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Aerial oblique photograph of the eastern end of the Isles Dernieres barrier island chain as of June 1986. (Photograph by S. Jeffress Williams.)
net offshore sediment transport, and gradients of sediment transport along the length of the shoreline. A series of experiments and modeling efforts are being undertaken; for example, direct measurements of the waves that wash over a barrier island during storms. • Assembling the research results in a form that can be used by coastal planners and engineers. Applications of the study results include developing better techniques for determining the rate at which artificially nourished beaches should be renourished and predicting future shoreline erosion so that coastal planners can properly locate new construction at a safe distance landward from the eroding shoreline.
The barrier islands known as Isles Dernieres (fig. 1) were studied in three surveys, done in 1890–1906, the 1930's, and 1986. Results of these surveys show considerable changes in the islands and nearby seafloor.
Bathymetry contours from each survey are shown in figure 2. At the turn of the century, the Isles Dernieres was a nearly continuous island. By the 1930's, the islands had become narrower and more inlets had been incised, dividing the island into four segments. By 1986, the islands had become fairly narrow, less than 0.6 mile at most locations, and the previously opened inlets had widened significantly. Historically, places along the Gulf of Mexico margin of the islands have eroded more than 65 feet per year.
Figure 3 shows changes in bathymetry over time. For the map comparing the 1890's to 1930's bathymetry (fig. 3, top), the elongated blue pattern represents erosion on the Gulf margin of the barriers as the barriers migrate landward. Some of the erosion is in excess of 8 feet. The blue pattern, adjacent to the purple/red pattern and in the vicinity of the major inlets, represents longshore migration of the inlet of several miles.
Between the 1930's and 1986, there is a similar pattern of erosion on the Gulf face of the barrier, associated with shoreface retreat, and further evidence for the continued longshore migration of inlets. An unexpected aspect of this comparison is the development of a broad elongate (6.5 feet thick, 12 miles long) accretional body, composed mostly of sand, seaward of the mouth of the inlets forming the entrance to Terrebonne Bay (between Isles Dernieres to the west and Timbalier Island to the east). This sand body has built from east to west over a 50-year period in water depths of 16 feet and distances offshore of a little over 2 miles. The sand body is the result of sand bypassing the inlet, and, as its westward growth continues, it could eventually provide a source of new sand to the severely eroding Isles Dernieres. This unexpected result suggests that the sediment budget for these islands cannot simply be viewed in the classic twodimensional sense of shoreline and shoreface response to rising sea level but must also include the effects of changes in the longshore movement of sand along the coast.
Proper management and protection of our Nation's coastal resources require knowledge of the patterns and rates of sediment movement, like that shown in
Proper management and protection of our Nation's coastal resources require knowledge of the patterns and rates of sediment
figures 2 and 3. For example, in order to determine the cost effectiveness of a beach nourishment project one needs to know how often to return and renourish the beach. Knowledge of the rates and patterns of movement of sediments will help determine the constraints of beach nourishment and other types of projects.
The results discussed here represent only one part of the overall study effort. By better understanding the geologic framework within which erosion occurs and improving our understanding of the processes of erosion, we hope to be able to improve our ability to forecast future erosion not only in Louisiana but also in other threatened coastal areas.
Figure 3. Amounts of vertical change between historical bathymetric surveys — 1890's-1934 (top) and 1934-1986 (bottom). Individual colors represent a 1.6-foot (0.5meter) interval of vertical change in bathymetry. The sign indicates erosion ( - ) or accretion (+). For example, green indicates erosion between 2.5 feet (0.75 meter) and 4.1 feet (1.25 meters), whereas red indicates accretion between 2.5 feet and 4.1 feet.
Hawaiian Submarine Lava Flows
By Robin T. Holcomb
A sonograph mosaic (facing page) of about 4,700 square nautical miles (16,000 km2) of the ocean floor 80 nautical miles (150 km) north of Oahu, Hawaii, shows a part of the newly recognized North Hawaiian Arch Volcanic Field. The image was produced from data gathered by the GLORIA long-range sidescan
By Wayne Thatcher
nautical miles (150,000 km2), much larger than the Hawaiian Islands. The thin tongues of lava have flowed southwest down the inner flank of the Hawaiian Arch toward the Hawaiian Trough, a depression created by downbowing of the crust under the weight of the Hawaiian volcanoes. Lava flows of the volcanic field are interbedded with deposits of landslides from the islands, indicating that the field has been intermittently active during the last few million years. Some flows that have very little sediment cover may be as young as a few thousand years.
These and other large submarine lava flows discovered recently east and south of Hawaii and elsewhere at low latitudes, for example, are thought by USGS scientists to rapidly deliver large amounts of heat into the ocean, possibly affecting weather patterns on a global scale. The weather phenomenon called El Niño occurs where the normal pattern of easterly driven wind and ocean circulation is altered to a westerly pattern. The cause of this switch is not clearly understood. The heating effect of the submarine lava flows, however, may be sufficient to warm the ocean currents, upsetting the balance between temperature and atmosphere and activating the reversal in the direction of motion, thereby triggering El Niño.
In the 175-year period from 1812 to 1987, California experienced at least 11 large earthquakes of about magnitude 7 or greater. Two of these events were the great earthquakes of 1857 and 1906, in which the San Andreas fault ruptured, affecting the areas of Los Angeles and San Francisco, respectively. Similar earthquakes will certainly occur in the future and can be expected to have significant impact on California and the Nation. The Federal Emergency Management Agency has estimated that property losses resulting from a repeat of the 1857 earthquake would be $17 billion, with estimated deaths of 3,000 to 14,000, depending upon the time of day of the earthquake. The 1857 earthquake (estimated magnitude greater than 8.0) occurred on the San Andreas fault, the nearest point of which is about 31 miles (50 km) northeast of the densely populated area of Los Angeles; a smaller earthquake, in the magnitude range of 7 to 7.5, occurring within an urban area may be expected to cause losses comparable to or greater than those for a repeat of the great 1857 earthquake.
Because of the increased public interest in and concern about expected losses from future earthquakes in California, the National Earthquake Prediction Council, a body of academic and government scientists advisory to the Director of the U.S. Geological Survey, recommended an evaluation of the probability of occurrence of large (magnitude 7 or greater) earthquakes in California. In response to this recommendation, the USGS formed a working group on California earthquake probabilities, which met several times in 1987–88 to review and assess the state of knowledge that would allow calculation of earthquake probabilities on specific segments of the San Andreas fault (fig. 4).
The probabilities were based on a model for time-dependent increase of earthquake probability. The model has its