CONTINUING EROSION IN SOUTHEASTERN COASTAL...
14
1 Summary of a Poster Presented at the: Sixty-Second Annual Meeting of the Mississippi Academy of Sciences in Biloxi, Mississippi February 26-27, 1998 =============================================================== CONTINUING EROSION IN SOUTHEASTERN COASTAL MISSISSIPPI - POINT AUX CHENES BAY, WEST GRAND BAY, MIDDLE BAY, GRANDE BATTURE ISLANDS: 1995-1997 Warren T. O’Sullivan G. Alan Criss Cooperative Intern Program between the Mississippi Gulf Coast Community College- Jackson County Campus Honors Biology Students and the USM Institute of Marine Sciences-Gulf Coast Research Laboratory INTRODUCTION This study was conducted to determine rates of erosion in the Point aux Chenes Bay area, located in the extreme southeastern coastal portion of the state of Mississippi and bordering Alabama (Figure 1). The study area contains one of the few remaining extensive stands of coastal marshgrass environments in the state. The Thematic Mapper Simulator (TMS) image in Figure 2 shows the coverage of this marshgrass environment in the study area. Figure 2 is a digital image which was taken on November 21, 1988 from a high-altitude aircraft using an instrument which simulates the spatial and spectral resolution of the Thematic Mapper on the Landsat-5 satellite (pixel resolution 30 m). The near-infrared band (.76-.90:m) was processed to produce this pseudocolor image in which the locations of Spartina and Juncus marshgrasses are correlated with red shades. Green shades indicate flooded marshgrass locations. The vertical stripe near the center of the image is a few lines of bad digital data, not a physical feature. Since this study began most of the study area has been purchased and designated as part of Grand Bay National Wildlife Refuge (Figure 3). Previous studies have documented the progressive nature of erosion in the study area. The shoreline silhouettes in Figures 4a and 4b were produced in a previous study
Transcript of CONTINUING EROSION IN SOUTHEASTERN COASTAL...
1
Summary of a Poster Presented at the:
Sixty-Second Annual Meeting of the
Mississippi Academy of Sciences
in Biloxi, Mississippi
February 26-27, 1998
===============================================================
CONTINUING EROSION IN SOUTHEASTERN COASTAL MISSISSIPPI -POINT AUX CHENES BAY, WEST GRAND BAY, MIDDLE BAY, GRANDE
BATTURE ISLANDS: 1995-1997
Warren T. O’SullivanG. Alan Criss
Cooperative Intern Program between the Mississippi Gulf Coast Community College-Jackson County Campus Honors Biology Students and the USM Institute of Marine
Sciences-Gulf Coast Research Laboratory
INTRODUCTION
This study was conducted to determine rates of erosion in the Point aux Chenes Bayarea, located in the extreme southeastern coastal portion of the state of Mississippi andbordering Alabama (Figure 1). The study area contains one of the few remainingextensive stands of coastal marshgrass environments in the state. The Thematic MapperSimulator (TMS) image in Figure 2 shows the coverage of this marshgrass environmentin the study area. Figure 2 is a digital image which was taken on November 21, 1988from a high-altitude aircraft using an instrument which simulates the spatial and spectralresolution of the Thematic Mapper on the Landsat-5 satellite (pixel resolution 30 m). Thenear-infrared band (.76-.90:m) was processed to produce this pseudocolor image inwhich the locations of Spartina and Juncus marshgrasses are correlated with red shades. Green shades indicate flooded marshgrass locations. The vertical stripe near the center ofthe image is a few lines of bad digital data, not a physical feature.
Since this study began most of the study area has been purchased and designated aspart of Grand Bay National Wildlife Refuge (Figure 3).
Previous studies have documented the progressive nature of erosion in the study area. The shoreline silhouettes in Figures 4a and 4b were produced in a previous study
2
(Eleuterius and Criss, 1991) using historical charts, aircraft photos, and satellite images todocument the historically dramatic erosion in the study area. Otvos (1990) concluded thata hurricane during the period of 1740-1766 bisected a long offshore barrier island intopresent day Dauphin and Petit Bois Islands. This allowed uninterrupted waves from theGulf of Mexico to impact the area around Point aux Chenes Bay with the accompanyingerosion. Note especially the destruction of the Grande Batture Islands.
METHODS
After noting the historical erosion in the study area, a series of studies was undertakento determine the linear loss of shoreline. In July 1992 reference markers were placed at20 station locations (Figure 5) along the shoreline perimeter of Point aux Chenes Bay, theremnants of the Grande Batture Islands, west Grand Bay, and Middle Bay. White PVCplastic pipes three inches in diameter were placed at the shoreline drop-off at each stationlocation. Another marker was placed 20 feet inland approximately perpendicular to theimmediate shoreline trend. Loran-C latitude-longitude were recorded for each marker tofacilitate relocation. (Additionally, reference markers had been installed earlier inFebruary 1990 at the position of station number 8.)
The stations were revisited in October 1995 to measure the change of the distance ofthe reference markers from the shoreline location of that date. These measurementsindicated the linear erosion over a period of three years and three months. This data wasreported by Wacker and Criss (1996). New reference markers were installed a distanceinland of markers remaining onshore at each station location.
In the present component of this series of studies reported here, the stations were againrevisited in November 1997 to measure the change of the distance of the referencemarkers from the shoreline location of that date (Figure 6: Note the original shorelinemarker in the water). These measurements indicated the linear erosion over a period oftwo years and one month. New reference markers were installed at the shoreline and adistance inland if no markers were found still on-shore. The distance each new referencemarker was located inland was based on the linear loss of shore at each station. Thedistance from shore data was recorded along with the GPS latitude-longitude of eachreference marker.
DISCUSSION
Two scenarios seem to be basically responsible for the processes of erosion in thecoastal area. At low water levels in relation to the beach scarp, the breaking or surging ofwaves against the marsh substrate for a sustained period of time results in undermining ofthe marsh substrate.
3
When the concavity is sufficiently deep, the weight of the unsupported marsh andsubstrate above falls away in large clumps (Figure 7) which are rapidly broken down bywave action.
When the water elevation is high relative to the beach, the breaking waves plunge ontop of the marsh substrate. This impact can break away large clumps undermined by theprevious scenario. Another important aspect of this scenario is that the sediment ladenwaters directed inland after impacting the marsh substrate may literally cut away themarsh vegetation leaving only stubble for 10-15 feet inland (Figure 8), beyond which afan of sediment is laid down (Figure 9). In some instances the first few inches of themarsh substrate may be removed (Figure 8). Narrow channels maybe cut well inland intothe marsh by the attack of waves following this scenario (Figure 8).
RESULTS
The results of the linear shoreline change measurements at each station during theperiod October 1995 - November 1997 are presented in Table 1(see also Figure 10). Aquestion mark indicated that no markers could be found at that station. The minimumlinear shoreline loss of 4' 4" occurred at station 16. The maximum linear shoreline loss of41' 5" occurred at station 14. The only station to gain shoreline was station 20 (4' 2"). The total linear shoreline loss at station 8, which was first measured in February 1990,was 44' 5" between 2/90-11/97 (7 years and 9 months).
4
TABLE 1: RESULTS
Station Shoreline Change 10 / 1995 - 11 / 1997
______________________________________________
1 ? 2 -26' 8" 3 -17' 3" 4 ? 5 -10' 7" 6 ? 7 -10' 7" 8 -13' 11" (44'5" since 2/90) 9 -17' 2"10 -12' 10"11 - 6' 1"12 - 5' 0"13 ?14 -41' 5"15 -26' 9"16 - 4' 4"17 -10' 2"18 -11' 4"19 - 7' 11"20 + 4' 2"
______________________________________________
Minimum loss = 4' 4" (station 16)Maximum loss = 41' 5" (station 14)Gain = 4' 2" (station 20)
5
Figure 1. Study Area
6
Figure 2. Thematic Mapper Simulator (TMS) pseudocolor image of the study area
7
Figure 3. Proposed acquisition boundary of Grand Bay National Wildlife Refuge
10 Figure 5. Station Locations
Figure 6 Figure 7
Figure 8 Figure 9 11
12
Figure 10
13
References
Criss, G. A. 1990. Evaluation of Airborne Thematic Mapper Simulator (TMS) Digital Data andHigh Altitude Aircraft Infrared Photographs for Assessment of Conditions in the Vicinityof Pt. aux Chenes Bay, Mississippi. Journal of the Mississippi Academy of Sciences35(Supplement): 73-74.
Eleuterius, C. K. 1979. Hydrology of Mississippi Sound North of Petit Bois Pass. MississippiMarine Resources Council. 57 pages.
Eleuterius, C. K. and G. A. Criss. 1991. Point aux Chenes: Past, Present, and Future Perspectiveof Erosion. Final Report, December 1991. Mississippi Department of Wildlife, Fisheries,and Parks, Coastal Division / Bureau of Marine Resources. 45 pages.
Eleuterius, C. K. and G. A. Criss. 1992. Coastal Erosion: Point aux Chenes, Mississippi.Proceedings: America's Sea - Keep It Shining! The Gulf of Mexico Symposium.Innisbrook, Tarpon Springs, Florida, 10-12 December 1992. Environmental ProtectionAgency's Gulf of Mexico Program. Technical Poster Session. p. 141.
Eleuterius, C. K. and G. A. Criss. 1992. Grande Batture Island / Point aux Chenes: A CoastalErosion Scenario. Journal of the Mississippi Academy of Sciences 37(1): 50.
Meyer-Arendt, K. J. and K. A. Kramer. 1991. Deterioration and Restoration of the GrandBatture Islands, Mississippi. Mississippi Geology 11(4).
Otvos, Ervin G. 1979. Barrier Island Evolution and History of Migration, North-Central GulfCoast. In: Stephen Leatherman (editor). Barrier Islands. Academic Press.
Otvos, Ervin G. 1981. Barrier Island Formation. Marine Geology 43: 238-243.
Wacker, K. P. and G. A. Criss. 1996. Erosional Rates in the Point aux Chenes Bay Area,Mississippi: 1990-1995. Journal of the Mississippi Academy of Sciences 41(1):67.
Reference this poster presentation as:
O'Sullivan, W. T. and G. A. Criss. 1998. Continuing Erosion in Southeastern Coastal Mississippi - Point aux Chenes Bay, West Grand Bay, Middle Bay, Grande BattureIslands: 1995-1997. Journal of the Mississippi Academy of Sciences 43(1): 58.
APPENDIX I
Station locations plotted on aerial imagery mosaic (Positive Systems, 4m resolution, acquired 1998, processed as false color infrared) A-1
