NATURE’S CONTRIBUTION TO FOREDUNE DEVELOPMENT ON DEVELOPED SHORELINES

Natural dunes achieve widths, heights and cross shore positions in accordance with natural cycles of beach change and delivery of wind-blown sand. Dunes on a developed coast are often fixed in position by sand fences, buildings and support infrastructure that can truncate dunes and make them variable in width, height, length and continuity. Restrictions in space and time on developed coasts place increased importance on supplementing natural processes through active human efforts.

A field investigation, conducted on the developed shoreline of New Jersey, USA, assessed the importance of natural sediment delivery to a foredune located in an enclave set back from the line of houses adjacent to it. Prevailing winds at the site are from the west, but northeasterly storm winds are most influential in delivering sediment from the beach. The setback at the site partially shelters the dune from the northeasterly winds.

Wind speeds were monitored at 0.25 m vertical increments on the beach berm crest, just seaward of the dune toe and the dune crest, where a recording wind vane was placed. Sediment in transport was measured using vertical cylindrical traps. Sediments are predominately moderately well sorted to well sorted medium to coarse sand.

Sediment transport was measured on 3 days of offshore winds and one day of longshore winds. Regional wind speeds were strong during offshore winds, but residential development reduced local speeds at the site. Rates of transport on the exposed berm crest during offshore winds were no greater than 1.62 kg m^-1 hr^-1 across fetch distances of up to 66 m associated with an average wind speed of 6 m s^-1 at 1 m elevation. Rates during the longshore winds were as high as 21.32 kg m^-1 hr^-1 on the active berm with an unrestricted fetch, associated with an average wind speed of 8 m s^-1 at 1 m elevation. Rates here and on the mid backshore were two orders of magnitude greater than on the dune crest and dune toe.

The system is transport-limited in the enclave due to human development. Longshore transport is normally an important input to coastal foredunes, but enclaves created by structures on the backshore decreases potential for transport. Management actions in these enclaves likely require artificial fill and bulldozing, to construct and maintain dunes to overcome vulnerability to storms.