Evaluation of long-term and storm event changes to the beaches of the Santa Barbara sandshed

Abstract

This research analyzed a 70-year record of historic beach width changes and shoreline position along 70 kilometers of the Santa Barbara and Ventura county coastlines to assess whether beaches were systematically narrowing in response to a reduction in sediment supply caused by natural factors or human alterations. Using a combination of rectified historic aerial photos and topographic lidar data sets, two shoreline reference features were used to identify the dry sand portion of the beaches. With two reference features, both shoreline change rates and beach width changes were calculated and compared. This beach width methodology provided additional information to more traditional shoreline change analyses on beach response, sand volumes, and human alterations.

Results demonstrated that beaches could be divided into two segments determined by the location relative to the Santa Barbara Harbor. Beaches updrift (northwest) of the harbor were found to oscillate with maximum widths measured during the negative Pacific Decadal Oscillation phase while minimum beach widths were measured following major El Niño events. Downdrift of the harbor (southeast) no such patterns were identified, indicating that harbor dredging and bypasses controlled beach widths. Littoral drift rates both updrift and downdrift were found to be about 1.7 km/year. Pulses of sand appear to move coherently through the littoral system and could be seen in transit on stable beaches that had a wide minimum beach width. Beaches between the stable beaches showed little signs of large volumes of sand passing suggesting that longshore transport in these reaches occurs offshore.

Beach width responses to large El Niños match long-term shoreline change patterns of shoreline reorientation indicating that El Niños play a major role in large-scale, long-term coastal evolution. El Niños were also shown to have significant impacts on nearly all aspects of sandy beach ecosystems, demonstrating the importance of physical process considerations in ecological studies.

While the long-term beach width changes did not identify any systematic long term narrowing attributable to sediment supply reductions, analyses revealed a pattern of beach narrowing resulting from shoreline armoring, illustrating passive erosion and placement loss effects. A sandshed concept linking watershed and coastal processes to maintain beaches for their economic, ecological, and hazard protection potential has significant implications for the management of our coasts at a variety of scales.