Seasonal patterns of onshore/offshore sediment movement

Measurements of beach profiles from southern California spanning a 5‐year period have been examined for temporal changes in beach configuration. On an annual time scale the data suggest two distinct seasonal pivotal points separating eroding and accreting regions. Empirical eigenfunction analysis of combined onshore and offshore profiles shows a pivotal point for seasonal onshore/offshore sediment movement at a depth of 2–3 m below mean sea level and suggests another at a 6‐m depth. Analysis of accurate reference rod measurements at 4‐, 6‐, 10‐, 14‐, and 20‐m depths supports the presence of the 6‐m pivotal point. A simple model of depth‐dependent seasonal sand movement suggests that during initial winter storms, sand is eroded from both the foreshore and from depths of 6–10 m and is deposited in water depths from 2 to 6 m. During less energetic periods, sediment migrates both shoreward (to the beach face) and seaward (to depths of 10 m) from its winter site of deposition (water depths from 2–6 m). This observation of depth‐dependent motion contradicts the simple single pivotal point model previously suggested for nearshore seasonal onshore/offshore sediment motion and emphasizes the complexity of nearshore sediment transport. A sediment budget for seasonal onshore/offshore transport, based on the dual pivotal point model, consists of exchanges of 85 m 3 /m of beach length across the 3‐m pivotal point and 15 m 3 /m across the 6‐m pivotal point. Over a longer time scale (i.e., the entire 5 years of study) the beaches showed no net erosion or accretion, suggesting that this limited coastal region is stable over these short time scales.