Do Large Winter Storms Erode or Construct Beaches? Observations in Southern Monterey Bay, CA

Southern Monterey Bay receives most of its beach sand from sea cliff retreat that occurs sporadically during high surf conditions. Serial aerial photography indicates that coastal retreat in southern Monterey Bay ranges from 0 m/a to 2 m/a, with the maximum value located along the tall sea cliffs of former Fort Ord (Fort Ord Dunes State Park). On January 5, 2008, a winter storm brought 6.5 cm of rain, 80 km/h winds, and 10 m swells to Fort Ord Dunes State Park. Early morning high tide (1.5 m) brought the pounding surf to the toe of the sea cliff initiating a two-hour long episode of spectacular seacliff retreat and attendant beach replenishment. Both still photos and movies captured slope failure, at many different scales, that added an enormous volume of sand to the littoral zone. Before-after photo monitoring of the sea cliff face indicated that slope failure was wide spread. Before the storm, the sea cliff had developed a 1 to 3 m deep “regolith” of loose sand that draped the slightly more resistant sandstone substrate. This apron of sand was at the angle of repose, poised for large-scale failure when high surf undermined the toe of the cliff. After the storm, the sandy regolith had failed in broad shallow sand slips and minor slumps, bringing new sand to the beach environment. The beach had steepened, and the intersection of the cliff face and active swash zone had risen several meters. Clumps of ice-plant and geomorphically similar points are used to match photo pairs. Paradoxically, sea-cliff retreat is required for broad beaches in southern Monterey Bay. The sea cliff and beach move inland through graded time as a geomorphically stable system. Armoring the coast against retreat would starve the beaches of sand.