Evolution of an Ebb-Tidal Delta After an Inlet Relocation

  1. David G. Aubrey and
  2. Lee Weishar
  1. Timothy W. Kana and
  2. James E. Mason

Published Online: 23 MAR 2013

DOI: 10.1029/LN029p0382

Hydrodynamics and Sediment Dynamics of Tidal Inlets

Hydrodynamics and Sediment Dynamics of Tidal Inlets

How to Cite

Kana, T. W. and Mason, J. E. (1988) Evolution of an Ebb-Tidal Delta After an Inlet Relocation, in Hydrodynamics and Sediment Dynamics of Tidal Inlets (eds D. G. Aubrey and L. Weishar), Springer-Verlag, New York. doi: 10.1029/LN029p0382

Publication History

  1. Published Online: 23 MAR 2013
  2. Published Print: 1 JAN 1988

ISBN Information

Print ISBN: 9783540968887

Online ISBN: 9781118669242

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Keywords:

  • Captain Sams Inlet;
  • Channel development;
  • Ebb-tidal delta development;
  • Evolution of ebb-tidal deltas;
  • Patterns of sediment movement;
  • Seabrook Island;
  • Sediment budget;
  • Study plan and methodology

Summary

In March 1983, Captain Sams Inlet near Charleston, South Carolina, was intentionally relocated two kilometers (km) updrift of its most recent position at the terminus of a migrating barrier spit. The purpose of the project was for erosion control along the downdrift shoreline. The project afforded a unique opportunity to study the evolution of an ebb-tidal delta uncontrolled by coastal structures. A series of five surveys was completed between February 1983 and May 1985 encompassing the principal morphological units of the inlet and seaward shoals. All surveys were completed from a controlled baseline using an electronic distance measuring (EDM) device, rod and level, and survey fathometer. Isopach maps were developed to compare zones of accretion and erosion between surveys and to identify sediment compartments. Other key variables measured were tidal prism, throat cross-section, and channel migration.

The new inlet developed an equilibrium cross-sectional area, Ac, of approximately 210 square meters (m2) with a throat depth of ˜4.2 m mean sea level (MSL) within 250 days after relocation and a corresponding mean spring tidal prism, Tp, of 3.34 × 106 cubic meters (m3). The channel was found initially to shoal by wave erosion of adjacent spit and channel mouth sediments after the breach was made. This reduced the as-built crosssection of 112 m2 to 68 m2 (below MSL) during the first month after relocation. The initial period of shoaling was followed by a steady increase in cross-section and initiation of channel migration. Within the first 250 days, channel depth and Ac stabilized at the abovelisted rates, but the volume of the ebb delta continued to grow. Channel migration averaged 73 meters per year (m/yr) during the first 2.2 years following the project. Not surprisingly, the migration rate almost matched the historic rate during the most recent period, 1948-1983. The volume of shoals in the ebb delta reached 0.3 × 106 m3 by May 1985 (2.2 years after relocation). Unlike the channel cross-section and depth, ebb-delta volume after two years remained substantially below an estimated equilibrium value as predicted by the empirical model of Walton and Adams (1976) or by surveyed values for the tidal delta of the preexisting inlet (0.8-1.5 × 106 m3).

A sediment budget was estimated for the new inlet, delta, and adjacent barrier beaches. Although precise values for longshore transport and sand-bypassing rates are unavailable, comparisons between surveys indicate that erosion of the channel throat and channel mouth areas accounts for the net volume gained in the ebb-tidal delta compartment and downdrift accreting beach during the study period. This implies that the measured gain along the updrift spit is derived exclusively from longshore transport. The authors hypothesize that the principal, sand-transport pathway for migratory inlets such as Captain Sams is from the updrift littoral zone to the recurved spit platform; erosion of the downdrift barrier beach and channel shoreface provides the sand supply to the ebb-tidal delta and downdrift beach. Litde sand is believed to shift directly from the updrift beach to the terminal lobe for direct bypassing, in this case.