Topograph 98 Se 4.10 Download

[Author Metcalfe]

Manybeaches are currently threatened with land loss. Population growth and development along the coast, especially during the last several decades, 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.

Because the northern end of Wrightsville Beach has experienced significant erosion during the past several years, we will take a closer look at this area using both orthophotography and LIDAR data. Over the past century, Masons Inlet, separating Wrightsville Beach and Figure Eight Island, has been migrating south. Since 1938, the inlet has been moving at an average approximate rate of 110 feet per year. Rapid acceleration began in the early 1980s. Between the years of 1981 and 1993, the rate of southward movement of the south side of the inlet increased to about 250 feet per year. From November 1993 to November 1995, the south side of the inlet moved 650 feet to the south, representing an average annual rate between those years of 325 feet per year. Figure 4.8 shows the inlet movement from 1994 to1998.

Below is a closer look at the inlet's migration from 1994 to 1998. Over the last several years, the inlet has encroached on the resort property seen in the orthophotography below. Figure 4.9 shows the distance the inlet has moved in a four-year time span. The base image is a digital orthophoto taken in 1994. The red line overlaid on the photo is the south edge of Mason's Inlet as it stood in the spring of 1998. The red line was created by digitizing the south bank of Mason's Inlet from the 1998 orthophotography. Figure 4.10 shows the position of the inlet in 1998.

Although data sets from sources such as aerial photography can be used to measure inlet movement and shoreline change, LIDAR data can be used to accurately quantify beach erosion. To measure beach loss between two years, LIDAR-based grids can be compared. Grids are a geographic data model representing information as an array of equally sized square cells. Each grid cell has a value that corresponds to the feature or characteristic at that site, such as a soil type, vegetation class, or in this case, elevation. Values of the cell can be stored in a database file that is connected to its geographic x,y,z location. Shown below in figures 4.11, 4.12, and 4.13 are color-coded grids of northern Wrightsville Beach. The 1997 grid extends landward into development while the 1996 grid only covers the beach face. One notable difference between the grids is on the extreme northern tip of the island. The northernmost road shown on the 1996 grid is on land. On the 1997 and 1998 grids, however, it appears to be in the water.

Below are scenarios in which LIDAR data grids are used to detect and quantify beach erosion. Consecutive years of LIDAR data collected on Wrightsville Beach are put into grid format then one is subtracted from the other. The resulting grids (Figures 4.14 and 4.15) show difference, or change, between the years. Specifically, we are showing how much beach was lost on the northern end of Wrightsville Beach between the years of 1996 and 1997, and 1997 and 1998. To perform this calculation, the 1996 grid is subtracted from the 1997 grid. This results in a difference grid that quantifies sand loss from the beach face in vertical feet. The yellow, orange, and red colors indicate where significant erosion occurred. This type of analysis can be performed to show the effects of beach renourishment, major storm events, or change from year to year. In figure 4.14, the areas indicated by black show where severe erosion occurred between 1996 and 1997.

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 beaches. More than 200 beaches in the United States have undergone some renourishment effort. Beaches that have been recently nourished usually have a build-up in sand. In the spring of 1997, a beach renourishment project was initiated in Kure Beach, North Carolina. The images below show the condition of the beach face in North Kure Beach in October of 1996 (Figure 4.16), September of 1997 (Figure 4.17), and September of 1998 (Figure 4.18). There is much more sand on the beach face in 1997. This was in large part due to the renourishment project that occurred in the area.

The above section of beach can be further analyzed to show elevation and volumetric change to the beach face. Due to the beach nourishment project started in the spring of 1997, almost the entire section of beach experienced a gain or build-up in sand (Figures 4.19 and 4.20).

Figures 4.21 and 4.22 show change vertically and volumetrically on the northernmost part of Kure Beach from 1997 to 1998. Although there was a net gain in sand from September of 1997 to September of 1998, there was a considerable amount of sand loss in the intertidal zone (Figure 4.20). This type of analysis could provide an effective way to observe beach dynamics.

It is important to note that LIDAR data can only be used to measure change landward of the land-water interface. To accurately assess the performance of a beach nourishment project, the entire sand budget needs to be taken into account. This includes the beach face as well as the nearshore subtidal zone.

Grid data sets of the four islands in New Hanover County are large, requiring a substantial amount of hard drive storage space. Additionally, the user must have GIS software, such as ArcView Spatial Analyst, ARC/INFO GRID, or MapInfo Vertical Mapper that can create and analyze grid data.

However, if you are interested in acquiring the grids, contours, or point data for the islands in New Hanover County, contact the NOAA Coastal Services Center via e-mail at cleari...@csc.noaa.gov.

We present a numerical study on how tidal force and topography influence flow dynamics, transport and mixing in horizontal convection. Our results show that local energy dissipation near topography will be enhanced when the tide is sufficiently strong. Such enhancement is related to the height of the topography and increases as the tidal frequency $\omega$ decreases. The global dissipation is found to be less sensitive to the changes in $\omega$ when the latter becomes small and asymptotically approaches a constant value. We interpret the behaviour of the dissipation as a result of the competition among the dominant forces in the system. According to which mechanism prevails, the flow state of the system can be divided into three regimes, which are the buoyancy-, tide- and drag-control regimes. We show that the mixing efficiency $\eta$ for different tidal energy and topography height can be well described by a universal function $\eta \approx \eta _HC/(1+\mathcal R)$, where $\eta _HC$ is the mixing efficiency in the absence of tide and $\mathcal R$ is the ratio between tidal and available potential energy inputs. With this, one can also determine the dominant mechanism at a certain ocean region. We further derive a power law relationship connecting the mixing coefficient and the tidal Reynolds number.

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The Boston Harbor area has been sculpted by multiple glaciations during the Quaternary Period, the last 1.8 million years. The most recent Ice Age reached its maximum extent south of Cape Cod about 21,000 years ago. Glaciers retreated northward across the study area as the climate warmed, passing the present coast about 14,500 years ago (Kaye and Barghoorn, 1964). Most of the islands in the inner harbor are drumlins, oblong hills of glacial till that formed beneath the ice sheets. Numerous other drumlins, eskers, kettle lakes and moraines are found around the Boston area (Newman and others 1990) and in submerged areas of Massachusetts Bay (Oldale and others, 1994). The retreating glaciers left behind two drifts of glacial sediment in Massachusetts Bay, the older described as a compacted till of cobbles and boulders, and the younger consists of till, outwash sand, gravel, and glacial-marine mud (Knebel and Circe 1995). The glacial-marine sediment was deposited contemporaneously with ice retreat, blanketing wide areas of the coast and inner shelf in northeastern Massachusetts. Known as the "Boston Blue Clay" (Kaye and Barghoorn, 1964), this glacial-marine sediment unconformably overlies older glacial deposits and bedrock. It typically consists of well stratified sand and mud with scattered dropstones of ice-rafted material, and constitutes much of the harbor bottom (Knebel and others, 1992).

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