Evolution of an Ebb-Tidal Delta after an Inlet Relocation
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 cross-section 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 above-listed 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. Little sand is believed to shift directly from the updrift beach to the terminal lobe for direct bypassing, in this case.
KeywordsTidal Inlet Sediment Budget Tidal Prism Channel Migration Terminal Lobe
Unable to display preview. Download preview PDF.
- Brown, P.J., 1977. Variations in South Carolina coastal geomorphology. Southeastern Geol., 18(4):249–264.Google Scholar
- Bruun, P., 1978. Stability of tidal inlets: theory and engineering. New York: Elsevier, 510 pp.Google Scholar
- Dean, R.G. and Walton, T.L., Jr., 1975. Sediment transport processes in the vicinity of inlets with special references to sand trapping. In: Cronin, L.E. (ed.), Estuarine Research, vol. 2, New York: Academic Press, p. 129–150.Google Scholar
- Finley, R.J., 1976. Hydraulics and dynamics of North Inlet, South Carolina: 1974–75. GITI Rept. 10, Coastal Eng. Res. Cent., U.S. Army Corps of Engineers, Ft. Belvoir, VA, 188 pp.Google Scholar
- Finley, R.J., 1978. Ebb-tidal delta morphology and sediment supply in relation to seasonal wave energy flux, North Inlet, South Carolina. J. Sediment. Petrol., 48(1):227–238.Google Scholar
- Hayes, M.O., 1975. Morphology of sand accumulation in estuaries: an introduction to the symposium. In: Cronin, L.E. (ed.), Estuarine Research, vol. 1, New York: Academic Press, p. 3–22.Google Scholar
- Hayes, M.O., 1977. Development of Kiawah Island, South Carolina. In: Proc. Coastal Sediments 77, New York: ASCE, p. 828–847.Google Scholar
- Hayes, M.O., Kana, T.W. and Barwis, J.H., 1980. Soft designs for coastal protection at Seabrook Island, SC. In: Proc. 17th Conf. Coastal Eng., New York: ASCE, p. 897–912.Google Scholar
- Hayes, M.O., Sexton, W.J., Domeracki, D.D., Kana, T.W., Michel, J., Barwis, J.H. and Moslow, T.M., 1979. Assessment of shoreline changes, Seabrook Island, South Carolina. Tech. Rept., Research Planning Inst., Inc., Columbia, SC, 82 pp.Google Scholar
- Jarrett, J.T., 1976. Tidal prism-inlet area relationships. GITI Rept. 3, Coastal Eng. Res. Cent., U.S. Army Corps of Engineers, Ft. Belvoir, VA, 76 pp.Google Scholar
- Kana, T.W., 1977. Suspended sediment transport at Price Inlet, SC. In: Proc. Coastal Sediments 77, New York: ASCE, p. 366–382.Google Scholar
- Kana, T.W., Mason, J.E. and Williams, M.L., 1987. A sediment budget for a relocated tidal inlet. In: Coastal Sediments’ 87, New York: ASCE, p. 2094–2109.Google Scholar
- Kana T.W., Sexton, W.J., Thebeau, L.C. and Hayes, M.O., 1981. Preliminary design and permit application for breaching Kiawah spit north of Captain Sams Inlet. Tech. Rept., Research Planning Inst., Inc., Columbia, SC, 39 pp.Google Scholar
- Kana, T.W., Siah, S.J. and Williams, M.L., 1984. Alternatives for beach restoration and future shoreline management, Seabrook Island, South Carolina. Tech. Rept., RPI Coastal Science & Engineering, Inc., Columbia, SC, 118 pp.Google Scholar
- Mason, J.E., 1986. Morphologic evolution of a relocated tidal inlet: Captain Sams Inlet, South Carolina. Tech. Rept., Dept. Geol., Univ. South Carolina, and SC. Sea Grant Consortium, Columbia, SC, 149 pp.Google Scholar
- Moslow, T.F., 1980. Stratigraphy of mesotidal barrier islands. Ph.D. Dissertation, Dept. Geol., Univ. South Carolina, Columbia, 186 pp.Google Scholar
- Nayak, I.V., 1971. Tidal prism-area relationship in a model inlet. Tech. Rept. HEL 24–1, Hydraulic Engineering Lab., Univ. California at Berkeley, 72 pp.Google Scholar
- Nummedal, D. and Humphries, S.M., 1978. Hydraulics and dynamics of North Inlet, South Carolina, 1975–1976. GITI Rept. 16, Coastal Eng. Res. Cent., U.S. Army Corps of Engineers, Ft. Belvoir, VA, 214 pp.Google Scholar
- O’Brien, M.P., 1931. Estuary tidal prisms related to entrance areas. Civil Eng., 1(8):738–739.Google Scholar
- O’Brien, M.P., 1969. Equilibrium flow areas of inlets on sandy coasts. J. Waterways and Harbors Div., New York: ASCE, 95:43–52.Google Scholar
- Oertel, G.F., 1972. Sediment transport of estuary entrance shoals and the formation of swash platforms. J. Sediment. Petrol., 42:857–863.Google Scholar
- Sexton, W.J., 1981. Natural bar-bypassing of sand at Captain Sams Inlet, South Carolina. M.S. Thesis, Dept. Geol., Univ. South Carolina, Columbia, 101 pp.Google Scholar
- Sexton, WJ. and Hayes, M.O., 1983. Natural bar-bypassing of sand at a tidal inlet. In: Proc. 18th Conf. Coastal Eng., New York: ASCE, p. 1179–1195.Google Scholar
- U.S. Dept. Commerce, 1984. Tide tables, east coast of North and South America. NOAA, National Ocean Survey, Rockville, Md., 288 pp.Google Scholar
- U.S. Naval Weather Service Command, 1970. Summary of synoptic meteorological observations, Atlantic and Gulf coasts. Charleston, SC: vol. 3, area 10.Google Scholar
- Walton, T.L., Jr. and Adams, W.D., 1976. Capacity of inlet outer bars to store sand. In: Proc. 15th Conf. Coastal Eng., New York: ASCE, 2:1919–1937.Google Scholar