Center for Coastal Geology
Short and Long-Term Variability of Ebb-Tidal Deltas:
Management Implications
Mark Hansen
US Geological Survey, St. Petersburg, Florida
Paul Work
Clemson University, Clemson, South Carolina
ABSTRACT:
With the increasing demand for suitable beach fill material,
coastal planners often covet ebb-tidal shoal sands due to their
(typically) coarse grain size and proximity to the beach.
However, these sand bodies are rarely mined because of potential
adverse effects on adjacent shorelines. The quantification of
sediment volumes in an ebb-tidal delta over short and long time
spans can be used to identify the system's natural variability.
Understanding this variability may help planners make wiser
decisions regarding the potential use of ebb-tidal delta
material. To address the issue of variability, this study has
investigated two unstructured inlets in South Carolina over time
spans of years and decades using modern and historical
bathymetric surveys. Detailed modern inlet surveys have been
conducted by the US Geological Survey over the past four years
(1994-1997) and compared to historical data. Results suggest
that limited mining of ebb-tidal delta material, on an annual
basis, may have minimal impacts on the long-term stability of
ebb-tidal delta complexes.
DISCUSSION:
Mining ebb-tidal delta material as a source material for beach
nourishment projects is often considered, but rarely utilized,
because of potential adverse impacts on adjacent beaches.
However, it is likely that there will be increasing pressure to
utilize these sand bodies in the future as there is a limited
quantity of quality sediment elsewhere in this part of South
Carolina. This study investigates the change in ebb-tidal delta
sediment quantities at Dewees and Capers Inlets, located just
north of Charleston, SC. The intent is to determine ebb-tidal
delta sediment flux on short and long term bases in order to
assess if a portion of the delta could be mined without adversely
impacting adjacent shorelines.
The movement of sediment in and around these inlets has been well
documented by Hayes, Fitzgerald, Hubbard and others.
Simplistically, sediments are transported by longshore currents
and forced by regional and locally refracted wave fields, with a
percentage of the material deposited in the ebb-tidal delta and
most of the remaining portion transported past the inlet to down
drift beaches. With time, the build-up of sediments in the
ebb-tidal delta cause the inlet to become hydrodynamically
unsteady. The instability is naturally corrected during a major
storm event with the main ebb-channel breaching the ebb-delta and
shifting to a more hydrodynamically stable position. This shift
usually permits a large quantity of ebb-tidal delta material (on
the down drift side) to break free of (or bypass) the delta and
eventually be deposited on the adjacent down drift beach. The
quantity of material bypassed and the time interval between
events is probably most dependent upon the size of the tidal
prism, e.g. the ebb-tidal delta at Dewees Inlet probably releases
more than a million cubic meters of material to Isle of Palms every 5-10
years.
The above describes processes which transport large quantities of
sediment at irregular time intervals. More subtle changes of the
ebb-tidal delta, which occur on an annual basis, are less well
known. It is these changes which this study has focused upon.
Quantifying changes on both a short and long term basis may help
to identify the natural variability or flux of inlets, which will
in turn aid coastal policy maker on management decisions
regarding the mining of ebb-tidal delta material. If the short
term natural flux is significant, it would probably be acceptable
to artificially bypass that quantity on an annual basis.
Conversely, a small flux probably indicates that the inlet is
very dynamically stable and that any mining would upset the
inlets natural balance with nearshore processes.
Inlet Bathymetry Series
RESULTS:
Using bathymetric survey data, volumetric comparisons were made
between time periods at Dewees and Capers Inlets (Figure 1). The
polygons (indicated in red) identify the boundaries in which
volumes were calculated. In order to account for potential
survey error, (+/- 0.12 m for recent surveys, greater for the
1965 and 1920), the change volume between -0.25 and +0.25 was
subtracted from the overall volume measurement.
The time intervals between 1920-1965, 1965-1994 and 1994-1995
were used for Dewees Inlet, and intervals between 1920-1965,
1965-1995, 1995-1997 were used for Capers Inlet. Additional
comparisons would certainly increase confidence to the results;
however, there appears to be a definite trend with this limited
data set. Results indicate there is a significant flux of
material exchanged between the ebb-tidal delta and adjacent areas
over short time periods at both inlets. It is interesting to
note that the short term fluxes are of the same magnitude as
estimated sediment transport rates (150,000 m3/yr) for the
Charleston area (Kana). A larger flux occurs at Dewees Inlet
which correlates well with its size compared to Capers Inlet.
Over longer time periods (1965-1995 and 1920-1965), the fluxes
are very small suggesting the volume of the ebb-tidal deltas are
in dynamic equilibrium with local coastal processes. However,
there appear to be significant geomorphic changes which occur
within and are confined to the delta itself, as indicated in the
Dewees maps between 1965 and 1994. This is probably due to main
ebb-channel realignment and bypassing processes as describe above.
Time Interval Erosion
| 1920-65 |
1965-94 |
1994-95 |
1995-97 |
 |
 |
 |
 |
CONCLUSIONS:
What are the coastal management implications of this study? The
use of ebb-tidal delta sediment as a source for beach
nourishment material has and will be controversial. However,
this study indicates that there is a natural variability to these
inlets systems, and if artificially bypassing practices mimic the
natural processes by removing a small percent of the delta on an
annual basis, there is likely to be minimal adverse impact to
adjacent shorelines. Mining the seaward edge of the delta over a
large area would maintain the inlets overall geomorphology and
would reduce the possibility of severely altering nearshore
refraction and sediment transport patterns.
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Updated: 18 March, 1999 @ 03:52 PM
(THF)
