Understanding processes controlling sediment transports at the mouth of a highly energetic inlet system (San Francisco Bay, CA) Edwin P.L. Elias a, ⁎, Jeff E. Hansen a, b a U.S. Geological Survey, Pacific Coastal Marine Science Center, 400 Natural Bridges Drive, Santa Cruz, CA 95062, USA b University of California Santa Cruz, Department of Earth and Planetary Sciences, 1156 High St., Santa Cruz, CA 95064, USA abstract article info Article history: Received 23 September 2011 Received in revised form 3 July 2012 Accepted 5 July 2012 Available online 16 July 2012 Keywords: San Francisco Bay Delft3D process-based model sediment transport San Francisco Bay is one of the largest estuaries along the U.S. West Coast and is linked to the Pacific Ocean through the Golden Gate, a 100 m deep bedrock inlet. A coupled wave, flow and sediment transport model is used to quantify the sediment linkages between San Francisco Bay, the Golden Gate, and the adjacent open coast. Flow and sediment transport processes are investigated using an ensemble average of 24 clima- tologically derived wave cases and a 24.8 h representative tidal cycle. The model simulations show that with- in the inlet, flow and sediment transport is tidally dominated and driven by asymmetry of the ebb and flood tides. Peak ebb velocities exceed the peak flood velocities in the narrow Golden Gate channel as a result of flow convergence and acceleration. Persistent flow and sediment gyres at the headland tips are formed that limit sediment transfer from the ebb-tidal delta to the inlet and into the bay. The residual transport pattern in the inlet is dominated by a lateral segregation with a large ebb-dominant sediment transport (and flow) prevailing along the deeper north side of the Golden Gate channel, and smaller flood dominant transports along the shallow southern margin. The seaward edge of the ebb-tidal delta largely corresponds to the seaward extent of strong tidal flows. On the ebb-tidal delta, both waves and tidal forcing govern flow and sediment transport. Wave focusing by the ebb-tidal delta leads to strong patterns of sediment con- vergence and divergence along the adjacent Ocean Beach. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Tidal inlets are a common geomorphic feature along the world's coast- lines and are found in a variety of coastal settings (Glaeser, 1978). These settings range from highly mobile cuts through barrier islands as ob- served along the U.S. East Coast, bedrock defined drowned river valleys such as the Hudson River Estuary and Chesapeake Bay, and glacially carved embayment's (e.g. the Puget Sound in the Pacific Northwest). San Francisco (SF) Bay is a unique example of an over 100 m deep bed- rock defined inlet formed due to recent tectonic activity. Numerous conceptual models have been formulated to explain sediment dynamics and interactions at barrier island type inlets (Hubbard et al., 1979; FitzGerald, 1988, 1996; Oertel, 1988). However these models may not be applicable to considerably larger and deeper inlets such as SF Bay that greatly differ in dimensions, geographic and morphologic setting, and hydrodynamic forcing regime. It is estimated that anthropogenic activities in SF Bay and its coastal system, such as channel dredging, sand mining and development, have re- moved or displaced over 200 million m 3 of sand sized-sediment in the last century alone (United States Army Corps of Engineers, 1996; Chin et al., 2004). The impact of these disturbances on the coastal system has not been quantified, but severe hot-spot erosion at Ocean Beach, the shoreline south of the inlet, and shrinkage of the ebb-tidal delta are certainly related (Hansen and Barnard, 2010; Dallas and Barnard, 2011; Hansen et al. (2013–this issue)). Understanding the physical processes that govern water and sediment exchange between San Francisco Bay and the open coast through the Golden Gate inlet is essential for understanding the ob- served changes and future sustainable management of the coasts. Understanding sediment dynamics in large and energetic coastal sys- tems like SF Bay is notoriously difficult as flows and sediment transports are often spatially and temporally complex. Collecting in situ field data with the required spatial and temporal resolution is extremely challeng- ing and expensive. Numerical process-based models have reached a stage that they can be used to investigate the circulation dynamics and greatly improve our fundamental understanding of the processes driving sediment transport (Elias, 2006; Lesser, 2009; van der Weegen, 2009). Van der Weegen (2009) illustrated that long term (centuries) morphodynamic simulations are capable of reproducing concepts and equilibrium relations based on measurements and laboratory experi- ments. Further, Lesser (2009) demonstrated, through agreement be- tween modeled and measured morphodynamic behavior of Willapa Bay (WA), that a process based numerical model could reproduce the most important physical processes in the coastal zone over medium Marine Geology 345 (2013) 207–220 ⁎ Corresponding author at: Deltares, P.O. Box 177, 2600 MH Delft, Rotterdamseweg 185, 2629DH Delft, Netherlands. Tel.: +31 88 335 8273; fax: +31 883358582. E-mail addresses: Edwin.elias@deltares.nl (E.P.L. Elias), Jeff_hansen@usgs.gov (J.E. Hansen). 0025-3227/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2012.07.003 Contents lists available at ScienceDirect Marine Geology journal homepage: www.elsevier.com/locate/margeo