Aubrey and Gaines (1982) describe three mechanisms by which spits are created
and grow. In the present paper, discussion is restricted to the commonly observed
mechanism of spit elongation by down-drift accumulation of sand introduced by
longshore transport. Kumar and Sanders (1974) summarize the nomenclature and
early work on spit processes (see also, Schwartz 1972), including a case study of the
famous spit (Democrat Point) at Fire Island Inlet on the south shore Long Island,
New York. Based on maps and surveys compiled by the U.S. Army Corps of
Engineers (Gofseyeff 1953), Kumar and Sanders found that Democrat Point and Fire
Island Inlet had moved westward (toward New York City) at an average rate of
63.6 m/year for the period 1834-1955, with the range varying between 5.4 and
251.4 m/year. Many geomorphologic and engineering studies have investigated spit
growth (e.g., Schwartz 1972, Jimenz et al. 1997; Orford et al. 1996; Tanaka et al.
1995; Uda and Yamamoto 1991). Moore and Cole (1960) appear to be the first to
relate measured volume change of an elongating spit to wave-induced longshore
Inlet spit dynamics have received little attention, yet as an organized motion the
possibility exists for developing tractable quantitative models describing spit
evolution. This paper presents such an analytical approach through a process-based
model for predicting spit elongation and deformation under simplified conditions.
The model can be extended to describe complex physical situations, both analytically
and numerically. The model is process-based in calculating evolution through time-
averaged quantities representing meso- to macro-scale motion over intervals on the
order of hours to days and over tens to hundreds of meters alongshore. Model
predictions are examined by reference to the behavior of a spit along Corpus Christi
Bay, Texas, and to spits generated in a laboratory study performed as part of the
Coastal Inlets Research Program of the U.S. Army Corps of Engineers.
CLASSIFICATION OF PROCESSES GOVERNING SPIT EVOLUTION
The model described below derives from observations of spit evolution and
quantification of morphological change based on modern understanding of the acting
coastal processes. For example, spits in particular bodies of water (for example,
lakes, lagoons, bays, and ocean) or along the same coast are expected to have the
same characteristic width, elevation above mean sea level, and elongation speed.
Table 1 presents a classification of spit macro-properties and processes by time scale.
Several dependencies listed in Table 1 emerge in development of the analytical
model. Some authors have emphasized the influence of wave height on spit width
and elevation. Elevation of a berm depends primarily on wave uprush or runup. By
Hunt' formula, runup depends linearly on wave period and only as the square root
of wave height. Therefore, wave period enters prominently in creating spit
"thickness" its width and elevation. Greater tidal range carries the runup to greater
elevation, increasing spit elevation.
Temporal variability in forcing includes weather cycles on scales from days to
global weather patterns over decades, such as El Nio events, and intermittence in
sediment supply as from river discharges and variations in direction of longshore