CoastWise: Rethinking Tidal Road Crossings

Backhoe on road, photo by Slade Moore

The Challenge

Safe, dependable roads are crucial for Maine's economy, access to critical services, and a way of life valued by citizens and visitors alike. Maintaining roads is a challenge for most communities in Maine, especially in coastal areas experiencing rapid change due to accelerated sea level rise. Where roads cross tidal wetlands at over 800 locations in Maine, the challenges are considerably magnified.

Tidal wetlands can provide coastal storm and flood damage protection, pollutant filtration and break-down, fish and wildlife habitat, commercial harvesting, and recreational opportunities. Some wetlands, like salt marshes, store atmospheric carbon that would otherwise contribute to sea level rise and other climate shifts. To deliver these benefits, tidal wetlands must remain healthy and resilient to sea level rise. That requires unimpaired tidal flow, but about 90% of Maine's tidal road crossings are tidal restrictions. These crossings put Maine's tidal wetlands at risk, but are also more apt to experience flooding, higher maintenance costs, and interrupted access to emergency services.

Closed bridge crossing with barriers, photo by Slade Moore

The CoastWise Approach

Traditional practices for designing tidal road crossings don't adequately address the unique complexities, uncertainties, risks, or benefits associated with tidal environments. In response, the Maine Coastal Program and the CoastWise Steering Committee convened experts in the field and marshaled the input of over 30 organizations to develop the Coastwise Approach for Tidal Crossing Design (pdf file, 7.3 MB). CoastWise provides a voluntary set of best practices, decision-making tools, and path for designing safe, cost-effective, ecologically supportive, and climate-resilient tidal crossings.

CoastWise Principles

Tidal wetlands are dynamic systems influenced by a wider range of interacting social and environmental factors than most non-tidal streams. Tidal road crossings require a design approach that adequately addresses complexity and risk, now and in the future. CoastWise Approach principals include:

  • Know your tidal crossings: Learn which crossings are tidal now or likely to be in the coming decades; use the Maine Coastal Program's Tidal Restriction Atlas or other available tools.
  • Start early: Tidal crossing design is complex and requires ample time to seek funding, collect and analyze diverse data types, establish clear objectives, and develop design alternatives.
  • Ask for advice: Contact a CoastWise Technical Partner to help with project planning, connecting with the right resources, and providing other support.
  • Engage qualified engineering: Expertise in data collection, analyses, sea level rise, tidal hydrodynamic modeling, and crossing design specific to tidal environments is the best fit.
  • Encourage local participation: Crossing design requires value judgements having lasting impact. A transparent, participatory design process facilitates outcomes that serve communities best.
  • Identify risk factors: Consider wetland condition, vulnerable species, and low-lying infrastructure and resource-uses within the crossing's influence, now and in the future.
  • Plan for coastal change: Selecting an appropriate sea level rise scenario is essential to effectively plan for flooding, emergency access needs, and long-term cost effectiveness.
  • Establish objectives: Clear, measurable objectives streamline the design process and save costs.
  • Size crossings for resilience: To survive, marshes and other tidal wetlands upstream of crossings need unrestricted tidal exchange during the highest tides of the year for the life of the crossing.

New bridge, photo by Slade Moore

Learn More

Downloads are PDF files less than 1 MB in size unless otherwise indicated, and may require free Adobe Reader software (download here) to view or print.


For information about the CoastWise Program, please contact Kathleen Leyden (below). For technical support on individual crossing projects, contact any of the providers listed below according to their service areas.


Kathleen Leyden
Director, Maine Coastal Program

Bill Bennett
Fish and Wildlife Biologist
Gulf of Maine Coastal Program
U.S. Fish and Wildlife Service
207-781-8364 x15

Southern Maine

Jacob Aman
Project Manager
Wells National Estuarine Research Reserve
207-646-1555 x112

Casco Bay

Matt Craig
Habitat Program Manager
Casco Bay Estuary Partnership


Jeremy Gabrielson
Senior Conservation Planner
Maine Coast Heritage Trust

Participants from these Organizations Contributed to the Development of CoastWise

Logos of CoastWise development contributor organizations

  • Acadia Civil Works
  • Bates College
  • Bearcreek Wildlife Sanctuary
  • Bowdoin College
  • Casco Bay Estuary Partnership
  • Columbia University
  • Downeast Salmon Federation
  • Goddard Space Flight Center
  • Great Bay National Estuarine Research Reserve
  • Inter-Fluve
  • Kennebec Estuary Land Trust
  • Maine Audubon
  • Maine Coast Heritage Trust
  • Maine Coastal Program, Maine Dept. of Marine Resources
  • Maine Dept. of Inland Fisheries and Wildlife
  • Maine Dept. of Transportation
  • Maine Geological Survey, Maine Dept. of Agriculture, Conservation, and Forestry
  • Massachusets Dept. of Fish and Game, Ecological Restoration Division
  • Massachusetts Office of Coastal Zone Management
  • Midcoast Conservancy
  • Natural Areas Program, Maine Dept. of Agriculture, Conservation, and Forestry
  • The Nature Conservancy
  • New Hampshire Coastal Program
  • New Hampshire Dept. of Enviromental Services
  • New Hampshire Dept. of Fish and Game
  • US Army Corps of Engineers
  • US Federal Highway Administration
  • US Fish and Wildlife Service
  • US Geological Survey
  • US National Aeronautics and Space Administration
  • US National Oceanic and Atmospheric Administration
  • University of Copenhagen
  • University of Maine
  • University of New Hampshire
  • University of New Orleans
  • University of Rhode Island
  • VHB
  • Wells National Estuarine Research Reserve
  • Woods Hole Group