ERDC/CHL CHETN-IV-30
December 2000
Natural Mechanisms of Sediment
Bypassing at Tidal Inlets
by D. M. FitzGerald, N. C. Kraus, and E. B. Hands
herein describes mechanisms by which sediment bypasses both natural and improved tidal inlets.
The note pertains principally to inlets on alluvial or sandy shores and is a product of activities of
the Coastal Inlets Research Program (CIRP), which is conducting both applied and basic studies
a t co a s t a l in l e t s (fo r fu r t h e r in f o r m a t i o n o n th e C I R P , se e h t t p : / / c i r p . w e s .a r m y . m i l / c i r p / c i r p .h t m l ) .
BACKGROUND: Sediment transport in the vicinity of tidal inlets is complex, making it one of
the most difficult systems in the coastal environment to quantify. At tidal inlets, sand moves
under the combined action of waves and currents, superimposed on highly variable bathymetry
with constantly changing water levels. Characterization of patterns of sand transport at inlets
necessitates consideration of a wide range of temporal and spatial scales covering movement of
individual sand grains (centimeters/second), migration of bed forms (meters/day), and
displacement of large bars (hundreds of meters/year). This complexity extends to the process in
which sand bypasses an inlet.
The dominant variables controlling the processes and rates of inlet sand bypassing have been
documented from numerous case studies. The governing variables include tidal prism, inlet
geometry, wave and tidal energy, sediment supply, spatial distribution of backbarrier channels,
regional stratigraphy, slope of the nearshore, and engineering modifications. Engineering
modifications at inlets are usually to improve navigation and typically involve a combination of
jetties and maintenance of a dredged channel. Between 50 and 100 million m3 of sand are
dredged annually from Federal channels at a cost of more than 0 million (Rosati and Kraus
2000). Potential decrease in dredging needs by exploiting natural breaching processes could
offer substantial reduction in annual dredging costs. As examples, reductions in dredging
frequency and/or dredged volume might be possible by aligning navigation channels with natural
breaches through the ebb-tidal delta or by reconnaissance through periodic wide-area surveying
to anticipate movement of large sediment bodies (Rosati and Kraus 2000). Knowledge of shoal
formation and channel migrational trends at inlets can aid in the design and modification of
jetties and breakwaters.
This CETN presents examples of sediment bypassing at natural and modified inlets to serve as
qualitative models that can be related or adapted to other study areas. At many inlets, the
dominant mode of sand bypassing can be identified from sequential aerial photographs and
bathymetric maps. These types of analyses have been facilitated in recent years by improved
access to Geographic Information System technology and large aerial photograph databases.
BASIC SAND-TRANSPORT PROCESSES AT INLETS: Inlet sediment bypassing is the
process by which sediment moves from the updrift to the downdrift side of the inlet, involving
the inlet channel and ebb-tidal delta (also referred to as the ebb-tidal shoal). Along most barrier
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