- Beaches and Costs: Tidal
1976, Barrier beaches of the east coast
1979, Barrier island morphology as a function of tidal and wave regime
tidal inlet is a channel, maintained by tidal flow, connecting the ocean
with a bay, lagoon or marsh and tidal creek
system. Tidal inlets typically cut through shore-parallel barriers and
- Inlets are the most dynamic
and ephemeral features of a barrier beach system. In addition, they are
often the only access to the sheltered waters behind the barrier. Once
formed there is often pressure to stabilize an inlet with jetties, either
to keep it open so the access is maintained, or to prevent it from
migrating and eroding the island downdrift. Once jetties are
dynamics of the barrier island system is altered.
Definition: An accumulation of
sediment on the shoreward or oceanward side of an inlet (figure 1).
Types of tidal deltas
Figure 1. Flood
and ebb tidal deltas are deposited on the shoreward and
oceanward side of an
inlet (mesotidal/mixed energy). Channels occupied by the ebb and flood
tidal flows are shown by light and dark blue arrows respectively.
EM 110-2-1810 figure 4-15. The flood ramp throat and main ebb
channel form the inlet throat, the deepest portion
of the channel. Sand moved into the inlet by waves and flood currents
deposited on the flood tidal delta or circulated back out to the
delta. Waves reorganize sediment in a series of swash bars which
eventually migrate and attach to the ends of the barrier. Typically
the down drift end receives the most sediment.
Ebb tidal deltas:
- An ebb tidal delta is an
accumulation of sand on the seaward side of an inlet formed by the ebb
- Weak tidal currents relative
to wave activity inhibits the formation of ebb tidal deltas along microtidal
coasts. Macrotidal coasts typically lack shore-parallel barriers, hence
inlets and their deltas.
- Ebb tidal deltas
most commonly occur in front of mesotidal (or mixed energy) inlets where
they interact with waves and longshore drift. Barriers are often drumstick-shaped
with down drift offsets. Think: Why?
- The length (degree of protrusion)
of an ebb tidal delta reflects the relative strengths of tidal flow and
The asymmetry, or lean, of the delta is determined by the direction longshore
Flood tidal delta:
- A flood tidal delta is an
accumulation of sand on the shoreward sided of an inlet. These deltas are
initially formed during storm surges and maintained by flood currents.
- Flood tidal deltas become stabilized
when salt marshes establish on them.
- Salt-marsh grasses cause the
land to be built up to high-tide level and new land is added to the island
- In mesotidal systems, flood
tidal deltas transform into marshland causing the inlet to migrate to a new
position. New tidal deltas are created along the entire area of inlet migration
- Widening by inlet migration
is an important process on the North Carolina Islands.
- Inlets offer access to sheltered
bays and lagoons.
- Inlets enable the exchange of water and nutrients between the ocean and
back barrier region required for finfish and shellfish reproduction.
- Tidal deltas are major sediment
sinks, capturing a large amount of sediment in the littoral system.
- Ebb tidal deltas refract ocean
waves, thereby altering the local littoral dynamics.
- Flood tidal deltas aid in
widening the back barrier region allowing the barrier system to migrate
- Stabilization of an inlet
require an understanding the circulation of sediment in and around an inlet
as well as the relationship between inlet throat dimensions and tidal prism
and back barrier area.
of Geological Sciences, Salem State
College, Salem, MA
last modified 11/25/03