Coastal Sediment Budgets and the Littoral Cutoff Diameter: A Grain Size Threshold for Quantifying Active Sediment Inputs

Abstract

The use of coastal sediment budgets has garnered wide acceptance since its inception nearly 40 years ago. Since then, many researchers have used sediment budgets to quantify littoral transport rates and understand coastal processes on diverse coastlines including the high-energy Pacific coast of North America, the Black Sea, the Nile Delta and beyond. Here, we suggest further improvement on an already successful conceptual tool by questioning the broad definition of sand set forth by the classic Wentworth grain size scale (63–2000 microns) that is often used in quantifying coastal sediment budget inputs from sources such as coastal-draining rivers and eroding sea cliffs. A smaller range of sediment sizes is found on many beaches in California. This range is defined by a minimum grain size threshold, termed the littoral cutoff diameter. Sediment contributed to the littoral system that is smaller than this threshold, even if defined as sand by the Wentworth scale, may not remain on the beach in any significant quantity. The littoral cutoff diameter ranges from 88 to 180 microns on the California beaches studied herein, and results from a variety of locations show that yearly littoral sediment flux from coastal-draining rivers and eroding sea cliffs can be overestimated by 16–300% percent if the littoral cutoff diameter is not considered. The presence of the littoral cutoff diameter suggests that quantifying sediment inputs within the context of preexisting littoral sediments is of first-order importance when constructing sediment budgets in California and in other analogous coastal environments.