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Geology & Morphodynamics of the Santa Monica Bay: The Fight Against Erosion (Page 1 of 3) Geology & Morphodynamics of the Santa Monica Bay: The Fight Against Erosion With the hope of clarifying some issues surrounding the loss of beaches along the Santa Monica Bay, this chapter introduces some basic geology of the Los Angeles basin, factors which have contributed to starving the local beaches of sand, the morphodynamics of the El Segundo area and some typical engineering structures created to combat coastal erosion [See Figure 2.1]. Most sand that is found on the beaches of Southern California has traveled from inland mountains down streams and rivers which then run into the sea (Miller, 1964). The erosion problems along the beaches of the Southern California Bight can be directly attributed to the damming of many Southern California rivers which cut off the sand supply to the beaches. Without a constant source of sand input into the system, the beaches began to erode as the steady littoral drift moved sand south and sometimes out to sea. Concurrent with the damming of many of the rivers that drain into the Santa Monica Bay, development pressures steadily increased and construction directly adjacent to the coastal beaches was the norm. As the beaches thinned, numerous coastal engineering structures were erected with the hope of preventing the immanent erosion. The El Segundo Groin, discussed above, started as a classic case of erosion protection along the Santa Monica Bay. Examination of the physical setting, some efforts to battle erosion along the Santa Monica Bay, and discussion of some coastal processes will help to better interpret coastal management decisions along the Santa Monica Bay. |
In geologic terms the western margin of the United States is an extremely active zone. The geology of the coast of Southern California has been dominated by evolution of the San Andreas transform zone as the boundary between the Pacific and North American plates. The Santa Monica Bay area and the Los Angeles basin, immediately east of the shoreline, are located at the juncture of three primary physiographic provinces of coastal southern California: The Peninsular Ranges, the Transverse Ranges, and the Continental Borderland [See Figure 2.1] .
The north and east sides of the basin are bordered by the Transverse Ranges which include the Santa Monica and San Gabriel Mountains. These ranges are separated from the basin by the Santa Monica Fault to the north and the Whittier Fault to the northeast. A distinctive feature of the northern border of Santa Monica Bay are the Pleistocene alluvium cliffs of Malibu that lead northward to Point Dume. To the south, the Peninsular Ranges extend southeastward from southern California down the length of the Baja Peninsula (Biddle, 1991). A notable feature at the southern end of Santa Monica Bay is the prominent Palos Verdes Peninsula which is an uplifted marine terrace (Sharp, 1978). The Continental Boarderlands are characterized by northwest-trending basins and ridges which formed as a result of differential vertical displacements and rotation of local tectonic blocks. Many of these basin and ridge features are submerged off the coast (Biddle, 1991). One such example is the submerged Santa Monica basin which is 3000 ft deep, 20 miles wide and extends 45 miles from Palos Verdes Peninsula to Anacapa Island. At southern end of the Santa Monica Bay lies the Redondo Submarine Canyon, which provides a swift path for sand to drift offshore and into the Santa Monica Basin (Sharp, 1978)[See Figure 2.2]. 
One of the most notable coastal features in southern California are the Channel Islands. These islands are a relic of the differential vertical displacements in the Continental Boarderlands. Santa Cruz and Anacapa Islands were once a westward extension of the Santa Monica Mountains, called the Cabrillo Peninsula (Sharp, 1978). The Channel Islands play an important role in the coastal processes that effect Santa Monica Bay because of their wave shadowing effects. Many rivers of varying size flow through this basin and drain to the Santa Monica Bay carrying sand vital to the health of the beaches. Many of the important sand supplying streams of Southern California are affected by dams built in the late 1930's and early 1940's. In some regions up to 90 percent of the drainage basin has been blocked by dams (Miller, 1964). Not only has the sand transporting ability of rivers been affected by dams, many rivers in Southern California have been routed into concrete-lined channels to minimize the risk of storm flooding. These channels often begin with a flood control dams or other sediment trapping structures and also prevent natural erosion of the stream channel, thereby eliminating much of the sand supply that would reach the beach under natural conditions (Miller, 1964). The only other sources of sand for the region are sand flowing around Point Dume and local bluff erosion, both of which are relatively small contributors (Leidersorf et al, 1993). It is interesting that concurrent with this reduction of sand reaching the beach, sand dredged from the construction of harbors and marinas was placed on the beaches at approximately the same rate the sand that would have been supplied naturally by the streams (Inman & Brush, 1973). This temporary balance created a false sense of security during a time of intense development after World War II. The beach widths appeared static as the dredged material compensated for the loss of sediment supply from the rivers, resulting in some poor development plans along the coast. This misjudgment has resulted in a long and continued battle against a slowly diminishing supply of sand along the California coast. |
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