Restricted Tidal Flow

Coastal road crossings often restrict tidal flow, degrading coastal habitat and imperiling vulnerable species. The Wells Reserve and its partners assist coastal communities that are working to improve bridges and culverts threatened by sea level rise and coastal flooding. In the process, projects can restore ecological functions to tidal wetlands. 

Habitat restoration and coastal resilience planning require good design and good data. For design, we have worked with our partners in developing the CoastWise Approach to Tidal Road Crossing Design. For data, a lot of field work is required.

This year the Wells Reserve partnered with the Maine Coast Heritage Trust to hire two coastal stewardship interns, Phoebe Scott and Casey Wood, to collect data on tidally influenced road crossings. Their work will update the Maine Tidal Restriction Atlas maintained by the Maine Coastal Program.

Summer as Coastal Stewardship Interns

We spent the summer surveying tidal structures up and down the coast of southern Maine and around Casco Bay, visiting more than 100 bridges and culverts in our travels. The main intent of our surveys was to document the location and condition of crossings that may need updating or replacement due to sea level rise in the near future. We managed to survey over half of the structures we visited, but many were inaccessible due to site conditions, safety concerns, or private ownership.

Most people probably don't even notice these structures as they pass over them, though they are probably enjoying the views of the coast they get as they drive along. Our work was important because sea level rise is a threat to coastal infrastructure and Maine is particularly at risk with 55% of the state population living in coastal areas. Many tidal road crossings are located along the sole entry/exit route for getting to homes and businesses, so its crucial that we determine which crossings must take highest priority in the coming years.

Survey Methods

We used a simplified version of methods developed in New Hampshire to assess culverts and bridges from Kittery to Brunswick. Our field protocol called for measuring or estimating structural dimensions and condition, the elevation of tidal flooding, and impacts to habitat and wildlife.

We took multiple photos of each side of the crossing as well as upstream and downstream habitats. This will allow future scientists and planners to see for themselves any evidence of tide-related structural deterioration or habitat impacts. To collect these pictures, we often had to wear our chest waders and get into the mud and tidal creeks. One of the required images is a wide-angle view of both sides, so we typically needed to walk far into the marsh or stream to get the desired photos.

We also took measurements of the physical dimensions of bridges and culverts, which determine their capacity to pass tidal flow, as well as the daily water-level height in relation to the structures. These measurements will allow planners to make rough estimates of the structures' performance and the potential scale of needed upgrades.

We entered data and photos into a mapping database using ESRI's Survey123 application on an iPad, then verified our records when we returned from the field.

Plans Versus Reality

This project proved to have its fair share of challenges. Planning field days was a very fluid sort of process, with change becoming a regular part of our routine. Field days were scheduled for 4-hour windows around low tide, but many times we would need to adjust while in the field. Some sites would appear to be accessible from aerial images, but when arriving on scene we would find busy roads, steep embankments, deep mud, or other factors jeopardizing our safety. Sometimes these challenges would merely make a site more difficult to survey and increase the time we spent there. Other times the risks would make the site a poor candidate for surveying.

Ensuring Quality Data

The information we collected will help coastal planners prepare for best- and worst-case sea level rise scenarios. This is why a big portion of our project was focused on data quality assurance. We were responsible for keeping track of what sites we visited, any problems we ran into, and that the information itself was error-free and descriptive enough so future users find it reliable. Our work also helped to highlight knowledge gaps for later workers to investigate. Data assurance is a time-consuming process. While it is not glamorous, it is critical to making the data a reliable resource.

We had a great time doing surveys this summer, seeing the coast of Maine from a different perspective. We are glad this project will continue to build on our efforts and expand to include hundreds more sites that we were not able to survey this year. We are gratified that our work will enable coastal communities to make informed decisions regarding crossing maintenance for years to come. 

We will be building on these experiences as we pursue our academic and professional careers in coastal science, and we will never look at a culvert or a bridge in quite the same way.