Changes in surfzone morphodynamics driven by multi-decadel contraction of a large ebb-tidal delta

Marine Geology
By: , and 
Edited by: P.L. BarnardB.E. Jaffee, and D.H. Schoellhamer



The impact of multi-decadal, large-scale deflation (76 million m3 of sediment loss) and contraction (~ 1 km) of a 150 km2 ebb-tidal delta on hydrodynamics and sediment transport at adjacent Ocean Beach in San Francisco, CA (USA), is examined using a coupled wave and circulation model. The model is forced with representative wave and tidal conditions using recent (2005) and historic (1956) ebb-tidal delta bathymetry data sets. Comparison of the simulations indicates that along north/south trending Ocean Beach the contraction and deflation of the ebb-tidal delta have resulted in significant differences in the flow and sediment dynamics. Between 1956 and 2005 the transverse bar (the shallow attachment point of the ebb-tidal delta to the shoreline) migrated northward ~ 1 km toward the inlet while a persistent alongshore flow and transport divergence point migrated south by ~ 500 m such that these features now overlap. A reduction in tidal prism and sediment supply over the last century has resulted in a net decrease in offshore tidal current-generated sediment transport at the mouth of San Francisco Bay, and a relative increase in onshore-directed wave-driven transport toward the inlet, accounting for the observed contraction of the ebb-tidal delta. Alongshore migration of the transverse bar and alongshore flow divergence have resulted in an increasing proportion of onshore migrating sediment from the ebb-tidal delta to be transported north along the beach in 2005 versus south in 1956. The northerly migrating sediment is then trapped by Pt. Lobos, a rocky headland at the northern extreme of the beach, consistent with the observed shoreline accretion in this area. Conversely, alongshore migration of the transverse bar and divergence point has decreased the sediment supply to southern Ocean Beach, consistent with the observed erosion of the shoreline in this area. This study illustrates the utility of applying a high-resolution coupled circulation-wave model for understanding coastal response to large-scale bathymetric changes over multi-decadal timescales, common to many coastal systems adjacent to urbanized estuaries and watersheds worldwide.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Changes in surfzone morphodynamics driven by multi-decadel contraction of a large ebb-tidal delta
Series title Marine Geology
DOI 10.1016/j.margeo.2013.07.005
Volume 345
Year Published 2013
Language English
Publisher Elsevier
Contributing office(s) Pacific Coastal and Marine Science Center
Description 14 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Marine Geology
First page 221
Last page 234
Country United States
State California
Other Geospatial San Francisco Bay
Online Only (Y/N) N
Additional Online Files (Y/N) N
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