Observations of flow and sediment transport at Pt. Dume, Malibu

Many assumptions must be made in characterizing relationships between rocky headlands, sediment flux, and morphology. Despite extensive study off headland-embayed beaches, flow and sediment transport around headlands are not commonly observed in the field. To address this, a field-based observational study was conducted of Pt. Dume, Malibu, which is the most prominent headland feature of Santa Monica Bay near Los Angeles and is situated in the middle of the Santa Monica Littoral Cell. Current, wave, and turbidity data were collected for 75 days during which several large-scale events occurred, including Hurricane Simon and the first winter storm of 2014; sediment grabs were also collected to quantify grain-size distributions. Mean current patterns show consistent flow towards the headland apex from both sides that develop convergence zones on either side of the apex. Tidally-reversing alongshore current velocities on the order of 0.5 m/s were observed at the apex, with short-lived offshore jets in both directions. Waves affected the general circulation by compressing the flow on the exposed (west) side of the apex whereas on the leeward side, wave refraction and blocking limit their flow altering influence. Bed shear stress was dominated by the waves compared to the currents; as wave energy increased during weather events, bed shear stress and observed turbidity increased. The spatial pattern of shear stress is inferred from the coarser sand bed on the more exposed side of the headland. Conceptually, the converging flows at the headland that produced the offshore jets would eject sand suspended by wave action and deposit it in deeper water. Through this process, sediment may accumulate offshore until large wave events mobilize the bed and the prevailing currents transport it across the shelf or down Dume Submarine Canyon. Stepwise linear regression on the physical forcings associated with the circulation patterns indicate that regional hydrodynamic pressure fields, tidal water levels, and basin-wide wind speeds are the most important for the flow. The findings have large regional implications for sediment management and littoral cell boundaries as defined by headlands.