Geology Reference
In-Depth Information
12.2.1 Tidal Delta Morphodynamics
function of inlet size. Along the mixed-energy coast of
Maine, where tidal inlets are comparatively small
(width < 100 m), fl ood-tidal deltas are correspondingly
small and stacked in an alternating pattern along the
main tidal creek (FitzGerald et al.
1990
) . Tidal inlets
along the barrier coasts of central South Carolina and
Georgia have no fl ood-tidal deltas because the back-
barrier has been almost completely fi lled with fi ne-
grained sediment and marsh deposits, resulting in tidal
channels that are too narrow and deep for delta devel-
opment. Along some mixed-energy coasts (Hayes
1979
) , fl ood-tidal deltas are indistinguishable from the
surrounding marsh due to sedimentation and infi lling
of backbarrier followed by the colonization of sand
shoals by saltwater vegetation. This may be the case
for some inlets in central South Carolina. At other
sites, portions of fl ood-tidal deltas are dredged to pro-
vide navigable waterways and thus they become highly
modifi ed by human activity.
Flood-tidal deltas are best developed in areas with
moderate to large tidal ranges (1.5-3.0 m) because
in these regions they are well exposed at low tide. As
tidal range decreases, fl ood deltas become largely sub-
tidal shoals. Most fl ood-tidal deltas have similar mor-
phologies consisting of the following components:
1. Flood ramp
-
Landward shallowing channel that
slopes upward toward the intertidal portion of the
delta. The ramp is dominated by strong fl ood-tidal
currents and landward sand transport.
2. Flood channels
-
The fl ood ramp splits into two
shallow fl ood channels. Like the fl ood ramp, these
channels are dominated by fl ood-tidal currents.
Sand is delivered through these channels onto the
fl ood delta.
3. Ebb shield
-
This defi nes the highest and landward-
most part of the fl ood delta and may be partly cov-
ered by marsh vegetation. It shields the rest of the
delta from the effects of the ebb-tidal currents.
4. Ebb spits
-
These spits extend from the ebb shield
toward the inlet. They form from sand that is eroded
from the ebb shield and transported back toward the
inlet by ebb-tidal currents.
5. Spillover lobes
-
These are lobes of sand that form
where the ebb currents have breached through the
ebb spits or ebb shield, depositing sand in the inte-
rior of the delta.
In wave-dominated settings, fl ood-tidal deltas are
commonly present at small to large inlets exhibiting a
variety of forms from sub-tidal shoals to multi-lobate
Tidal deltas were fi rst described in detail by Hayes
(
1975
) and then incorporated into his coastal classifi -
cation based on the relative magnitude of wave versus
tidal energy (Hayes
1979
) . Wave-dominated coasts
have long barriers with few inlets, whereas mixed-
energy coasts have short stubby barriers with numer-
ous tidal inlets. Later, it was shown that this scheme
could also be infl uenced by tidal prism, which is the
volume of water fl owing through a tidal inlet during a
half tidal cycle (Davis and Hayes
1984
) . They showed
that regardless of coastal setting, barrier coasts with
large tidal prisms tend to have larger and/or greater
number of inlets to accommodate this tidal exchange.
The formation of ebb- and fl ood-tidal deltas is a prod-
uct of sand deposition by the ebb- and fl ood-tidal jets,
respectively. As tidal waters fl ow beyond the constric-
tion of the barriers, the currents expand laterally, los-
ing their velocity and their capacity to transport sand.
Flood deltas are commonly built or enlarged during
storm events. Their size, geometry, and facies relation-
ships are a function of tidal range, tidal prism, storm
processes, and accommodation space. The volume,
morphology, and sedimentary sequences comprising
ebb-tidal deltas are a product of tidal prism, nearshore
slope, sand bypassing processes, as well as interac-
tions between wave and tidal energy.
12.2.1.1 Flood-Tidal Deltas
These shoals exhibit a characteristic horseshoe-shaped
morphology worldwide (Figs.
12.1
and
12.2
). Their
presence or absence is related to the availability of
sediment and extent of open water in the backbarrier.
Along mixed-energy coasts, tidal inlets are connected
to a broad marsh and tidal creek system containing a
single, relatively large fl ood-tidal delta, if space per-
mits (i.e., Essex Inlet, Massachusetts, Fig.
12.1
; Hayes
1979
). Contrastingly, inlets such as Drum Inlet along
the Outer Banks of North Carolina (wave-dominated
coast), that are backed by large shallow bays, may con-
tain multiple fl ood-tidal deltas. Along some microtidal
coasts, such as Rhode Island, fl ood deltas form at the
end of narrow inlet channels cut through the barrier.
Changes in the locus of deposition at these deltas pro-
duce a multi-lobate morphology resembling a lobate
river delta (Boothroyd et al.
1985
) .
To some extent, delta size is related to the amount
of open water area in the backbarrier, which is a
