Herring River Study Examines Docks' Environmental Impacts

 HARWICH – In places like the Herring River, docks and piers can have a negative impact on salt marshes. That’s the conclusion of a study backed by the conservation commission.

The commission has been looking at ways to better protect wetland resources and shellfish grounds in the Herring River for several years. In 2023 the commission proposed a warrant article to impose a moratorium on new structures in the river while amendments were made to the bylaws that regulate them.

But that article was pulled by the select board, citing the absence of public participation in the proposal. The board concluded that a study should be done before any regulations are changed, and it approved the commission’s request to use $72,640 for that purpose. The money, from the wetland revolving fund, is generated by the fees the conservation department charges for regulatory applications. TRC Environmental Corp. of East Providence, RI was hired to conduct the research.

The study concluded piers and docks have a negative impact on salt marsh vegetation density and abundance, most significantly directly underneath structures. It also found that newly installed monopile structures have temporary construction impacts, specifically from pile-driving activities, and that the impacts may linger for more than a single growing season after construction.    

 However, the study couldn’t identify the specific structural design characteristics that harm salt marsh vegetation, given variables of topographic conditions and dock age.

 The study also determined that it was difficult to quantify the impacts of water-dependent structures on areas containing shellfish; the document calls for more shellfish mapping in the river. Detailed baseline mapping of shellfish habitat in the river would be a useful tool for guidance in siting of future structures and alterations, said Matt Ladewig of TRC. He said important resource areas would be more clearly delineated and help to avoid impacts in most sensitive locations.

 Present regulations require a mandatory shellfish survey when water-dependent structures are proposed and the study recommends three baseline surveys be conducted, spaced three months apart, to assess seasonal variations of shellfish abundance, as well as a post-construction survey to identify the impact of docks.

Researchers noted that these areas are unlikely to be static, particularly in the face of climate change and rising sea levels.

Ladewig said there was not a lot of development along the river in the 1940s, but by the 1990s there was heavy development there. 

He said an experimental study in Marshfield in 2017 found that taller structures cause less shading directly underneath them; increased deck spacing and the use of metal-grated decks also cause less shadow than wooden docks. The absence of sunlight impacts salt marsh vegetation growth, the study found.

The study also found that topographic conditions along the river significantly affected salt marsh abundance and density around structures. Steep sections of bank have less marsh vegetation whereas flat sections of bank support more salt marsh.

There are very few locations along the river where new water-dependent structures can be located. While a few property owners have recently built shared docks and piers, the study estimates that fewer than five structures could be built there. Recommendations call for new structures to have a 1.5:1 height-to-width ratio to minimize shading impacts underneath structures. The recommendations also call for use of monopile installations to limit pile impacts.

For old dock alteration, recommendations include that if pilings are still in good condition, the deck should be changed from wooden to grated metal to increase light, and if pilings need to be replaced, the recommendations for new docks should be followed.

Conservation Commission Chair John Ketchum praised the work and presentation provided by TRC. He further said the commission will need to have discussions on how they will use this information.