5
Sediment Pathways
A third task was to review the past studies on identification of sediment trans-
port pathways at Chatham Inlet and to identify present pathways based on devel-
opments in inlet morphology. The processes driving the system can be inferred
based on the evolution of the inlet and shoal morphology. Spatial changes and
directions of movement observed in the inlet features identify the predominant
sediment transport directions.
Initial Inlet Development (1987-1991)
For the first three years of inlet evolution, the trend was for the throat to
expand and the shoal features to form and establish typical inlet features. This can
be illustrated by summarizing the movement of the main ebb channel, the ebb
shoal, and adjacent spits (Figure 47). The ebb shoal has grown seaward and the
edge migrated 2,500 ft (762 m) to the south while a large channel-margin swash
platform has formed on the north and a marginal linear bar has formed on the
south. From 1987 to 1991 the main ebb channel center line has moved some
1,800 ft (549 m) southward. The South Beach spit initially moved south 1,400 ft
(427 m) then westward incorporating the south flood shoal and virtually closing
off the south Chatham Harbor by 1991. Nauset Spit has migrated 2,000 ft (610 m)
north and west into Chatham Harbor. The growth of the north flood shoal in both
width and length up-estuary caused the west flood channel to move westward,
while the eastern channel experienced shoaling.
Seasonal wave climate and longshore drift patterns have played a role in the
evolution of the adjacent spits and ebb shoal (Liu et al. 1993). Summer seasonal
wave approach from the southwest and northward drift influence the northward
movement of the north spit. The more energetic winter wave climate dominated
by northeasters with the dominant northeast wave approach and southward long-
shore drift cause the southward migration of the ebb shoal/swash platform, ebb
shoal and South Beach spit. The retreat of the South Beach shoreline indicates
that the inlet is acting as a littoral block and insufficient sand is bypassing the
inlet. Most of the southward sediment transport is being incorporated into the
growing ebb and flood shoals.
Ebb dominance of the tidal currents was established based on measurements
in this early phase of inlet development (Liu et al. 1993; FitzGerald and Montello
1993). FitzGerald and Montello (1993) found that the maximum flood currents
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Chapter 5 Sediment Pathways
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