About 80-90% of U.S. East Coast barrier beaches have experienced erosion in the last 100 years. South Carolina’s coastline forms no exception, a third of its developed shoreline experiences erosion. Among these eroding shorelines is Hilton Head Island, the second largest barrier island on the U.S. East Coast. Until now, erosion here has been addressed through traditional local beach nourishments. An alternative approach to the traditional nourishment method, are so-called feeder nourishments or feeder beaches. The potential advantages of the feeder nourishment concept over the traditional method are reduction of the nourishment frequency, containment of the ecological stress in a relatively small area, and a short to medium term increase of local available space for recreation and the environment. Given the potential advantages above, the residents of Hilton Head Island asked TU Delft to investigate the possibility of applying a feeder nourishment at their shoreline. Currently, a pilot project known as “The Sand Engine” is examined along the Dutch coast. Several studies into its morphological behaviour show that this feeder nourishment can be beneficial to the sediment budget of a larger coastal cell. Because of the promising results at the Sand Engine pilot project, it is tempting to state that a feeder-nourishment could also be applied at Hilton Head Island. The problem, however, is that the conditions at Hilton Head Island and the Sand Engine are different. There are two main differences between Hilton Head Island and the Sand Engine. First, Hilton Head is subjected to a relative calm wave climate in comparison to the Sand Engine. Second, the presence of two tidal inlets at Hilton Head, compared to a relative straight and uninterrupted coastline at the Sand Engine. As a result, the conclusions drawn from the Sand Engine pilot project do not necessarily hold for Hilton Head Island as well. The main objective of this thesis is to analyse the morphological behaviour of a feeder nourishment located at Hilton Head Island. First, to study its potential as a measure against erosion at Hilton Head. Second, to compare its morphological behaviour to that of the Sand Engine. And third, to be able to examine the potential of the concept for the Atlantic southeast coast of the U.S. in general. The morphological development of a feeder nourishment at Hilton Head Island was simulated with Delft3D over the course of 1 year for different model scenarios, with varying forcing conditions and varying bathymetric features. The effect of the relative calm wave climate at Hilton Head Island in comparison to the Sand Engine is twofold. First, the contribution of wave forcing to the total erosional volume of the feeder nourishment after 1 year is smaller as compared to the Sand Engine. Eliminating all driving forces besides wave forcing reduces the total erosional volume to 58% at Hilton Head, in comparison to 75% at the Sand Engine. Second, the contribution of storm events to the total erosional volume after 1 year from the feeder nourishment is smaller at Hilton Head compared to the Sand Engine. It measures 23% at Hilton Head, in comparison to 60% at the Sand Engine. To assess the impact of the two tidal inlets on the feeder nourishment, they were closed off. Closing of the tidal inlets eliminates any (potential) residual currents. This reduces the total amount of sediment that is eroded from the feeder nourishment by 7% compared to a reference scenario with open tidal inlets. Before construction of the feeder nourishment the coastline south of the nourishment experienced a net sediment outflux of approximately 4000 m3/year. After construction of the feeder nourishment, the southern section experiences a net import of sediment of approximately 100.000 m3/year. Meaning that the southern section, on average, has transitioned from being erosive to accreting. Up to 500 meter away from the nourishment the cross-shore profile shows a seaward movement of the shoreline position of approximately 25 m compared to the original situation without nourishment. Before construction of the feeder nourishment the coastline north of the nourishment experienced a net sediment outflux of approximately 40.000 m3/year. After construction of the feeder nourishment, this net outflux of sediment has decreased to approximately 25.000 m3/year. This shows that the feeder nourishment is feeding sediment to the northern section, but at a rate that is not sufficient to keep up with the underlying erosion rate. The northern domain, on average, still experiences a sediment outflux and stays erosive. Roughly 50 m of coastline directly north of the feeder nourishment experiences a seaward movement of the shoreline position. However, moving further away from the nourishment, the shoreline remains erosive. The Atlantic southeast coast of the United States is made up of North Carolina, South Carolina, Georgia and Florida’s east coast. The South Carolina and Georgia coastline are comparable in both hydrodynamic conditions and geomorphological setting. They are mixed-energy coasts, broken up by numerous tidal inlets, and home to short barrier islands with complex sediment transport patterns. North Carolina’s and Florida’s east coast are wave-dominated, with relative straight shorelines. Which is distinctly differences from the conditions found at Hilton Head Island. Therefore, the potential of the feeder nourishment concept is only analysed for South Carolina’s and Georgia’s coastline. The presence of numerous tidal inlets leads to strongly varying conditions along the coastlines of both states. The developed locations along South Carolina’s coastline that require erosion mitigating measures are south Debidue beach, North Island, Hunting Island and Daufuskie Island. Along Georgia’s coastline there are only some erosion hotspots along Sea Island’s coastline that require erosion mitigation measures. The wave climate at all the above mentioned location is similar to Hilton Head. A southeast swell, with a narrow range of directions and an annual wave height of roughly 1,0 m. The same goes for the tidal range. The results at Hilton Head show that erosion on adjacent coastal sections can be lessened and/or prevented by constructing a feeder nourishment. Given that these locations are subjected to similar conditions, the construction of a feeder nourishment could potentially be an effective measure to prevent or lessen the occurring erosion.

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.