Hepburn Family Preserve Living Shorelines Case Study Briefing

Challenge

Beach and dune erosion have been an ongoing issue along the approximately 450 linear foot barrier spit that makes up the Hepburn Dune section of the Old Saybrook shoreline. The Borough of Fenwick and Lynde Point Land Trust had expended significant effort and cost to stabilize the shoreline and Lynde Point Marsh during the last 10 to 15 years. Crab Creek, located landward of the dune, was blocked by sediment and the Connecticut Department of Energy and Environmental Protection (DEEP) had made emergency repairs. Shoreline erosion continued and in 2017 there was concern that future storm events may cause a breach of the barrier spit. A breach would have negative consequences for both improved property in the area and the recently restored Lynde Point Marsh and Crab Creek located upland of the barrier spit. GZA was retained in August 2017 by the Borough of Fenwick and Lynde Point Land Trust to design a solution.

Solution

GZA proposed designing a Living Shoreline approach to mitigate ongoing erosion and reduce the potential for a breach of the barrier spit. Additional funding was necessary to complete the design process and CIRCA awarded Fenwick a Municipal Resilience Grant. Several alternatives were developed and evaluated relative to system performance, likelihood of permit acceptance and cost. GZA evaluated the performance of the Living Shoreline under prevailing wind, tide, and wave conditions (to establish requirements for new wetlands survivability) and under storm conditions (storm surge and waves) representing different recurrence intervals. The recommended alternative utilizes: 1) a nearshore wetland sill to attenuate prevailing wave heights; 2) new fill and tidal wetlands between the sill and the beach; 3) reshaping of cobble beach; and 4) dune restoration with a quarrystone-reinforced, planted dune (above the Coastal Jurisdiction Line).

Benefit

A Living Shoreline provides the benefit of a nature-based approach to shoreline stabilization, consistent with the existing barrier spit and marsh setting and with only temporary habitat impact. This approach also addresses the erosive effects of existing, historical coastal structures located near the site. GZA assisted the Borough with an application for a CIRCA Municipal Resilience Grant which was awarded. The project was designated by DEEP as a pilot demonstration project with long term performance monitoring. GZA collaborated with CIRCA and the University of Connecticut to establish empirical Living Shoreline performance criteria applicable to Long Island Sound. The project was also recently presented by CIRCA as part of a Living Shoreline permit workshop.

Project Design

GZA is at the forefront of Living Shoreline and hybrid system design for coastal shoreline stabilization. GZA’s approach has been to integrate ecology and coastal engineering technologies to develop a rational method for design of Living Shorelines, and utilizes:

1. Detailed statistical analysis of wind, water level and wave data; 
2. Assessment of the environmental stress conditions required for new marsh survivability (based on tidal elevation and prevailing wave height and frequency); 
3. Use of high-resolution numerical wave and surge modeling; 
4. Risk-based design which evaluates the system response under a range of conditions (from prevailing conditions to different recurrence interval coastal flood conditions), and;
5. Consideration of the state and local regulatory agency goals for living shorelines during design.

GZA used a very high resolution, 2-dimensional numerical wave model (SWAN) to characterize nearshore waves. The results of the wave modeling were used to: 1) determine the need for wave attenuation (for new marsh survivability); and 2) inform placement of the Living Shoreline features (e.g., wetlands and rock sills) to conform to the existing shoreline geomorphology and tidal and wave currents. This design approach enhances shoreline stabilization while avoiding significant habitat impact and creating additional erosion in the vicinity. 

GZA’s SWAN modeling results of the prevailing wave conditions confirmed that the prevailing wave heights exceed marsh survivability criteria and new wetland sills will be required for wave attenuation. The project is on the southern facing shoreline, with a long fetch across Long Island Sound. GZA’s risk analysis established a probability-based design basis, which defines the performance of the system in terms of likelihood of failure and predicted frequency of maintenance.

GZA completed the permitting in one year. Construction began in 2020 and was completed on May 15, 2021.