Category Archives: Sandshed Science

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What is the littoral cut-off diameter?

Very fine-grained sand, ranging from 0.0625 to 0.125 mm in diameter (4ø to 3ø), typically doesn’t remain on the exposed (dry) portions of most California beaches due to the high-energy wave environment. An investigation of littoral transport processes and beach sand in northern Monterey Bay (Hicks, 1985), discovered that there is a littoral cut-off diameter, or a grain-size diameter, characteristic of any particular segment of coast. The cut-off diameter serves as a functional grain size boundary in that very little material finer-grained than this diameter actually remains on the exposed beach. The cut-off diameter along any particular beach or stretch of coast is primarily a function of wave energy at that location. Studies along the coast of northern Santa Cruz County, which is a relatively high-energy, exposed coast, determined a littoral cut-off diameter of ~0.18 mm (2.5ø) for this stretch of coast, with very little finer sand remaining on the exposed beaches. In southern California, where much of the coast is protected from strong wave action by the sheltering effect of the Channel Islands, the littoral cut-off diameter is smaller, typically around 0.125mm (3ø). When estimating or calculating inputs to a sand budget or planning a beach nourishment project, it is important to consider the littoral cut-off diameter. Sand placed on the beach or entering a littoral cell that is finer than the littoral cut-off diameter will not remain on the dry beach.

What is Littoral Drift?

Researchers have learned that sand is in constant motion along California’s coastline, and only resides “temporarily” on an individual beach. An alongshore or littoral current is developed parallel to the coast as the result of waves breaking at an angle to the shoreline. This current and the turbulence of the breaking waves, which serves to suspend the sand, are the essential factors involved in moving sand along the shoreline. As waves approach the beach at an angle, the up-rush of water, or swash, moves sand at an angle
onto the shoreface. The backwash of water rushes down the shoreface perpendicular to the shoreline or a slight downcoast angle, thus creating a zigzag movement of sand. This zigzag motion effectively results in a current parallel to the shoreline. Littoral drift refers to the movement of entrained sand grains in the direction of the longshore current.

Littoral drift can be thought of as a river of sand moving parallel to the shore, moving sand from one coastal location to the next and so on until the sand is eventually lost to the littoral system. Littoral drift or transport in California can occur alongshore in two directions, upcoast or downcoast, dependent on the dominant angle of wave approach. Along the California coast, southward transport is generally referred to as downcoast and northward transport is considered upcoast. If waves approach perpendicular to the shoreline, there will be no net longshore movement of sand grains, no littoral current, and thus no littoral drift. Longshore transport for a reach of coast will typically include both upcoast and own coast transport, often varying seasonally.

What is a Sandshed?

Sandsheds form the framework for our understanding of the sources, sinks, transport, and storage of sand along the coast in conjunction with the processes and factors shaping the economic, recreational and ecological health of beaches. Similar to a watershed which delineates the boundary of land draining into a particular river, a sandshed physically encompasses the land and nearshore zone affecting the sand supply and overall health of our sandy beaches in a particular beach compartment or littoral cell. In California, a beach compartment or littoral cell typically begins at a rocky headland and terminates into a submarine canyon with sand transported along the shoreline in the direction of the predominant waves. Managing the coastline with a sandshed-based approach is a beach-centric, interdisciplinary method that assesses sand supply and processes within a regional landscape to evaluate the stability and condition of our beaches in a holistic way to minimize hazards and maximize the recreational, geomorphological, ecological, and cultural value of the beach.

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