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Assessing Human Alterations to Fluvial Sediment Delivery and the Impacts to Beach Sustainability, Central California

  • Author(s): Willis, Cope M.
  • et al.
Abstract

With the passage of Assembly Bill 64, the California Public Beach Restoration Act, in 1999, the State of California made its first enduring commitment to public beach nourishment. While ephemeral beach erosion and storm wave inundation of coastal properties occur frequently, particularly during the last two major El Niño events, there have been no comprehensive studies to date of regional beach changes documenting long-term trends of erosion that would justify artificially adding sand to California's beaches. Before the state embarks on an expensive program to augment the natural sediment supply, we need to understand how watershed development has altered natural sand delivery to the coast and how beaches have responded historically to variations in sand supply. In the watersheds draining to the central California coast between San Francisco and Monterey, 58 dams impound more than 2,600 km2 (or 15%) of watershed area. In the Santa Cruz Mountains, nearly 500 km2 of redwood and douglas-fir forest (.-50% of the historical extent) has been logged over the twentieth century. This study seeks to quantify the impacts of coastal dams and timber harvesting on coastal sediment supply by (1) using historical records of stream sediment flux to quantify the volume of sand that has been impounded by coastal dams and (2) measuring current stream sediment loads within watersheds with different logging histories to assess its effects on sediment yield. Concurrently, the first high-resolution time series of regional beach change along central California will be compiled from historical aerial photographs. The results of this study should reveal the magnitude of human alterations to fluvial sand supply, the primary source of beach sand, and the resulting beach response between 1928 and 2000, providing the scientific background required to evaluate the need and effectiveness of costly beach nourishment activities along the central coast.

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