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Littoral Cell, Sand Budgets and Beaches

People have been interested in beaches and coastal processes for many years. It has been observed that beach width can change significantly over a range of time periods, from hours and days to years and decades. Long-term erosion/narrowing of California’s beaches is of great concern to coastal managers as well as the general public, since habitat, recreational opportunities and shore protection are adversely affected by such loss.

The coastline of California can be separated into distinct, essentially self-contained regions or cells that are geographically limited. For example, beach sand in the Santa Barbara area originates from the watersheds and the coastline in the Santa Barbara area, while beach sand in San Diego or Santa Cruz comes from within those geographic areas.

Coastal geologists and engineers call these self-contained coastal units “littoral cells”. These cells are geographically limited and consist of a series of sand sources (such as rivers, streams and eroding coastal bluffs) that provide sand to the shoreline; sand sinks (such as coastal dunes and submarine canyons) where sand is lost from the shoreline; and longshore transport or littoral drift that moves sand along the shoreline. Understanding this setting allows researchers to focus on the major elements influencing specific beach or shoreline areas. This report illustrates how sand moves from one location to another within littoral cells.

It is the balance between the volumes of sand entering and leaving a littoral cell over the long-term that govern the long-term width of the beaches within the cell. Scientists use the concept of sand budgets to identify and quantify, to the degree possible, additions and losses of sand that influence beach width. Where sand supplies have been reduced through the construction of dams or debris basins in coastal watersheds, through armoring of the seacliffs, by mining sand or restricting littoral transport through large coastal engineering structures, the beaches may temporarily or permanently narrow. Sand budgets are illustrated and then summarized for California’s major littoral cells.

Beach nourishment or beach restoration is the placement of sand on the shoreline with the intent of widening a beach that is naturally narrow or where the natural supply of sand has been significantly reduced through human activities. Nourished shorelines provide a number of benefits including increased area for recreation, increased revenue from tourism, habitat improvement for shore dependant species, greater protection of the coastline from coastal storms, reduced need for armor, and increased public access. While nourishment may appear to be an attractive alternative to coastal armoring or retreat, this report describes a number of issues or considerations that need to be carefully considered and addressed. Sand retention systems have been used effectively at a number of sites in California as a way to significantly extend the lifespan of a beach nourishment project.

The CSMW and the University of California at Santa Cruz, have developed this report as an educational tool as part of their public outreach effort associated with the Sediment Master Plan. A more detailed report on specific sand budgets for California’s major littoral cells is a complement to, and resource for this more general discussion. The document was prepared with significant input from CSMW personnel, but does not necessarily represent the official position of member agencies.