Temporal and spatial variations in sand budgets with application to southern Monterey Bay, California

• Littoral sand budget constrained to medium to coarse grain sand.
• Natural and anthropogenic changes resolved by sand budgets over long time period.
• High erosion rates in southern Monterey Bay, California are due to shoreline sand mining.

Often in calculating a sediment budget there is an unknown contribution, which is ascribed to the residual of the balance, and the budget cannot be fulfilled. The temporal and spatial variations and sediment size within a littoral cell provide additional information to define sediment budgets. The approach here is to consider only medium to coarse sand (grain size > 0.25 mm) over a long time period during significant changes of natural and anthropogenic inputs and losses that allow calculating several budgets to isolate the unknown inputs and outputs to close the system. Considering only medium to coarse sand eliminates including the cross-shore transport of fine sand by waves, which is poorly quantified. To demonstrate these concepts, shoreline recession rates measured over a 101-year time period are used to calculate sediment budgets for southern Monterey Bay, California, littoral cells. The coarse sandy shoreline is backed by extensive bluffs and dunes that reach 44 m in elevation. Intensive sand mining of coarse sand derived directly from the beach and surfzone started in 1927 and continues today. A contribution of about 100 k m3/year of medium to coarse sand from the Salinas River is calculated from measured shoreline accretion for the period 1910–1945 starting when the river first flowed into the littoral cell, prior to damming of the river and significant losses owing to sand mining. Sediment budgets are calculated for 1940–1989 and 1989–2011 to spatially identify the loss of about 200 k m3/year attributed to different mining operations that captured the littoral transport. The primary contributions of medium to coarse sand to the littoral system is approximately 180 k m3/year from the eroding dunes and beaches. Only a quarter of the dune sand is found to be compatible with the coarser beach sand with the finer fraction carried offshore. A conclusion is that sand mining is the cause of the observed high recession rates.