prediction of Walton and Adams (1976) was examined. The coefficient value
CE = 8.7 105 (Equation 6-5) for mildly exposed coasts was employed in the
calculation. For the tidal prism of 4.32 107 cu ft, the volume of the shoal is
predicted to be 340,000 cu yd. The actual volume of this feature was calculated
to be 460,000 cu yd (Chapter 4).
The difference between the actual and predicted volume of this feature
supports a conclusion that the feature is not a relic or active ebb-tidal shoal.
Implicit in the estimation procedure of Walton and Adams (1976) is the
requirement that the sediment at a site be predominantly fine to medium sand.
Because the offshore shoal considered contains a significant percentage of coarse
sand and gravel, it is expected that the Walton and Adams (1976) empirical
relation would overestimate the volume if the feature were an ebb shoal. The
underestimation of the shoal volume by the Walton and Adams (1976) relation
supports the conclusion that this feature is not an ebb shoal.
Other aspects of this study further support this conclusion. The shoal is
located too far east (outside the ebb-tidal jet) and possesses large gravel content,
making transport of these sediment grains from the inlet to the feature doubtful.
Tidal circulation modeling of Mattituck Inlet (Chapter 5) indicates that the
feature is not hydraulically connected to the inlet in its present condition or in its
natural condition. Finally, the feature experienced minimal growth between 1927
and 2002 (Chapter 4).
Mattituck Inlet flood shoal
The volume of the flood shoal at Mattituck Inlet can be estimated directly
through bathymetry difference calculations in a GIS and compared to the
empirical relation derived by Carr de Betts (1999).
The major area of shoaling at Mattituck Inlet is found on the east bank
adjacent to the end of the east jetty, and there is also considerable shoaling on the
opposite (west) bank. The shoaling refers to locations above the navigation
channel and does not include deposition along the walls of the channel (bank
encroachment). Shoaling inferred to be caused by the transport of sediment into
Mattituck Inlet continues along both banks to a distance of about 2,000 ft beyond
the landward end of the inlet.
A polygon that encompasses this area was created in a GIS, and a TIN was
generated (Figure 6-6). The extent of the flood shoal considered here is greater
than the preceding analysis of flood shoal morphology change. Morphology
change analysis was based on the area of the flood shoal included in typical New
York District condition surveys. Because the bathymetry survey of 6-8 October
2002 covered all of Mattituck Creek, the area of analysis is extended here. An
estimate of the volume of this portion of the flood shoal was obtained by
calculating the volume of sediment found above a reference datum. Two
volumes with respect to two reference datums (-6 ft NAVD88 and 4 NAVD88,
which correspond approximately to 3 ft mlw and 1 ft mlw) were calculated to
provide an estimated range of the volume of this portion of the flood shoal.
The area considered to be part of the flood shoal included dry land to an
elevation of up to +1.5 ft NAVD88. This identification was made because
analysis of historical aerial photographs indicated that these areas are new
features created by shoaling and landward bypassing accumulation (Figures 4-24
Chapter 6 Inlet Morphology and Stability