Nearshore placement of dredged material to support shoreline stabilisation

Abstract

Navigation channels transecting ebb shoals can influence the shoreline of adjacent beaches. Channels influence ebb shoal formation and sediment bypassing, which includes attachment bars where the shoal abuts the shoreline. Interruption or modification of sediment bypassing mechanisms at coastal inlets alters sediment supply to adjacent beaches, frequently resulting in shoreline erosion. Historically, structures and/or periodic beach fill have been implemented on impacted shorelines. The US Army Corps of Engineers maintains an entrance channel on the Savannah River near Tybee Island, GA, USA. This deep-draught navigation channel has influenced sediment bypassing by the ebb shoal and appears to have contributed to shoreline recession on the north part of Tybee Island. The Corps has addressed this recession issue by beach-placement of sand from an offshore borrow site. The Corps also dredges a significant amount of material from the entrance channel each year to maintain navigability. This dredged material is often greater than 70% sand. However, it is not considered suitable for beach placement. The Corps is investigating nearshore placement of this dredged material along Tybee Island. The goal of nearshore placement is to indirectly nourish the beach by increasing sand supply in the littoral zone. The proposed method of using dredged material will potentially provide a lower-cost alternative to expensive sand-mining and direct beach placement. Nearshore placement of this non-beach quality material will permit natural surf-zone processes to winnow the unwanted fine-grained material and transport it offshore. The Corps initiated a study to investigate the feasibility of nearshore placement for beneficial use of dredged material to nourish the littoral zone. This paper describes a portion of the issues and study methods. Specifically, hydrodynamic, wave and sediment transport models were utilised to assess the fate of dredged material placed in the nearshore. Optimal placement sites were selected based on the study findings.