Surfrider Foundation: Mitigation Through Surf Enhancement

CHAPTER 2
(Page 2 of 3)

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)

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