Conclusions

This study demonstrated that out of HS and LS, HS were the most effective of the two protection shoreline strategies at resisting erosion over the long-term. However, this study did not test for neighboring shoreline erosion, which is a known consequence to implementing HS as it can strongly affect longshore sediment transfer. Employing HS could have detrimental effects on surrounding ecosystems by exacerbating local erosion and by destroying intertidal habitats with its construction. These morphology changes to the shoreline have the potential to cause negative changes to natural abundances and diversity of flora and fauna in the area. Additionally, while HS do protect the shore against wave action they are often vulnerable to severe storm conditions that have the power to destroy the hardened structure. The breaching of the HS structure is not only expensive to fix but can also cause major shifts to the surrounding sediment, furthering damage to the shoreline. In comparison vegetated shores have the ability to recover naturally after a storm. Considering these factors, while HS are effective at protecting the shoreline against erosion directly behind the structure, HS might not be the best choice when considering shoreline protection as they can cause negative effects to surrounding areas. This study also demonstrates that alternative methods to armoring the shoreline are effective against erosion. The LS, while not able to reduce erosion as much as HS, were shown to be successful at resisting erosion when compared to NS. Combining the facts that LS can increase natural functions and have the ability to adapt to changing sea levels, LS could be a better long-term investment when considering shoreline protection strategies.

We comprehensively evaluated how hydrological, geological, and biological parameters can affect different shoreline types (natural marsh, living shorelines, and hardened shorelines). This information can help coastal managers better identify conditions at potential sites where a living shoreline project may be effective. Based on this study, we conclude that the amount of energy impacting a shoreline can serve as a good proxy for the environmental conditions that can benefit the potential implementation of a living shoreline. Depending on research tools available, methods for obtaining this data would be either field sampling to measure the average wave power, or computer calculations to derive the relative exposure from fetch distance, however, neither method alone is completely accurate and other factors are involved in affecting project success. The low energy coastline groups exhibited less turbidity, less erosion, sediments with a higher percent of silt/clay, more sediment organic matter, and a higher diversity and percent cover of vegetation. We also found that high energy sites can cause a greater variability in the responses of the factors measured among the three different shoreline types than there was at the low energy sites.

The main difference found between NS and LS was that the LS had a slower erosion rate. If a project goal is to stop erosion of a shoreline, then a HS would be the best method, however, if the project involves maintaining a similar ecosystem to a NS, then a LS is a better alternative. The LS in this study did not stop erosion, but lessened the rate compared to the adjacent NS control. Other than erosion, the LS and NS were similar in slope, sediment grain size, soil BD, organic matter content, percent cover of vegetation, and the percent of dominant marsh vegetation. This study showed that a LS is a potentially good alternative to help maintain a similar ecosystem to the NS while also slowing erosion rates.

Only two of the LS in this case study had been implemented longer than five years at the time of sampling. It is important to understand that restoration projects undergo succession, and this is a short time frame. With increased and more frequent sampling of LS after they are implemented the rate of succession could be identified for the different factors. With that in mind there are studies that have looked at LS after longer periods of implementation.

Living shorelines are better for the environment and help lessen the rate of erosion, however, many people still implement HS, especially on small scale projects. This often happens on personal property where people want immediate access to the water instead of multiple meters of tall vegetation separating them. Unfortunately, many of these property owners are unaware of the benefits that marshes provide. Another reservation cited by property owners on the use of LS for shoreline protection is the uncertainty about the cost.

This research is important because the elevation, currents, and substrate of the U.S. Eastern and Gulf coasts makes these regions particularly vulnerable to storms and sea level rise. This research has increased our knowledge on what environmental conditions may be most suitable for living shorelines to help to decrease erosion rates. Further research for the implementation of LS could focus on: (1) following the data from LS sites after implementation for multiple years, (2) differences in the responses of the various types of LS constructions, (3) whether using alternate types of sediment fill for a LS can make a difference (4) and the cost effectiveness of the most successful of the different types of LS constructions over the long term (> 5 years). Those studies could help weigh the benefits of maintaining the different types of shorelines to the cost of erosion losses to habitats or property.