Sandy Beaches
I. Beach Structure and Formation

(also see Geology and Physical Oceanography sections)

Sand beaches represent half the intertidal habitat in the MBNMS. Included are long exposed beaches, protected pocket beaches, and transient beaches which are eroded to bedrock in the winter, then reappear during summer when wave energy is reduced.

Central California beaches exhibit the classic beach structure: they are backed either by dunes or cliffs, followed to seaward by the berm, beach flat, trough, and bar. There is a seasonal onshore-offshore movement of sand, with steeper beach slopes and offshore bars in the winter, and gradual consistent slope in the summer (Bascom 1964, Harlett 1967).

Sand in the MBNMS is derived from several sources, including transport from San Francisco Bay and locations to the north, local erosion of cliffs, and transport down local rivers (Chipping and McCoy 1982, Clark 1980, Clark and Osborne 1982, Combellick and Osborne 1977, Griggs and Hein 1980, Porter et al. 1979). Transient fans often form at the mouths of rivers in response to major flood events (Hicks and Inman 1987). The sand is predominately quartz, but has significant quantities of magnetite, garnet and other heavy minerals which can be used as tracers to discrete sources (Hutton 1959, Judge 1970). Sand transport along the open coast is generally from north to south, as a result of the prevailing northerly winds (Cherry 1966, Johnson 1966). However, this is only an average trend, as periodic reversals of longshore transport in response to storms from the south can result in significant sporadic northward transport. Much of the sand transport over the course of a year may be the result of a few large storms (Konar et al. 1990).

Within Monterey Bay, the sources of sand are 27% from local cliff erosion, 54% from river discharge, and 19% from longshore drift (Oradiwe 1986, also Dittmer 1972). The Monterey Submarine Canyon acts as a sediment sink, with sand accumulating in the canyon head, then being flushed down the canyon as a result of storms or earthquakes (Greene et al. 1991, Arnal et al. 1973). The canyon head is in the sub-littoral zone at Moss Landing, and pieces of the Moss Landing pier have occasionally been transported down-canyon along with the sediment. Longshore transport in the Bay is generally from north to south, but there are several small cells with northward transport, particularly in the southern half of the Bay (Oradiwe 1986).