Sources, Dispersal and Fate of fine Sediments

Regional Mud Budget

State and federal regulatory staff desire regional sediment budget information along the California coastline, so that they could, for example, compare the expected input from a beach nourishment project to assess whether or not such a project would "overload" the regional system. Much of their concern relates to fine-grained sediment (silt & clay) both transported by the natural system and included in nourishment materials. Regional sand budgets for California's major littoral cells have been compiled as part of the "Regional Sand Budget" study. However, these budgets do not address the movement of fine-grained materials through the coastal system. Such migration of fines is poorly understood, which leads to a cautionary approach by regulatory personnel when evaluating potential project impacts. While site-specific studies evaluating migration of fine grained materials from sediment management activities will continue to be needed, that process could be streamlined by having a science-based assessment on what is known about such migration on a regional scale.

On behalf of the CSMW, the US Geological Survey, Coastal and Marine Section, proposed to and has investigated the sources, dispersal and fate of fine sediment supplied to the coastal waters of California. This study documented the rates and characteristics of these processes. The report concludes that:

  • large amounts of fine sediment are supplied to California coastal waters, primarily through rivers;
  • spatial and temporal variation in this sediment supply exist as a result of geographic province and intensity of winter storms;
  • dispersal occurs primarily as suspended transport due to wave velocities at the seafloor and through downslope gravity flows along the seafloor when sufficient concentrations of fines are present;
  • fines are continually resuspended on their way to offshore mudbelts and other final resting places along the continental shelf;
  • significant amount of fines are deposited and retained in prominent offshore mudbelts, but are also retained inshore and offshore of these conspicuous features, and;
  • inner shelf accumulation of fines appears to be greater in southern California than in central or northern California.