Geology Reference
In-Depth Information
contains short-term and long-term reservoirs of sand
varying from the relatively small sandwaves fl ooring
the inlet channel that migrate meters each tidal cycle,
to the large fl ood-tidal delta shoals where some sand is
recirculated but the entire deposit may remain stable
for hundreds of years. Sand dispersal at tidal inlets
is complicated because, in addition to the onshore-
offshore movement of sand produced by tidal and
wave-generated currents, there is steady delivery of
sand to the inlet and removal by the longshore trans-
port system. In the discussion below, the patterns of
sand movement at inlets are described, including how
sand bypasses tidal inlets because many of these pro-
cesses produce the major architectural units and large-
scale stratigraphic bounding surfaces comprising
ebb-delta deposits.
countered by landward sand transport in the marginal
fl ood channels and across the swash platform. A varia-
tion of this pattern occurs along the East Friesian
Islands where the updrift swash platform consists of
alternating fl ood- and ebb- dominant channels in which
sediment moves onshore-offshore, but in an overall
easterly direction coincident with the dominant wave
energy fl ux and net longshore transport direction
(Fig. 12.4 , Nummedal and Penland 1981 ) .
12.5.2 Inlet Sediment Bypassing
Along most open coasts, particularly in coastal plain
settings, angular wave approach causes a net move-
ment of sediment, which along the East Coast of the
United States varies from 100,000 to 500,000 m 3 /year.
The manner whereby sand moves past tidal inlets and
is transferred to the downdrift shoreline is called inlet
sediment bypassing (Bruun and Gerritsen 1960). The
primary mechanisms of sand bypassing natural inlets
include: (1) Stable inlet processes, (2) Ebb-tidal delta
breaching, and (3) Inlet migration and spit breaching
(FitzGerald et al. 2001a ) . These mechanisms involve
channel shifts, and the landward migration and
attachment of large bar complexes to the inlet shore-
line that produce a distinctive set of tidal inlet facies
(Fig. 12.5 ).
12.5.1 General Sand Dispersal Trends
Ebb-tidal deltas consist of segregated areas of land-
ward versus seaward sediment transport that are con-
trolled primarily by the way water enters and discharges
from the inlet as well as the effects of wave-generated
currents. During the ebbing cycle, the tidal fl ow leav-
ing the backbarrier is constricted at the inlet throat,
causing the currents to accelerate in a seaward direc-
tion. Once out of the confi nes of the inlet, the ebb fl ow
expands laterally and the velocity slows. Sediment in
the main ebb channel is transported in a net seaward
direction and is eventually deposited on the terminal
lobe due to this decrease in current velocity.
In the beginning of the fl ood cycle, the ocean tide
rises while water in main ebb channel continues to
fl ow seaward as a result of momentum. Due to this
phenomenon, water initially enters the inlet through
the marginal fl ood channels that are the pathways of
least resistance. Generally the fl ood channels are dom-
inated by landward sediment transport and are fl oored
by fl ood-oriented bedforms. On both sides of the main
ebb channel, the swash platform is most affected by
landward fl ow produced by the fl ood-tidal currents and
breaking waves. As waves shoal and break, they gener-
ate a landward fl ow, which augments the fl ood-tidal
currents but retards the ebb-tidal currents. The interac-
tion of these forces acts to transport sediment in a net
landward direction across the swash platform. In sum-
mary, at many inlets there is a general trend of sea-
ward sand transport in the main ebb channel, which is
12.5.2.1 Stable Inlet Processes
This mechanism of sediment bypassing occurs at inlets
that do not migrate and have main ebb channels that
remain approximately in the same position (Fig. 12.5b ).
Sand entering the inlet via tidal and wave processes is
transported to the terminal lobe due to the dominance
of ebb-tidal currents in the main channel. Swash bars
form in the periphery of the delta (50-150 m long,
50 m wide) and move onshore due to the dominance of
landward fl ow across the swash platform. The coales-
cence of landward-migrating swash bars forms large
bar complexes that may be more than a kilometer in
length and up to 3 m in height. The welding of these
bar complexes to the landward beach completes the
inlet sediment bypassing process. The symmetry of the
ebb delta and its overlap along the inlet shoreline con-
trol the location of landward bar migrations. As seen at
Price Inlet, South Carolina the welding of bar com-
plexes to the beach is responsible for a progradation of
the shoreline at specifi c locations (Fig. 12.6 ).
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