Jim Oakden
Moss Landing Marine Laboratories
P.O. Box 450, Moss Landing, CA 95039
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).
Next - Section II. Beach fauna
Sandy Beaches Table of Contents
