profile data (3 to 5 profiles per compartment, up to 305 m apart) and existing morphological
features (e.g., inlets). A net LST rate as inferred from impoundment rate at the Fire Island
Inlet east jetty (1940 to 1954; 420,000 m3/yr) formed the basis for calculation of LST rates
at each alongshore compartment. A reduced value equal to 306,000 m3/yr was applied at a
location 3.3 km east of the Fire Island east jetty (Table 1). The value was decreased from
impoundment and (b) the change in shoreline orientation to one more parallel with incoming
waves. The compartments did not extend offshore to depth of closure; thus, a component of
profile adjustment was included to reflect changes due to offshore losses.
Kana (1995). Kana updated RPI' (1983) sediment budget by extending the profile
s
calculations to depth of closure, revising the dredging and beach fill placement records, and
modifying the Montauk Point bluff erosion calculation. The middle portion of Fire Island
(20 km east of Fire Island Inlet) had a lower net LST rate than expected. During this period,
severe erosion of eastern Fire Island was feeding the central portion. Relic Fire Island Inlet
shoals appeared to have been a significant source of sediment to the central and western Fire
Island beaches through the early 1900s. However, because of the erosion of west Fire Island
beaches apparent in the 1955 to 1979 profile data, this source appeared to be largely
diminished. To solve the budget, a reversal in net LST was determined to occur west of the
Westhampton Groin Field, resulting in 85,000 m3/yr net LST to the east (6.7 km east of inlet).
Study Objectives
A regional sediment budget was formulated to characterize sediment transport pathways,
magnitudes, sources, and sinks, and engineering activities (dredging and beach fill placement)
characteristic of the 1979 to 1995 period for the Fire Island to Montauk Point study area.
With this regional sediment budget, the significance of recent dredging and beach fill practices
to the littoral transport system was assessed, specifically:
Have these engineering activities been critical in maintaining the barrier island system in
its present state?
Is the postulated source of littoral sediment offshore of central Fire Island required to
satisfy the sediment budget?
SEDIMENT BUDGET METHODOLOGY
A sediment budget is a model of sediment gains and losses, or sources and sinks, within
a specified control volume (or cell), or series of connecting cells, over a given time. There
are numerous ways of formulating a sediment budget (e.g., SPM 1984, Jarrett 1991). The
difference between the sediment sources and the sinks in each cell, hence for the entire
sediment budget, must equal the rate of sediment volume change occurring within that region
accounting for pertinent engineering activities. The sediment budget equation can be
expressed as,
∑Q
∑Q
-
- ∆V + P - R = residual
(1)
source
sink
5
Rosati et al.
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