Surfrider Foundation: Mitigation Through Surf Enhancement

CHAPTER 2
(Page 3 of 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).

Santa Monica pier and breakwater

Figure 2.4: Sediment response to the construction of a offshore breakwater near the Santa Monica Pier (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].

Graphic display of sediment response

Figure 2.5: Typical sediment response to the construction of a groin (Komar, 1983).

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.

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