Evaluating Coastal Erosion Structures

Sponsor: Nantucket Natural Resources Department DSCN3747
Sponsor Liaison: Jeff Carlson, Natural Resources Coordinator
Lauren Hunt, Christopher Sample, Kathleen Sullivan
Abstract: The island of Nantucket experiences extreme coastal erosion and property owners have built various types of coastal erosion structures to try and limit adverse impacts. The goal of this project was to find, categorize, and conduct a preliminary evaluation of the effectiveness of existing structures. Through reviews of permit information and aerial photographs, site visits, stakeholder interviews, we created a comprehensive database and an interactive map of 72 coastal erosion structures on the island. We conclude that ‘hard’ and ‘soft’ erosion structures may inhibit erosion in the short term over a limited spatial extent, but effectiveness varies dramatically by location and many structures have unintended proximal impacts.
Links: NRD IQP Final Report

Coastal Erosion Presentation

Executive Summary

Erosion is a severe problem on east coast of the US, including Massachusetts, but especially in Nantucket with erosion rates from 2 to 12 feet per year. In Nantucket, some of the methods to mitigate erosion that have been implemented include an array of ‘hard’ (e.g., bulkheads and groins) and ‘soft’ structures (e.g., beach nourishment and sand drift fencing). There is no comprehensive database with information regarding all the structures (such as type, condition, location, etc.) currently in use on the island. Therefore the goal of our project was to evaluate the current condition, impacts, and effectiveness of the various coastal erosion structures and techniques used on Nantucket. To achieve this goal, we:
• Identified and catalogued the variety of coastal erosion structures and methods used on the island to date and evaluated the impacts and effectiveness of the different structures and methods;
• Interviewed current stakeholders and officials on the island in regards to coastal erosion practices and policies; and
• Developed a set of recommendations for future policies and erosion strategies

We conducted a physical evaluation and took photographs of each coastal erosion structure we could identify on the island; reviewed past documentation (e.g., permit applications, newspaper reports, etc.) regarding the structures; interviewed key persons involved in the permitting, building, and maintenance of coastal erosion structures; devised a method of evaluating the effectiveness of each coastal erosion structure; and collated all the information in a comprehensive database and interactive map. We began by constructing the basic database structure and content categories in response to the needs of our sponsor and other town officials. Since the evaluation included an assessment of the effectiveness of the coastal erosion structures, we developed protocols guiding how we conducted this assessment. We populated the pilot database with information from the northwest section of the island to see if the database and protocols were adequate. We revised the database and protocols and populated the database with information on all the coastal erosion structures we could identify on the island based on various sources, including field studies and aerial photographs. Once the data collection was complete, we analyzed the data to identify our overall findings with regard to the impacts and effectiveness of coastal erosion structures on Nantucket.
Based on our site visits and analysis of Natural Resource Department files we identified 72 permitted erosion control structures on the island (see figure below), including 42 that we classified as hard structures (red squares) and 30 soft structures (blue circles). The geotubes that have been put in place in Siasconset are indicated with a blue square since they were permitted as emergency structures. Evidently, most of the structures are located in the harbor (50 structures) and northwest sections of the coastline (12 structures), although we also identified 5 structures in Siasconset and 5 on the south shore. We should note that the size, type and complexity of structures vary substantially across the island.


NRD Executive summary


The concentration of structures on the north coast probably reflects the density of population and the large number of houses that were built directly on beachfront property. For example, the majority of structures along Hulbert Avenue belonged to homes that were less than 100 yards from the beach. Records are incomplete, but it appears that many of the soft structures on the island were built more recently, reflecting the shift in emphasis from hard to soft structures that has been a predominant pattern nationally and, in particular, along the east coast. Twenty-three of the hard structures we identified were documented as being built prior to 1978 when more stringent oversight and regulations came into effect.
The database includes a broad array of information on each of these 72 structures, including:
• Map and parcel number;
• The date and time we visited the structure;
• Town address;
• If the structure was permitted;
• If the structure has a Chapter 91 license;
• The date the structure was installed;
• The most recent date that it was updated;
• How it was maintained;
• The name of property owner;
• If the structure is private or public;
• The condition of the structure;
• Permit and site visit notes;
• MORIS transect value; and our
• Effectiveness rating values.
Because some of the structures span more than one property the database includes 85 individual entries covering the 72 structures.

Measuring the effectiveness of coastal erosion structures is extremely complicated. We developed a relatively simple set of measures that we could apply in the field and these were summarized in the database. Based on our observations and measurements, we found that 31 out of the 72 structures scored a 5-6 on our rating scale and were deemed effective; we rated 36 structures as adequate since they scored between 3-4 and only 4 structures as ineffective with a score of less than 2. Surprisingly, we found that 48% of the hard structures were effective, while only 38% of the soft structures were effective. This is surprising because the Army Corps of Engineers, Nantucket Conservation Commission and others have been moving toward greater use of soft structures in preference to hard structures in the past three decades. This may be because our effectiveness rating scale focuses heavily on the proximal effects of coastal erosion structures coastal erosion structures, the immediate areas in front, behind, and to the sides of the structures and does not try to assess more distant impacts up and down the coast or offshore. It may also reflect the relatively narrow time horizon for our evaluation. Indeed, there are few traces of many structures that have been built in the past because they have been entirely destroyed by storm and wave action; such structures are by definition ineffective, but are not included in our assessment or database.

A preliminary assessment of structures suggests that the advantages and disadvantages of hard and soft structures are not so easily discernible, depending on the type of structure, its method of construction, and location. Soft structures may be successful at decreasing erosion and encouraging accretion while being a more environmentally friendly alternative; however they require more maintenance and upkeep. If a soft structure such as a sand fence is layered in such a way with a jute mesh bag that it forms a solid structure it may act more like a hard structure similar to a seawall. Hard structures may effectively protect the land immediately behind them; however they can cause scouring, the loss of beaches, and other distant impacts by limiting replenishing sand. Hard and soft erosion structures may inhibit erosion in the short term over a limited spatial extent, but effectiveness varies dramatically by location and many structures have unintended proximal impacts. We have observed structures here on island that use a combination of both hard and soft structures to try and make the best out of both techniques and we classified these structures as hard or soft by their primary feature.

From our observations we have made several recommendations to the town to help with the ongoing situation in regards to developing and maintaining coastal erosion structures. The information in our database was limited by time to what we deemed were the most important fields for evaluation.
• We recommend the town maintain and develop the database to include more comprehensive assessments of impacts and effectiveness. (This is recommended as a future IQP.) It is important the database be updated on a regular basis to ensure the information regarding all structures is accurate, while also including information on all new structures.
• We recommend the photographic database on the Google My Maps is maintained. This acts as a user-friendly location for the public to view the structures we found, along with a description of the structure and pictures from site visits. It is critical this be maintained to aid in tracking the upkeep of structures and conditions of the structure, surrounding land, and beaches.
• Due to the preliminary nature of our rating scale, we recommend that our scale be further developed and refined, specified to specific structure types (e.g. sand drift fencing and groins, which would each have their respective scales), and so future ratings are more accurate and consistent.
• Along with ensuring the information about each structure is centralized, we also recommend that the Conservation Commission continue to work with homeowners in a positive way when permitting new structures. It is important the Commission maintains an open line of communication when permitting structures to ensure homeowners meet the Commission’s requirements and environmental needs, while also getting the most effective structure for their property.