteristics, and identify opportunities for conservation and wetland restoration.

Balancing ecological and infrastructure needs, the project at Willard’s Woods included daylighting the previously
buried stream channel, installing a new headwall, creating a naturalized stream floodplain, and installing a rock vane.

Over the course of the next four years, the DPW evaluated the town’s drainage system in an effort to comprehensively understand the characteristics of Lexington’s waterways, associated wetlands and riparian areas, and drainage infrastructure. This included conducting drainage studies for all three watershed areas the town is situated in, which resulted in a number of findings that would prove to be crucial in developing a balanced drainage enhancement capital improvements plan. Among these findings were that several pipes and culverts required structural restoration that would involve engineering design and reconstruction, but that many of them were experiencing hydraulic or sediment obstruction issues and not structurally deficient. Downstream obstructions along low-gradient streams appeared to be causing sedimentation and hydraulic restrictions that were affecting many of these culverts and contributing to drainage problems upstream in the drainage system. The studies also identified natural areas within the town that had the potential for floodwater attenuation and management through targeted wetlands restoration that would not cause upstream drainage problems. The combination of identifying areas for further wetland enhancement, structural repairs, and targeted stream channel cleaning resulted in a balanced capital and operations plan that met the needs of both conservation and public works staff.

A Collaborative and Creative Approach for Drainage Enhancement Funding
Towns in New England face unique challenges simply due to their scale. New England has more than 1,500 individual political jurisdictions across six states, in an area roughly equivalent to the state of Washington in size. Many New England towns are so small that they can support only a few public works staff. These smaller municipalities often have smaller budgets to fund projects and fewer staff members to deliver them, and they often have to run funding requests through a town meeting vote for approval.

Willard’s Woods contains 3 miles of trails, meadows, pine groves, and wetlands, providing both an important habitat to
plants and wildlife and a recreational space for residents and visitors.

Funding is always a challenge for communities of any size, especially for projects whose benefits will mostly be seen over the long term, rather than immediately. Conservation projects typically fall into this category. The Lexington DPW and Conservation Division decided that it would be beneficial to separate each component of the improvement plan—which would include infrastructure restoration, drainage restoration, wetlands protection, and further drainage investigations—into individual projects rather than present them programmatically. This provided a better opportunity to show the town’s capital expenditures committee all the background information and project goals on a case-by-case basis. Because the town’s process for deciding on the allotment of funds to various projects comes down to a quantifiable decision-making matrix, presenting the plan for each individual project thoroughly was crucial in gaining approval.

Furthermore, the two departments realized that the projects within this plan were going to vary in terms of immediate outcomes and benefits. Infrastructure restoration projects would likely be seen as a higher priority than those geared toward wetlands protection and ecological drainage restoration. For that reason, they decided to separate the budgets for DPW infrastructure projects and Conservation Division projects instead of funding both from the same “drainage” account. This ensured that the conservation efforts wouldn’t be pushed to the bottom of the list every time decisions were made on the distribution of funding. While the DPW and Conservation Division collaborated to develop a real commitment to balancing the needs of both departments, this method of budgeting for individual projects was incredibly significant in making sure that a collaborative program was actually being implemented.

Projects for Ecological Benefit and Drainage Improvement
Several infrastructure projects identified during the investigation were placed into a category of “immediate action” in order to address obviously critical safety issues. Once these projects had been implemented and all of the watersheds were evaluated, a ranking system was developed to prioritize the remaining town-wide projects. This prioritization process was initiated using an objective screening criteria called a “pairwise comparison,” which allows several criteria to be matched against one another to identify the most important values of a project in relative terms. Pairwise comparison generally refers to any process of comparison in pairs to judge which factor of each entity is preferred, or has a greater amount of some value. The values considered in this case included health and safety, flood mitigation, coordination with other town projects, environmental benefit, social impact, improved operations and maintenance, and regulatory compliance. For example, health and safety (score of 5) may be more important in relative terms than the social impact (score of 1) of a given project. In this way, projects with equal values under the scoring criteria would be weighted toward public health and safety.

Willard’s Woods
One of the projects that was considered a high priority involved the daylighting of a stream in Willard’s Woods. Lexington boasts 26 town-owned Conservation Areas totaling more than 1,300 acres, of which Willard’s Woods makes up 100 acres. Willard’s Woods Conservation Area contains 3 miles of trails, meadows, pine groves, and wetlands, providing both an important habitat for plants and wildlife, and a recreational space for residents and visitors. Two streams run through the woods, one of which has historically discharged from Willard’s Pond into a 260-foot-long buried drainage culvert.

Vine Brook, a perennial 3-foot-deep stream with a bituminous bikeway on top of its east embankment, before the bank stabilization project got underway

When facing deteriorating infrastructure, many design engineers may simply replace the failing pipe. In this case—in which the stream’s drainage culvert had begun to fail—the town of Lexington decided that the benefits of naturalizing this stream would far outweigh the cost and challenge of doing so. Working with the Conservation Division, the DPW investigated options for responding to the needs present at this site while maintaining an awareness of the implications their actions would have on the surrounding environment. What the DPW ultimately decided to do was daylight the previously buried stream channel, install a new headwall using natural stream channel and bioengineering design principles, create a naturalized stream floodplain, and install a rock vane for energy dissipation and grade control. The project also included constructing a new pedestrian bridge to replace an existing stream crossing, and an extensive upland and wetland planting plan.

The entirety of the Willard’s Woods project was developed with a central focus on environmental considerations and a goal of improving the ecological conditions in this important Conservation Area. The town used a multidisciplinary approach during the design and development phases, bringing in a team of scientists and engineers who developed a stream restoration plan together that balanced the needs for drainage conveyance, flood control, water-quality improvements, and ecological restoration. Careful attention to the environment continued through the bidding and construction phases, as the town utilized a competitive, qualification-based approach to make sure the construction company it chose had the necessary ecological restoration skills to carry out the project successfully. To ensure that the area continued to see environmental benefits after the project was completed, Lexington worked with its partners to assess invasive species and develop a long-term invasive species management plan for the post-construction period. Together, the DPW, Conservation Division, and volunteer stewards will collaboratively execute the invasive species management plan to ensure that the plantings for the stream restoration have the opportunity to flourish.

A significant component of the stream daylighting effort was minimizing public inconvenience. Willard’s Woods is an important recreational space for Lexington with 3 miles of trails that allow members of the community to enjoy the meadows, pine groves, and wetlands present in the Conservation Area. As a result, keeping the public up to date on the work being done there was key to the success of the project. To ensure constant involvement and awareness, the town assembled a group of stakeholders that included conservation staff, watershed stewards, and other key figures from the community, with the intent of educating and involving the public in the planning and design of the project. After work began, the town provided ongoing public outreach as well. Fact sheets were developed and made available through Lexington’s website, and a sign with information about the benefits of the project was posted in several locations: at the park entrance, the job site, and online. When trails were affected by project activities, alternate paths were always provided, ensuring that the woods’ trail networks could still be used by walkers, runners, and bicyclists.

Unlike those of typical engineering projects, the construction documents for this project were developed to allow for flexibility during construction so that the final stream channel would closely mirror the irregular conditions of a natural stream. This was largely accomplished by including performance standards in the design that were based on an assessment of a “reference reach”—a naturalized, stable portion of the stream that demonstrates the target characteristics of restoration work—located immediately downstream of the project area. Taking the unique nature of natural stream restoration into account, as well as the necessary irregularity of the design, Lexington thought it best to retain the design team for construction oversight services. By establishing means for the design engineer to make onsite decisions on rock placement and channel construction, the town was able to provide the flexibility it had hoped for throughout the construction phase while ensuring the project was continuing along its course with a focus on sustainability and ecological responsibility.

Daylighting the stream makes it possible for the waterway to function in its ecosystem as it naturally would and restores an important riparian habitat on the town’s conservation land. This ecologically responsible project approach improves water quality while minimizing future infrastructure maintenance, making it an overall win-win for both Conservation and the DPW.

Vine Brook just after construction

Lower Vine Brook
In addition to the Willard’s Woods project, the town also undertook a bank stabilization project at another Conservation Area: Lower Vine Brook, a 110-acre forested area home to 4.5 miles of trails. Vine Brook, a tributary to the Shawsheen River, is a perennial 3-foot-deep stream running south to north with a bituminous bikeway on top of its east embankment. A portion of this embankment, having weathered repeated storm events and their associated flooding over the course of many years, had begun to fail, with the embankment left scoured and organic debris deposited into the waterway. Just as in the Willard’s Woods project, Lexington’s DPW and Conservation Division worked together to decide on the best methods for remedying the situation.

Vine Brook one year after construction

The failing embankment could have been approached with more traditional methods of erosion control, such as placing interlocked concrete blocks over the surface of the bank or creating a stone revetment, but having seen the negative impacts these techniques can have on streams, the project team decided on an approach that would stabilize the bank without harming the waterway. This did involve some dredging—60 cubic yards of eroded organic debris and sediment needed to be removed to create a stable slope foundation. Excavation was also conducted 3 feet below the toe of the embankment so that riprap with a filter fabric could be installed as a compacted foundation for new slope fill. The toe of the slope up to the mean annual high-water line was also covered with semi-angular riprap to control groundwater seepage from the embankment and make the toe more resilient to seasonal water level changes.

After the foundation of the slope was prepared, the embankment was reconstructed using clean fill and ­erosion control matting. In con­sideration of the existing bikeway on top of the embankment, the new slope grades tied into the existing grades there so as to avoid any impact on this section of the trail system. Lastly, the riprap toe was loamed and seeded for two rows of wetland plantings, and the rest of the embankment slope was revegetated with live fascines, shrubs, and trees. The combination of revegetation and stone armoring illustrate how stability needs and environmental benefits can both be addressed effectively, if the town uses a compressive, collaborative project approach.

The project team did consider an alternative solution to this issue that involved cutting back the existing slope after dredging the channel and installing the riprap foundation. This alternative would have achieved the same outcomes as the option that was ultimately chosen—namely, stabilizing the failed embankment and preventing erosion and sedimentation pollution—but this alternative would widen 80 feet of the brook and require rerouting the existing bikeway. It was decided that this was less desirable, as a wider, shorter section of the brook could have an impact on hydraulics and possibly create an area that would be susceptible to eddying and scouring. It could also reduce the brook’s flow velocity, leading to sedimentation in the channel—an issue the project was trying to solve in the first place. Though this was a viable solution, the DPW and Conservation Division went with the approach they did out of a commitment to restoring Vine Brook to its pre-failure conditions without having any significant effect on its hydraulics.

Benefits of Using the Original Green Infrastructure
Overall, the techniques used in these projects improve water quality, eliminate flow restrictions, ensure long-term bank stability, control migration of nutrients and sediment, and mitigate potential future emergency response measures. These are, by and large, the same outcomes many projects attempt to achieve with newer green infrastructure technologies. Through interdepartmental collaboration and by accepting (and understanding) the place of streams in the town’s overall drainage system management planning, Lexington has developed a drainage system capital improvement program that is ­progressive, sensible, and environmentally friendly.

About the Author

Zach Henderson & Denise Cameron

Zach Henderson is a water resources technical manager. Denise Cameron, P.E, is a civil ­engineer. They were integral in the ­execution of these projects in ­Lexington, along with their colleagues at Woodard & Curran, Regina S. ­Leonard Landscape Architecture & Design, and SumCo Eco-Contracting.