ERDC/CHL CHETN-IV-30December 20001Natural Mechanisms of SedimentBypassing at Tidal Inletsby D. M. FitzGerald, N. C. Kraus, and E. B. HandsPURPOSE: The Coastal and Hydraulics Engineering Technical Note (CHETN) describedherein 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 ofthe Coastal Inlets Research Program (CIRP), which is conducting both applied and basic studiesat coastal inlets (for further inform ation on the C IRP, see http://cirp.wes.army.mil/cirp/cirp.htm l).BACKGROUND: Sediment transport in the vicinity of tidal inlets is complex, making it one ofthe most difficult systems in the coastal environment to quantify. At tidal inlets, sand movesunder the combined action of waves and currents, superimposed on highly variable bathymetrywith constantly changing water levels. Characterization of patterns of sand transport at inletsnecessitates consideration of a wide range of temporal and spatial scales covering movement ofindividual sand grains (centimeters/second), migration of bed forms (meters/day), anddisplacement of large bars (hundreds of meters/year). This complexity extends to the process inwhich sand bypasses an inlet.The dominant variables controlling the processes and rates of inlet sand bypassing have beendocumented from numerous case studies. The governing variables include tidal prism, inletgeometry, wave and tidal energy, sediment supply, spatial distribution of backbarrier channels,regional stratigraphy, slope of the nearshore, and engineering modifications. Engineeringmodifications at inlets are usually to improve navigation and typically involve a combination ofjetties and maintenance of a dredged channel. Between 50 and 100 million m3 of sand aredredged annually from Federal channels at a cost of more than $100 million (Rosati and Kraus2000). Potential decrease in dredging needs by exploiting natural breaching processes couldoffer substantial reduction in annual dredging costs. As examples, reductions in dredgingfrequency and/or dredged volume might be possible by aligning navigation channels with naturalbreaches through the ebb-tidal delta or by reconnaissance through periodic wide-area surveyingto anticipate movement of large sediment bodies (Rosati and Kraus 2000). Knowledge of shoalformation and channel migrational trends at inlets can aid in the design and modification ofjetties and breakwaters.This CETN presents examples of sediment bypassing at natural and modified inlets to serve asqualitative models that can be related or adapted to other study areas. At many inlets, thedominant mode of sand bypassing can be identified from sequential aerial photographs andbathymetric maps. These types of analyses have been facilitated in recent years by improvedaccess to Geographic Information System technology and large aerial photograph databases.BASIC SAND-TRANSPORT PROCESSES AT INLETS: Inlet sediment bypassing is theprocess by which sediment moves from the updrift to the downdrift side of the inlet, involvingthe inlet channel and ebb-tidal delta (also referred to as the ebb-tidal shoal). Along most barrierERDC/CHL CHETN-IV-30December 20002coasts, sand enters the main inlet channel along the beach by wave action or through marginalchannels by tidal currents. Additional sand is transported into the inlet by flood-tidal and wave-generated currents over the shallow swash platform that flanks both sides of the main channel.At jettied entrances, sand may enter the inlet directly from the beach and/or shoreface insituations where the updrift jetty is short or where the beach has accreted to approach theseaward end of the jetty. In other cases, rip currents move sand toward the seaward end of jettieswhere flood currents can transport sediment into the inlet. The net movement of sand into thebackbarrier or out the main channel or jettied channel is controlled by the dominance of theflood- versus ebb-tidal currents, respectively.At most inlets, sand is transported into the bay during storms, when large waves increase thedelivery of sand to the inlet and the storm surge produces strong flood currents. Under normal ornon-storm conditions, sand in the inlet channel is moved in a net seaward direction and isultimately deposited on the terminal lobe (outer bar). Waves shoaling and breaking on theterminal lobe generate currents, which augment flood-tidal currents and retard ebb-tidal currents.Because of the wave current interaction, sand on the terminal lobe is transported landward acrossthe swash platform or along the periphery of the delta toward the adjacent beaches. Sedimentmovement onshore typically takes place in the form of large, landward migrating swash barshundreds to thousands of meters long, 50 to 100 m wide, and 1 to 3 m in height. Along thedowndrift beach, sand may be recirculated back toward the inlet or transferred farther down thebarrier depending upon the morphology of the ebb-tidal delta and wave approach. These generalpatterns of sand transport result in sediment bypassing at inlets and are described in thefollowing section.MECHANISMS OF INLET SEDIMENT BYPASSING: The following examples demonstratemechanisms by which sand is transferred to the downdrift shoreline. These conceptual modelsbuild on the pioneering work of Bruun and Gerritsen (1959) and Bruun (1966) and later researchby FitzGerald (1982, 1988). Additional models are presented here based on more recent tidalinlet investigations. Both natural and structured inlets are considered.Model 1. Stable Inlet Processes. A stable inlet is one that has a stable inlet throat (non-migrating) and a stable main ebb channel position through the ebb-tidal delta. The stability ofthe inlet is usually related to the channel being anchored in a substrate resistant to erosion.Bypassing at these inlets occurs through the formation, landward migration, and attachment oflarge bar complexes to the downdrift shoreline (Figure 1a). The development of bar complexesresults from the stacking and coalescence of swash bars on the delta platform. Wave-built swashbars move onshore due to the dominance of landward flow created by wave swash. Theirstacking results from a decrease in the rate of onshore migration. As the bars migrate up theshoreface, they gain a greater and greater intertidal exposure. Consequently, wave swash, whichcauses their onshore movement,