Geology topics

Douglas A. George

Publications and source records attributed to Douglas A. George.

5 recordsLinked to original sources

USGS2019

Modeling sediment bypassing around idealized rocky headlands

Alongshore sediment bypassing rocky headlands remains understudied despite the importance of characterizing littoral processes for erosion abatement, beach management, and climate change adaptation. To address this gap, a numerical model sediment transport study was developed to identify controlling factors and mechanisms for sediment headland bypassing potential. Four idealized headlands were designed to investigate sediment flux around the headlands using the process-based hydrodynamic model Delft-3D and spectral wave model SWAN. The 120 simulations explored morphologies, substrate compositions, sediment grain sizes, and physical forcings (i.e., tides, currents, and waves) commonly observed in natural settings. A generalized analytical framework based on flow disruption and sediment volume was used to refine which factors and conditions were more useful to address sediment bypassing. A bypassing parameter was developed for alongshore sediment flux between upstream and downstream cross-shore transects to determine the degree of blockage by a headland. The shape of the headland heavily influenced the fate of the sediment by changing the local angle between the shore and the incident waves, with oblique large waves generating the most flux. All headlands may allow sediment flux, although larger ones blocked sediment more effectively, promoting their ability to be littoral cell boundaries. The controlling factors on sediment bypassing were determined to be wave angle, size, and shape of the headland, and sediment grain size.

Journal of Marine Science and Engineering↗

USGS2018

Currents, waves and sediment transport around the headland of Pt. Dume, California

Sediment transport past rocky headlands has received less attention compared to transport along beaches. Here we explore, in a field-based study, possible pathways for sediment movement adjacent to Point Dume, a headland in Santa Monica Bay, California. This prominent shoreline feature is a nearly symmetrical, triangular-shaped promontory interior to the Santa Monica Littoral Cell. We collected current, wave, and turbidity data for 74 days during which several wave events occurred, including one associated with a remote hurricane and another generated by the first winter storm of 2014. We also acquired sediment samples to quantify seabed grain-size distributions. Near-bottom currents towards the headland dominated on both of its sides and wave-driven longshore currents in the surf zone were faster on the exposed side. Bed shear stresseswere generated mostly by waves with minor contributions from currents, but both wave-driven and other currents contributed to sediment flux. On the wave-exposed west side of the headland, suspended sediment concentrations correlated with bed stress suggesting local resuspension whereas turbidity levels on the sheltered east side of the headland are more easily explained by advective delivery. Most of the suspended sediment appears to be exported offshore due to flow separation at the apex of the headland but may not move far given that sediment fluxes at moorings offshore of the apex were small. Further, wave-driven sediment flux in the surf zone is unlikely to pass the headland due to the discontinuity in wave forcing that causes longshore transport in different directions on each side of the headland. It is thus unlikely that sand is transported past the headland (specifically in a westerly direction), although some transport of finer fractions may occur offshore in deep water. These findings of minimal sediment flux past Point Dume are consistent with its role as a littoral cell boundary, although more complex multi-stage processes and unusual events may account for some transport at times.

California↗

USGS2015

Classification of rocky headlands in California with relevance to littoral cell boundary delineation

Despite extensive studies of hydrodynamics and sediment flux along beaches, there is little information on the processes, pathways and timing of water and sediment transport around rocky headlands. In this study, headlands along the California coast are classified to advance understanding of headland dynamics and littoral cell boundaries in support of improved coastal management decisions. Geomorphological parameters for 78 headlands were quantified from geological maps, remote-sensing imagery, navigational charts, and shoreline geospatial databases. K -means cluster analysis grouped the headlands into eight distinct classes based on headland perimeter, bathymetric slope ratio, and the headland apex angle. Wave data were used to investigate the potential for sediment transport around the headland types and determine the efficacy of the headland as a littoral cell boundary. Four classes of headland appear to function well as littoral cell boundaries, with headland size (e.g., perimeter or area) and a marked change in nearshore bathymetry across the headland being relevant attributes. About half of the traditional California littoral cell boundaries align with headland classes that are expected to perform poorly in blocking alongshore sediment transport, calling into question these boundaries. Better definition of these littoral cell boundaries is important for regional sediment management decisions.

California↗

USGS2011

Shallow stratigraphy of the Skagit River Delta, Washington, derived from sediment cores

Sedimentologic analyses of 21 sediment cores, ranging from 0.4 to 9.6 m in length, reveal that the shallow geologic framework of the Skagit River Delta, western Washington, United States, has changed significantly since 1850. The cores collected from elevations of 3.94 to -2.41 m (relative to mean lower low water) along four cross-shore transects between the emergent marsh and delta front show relatively similar environmental changes across an area spanning ~75 km 2 . Offshore of the present North Fork Skagit River and South Fork Skagit River mouths where river discharge is focused by diked channels through the delta, the entire 5–7-km-wide tidal flats are covered with 1–2 m of cross-bedded medium-to-coarse sands. The bottoms of cores, collected in these areas are composed of mud. A sharp transition from mud to a cross-bedded sand unit indicates that the tidal flats changed abruptly from a calm environment to an energetic one. This is in stark contrast to the Martha's Bay tidal flats north of the Skagit Bay jetty that was completed in the 1940s to protect the newly constructed Swinomish Channel from flooding and sedimentation. North of the jetty, mud ranging from 1 to 2 m thick drapes a previously silt- and sand-rich tidal flat. The silty sand is a sediment facies that would be expected there where North Fork Skagit River sedimentation occurred prior to jetty emplacement. This report describes the compositional and textural properties of the sediment cores by using geophysical, photographic, x-radiography, and standard sediment grain-size and carbon-analytical methods. The findings help to characterize benthic habitat structure and sediment transport processes and the environmental changes that have occurred across the nearshore of the Skagit River Delta. The findings will be useful for quantifying changes to nearshore marine resources, including impacts resulting from diking, river-delta channelization, shoreline development, and natural variations in fluvial-sediment inputs. These results also provide important quantitative data on the amount of sediment delivered to the nearshore from the Skagit River for use in calculating sediment budgets for application to watershed planning and wetland and coastal-ecosystem restoration.

Washington↗

USGS2007

Beach morphology monitoring in the Elwha River Littoral Cell, 2004-2009

This report describes the methods used, data collected, and results of the Beach Morphology Monitoring Program in the Elwha River Littoral Cell, starting in 2004. The U.S. Geological Survey and the Washington State Department of Ecology collaborated in the data collection with the support of the local Lower Elwha Klallam Tribe. Beach monitoring efforts consisted of collecting topographic and bathymetric horizontal and vertical position data by using a Real Time Kinematic Differential Global Positioning System (RTK-DGPS). The monitoring program was designed to characterize the littoral system of the Elwha River before the scheduled removal of two large dams in 2012. A primary objective of this work is to quantitatively describe the topography and bathymetry of the Elwha River littoral system so that the effects of dam removal may be quantified. Sediment inputs following dam removal are hypothesized to result in (A) larger amounts of fine sediment grain-sizes entering the littoral system and, (B) a reduction or reversal of coastal erosion.

Washington↗