44-1 From Waves to Grains: Linking Wave Characteristics to Sediment Sorting Across the Drakes–Limantour Littoral Cell

Abstract:

Understanding how waves transport sediment is critical for predicting shoreline stability and erosion, especially in environmentally sensitive zones such as Drakes Beach and Limantour Beach at the southern tip of the Point Reyes peninsula in Northern California. These two beaches occupy a semi-sheltered littoral sub-cell bounded by coastal headlands that limit sediment exchange with adjacent cells. Given this limited exchange, we suspected that wave-driven processes within this embayment could be creating spatial gradients in sediment characteristics due to isolation from other sediment transport systems. Specifically, we hypothesized that grain size decreases and sediment sorting improves in the direction of net longshore transport due to selective sorting and cumulative abrasion.

Sediment grain-size distribution was quantified and paired with nearshore wave data to evaluate how wave climate influences sediment transport and beach morphology within the Limantour–Drakes littoral sub-cell. Spatial variability in grain size was compared with modeled wave energy and wave approach angle to assess patterns of longshore transport and sorting across the study area.

Results support the hypothesis at Limantour Beach, where wave direction was the strongest predictor of median grain size (D50; R² = 0.420, p < 0.001), indicating that wave-driven longshore transport exerts a primary control on sediment distribution and produces predictable coarse-to-fine sorting associated with prevailing northwest swell and shoreline orientation. In contrast, no wave parameter at Drakes Beach correlated significantly with sediment characteristics, suggesting that local sediment inputs from adjacent bluffs and Drakes Estero overprint the broader regional wave-driven transport signal.

These findings demonstrate that beach dynamics within the same littoral system can vary depending on the balance between wave forcing and localized sediment supply, improving understanding of how sediment is redistributed along high-energy coastlines. Because grain-size distributions influence beach slope, sediment movement, and erosion susceptibility, identifying where wave climate dominates sediment transport versus where local sediment sources control beach morphology provides a useful framework for predicting shoreline response and long-term coastal vulnerability under changing wave climates, increasing storm intensity, and sea-level rise.

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