NUMERICAL SIMULATION OF SEDIMENT PATHWAYS AT AN IDEALIZED INLET AND EBB SHOAL

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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.

INTRODUCTION

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. .