- Division of Ecological Restoration
The use of structures such as seawalls, dikes and dams to protect homes and business from the effects of flooding is certainly not a new concept. For example, historians point back to the Byzantium seawalls, built primarily for protection but maintained and expanded on the seaward side according to early written records almost 400 years after initial construction attributed to Constanine I. Other well-known examples include The Dutch windmills, pumps and dikes to control coastal flooding for lands below sea level and the levees and dikes in Louisiana, brought to national attention more recently after Hurricane Katrina.
Massachusetts has its own rich history and relationship with flood protection measures such as the New Bedford Hurricane Barrier. However, there are many examples across Massachusetts that are smaller in size and scale, such as Town Creek in Salisbury. Dating back to the late 1800’s, a rail line was constructed across the marsh and creek system, effectively creating a dike. Where the rail line crossed the creek, a granite culvert with a wooden flap-style tide gate was installed in the rail bed to help protect upstream areas against flooding from the Merrimack River and from storm tides. However, the tide gate also prevented salt water from entering upstream tidal wetlands which resulted in severe impairment of this coastal habitat. The prolonged restriction of tidal flow to the salt marsh upstream was akin to cutting off the circulatory system of this complex habitat, impeding freshwater drainage and “clogging” the upstream marsh with impounded waters that also reduced storage capacity that can aid in reducing flooding in the event of storms. The impaired hydrology also facilitated the decline of native salt marsh grasses, and the spread of invasive Phragmites, and cut off access to nursery and feeding grounds for important commercial fisheries.
After major flood events in 2005, 2006, and 2007 overtopped the rail bed, flooded out several business and closed Route 1, a major thoroughfare, for days, the Town responded by implementing the Town Creek Flood Hazard Mitigation & Wetland Restoration Project. Completed and initially opened in 2014, the new culverts and tide gates were designed to improve flood control while also allowing reintroduction of more natural tides that had been blocked from the system for more than 150 years. The approach, developed through partnership with the Town, State and Federal agencies, was designed with joint goals of flood protection and habitat restoration, relying on management of the tide gates to achieve a delicate balance.
It is imperative to recognize that while the project was completed in 2014, the work does not end with installing the tide gates and setting an opening. Tide gates and other control structures require continued attention. The benefits of habitat restoration and flood protection can be significant, but these structures cannot be managed with a set-it and forget-it approach, instead they require diligent observation, monitoring and management. The removal of restrictions and tide gates is the most preferred option for restoration of ecological processes and habitat but full removal may not always be practicable due to constraints created by pre-existing low lying infrastructure. Such is the case at Town Creek, where pre-existing upstream development required a balanced management approach.
The Town of Salisbury continues to monitor response upstream and DER is supporting the Town in its efforts to find balance and restore this system. This Spring, DER, through a consulting contract with the Woods Hole Group, assisted the town with hydrology study of Town Creek, including comprehensive tidal monitoring, comparing the upstream response of tides and performance of the tide gates with downstream tidal conditions. Initial results suggest that while a natural tidal flow pattern has been reestablished, there may be room for additional adjustment of the gates. Pending completion of the Town Creek hydrology study, the Town, DER, and partners will review findings and assist the Town in developing management actions going forward.
Photos Left: Location of tidal monitoring equipment at Town Creek. Middle and Right: Measuring opening size and elevation at the Town Creek Tide Gate (Figure and photos by Woods Hole Group, Inc.).