COASTAL EROSION Many people dream of building their home on the California Coastline with its picturesque vistas of the setting sun and the gentle sounds of waves breaking. However, these waves are not always so gentle! Coastal erosion is a significant problem for many coastal communities. In this exercise, we ll: 1. Measure how quickly waves can erode the coastline 2. Discover factors that accelerate coastal erosion 3. Discuss methods for protecting coastal communities from rapid erosion 1 Getting Oriented The image at right shows a typical view of the California coastline in the city of Pacifica. Like much of the California coast, Pacifica lies on an elevated terrace and there is a small cliff separating the community from the ocean and beach below. If waves erode the cliff, the houses can be endangered. We will use historical air photos from 1946-2000 to map the coastline to see how its shape changes with time. The Pacific Ocean shows up as a grey patch on the left. As waves break, they appear white in both the photograph and the air photo. The beach shows up as a dark grey, but is not uniform in color some parts are darker and some parts are lighter. We will map the top edge of the cliff on the air photos. It shows up as a change in color/brightness and sometimes has a dark shadow below it (to the left). Sometimes, it can be difficult to determine the precise position of the cliff top from air photos. Figure 1: Cliffs 1
2 Instructions (A) Examine each of the historical air photos attached to this handout. Think about which features have changed? Which are the same? (B) Using a colored pencil, draw a line along the top edge of the coastline cliff on each of the air photos. This will make it easier to see the shape of the cliff through the tracing paper. (C) Place the tracing paper over the oldest air photo and do not move it. (D) Trace the border of the air photo and the shape of the highway and a few roads onto the tracing paper for reference so that you can align all the future air photos. (E) Trace the top of the cliff edge from the oldest photo. (F) Move the tracing paper onto each of the other air photos. Line up the highway as best you can. Note that new lanes have been added to the highway and they don t line up perfectly. (G) Trace the top of the cliff edge in a different color for each photo. (H) Notice where erosion has been the most severe and where it has not. Figure 2: The view from the beach, 1998 http://www.northcoastjournal.com/120398/cover1203.html Figure 3: Houses on the edge, 1998 The U.S. Geological Survey and NASA use a sophisticated laser mapping system to determine the elevation of points on the Earth s surface. They fly a small airplane over an area and collect data (See Figure 4). They captured beach erosion in action during the El Niño winter of 1997-1998. In order to compare our historical air photo data with the NASA Laser data, we ll have to make some measurements. 2
(I) Locate Profiles A and B in the image shown in the inset on the left side of Figure 5. Sketch the profile onto your tracing paper, using the houses and roads to help determine where it is located. (J) Line up the edge of the scale bar so that the 1946 cliff line on your tracing paper is at zero and the scale forms a 90 degree angle to the coastline. Record the position of each of the other cliff lines on the table below. Year Profile A Profile B Erosion (m) Erosion (m) 1946 0 0 1973 1983 2000 (K) Plot these values on graph paper. Let the x-axis be year and the y-axis be erosion. Draw points for each profile and connect them with a line. Use a different color for each profile. 3 Questions (1) Calculate the average annual erosion rate observed for Profiles A and B for each of the time spans (1946-1973; 1973-1983; 1983-2000). The annual erosion rate is equal to the total erosion during that time period divided by the number of years. (2) Calculate the average annual erosion rate for the entire time period 1946-2000. (3) Your aunt Mathilda is considering buying a new house in Pacifica and she heard that you are taking a geology class this semester. The realtor showed her the white house across the street from the cliff in Figure 7, and she wants to know if it is safe from coastal erosion. Assuming that erosion will continue at the long-term average rate you calculated above, in what year will the top of the cliff erode across the street and threaten the house? Should Aunt Mathilda buy it? (4) Looking at your graph and the calculations from problem 1, is the erosion rate constant or does it change much? Why might this be? Figure 4: Collecting LIDAR Data http://coastal.er.usgs.gov/lidar/agu fall98/ 3
Figure 5: LIDAR data in Pacifica http://coastal.er.usgs.gov/lidar/agu fall98/ (5) According to the LIDAR data, how much did the top edge of the cliff erode in each Profile? How does this compare to your own observations that span the similar time interval? (6) Examine the photo in Figure 6, taken just after the storms in 1998. Other than the grey boulders, what do you see at the base of the cliffs? Can you explain these features? How did they get there? (7) The two photos in Figure 7 show the area immediately north of the trailer park. Both were taken after the storm, but the larger image was taken after engineers performed several modifications to the cliff face using bulldozers and backhoes. Describe the differences between the cliff face in the two photos. (8) Explain why you think these changes were made. (9) Was the area shown in these pictures a zone of high erosion or low erosion? How about the area immediately to the south (near the trailer park)? Even though these two locations are right next to one another, the erosion rate is quite different. The large picture below gives two hints about why there might be such a difference. 4
Figure 6: Just after the storms, February 1998 http://svs.gsfc.nasa.gov/vis/a000000/a000600/a000645/ Figure 7: View immediately after the storms (Inset in Upper Right; February 1998 http://svs.gsfc.nasa.gov/vis/a000000/a000600/a000645/) and several years later (Bottom; September 2002; http://www.californiacoastline.org/cgi-bin/image.cgi?image=5781&mode=sequential&flags=0) 5
Figure 8: 1946 29-July; Jack Ammann Photogrammetric Engineers/ U.S. Geological Survey, U. C. Berkeley Air Photo Collection GS-CP 3-2) 6
Figure 9: 1973 July 1973 (Reviewed 1977), Real Estate Data, Inc., U. C. Berkeley Air Photo Collection G1528.S28R4 1977 Sheet 3C - Case D) 7
Figure 10: 1983 6-June; Pacific Aerial Surveys, U. C. Berkeley Air Photo Collection AV-2265-02-17) 8
Figure 11: 2000 21-March; WAC Corporation Inc., U. C. Berkeley Air Photo Collection WAC-C-00-CA 3-37) 9