Geology and Earth ScienceSedimentation at Democrat Point- John Bucaro
To the south, Long Island is bordered by Fire Island. Fire Island is a series of Barrier beaches that provide the south shore of Long Island with some protection from hurricanes and other storms. The formation of these barrier beaches can be approached in two different theories.
“The most widely held hypothesis is that of ‘shoreface retreat’. This view states that as sea level rises, the barriers migrate continuously landward, mainly through the combined effects of shoreface erosion and washover on the landward sides of the barriers. During the migration, the breaker zone traverses the entire area submerged... The contrasting view, that of in place ‘drowning’, states that as sea level rises, the barrier may remain in place, while the lagoon on it’s landward side deepens and widens.
Eventually, the breaker zone reaches the level of the top of the barrier, the sea drowns the barrier, and the breaker zone oversteps landward to form a new barrier shoreline along the landward edge of the former lagoon” (Rampino and Sanders, 1981).
The barrier beaches of Long Island are often undergoing analysis to understand how much they are eroding and at what rate. It is a difficult task because often times the sand may not be gone from the barrier simply moved to another are of the island. Currents and long shore transport move sediment along the beach and in and out of inlets.
“Circulation patterns are specific to each inlet, but certain properties are common to many inlets. Common properties include ebb or flood dominance, preferred channel on ebb and flood tide, eddy formation and migration, and jetty control on flow patterns” (Militello and Hughes, 2000). At Democrat
Point, near the Fire Island inlet, a large jetty was constructed to prevent sediment being transported into the inlet and filling in the inlet. Since the predominant sediment transport direction is from east to west the east side of the jetty has filled in with sediment.
There is also a jetty across the inlet from Democrat Point. The two jetties are not parallel and can thus be classified as offset jetties. In this situation the current is affected and becomes strongest on the sides with the shorter jetty and water enters the inlet at an angle to the inlet center line Militello and Hughes, 2000). Since the tidal currents combined with longshore transport effect sediment distribution one would think that the jetties effect on the currents would result in unexpected sedimentation patterns.
This study attempts to analyze the sedimentation patterns around the jetty at Democrat Point through grain size analysis. As currents slow down the larger, heavier grains will fall out of solution first. Smaller, lighter grains are still able to be transported by a slow moving current. Since the jetty effects the currents around the inlet a grain size analysis should allow one to determine the jetties effect on sedimentation processes.
Results
Phi size is a useful measurement when doing grain size analysis. The larger phi sizes correlate with smaller grains; therefore a phi size measurement of
4 is a smaller grain than a phi size measurement of 0. Sorting by phi size also allows one to separate the grains out and measure the percentage of the sample within each phi size. By dividing the weight of the sediment in a particular phi size by the total weight of the sediment in the sample one can determine the percentage of the sample in that grain size. Using the percent composition of each phi size is useful because it allows us to see what size most of the grains are. If a higher percentage of the grains are large than we know we’re looking at a coarse sediment where as if the majority of the grains are smaller than we have a finer sediment. Whether the sediment is coarse or fine allows one to determine the type of depositional environment.
In conclusion, we can see that all of the samples are dominated by grains in the larger phi sizes. However, it is worth noting that in the sample furthest east we saw dominance in the top two phi sizes and very little grains in the smaller phi sizes. As one moves further west the grains are more evenly distributed throughout the phi sizes although still the majority are in the larger phi sizes. (See attached spread sheet and graph).
There is also a pattern that is noticeable in the smaller phi sizes. As one moves from location 1 to location 4 (east to west) there is an increase in the percent of grains contained in the smaller phi size category (See attached spread sheet and graph). One would think that the locations closer to the inlet would have a small percentage of grains in the small phi size category because of the increased current. However, this did not occur in this study. One possible reason for this could be that the locations were still too far from the inlet to be greatly effected by the currents.
One might think that the large jetty built at Democrat Point would have an effect on the distribution of sediment and grain sizes. In this study there is very little evidence that the jetty had much effect at all on the distribution of grain sizes. There is an overall general pattern that as one moves from location 1 to location 4, east to west and closer to the inlet that the grain sizes become more evenly distributed throughout the phi sizes.
The jetty does not seem to have any significant effect on this general pattern. The jetty was originally built to keep sediment out of the inlet and prevent the filling in of the inlet. It seems to be succeeding in some degree in this area. The beach on the east side of the jetty (away from the inlet) is significantly larger than the beach on the west side of the jetty. On the east side of the jetty the beach extends approximately 10-15 meters further seaward.
References
Militello, A. and Hughes, S.A. (2000). “Circulation Patterns at Tidal Inlets
with Jetties,” ERDC/CHL
CETN-IV-29, U.S. Army Engineer Research and Development Center. Vicksburg,
MS.
Rampino, R. and Sanders, JE. (1981). Evolution of the Barrier Islands of
Southern Long Island, New York.
Sedimentology 28. pp 37-47.
“The most widely held hypothesis is that of ‘shoreface retreat’. This view states that as sea level rises, the barriers migrate continuously landward, mainly through the combined effects of shoreface erosion and washover on the landward sides of the barriers. During the migration, the breaker zone traverses the entire area submerged... The contrasting view, that of in place ‘drowning’, states that as sea level rises, the barrier may remain in place, while the lagoon on it’s landward side deepens and widens.
Eventually, the breaker zone reaches the level of the top of the barrier, the sea drowns the barrier, and the breaker zone oversteps landward to form a new barrier shoreline along the landward edge of the former lagoon” (Rampino and Sanders, 1981).
The barrier beaches of Long Island are often undergoing analysis to understand how much they are eroding and at what rate. It is a difficult task because often times the sand may not be gone from the barrier simply moved to another are of the island. Currents and long shore transport move sediment along the beach and in and out of inlets.
“Circulation patterns are specific to each inlet, but certain properties are common to many inlets. Common properties include ebb or flood dominance, preferred channel on ebb and flood tide, eddy formation and migration, and jetty control on flow patterns” (Militello and Hughes, 2000). At Democrat
Point, near the Fire Island inlet, a large jetty was constructed to prevent sediment being transported into the inlet and filling in the inlet. Since the predominant sediment transport direction is from east to west the east side of the jetty has filled in with sediment.
There is also a jetty across the inlet from Democrat Point. The two jetties are not parallel and can thus be classified as offset jetties. In this situation the current is affected and becomes strongest on the sides with the shorter jetty and water enters the inlet at an angle to the inlet center line Militello and Hughes, 2000). Since the tidal currents combined with longshore transport effect sediment distribution one would think that the jetties effect on the currents would result in unexpected sedimentation patterns.
This study attempts to analyze the sedimentation patterns around the jetty at Democrat Point through grain size analysis. As currents slow down the larger, heavier grains will fall out of solution first. Smaller, lighter grains are still able to be transported by a slow moving current. Since the jetty effects the currents around the inlet a grain size analysis should allow one to determine the jetties effect on sedimentation processes.
Results
Phi size is a useful measurement when doing grain size analysis. The larger phi sizes correlate with smaller grains; therefore a phi size measurement of
4 is a smaller grain than a phi size measurement of 0. Sorting by phi size also allows one to separate the grains out and measure the percentage of the sample within each phi size. By dividing the weight of the sediment in a particular phi size by the total weight of the sediment in the sample one can determine the percentage of the sample in that grain size. Using the percent composition of each phi size is useful because it allows us to see what size most of the grains are. If a higher percentage of the grains are large than we know we’re looking at a coarse sediment where as if the majority of the grains are smaller than we have a finer sediment. Whether the sediment is coarse or fine allows one to determine the type of depositional environment.
In conclusion, we can see that all of the samples are dominated by grains in the larger phi sizes. However, it is worth noting that in the sample furthest east we saw dominance in the top two phi sizes and very little grains in the smaller phi sizes. As one moves further west the grains are more evenly distributed throughout the phi sizes although still the majority are in the larger phi sizes. (See attached spread sheet and graph).
There is also a pattern that is noticeable in the smaller phi sizes. As one moves from location 1 to location 4 (east to west) there is an increase in the percent of grains contained in the smaller phi size category (See attached spread sheet and graph). One would think that the locations closer to the inlet would have a small percentage of grains in the small phi size category because of the increased current. However, this did not occur in this study. One possible reason for this could be that the locations were still too far from the inlet to be greatly effected by the currents.
One might think that the large jetty built at Democrat Point would have an effect on the distribution of sediment and grain sizes. In this study there is very little evidence that the jetty had much effect at all on the distribution of grain sizes. There is an overall general pattern that as one moves from location 1 to location 4, east to west and closer to the inlet that the grain sizes become more evenly distributed throughout the phi sizes.
The jetty does not seem to have any significant effect on this general pattern. The jetty was originally built to keep sediment out of the inlet and prevent the filling in of the inlet. It seems to be succeeding in some degree in this area. The beach on the east side of the jetty (away from the inlet) is significantly larger than the beach on the west side of the jetty. On the east side of the jetty the beach extends approximately 10-15 meters further seaward.
References
Militello, A. and Hughes, S.A. (2000). “Circulation Patterns at Tidal Inlets
with Jetties,” ERDC/CHL
CETN-IV-29, U.S. Army Engineer Research and Development Center. Vicksburg,
MS.
Rampino, R. and Sanders, JE. (1981). Evolution of the Barrier Islands of
Southern Long Island, New York.
Sedimentology 28. pp 37-47.
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