Mitigation Through Surf Enhancement/Chapter 2
|Mitigation Through Surf Enhancement|
An Early History of Pratte's Reef
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.
Once sand is supplied to the beaches a new physical regime is entered and the sand motion becomes dominated by wave and wind driven nearshore processes. These processes can move sand both in the longshore direction (parallel to the beach) and in the cross-shore direction (normal to the beach). On small spatial and temporal scales sand movement may appear isotropic, however when these processes are observed over larger scales patterns become apparent and net transport of sand can often be estimated.
As ocean gravity waves generated offshore approach the beach, the energy they carry is dissipated as the waves break in the surf zone as heat, turbulence of the water, and through the movement of sand. Although theoretically waves should refract (bend) as they shoal and approach the shore with there crests parallel to the shoreline, this is rarely the case in reality. Propagating waves carry momentum, as well as energy. This momentum flux is estimated via the concept of radiation stress which as described as the excess transfer of momentum due to the waves (Longuet-Higgins and Stewart, 1964). As waves strike the coast at an oblique angle, momentum is imparted along the shore (in the longshore direction). The alongshore directed momentum sets up longshore currents which play a critical role in the movement of sand. The acceleration of these currents is balanced through momentum lost to turbulence in the bore (foamy part of wave) and bottom friction. The strength of the longshore current is correlated to the size of the waves and the angle of incidence to the beach so that larger waves will create a stronger longshore flow (Guza and Thorton, 1986). It is important to remember that while it is the velocity of the current that affects longshore transport of sand, it is the gradients in transport resulting from changes in velocity that lead to deposition or erosion.
Nearshore currents can also flow in the cross-shore direction (out to sea). These currents are known as rip currents and undertow. Rip currents, long recognized as a threat to swimmers, are also forced by incident waves. Longshore variation in wave height creates an imbalance in the radiation stress alongshore. In an attempt to balance this inequality, water flows to levels of lower radiation stress creating seaward currents where wave heights are low (Bowen, 1969). In addition to rip currents, undertow can move water in the cross-shore direction. Undertow is a interior flow of water below the breaking waves. This flow is forced by an attempt at balancing the momentum carried on shore by waves (Haines and Sallenger, 1994). These cross-shore flows act to move sand offshore and are also proportional to wave height.
Upon observation, these cross-shore flows in combination with longshore currents "showed that nearshore movement of water could be described in terms of a circulation cell consisting of (1) a shoreward mass transport due to wave motion carrying water through the breaker zone in the direction of wave propagation, (2) a movement of this water parallel to the coast as a longshore current, (3) a seaward flow along a concentrated lane, known as a rip current, and (4) longshore movement of the expanding rip head" (Bowen, 1969). Circulation cells, also known a littoral cells, can occur at many different scales; from hundreds of meters to kilometers.
The geology of the Santa Monica Bay controls a large littoral cell that starts near Malibu and is forced offshore several kilometers to the south by the Palos Verdes headlands. This cell is known as the Santa Monica Cell (Inman & Brush, 1973). The long-shore mean littoral flow along the Santa Monica Bay beaches is southerly because predominant winter waves from the northwest dominate the annual longshore current average. The sand is transported southward along the coast until it is eventually directed offshore at the Palos Verdes headlands. Some of this sand is intercepted by the Redondo Submarine Canyon and is diverted out of the system and into the deep water of the Santa Monica basin (Inman and Brush, 1973) [See Figure 2.2].
Although the sand supply has been cut off by the urbanization of the Los Angeles basin and the damming of many rivers, the Santa Monica littoral cell has continued to transport sand to the south and down the Redondo Submarine Canyon. A Corps of Engineers' study (US. Army 1948) estimated an annual net littoral transport of 162,000 cubic yards in a southerly direction, based on average annual sand accretion at the Grand Avenue Groin. This imbalance between deposition and erosion has resulted in a net loss of sand along the coast and created erosion problems along most of the Santa Monica Bay (Griggs and Savoy, 1984) [See Figure 2.3].
Humans have repeatedly built shoreline structures in an attempt to control erosion and reduce the impact of waves with varying levels of success. "Every coastal structure will have an effect on the coastal zone, and it is only the magnitude of the effect that is not predictable" (Griggs and Savoy, 1984). Coastal structures have been constructed along the Santa Monica Bay in efforts to thwart coastal erosion include groins, breakwaters, and jetties. These structures, depending on their orientation, usually create deposition upcurrent (to the north along Santa Monica Bay) and erosion downcurrent ( to the south along Santa Monica Bay).
Jetties are structures built in pairs to improve or maintain a river or harbor entrance so that it is safely navigable by boats. Jetties have many problems due to the manner that they alter the littoral flow. The jetties built at Marina del Rey, the world's largest recreational harbor, have experienced many of these problems; from increased wave heights in the harbor resulting from positive interference of reflected waves off the jetties, to deposition in the harbor entrance and erosion down coast (Griggs & Savoy, 1984).
Breakwaters are structures built parallel to the shore intended to reduce wave energy and shoreline erosion[See Figure 2.4].
Some breakwaters are exposed at the highest of tides, whereas other are submerged. The artificial surfing reef is analogous to a submerged breakwater, however it is not intended to affect sand transport (Skelley Engineering, 1995). Breakwaters are intended to reduced wave energy and thus reduce the mean flows responsible for moving sand. A submerged breakwater just north of the Santa Monica pier was built in 1934. Immediately following its construction sand began to deposit in the area of reduced energy behind the breakwater. The notion was that this design would permit the littoral drift to "slip through" while providing an area sheltered from wave attack. However, the depositional effects were stronger than anticipated and has necessitated repeated dredging to prevent the shoreline from attaching to the structure forming a tombolo (Komar, 1983).
Another structure commonly used to halt erosion is a groin. A groin is a rib-like structure built perpendicular to the shore. This was the option that the Coastal Commission conditionally approved for Chevron. Groins act as a sand trap as the littoral flow of sand moves along the coast. The purpose of building groins is to widen the beach and thereby protect the landward property. Unfortunately, all of the sand that is caught by the groin effectively deprives beaches downcurrent. The movement of sand is the primary concern that goes into most permits required for construction of groins [See Figure 2.5].
Traditionally little or no effort has been made to look at offshore effects of coastal structures, as evidenced by the unpredicted wave phenomena as Marina del Rey and the degraded surf at El Segundo. It is commonly known that sand accumulated at the end of a groin can sometimes create sandbars which produce quality surfing waves. This was the case at the Grand Avenue groin which has now been buried by sand renourishment projects. In the case of the El Segundo groin, a 900 foot semi-permeable groin was constructed along with a sand renourishment program that added a half a million cubic feet of sand to the north side of the groin. This groin/renourishment project had a negative effect of surfing in the area (CCC, 1989). The popular theory for the degradation of the surf is that the renourishment in conjunction with the sand accretion north of the groin over-steepened the beach and altered wave shape, thereby reducing the quality of the surfing (Lissner, 1989). The creation of an artificial surfing reef is intended to ameliorate this problem. It is hoped that a submerged breakwater-like structure, discussed below in more detail, will create "surfable" waves.