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Because of the dynamic nature of all beaches, accurate shoreline measurements are necessary. By comparing measurements taken at different times, a beach's stability is determined.
Shoreline change data has many uses. In South Carolina, this information is needed to set construction setback lines and help engineers design beach nourishment projects. Officials use it to gauge the success of beach nourishment efforts, the impact of shore stabilization projects, and the imprints left by a storm. |
| A Washout Along the South Carolina Coast Image Provided by Charleston District Construction Division |
LIDAR, or laser beach mapping data, is technology that offers an exciting new way to efficiently document and measure shoreline change. In this section you will find information about LIDAR, an example of how LIDAR is used, and LIDAR data sets for many beaches in South Carolina.
Tourism is an important part of South Carolina's economy. Thousands of people visit South Carolina's coast each year. During the summer months, the population of Horry County swells to 10 times its winter population.
Unfortunately, many beaches are currently threatened with the persistent
problem of land loss.
Population growth and development of the coast, especially during the
last century, have
created a situation in which beach erosion can have severe economic
consequences. Estimates
reveal that approximately $3 trillion of U.S. coastal development is
potentially vulnerable
to erosion. It is also estimated that 70 percent of the world's beaches
are undergoing
erosion with percentages approaching 90 percent along the Atlantic
coastal plain.
There are two general categories of erosion control on sand beaches. The first type is hard stabilization or armoring where structures such as seawalls, groins, revetments, and off-shore breakwaters are built to help protect development along shorelines. However, these structures can be destructive to recreational beaches. South Carolina adopted state legislation, the 1988 Beachfront Management Act, preventing further armorment of the beaches. Currently, beach nourishment is a popular option for maintaining recreational beaches.
LIDAR data sets can aid coastal resource managers in understanding how, when, and where erosion and accretion are occurring along the shoreline. Below is a case study of a renourishment project being conducted in North Myrtle Beach.
Beach nourishment, renourishment, and replenishment are interchangeable terms for the process of placing sand on an eroding shore in order to restore and/or maintain recreational beaches. More than 200 beaches in the U.S. have undergone some renourishment effort. At least nine beaches in South Carolina have been replenished at some point in their history, with the earliest project in the state occurring in 1954 on Edisto Island.
The Grand Strand of Myrtle Beach is one of the beaches in South Carolina with a history of renourishment projects. A major nourishment effort was undertaken during the mid-1980s covering an 8.6-mile stretch of beach. At the time, the project was the second largest nourishment ever performed in the U.S. using an inland source of sand. Over 853,000 cubic yards of sand were hauled 24 hours a day, seven days per week. It took 59,539 truckloads of sand and the cost totaled $4.7 million. In 1989, Hurricane Hugo took a toll on the Grand Strand. The following spring more sand was added to the beach in an emergency nourishment effort.
In 1996, a second major nourishment project for the North Myrtle Beach area began in September and was completed in the fall of 1998. Several offshore sandbars served as the sediment sources and sand was hydraulically pumped onshore where it was spread with bulldozers. Total project length was about 25 miles and the cost was $54 million.
LIDAR surveys were flown over South Carolina's beaches in October 1996 and again in October 1997. This afforded a unique opportunity to view a beach renourishment process from a remote sensing perspective.
LIDAR data can be viewed in a geographic information system (GIS) as a point, grid, or contour data set. Viewing LIDAR beach elevation data in its native point or x, y, z format allows a user to understand the spatial dispersion of the data horizontally as well as vertically; but because each individual point is a discrete geographic entity, the point display limits its continuous nature.
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| LIDAR Data Viewed in ArcView Project | LIDAR Data Viewed as Profile | LIDAR Data Viewed as Elevation Maps |
A more aesthetically pleasing way of displaying elevation data is in a grid format. In this format, each cell is assigned a value based on surrounding elevation points. This gives the data an even and continuous distribution, resulting in a smooth surface when displayed. The grid or cell-based format also provide the best platform for analysis. Grids can be compared to show spatial, temporal, and volumetric change.
Once the LIDAR data are in a grid, other products such as a Digital Elevation Model (DEM) or a contour map can be created. Displaying the data as a DEM allows for a high resolution, color enhanced display of the LIDAR data. Another way of representing LIDAR data are as contour lines. Contours are isolines of elevation with a specified interval. Contours provide an effective way of showing gradient change. The closer the isolines are to each other, the steeper the elevation or beachface.
Different applications require LIDAR data to be displayed in a particular form. For example, grid data can provide detailed information about volumetric and surface changes on a beach. This is useful in understanding sediment movement after a renourishment project. The image below shows the 1996 and 1997 LIDAR data for North Myrtle Beach displayed as grid data.
The study area represented above is a small section of the North Myrtle Beach renourishment project that took place from September 1996 through May 1997. In the 1996 LIDAR elevation grid, the boundary where the renourishment project ended can be seen. However, in the 1997 LIDAR elevation grid, this line is not apparent. To determine what most likely happened to this beach between the sample dates during which the LIDAR data were collected, it is helpful to examine a difference grid.
Using a GIS software package, a difference grid can be created by subtracting two grids from one another. The difference grid displayed below was produced by subtracting the 1996 LIDAR data from the 1997 LIDAR data. The difference grid below shows the changes in beach elevations between October 1996 and October 1997, when the LIDAR data were collected.
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| 1996 to 1997 LIDAR Elevation Map Displayed as a Difference Grid Data Set for North Myrtle Beach, South Carolina |
The 1996 to 1997 difference grid shows that the region nourished in 1996 experienced a loss of sand. After a nourishment project, losses such as the one depicted above are to be expected as the beach profile adjusts to equilibrium. Typically, the nourishment design includes extra sand or "overfill" to account for sand movement. In this case, the long shore current and wave action redistributed sand southward. This gain in elevation is indicated by the dark green values.
Volumetric change can also be calculated by subtracting two grid data sets from one another.
Unfortunately, because of the large size of the grid files, grid data could only be provided as portable document file (PDF) maps on this CD-ROM. If you have Adobe Acrobat Reader® installed on your computer, from the root directory on this CD-ROM navigate to pdf/islands and click on a .pdf file. For users who do not have Adobe Acrobat Reader software, it has been provided on this CD-ROM. For more information click here.
For more information about creating your own grid data sets, click here.
The NOAA Coastal Services Center's technical report An Evaluation of Hurricane-induced Erosion along the North Carolina Coast Using Airborne LIDAR Surveys is a technical report that details the results of two airborne light detection and ranging (LIDAR) beach surveys of the North Carolina coast after Hurricane Bonnie. The full report can be found here.
"An Evaluation of Hurricane-induced Erosion along the North Carolina Coast Using Airborne LIDAR Surveys" is a technical report that details the results of two airborne light detection and ranging (LIDAR) beach surveys of the North Carolina coast after Hurricane Bonnie. The baseline survey was conducted over North Carolina in fall of 1997, and a second survey was conducted in fall of 1998 within days of Bonnie’s landfall. The very high density and accuracy of elevation measurements allows regional-scale beach volume calculations at an accuracy unavailable using traditional beach profiling survey methods. Geographic information system software is used to determine volumetric change of the dry beach for all North Carolina barrier beaches. From volumetric change calculations, the volume of sediment gain or loss by unit area and unit length of the beach are determined for each beach.
The northern barrier island beaches show greater average sediment loss over the length of the beach than the beaches in the middle and southern sections of the coast. The northern beaches generally show long erosional sections of beaches and dunes with smaller pockets of accretion. Overwash, when observed, is minor. Beaches in the central coast show a different response: alternating patterns of erosion and accretion with relatively little net sediment volume change. The areas of erosion are less severe than on northern beaches. More overwash is apparent, but generally it is limited, with the exception west of Cape Lookout where sediment was deposited on the barrier flats. The southern beaches exhibited a mixture of sediment gain and loss. Much of the gain can be attributed to a major beach renourishment project. Those beaches not renourished, however, have higher average losses of sediment over the length of the beach than the central section of coast. Complete islands and large sections of beaches were completely overwashed, significantly changing the beach morphology, but having a low impact on the total sediment volume.
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