INTERIM REPORT ON THE GRAND STRAND BEACH NOURISHMENT PROJECTS: BEACH FILL, BORROW SITE AND NEARSHORE REEF CHANGES

Beach nourishment projects placed 35 to 45 cu yds/ft of sand on North Myrtle Beach, Myrtle Beach and Surfside Beach between 1996 to 1998. Each project was approximately 10 kilometers in length and deposited 1.5 million cubic meters of sand between +3.0 and -1.5 meters NGVD. During the early post-nourishment period, these beaches exhibited varying amounts of redistribution of the fills. Though of similar design, the constructed beach fill at North Myrtle Beach lost very little sediment volume (<5%) from the design footprint during first year following construction. The early losses from the adjacent fills were more typically of constructed beaches (~15%loss at Myrtle Beach and ~20% at Surfside). At North Myrtle Beach a significant volume of sand (~700,000 cu meters) was deposited seaward of the design template during construction. This excess sand was lost from the shoreface during the early post-nourishment period and it is likely this sand moved onshore reducing the net loss from the shallower design template. This behavior was not observed at Myrtle Beach or Surfside. Regional geophysical surveys (seismic and side scan) document a sediment-poor inner shelf with extensive outcrops of older indurated deposits in the region. Abundant clasts derived from shoreface and inner shelf deposits are common on the beach and indicate that onshore transport of sediment from these areas does occur. Repetitive side scan sonar and beach profile surveys document significant movement of sediment locally on the shoreface and inner shelf. In this area, which receives no significant local river input, has few inlets or other sources of "new" sediment, this connection between the beach and shoreface/inner shelf may be an important part of the local sediment budget. The borrow sites used for the recent nourishments frequently encountered little or no sediment at the sea floor or utilized sand deposits > 5 kilometers offshore. Efforts to better understand sediment dispersal pathways within the shoreface are necessary to improving the efficiency of projects where long-term resources are so limited.