The evaluation of large-scale nourishment strategies for Duval County, Florida

Abstract

The coastline of the United States has been threatened by significant erosion for the past decades. A study in 2000 predicted that 25% of the houses located within 150 meters of the shoreline will be destroyed by erosion in 2060. The east coast of the US experiences an average erosion rate of roughly one meter per year. Moreover, the Atlantic coast can be classified as highly erosive as it is vulnerable to hurricanes in the summer as well as winter storm events. The erosive behavior is counteracted by the application of beach and shoreface (nearshore berm) nourishments, referred to as non-feeder nourishments.
To illustrate this problem on a local scale, a project side in the northeast of Florida is selected, Duval County. At this site numerous beach and shoreface nourishments have been applied with an average nourishment cycle of approximately 5 years. However, taking the effect of climate change and sea-level rise into account, the required nourishment volumes will increase in the future. This means that other solutions need to be investigated. In the Netherlands, the pilot experiment Sand Engine is carried out, involving a large-scale feeder nourishment. It is expected that this type of nourishment will be more beneficial as it reduces the nourishment frequency and contains a concentrated displacement area. The nourishment will spread along the adjacent coastlines in a natural fashion, reducing the impact on ecology. Lastly, large-scale nourishment can temporary lead to additional recreational and environmental area, with a potential of creating new ecological habitats. This leads to the following research question:
'How can a large-scale feeder nourishment be beneficial for highly erosive coastlines along the Atlantic coast of the US, and how can the effects of such nourishments be quantified on different timescales?'
In order to evaluate the effect of large-scale feeder nourishments on the coast of Duval County, an evaluation framework has been developed. This framework is based on ecosystem services, which describe the way humans are linked to and depend on nature. Three main ecosystem services have been identified, which have been divided into several sub-services indicated by quantifiable parameters. The first ecosystem service is coastal protection, which is evaluated in terms of flood protection and maintenance of the coastline position. The time-dependent indicators for these sub-services are the foreshore volume and the distance between the Coastal Construction Control Line and the Momentary Coast Line. Secondly, recreation is evaluated by the sub-services of beach leisure, swimming, kitesurfing and strolling. Beach leisure is indicated by the dry beach width, swimming by the offshore directed flow velocities around the nourishment, kitesurfing by the additional sheltered area, and strolling by the walkable beach length along the shoreline. Lastly, the ecosystem service of habitat provision is split into three sub-services, namely nursery area, turtle nesting and dune growth potential. The nursery area is quantified by mapping the existing ecotopes, turtle nesting is evaluated by the beach slope and the beach width and finally the dune growth potential is indicated by the intertidal beach width.
The researched nourishment alternatives differ from geometric shape and in nourishment frequency. Two shapes are connected to the beach and have a width to height ratio of (1:1) and (1:3), while one is detached from the beach in the form of an island. The first two shapes have been applied with a frequency of 1, 3, 5 and 10 years, and the island only for 5 and 10 years. The morphological development of the alternatives is predicted with the numerical model of Delft3D over a period of 10 years.
All nourishment alternatives have been evaluated for all the selected indicators. For coastal protection the most suitable nourishment alternative is an attached and elongated nourishment with a frequency of 10 years (the (1:3) nourishment alternative). For recreation it differs largely per sub-service, but considering all sub-services have an equal weighting, the (1:3) alternative in combination with a 1 year frequency and the offshore island alternative with a 10 year frequency perform the best. The (1:3) nourishment alternative with a 1 year frequency creates the most benefit for beach leisure and swimming, while the offshore island with a 10 year frequency does this for kitesurfing and strolling. Finally for the ecosystem service of habitat provision, the offshore island in combination with a 10 year frequency has the most potential. The most suitable nourishment alternative cannot be selected for Duval County as a whole since the weighting between the different ecosystem services is unknown, as it depends on the stakeholders involved.
Based on the analysis of the different nourishment strategies, the following conclusions have been drawn for the application of large-scale feeder nourishments:
•Large-scale nourishments can decrease the required coastline maintenance on the long-term as long as they are placed within the dynamic wave zone. However, shore connected shapes can cause initial downdrift erosion because of their protrusion into the ocean, which requires extra nourishments at these locations. The application involves a trade-off between applying a low nourishment frequency with a large volume, and being able to place all the sediment within the dynamic zone.
•The largest temporal additional recreational and environmental area is created by emerged alternatives. Shore connected nourishments provide the largest accessible beach area, while detached nourishments provide the largest sheltered area. In this study, the increase in sheltered area was up to 5 times as large compared to the original situation.
•Nourishment alternatives that are elongated and streamlined along the coastline have a larger region of influence after the simulation period. Here, the region of influence was 10-20\% larger compared to the other shapes.
•Large-scale feeder nourishments have the potential to transport sediment over the entire project area under sufficient tidal and wave forcing. It leads to a more gradual spread of sediment than small-scale nourishments, as they tend to pile up within the placement area.
•As the disturbance of large-scale feeder nourishments is less frequent and concentrated, the adjacent coastlines are fed in a natural fashion, reducing the stress on ecology.
In conclusion, large-scale feeder nourishment can be evaluated by the approach of ecosystem services. They can certainly be beneficial for highly erosive coastlines, but its optimal dimensions depend on the required wishes for the considered coastline.