Offshore Sand Sources at Fire Island, New York, on Decadal-to-Century Temporal Scales
Timothy W Kana1, Julie D Rosati 2, and Steven B Traynum1
(1) Coastal Science & Engineering, Inc., PO Box 8056, Columbia, SC, USA 29202 tkana@coastalscience.com straynum@coastalscience.com
(2) U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory, 109 St. Joseph Street , Mobile, AL 36628,
Julie.D.Rosati@usace.army.mil.
ABSTRACT
Determining the boundary of significant, measurable exchange of sediment between the beach
and offshore zone is a basic problem in coastal zone management. Delineating the
approximate depth of this boundary – the so-called depth of closure, DOC – is pertinent for
locating sand resources that are outside the active transport zone and could be mined for beach
placement. At decadal to century time scales, the boundary is likely to be some finite depth
in relatively shallow water in proximity to the existing beach. Wave energy, storm frequency,
and the profile geometry are generally recognized as the controlling factors. Along low wave
energy coasts, the DOC is shallower than along high wave energy coasts. The generally
accepted DOC at decadal scales for US East Coast beaches is less than 10 m. However, some
researchers have suggested that the DOC for central Fire Island, New York, may be much
deeper, in the order 10-20 m, which has limited sand shoals available for storm damage
reduction projects on Fire Island.
We review the evidence for measurable exchange of sand between these depths and the beach
using data from Fire Island developed from various sources. Five factors related to the
potential for a significant deep-water sand source in this setting were considered: (1) spatial
and temporal frames of reference necessary for this flux of sand; (2) studies of scour and
sediment transport over offshore features; (3) sediment size distribution across the foreshore;
(4) depth of closure; and (5) contribution of abandoned inlet shoals. Sediment budgets show
that the principal sand sources at decadal to century time scales are likely to be dominated by
littoral sediments derived from eroding beaches, bluffs, and cannibalization of inlet shoals in
shallower depths (i.e., <10 m). We acknowledge that there is a high degree of uncertainty
associated with previous sediment budgets, with the greatest uncertainty associated with ebb
tidal delta volumes. However, we conclude that evidence for an onshore flux of sediment
(i.e., order of 200,000 m3/yr) from water depths greater than 10 m at decadal scales is lacking
and suggest that reluctance to mine the offshore deposits (water depths between 10 m and
20 m) for beach nourishment is unfounded.