CETN IV-15
Rev. September 1999
Step 10: Conduct Sensitivity Testing. Once a sediment budget has been created, it can be
copied and modified to evaluate the impact of any assumptions or refinements to the underlying
data on the final sediment budget. Different data sets for the same project site can be applied to
evaluate seasonal variations in beach change and transport rate direction and magnitude. A
balanced budget representing a historical time period can be copied and altered to represent a
potential future with-project condition.
Example 1: Develop a conceptual sediment budget for the period 1938 to 1979 for the regional
littoral system of Shinnecock Inlet, Long Island, New York. This region extends east of
Shinnecock Inlet to Montauk Point and west of Shinnecock Inlet to Moriches Inlet.
Background Information: Shinnecock Inlet, located on the South Shore of Long Island, New
York, was formed during a hurricane in September 1938 (Figure 2). The west jetty was initially
constructed by New York State in 1947 and was extended from 1953 to 1955, and the east jetty
was constructed from 1952 to 1953. Shinnecock Inlet's littoral system is bounded to the east by
Montauk Point, a location at which net longshore sediment transport is negligible because of its
shoreline orientation and fetch distance from the mid-Atlantic coast. West of Montauk Point, 10-
to 21-m-high bluffs extend for 8 km and are a source of sediment roughly estimated as
35,000 cu m/year based on analysis of profile data. The U.S. Army Engineer District, New
York, formulated a sediment budget for the inlet (U.S. Army Corps of Engineers 1987).
Estimates are available for the net longshore sand-transport rate 1 km east of the inlet
(230,000 cu m/year), the ebb-shoal volume change (77,000 cu m/year), the flood-shoal volume
change (15,000 cu m/year), and the net longshore sand transport 1.8 km west of the inlet
(189,000 cu m/year).
Nersesian and Bocamazo (1992) developed a preliminary sediment budget in which the net
transport east of Shinnecock was 281,000 cu m/year, the ebb and flood shoal captured 77,000
and 15,000 cu m/year, respectively; and transport west of the inlet was 189,000 cu m/year. Kana
(1995) estimated net transport rates 3 km east and 2 km west of Shinnecock Inlet as 219,000 and
104,000 cu m/year, respectively. A seaward bulge located approximately 2 km downdrift of
Shinnecock Inlet is apparent in the 1979 shoreline position, indicating a possible region of
sediment exchange between the ebb-tidal shoal and the downdrift beach. West of Shinnecock
Inlet, the Westhampton barrier island extends for 25 km to Moriches Inlet. Moriches Inlet was
formed in March 1931 and migrated 1,200 m to the west before it closed naturally in May 1951.
Jetties were constructed in 1952 to 1953 at the position of the former inlet, and through dredging
and a minor storm, the inlet reopened. Taney (1961) estimated net transport rates at Moriches
Inlet as 229,000 cu m/year.
Conceptual Sediment Budget: Figure 2 shows the conceptual sediment budget developed from
the information presented. Applying this information with Equation 1 indicates that the beaches
between Shinnecock Inlet and Montauk Point and between Moriches and Shinnecock inlets most
likely have eroded during the subject period unless a significant quantity of beach fill was
placed. Some volumes are not quantified (e.g., beach losses because of relative sea-level rise
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