Transport of Sediment and Water Around Headlands: Observations and Modeling at Point Dume, With Implications for Santa Monica Bay Beaches and Littoral Cell

John Largier, Bodega Marine Laboratory, UC Davis
Douglas George, Bodega Marine Laboratory, UC Davis

Coastal environments are connected by alongshore flow. This is nowhere more evident than in the transport of sand between beaches, but also is important in the transport of biotic particles between habitat patches. Headlands protrude from an otherwise smooth shoreline, perturbing alongshore flow and disrupting alongshore transport of sediment and particles. Rocky headlands are prominent morphological features that may deflect or block alongshore currents, focus wave energy, shed eddies, and/or create retention zones down-current of the headland. Presently, many assumptions must be made in characterizing relationships between flux, sediment reservoirs and morphology – despite extensive study off beaches, flow and sediment transport along rocky shores and around headlands remains poorly studied. The essential question is whether in perturbing flow, headlands have an impact on alongshore fluxes of sediments and particles – and if so, whether the effect is local (deflecting flux and influencing erosion and accretion at the scale of the headland) or regional (disconnecting flux with an effect downstream at a scale much larger than the headland).

Drs. Largier and George have been funded by USC Sea Grant to study Point Dume, along the northern shore of Santa Monica Bay. They will use field observations and modeling to quantify transport past headlands and to identify the primary controls on the proportion of material that passes headland versus that which is either blocked or exported offshore. The specific objective of this proposal is to conduct these studies at Point Dume, resolving differences in transport as a function of particle type, transport direction and oceanographic conditions (currents, waves, wind). This work will be placed in the context of Santa Monica Bay beaches and Littoral Cell. There are four sub-objectives:

1. To quantify water and sediment transport rates around Point Dume.
2. To establish relationships between transport rate and parameters that describe headland morphology and oceanographic conditions.
3. To relate the impact of Point Dume on alongshore transport to the Santa Monica Littoral Cell, as presently defined.
4. To inform coastal management as it relates to sediment budgets, water quality and ecological connectivity.

The most notable outcome of the project will be the predictive conceptual model to estimate circulation and sediment transport around differently sized headlands on the coast of California. We expect that the predictive model will be used to improve sediment management on beaches and offshore by evaluating the efficacy of beach nourishment near headlands. This will benefit coastal zone managers in California including state and federal agencies (Coastal Commission, USACE), regional bodies (e.g., SANDAG), and local municipalities, as well as marine ecologists and biologists.

As coastal communities prepare climate change adaptation plans that consider beach nourishment, such as AdaptLA, the predictive model can aid in feasibility assessments for such approaches. Finally, a better characterization of the state’s littoral cells could influence the integration of physical processes into ecosystem protection zones (e.g., MPAs or ASBSs). The coastal processes field data specific to Point Dume will be useful to the City of Malibu to understand larvae transport, contaminant dispersion and recreational hazards (e.g., fecal matter distribution in the nearshore).

2017 Research Updates

Key outcomes:

1)   Researchers developed a model to estimate circulation and sediment movement around headlands of different sizes.

2)   Researchers created a classification system of rocky headlands in California, with a focus on identifying the boundaries of littoral cells.

3)   Research results are useful for coastal managers as they consider beach nourishment projects and prepare for the impacts of climate change.

Results show that the size and shape of the headland, together with incident wave angle, emerged as the dominant factors influencing the pathway of sediment. Sediment pathways around headlands varied by sediment grain size, and sediment grain size determined the volume of sediment flux. Another conclusion of this research is that a new set of parameters should be utilized to define littoral cell boundaries at headlands that take into account size, shape, and sediment.   

The most notable outcome of the project is a new model that estimates circulation and sediment transport around differently sized headlands on the coast of California.  The results are useful for coastal managers, including state and federal agencies, regional bodies, and local municipalities, as they consider beach nourishment projects and prepare for the impacts of climate change.

2016 Poster of project results

For more information on this project, contact Ms. Phyllis Grifman, Associate Director.