Evaluation of the effects of a demonstration project for restoring bayside sediment processes at Sailors Haven Marina

Nordstrom, Karl; Jackson, Nancy; Farrell, Eugene

Shoreline erosion is often exacerbated by reduction of sediment inputs because of armoring of upland sources or interference with sediment transport alongshore by shore-perpendicular structures. Establishing a feeder beach to reinstate the sediment budget adjacent to a bulkhead is evaluated here as a solution for addressing future erosion problems along sandy estuarine shores. A feeder beach was created adjacent to Sailors Haven Marina in Great South Bay at Fire Island, New York, using sand obtained from the entrance channel. The fill was placed along a 75 m length of upland at a height similar to the eroding bluff. Topographic surveys were conducted before and after fill and at half year intervals for 18 months. Daily changes in wind and wave processes and beach change were evaluated during an instrumented time-series field study conducted during the storm season within the period 11 March to 7 April 2012. Sediment transport rates and directions were gathered using sand traps and dyed sand tracers. The beach is microtidal (mean range 0.21 m). Fetch distances for wave generation are <15 km. The bulkhead is a wooden sheet pile structure that projects about 115 m into the bay. The sides of the bulkhead are comprised of a series of segments with different orientations relative to shoreline trend, resulting in a complex pattern of wave reflection off the structure. The shoreline is characterized by segments of forested upland alternating with Phragmites marsh. The quantity of fill placed was measured at 1,747 m3. Maximum shoreline advance due to fill emplacement was 20.7 m. The maximum volume placed at any transect was 28.6 m3 m 1 of shoreline length. Erosion of the fill occurred rapidly, with formation and landward migration of a conspicuous scarp. The largest significant waves during the time series study were 0.30 m, associated with a mean onshore wind speed of 11.3 m s-1 (maximum 18.2 m s-1), which caused a foreshore volume loss of 0.9 m3 m 1 of shoreline length. Sediment transport, monitored using a trap placed 55 to 62.5 m west of the bulkhead, was always to the west, away from the structure. The highest rate of measured sediment transport, 1.42 m3 h-1, was associated with a significant mean wave height of 0.26 m and mean current velocity of 0.19 m s-1. The edge of the upland 18 months after the fill was placed in front of it had retreated up to 4.6 m farther landward than the edge of the upland prior to fill emplacement. Movement of sediment alongshore, downdrift of the fill, occurred as wave-like pulses, extending the active foreshore bayward, causing accretion of the inner low tide terrace, burying marsh peat outcrops on the foreshore and creating a higher and wider washover platform on the marshes. Nourishment as a point source caused landforms downdrift to undergo stages, from erosion (pre-nourishment) to accretion to stability (with throughput of sediment alongshore) to erosion again. The time of each stage can be related to nourishment periodicity and the significance of the stage, in terms of topographic change, can be related to the volume of sediment added. Dredging of navigation channels can supply sediment and compensate for sediment starvation by structures built to protect marinas. At Sailors Haven, the presence of high quality beach sand in the channels and the requirement for periodic maintenance dredging provide a ready sediment source. The requirement to dredge makes losses in the dredged channel moot and provides a windfall benefit to dredging. The similarity of fill sediment to native sediment helps reduce negative impacts on fauna. Burial of biota on the low tide terrace by direct placement of the fill is a short term effect that would also occur under natural conditions if overwash and inlet formation occurred.

Evaluation of the effects of a demonstration project for restoring bayside sediment processes at Sailors Haven Marina

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