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The stunning shoreline of the Monterey Bay National Marine Sanctuary consists of long stretches of sandy beaches, tall dune fields, rocky cliffs with sediment-bearing coves, and sandy river and creek mouths. Understanding beach processes and the dynamic nature of sand transport provides insight into these different
coastal features and a context for effective resource management. Knowledge of sand sources, sinks, and transport mechanisms is necessary to understand and respond to coastal erosion as well
as to evaluate the impact of human activities on coastal systems.
Littoral cells represent segments of coastline within which
sand input, transport, and output are essentially self-contained. Littoral sand is defined as particles that are sufficiently small to
be moved by waves yet are large enough to remain in the beach system and not be carried offshore. In theory, the littoral sand budget in each cell is unaffected by changes in other cells. Hence, human activities such as construction of seawalls and other coastal protection structures (collectively known as coastal armoring), dredging, or beach nourishment that occur in one cell should
not affect sand dynamics in an adjacent cell. Understanding the location of littoral cell boundaries is important for evaluating the potential impact of various coastal management options.
At least four major littoral cells, some with sub-cells, exist
in the sanctuary. (See Figure 1.) The Santa Cruz cell is the longest; it is thought to extend from San Francisco Bay to the head of Monterey Canyon near Moss Landing. The Southern Monterey
Bay littoral cell begins at the Monterey Canyon and probably extends to the Monterey Peninsula. Little is known about littoral cells along the Big Sur coast. Submarine canyons that reach the shoreline (such as Monterey Canyon) intercept longshore transport,
funneling sand out of the littoral system and into the deep ocean. Therefore, Partington Canyon (south of the Big Sur River) is
likely a boundary subdividing the Big Sur coast into at least two littoral cells.
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| Figure 1. Littoral cells within the sanctuary |
The main sources of sand for sanctuary littoral cells are coastal streams and erosion of dunes and cliffs. Studies of the Santa Cruz cell indicate that coastal streams contribute about 75 percent and bluff retreat, about 20 percent of sand entering this cell. Dams
frequently trap sediment carried by streams. Hence, installation
of new dams could have a cumulatively important effect on sand
supply to this cell. Of the contribution from bluffs, nearly all the
sand comes from dunes and the marine terraces that cap many coastal cliffs. Thus, seawalls that armor marine terrace deposits
are likely to have a greater impact on local sand supply than
seawalls armoring other rock types.
Once sand enters the littoral system, it is carried along the
coast by longshore currents and waves (typically southward in
the sanctuary) toward its termination in a submarine canyon,
offshore shelf, or dune field. The transport system is like a
constantly moving conveyor belt that carries sand grains. For
example, Santa Cruz littoral sand transport rates range from 200,000 to 400,000 cubic meters per year. This is roughly
equivalent to between fifty and 100 dump trucks full of sand
gradually moving down the coast every day.
Compared with the Santa Cruz cell, analyses of the Southern Monterey Bay littoral cell indicate that longshore transport rates
are relatively low and suggest that only small amounts of sand
are mobilized to nourish down-coast beaches. The primary source of sand to the Southern Monterey Bay cell is the erosion of the
Fort Ord area dunes, with a small contribution by the Salinas River. Lack of information on the volume of historic sand loss due to mining provides uncertainty to sand budgets. A small sub-cell is thought to deliver sand predominantly from the Salinas River into Monterey Canyon. This sub-cell shares a sand source (the Salinas River) with the sub-region that extends from the Salinas River to Del Monte Beach but is otherwise independent. Monterey Peninsula beach sand is sourced from the breakdown of local granite and differs compositionally from sand found north of Del Monte Beach. The lack of sand exchange between sub-regions and low overall littoral transport rate imply that southern Monterey Bay beaches are likely to be very sensitive to changes in local sand supply.
Big Sur coast littoral system(s) have not been well studied. Coastal streams and frequent landslides transport large volumes
of sediment into the littoral zone of the Big Sur coast. Littoral
sand budgets, however, have not been compiled.
Understanding littoral cell extent and the relative importance
of factors affecting sand supply are first steps to understanding beach systems. The Coastal Armoring Action Plan, part of the sanctuary Joint Management Plan Review, aims to develop a regional, scientifically informed approach to these issues. Efforts are underway to further compile available information, build
partnerships, and encourage additional research.
Irina Kogan
Gulf of the Farallones National Marine Sanctuary
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