In: Proceedings Coastal Sediments '03. 2003. CD-ROM Published by World Scientific Publishing
Corp. and East Meets West Productions, Corpus Christi, Texas, USA. ISBN 981-238-422-7.
NUMERICAL SIMULATION OF SEDIMENT PATHWAYS AT AN
IDEALIZED INLET AND EBB SHOAL
Adele Militello and Nicholas C. Kraus
Abstract: Bypassing at inlets can occur across the ebb shoal, through tidal exchange, and by
episodic collapse of shoals. To examine ebb-shoal and tidal exchange bypassing in a systematic
way, we investigated sediment pathways at an idealized inlet with a coupled tide, wave, and
sediment transport-morphology change numerical modeling system. The idealized inlet, ebb
shoal, and channel were devised to test the coupled modeling system and isolate sediment
transport pathways driven by wave and tidal forcing. The inlet, channel, ebb shoal, and bay
dimensions approximate those of Shinnecock Inlet, New York. Five simulations consisting of
tide forcing, wave forcing (fair-weather and storm), and combined tide and wave forcing were
conducted. Patterns of calculated morphology change followed those found in nature.
Simulations with waves impounded sand against the updrift jetty and eroded the bottom in the
nearshore area on the downdrift side of the inlet. Wave breaking on the ebb shoal primarily
moved material updrift, but also flattened the shoal by eroding the top and depositing material
around its perimeter. For the forcing conditions examined, waves were the dominant transport
mechanism. Tidal currents modified the morphology change primarily at the inlet entrance and
on the updrift side of the ebb shoal by opposing the current during the flood tide.
Morphology change at tidal inlets is controlled by the net bypassing rate, tidal transport
through the inlet, and variations in features (bathymetry, structures, etc.) where localized scour
and shoaling take place. Common morphologic responses to wave and tide forcing are:
bypassing at the ebb shoal, skewing of the ebb shoal, migration of the inlet channel thalweg,
development of tip shoals, impoundment at the updrift jetty, beach erosion near the downdrift
jetty, and scour holes inside the inlet adjacent to the jetty tips. Shinnecock Inlet, Long Island,
New York is an example where all of these responses have been observed. The semi-circular
ebb-tidal shoal is skewed to the west owing to the transport driven by waves primarily from the
southeast (Militello et al. 2001). Militello and Kraus (2001a, b) found that advection of the inlet
ebb jet by the ocean tide contributes to realignment of the channel thalweg at Shinnecock Inlet.
Bruun and Gerritsen (1959) studied bypassing modes at tidal inlets and introduced the
concepts of (1) bypassing on the "offshore bar" or ebb shoal, and (2) bypassing through tidal
exchange. Episodic bypassing is a third mode (Gaudiano and Kana 2000), not discussed here.
Recently, an aggregate model of sediment bypassing and volume change of inlet features was
developed that is based on knowledge of sediment transport pathways (Kraus 2000; Militello and
Kraus 2001a). Here, we investigate sediment pathways at an idealized inlet with a coupled tide,
wave, and sediment transport-morphology change numerical modeling system. The idealized
inlet, ebb shoal, and channel were devised to test the coupled modeling system and identify
sediment transport pathways driven by wave and tidal forcing (bypassing modes 1 and 2).
1) Coastal Analysis LLC, 4886 Herron Road, Eureka, CA 95503. CoastalAnalysis@cox.net.
2) U.S. Army Engineer Research and Development Center, Coastal and Hydraulics Laboratory,
3909 Halls Ferry Rd., Vicksburg, MS 39180. .