Geology topics

Jessica R. Lacy

Publications and source records attributed to Jessica R. Lacy.

43 records · Page 3Linked to original sources

USGS2011

Wave exposure of Corte Madera Marsh, Marin County, California: A field investigation

Tidal wetlands provide valuable habitat, are an important source of primary productivity, and can help to protect the shoreline from erosion by attenuating approaching waves. These functions are threatened by the loss of tidal marshes, whether due to erosion, sea-level rise, or land-use practices. Erosion protection by wetlands is expected to vary geographically, because wave attenuation in marshes depends on vegetation type, density, and height and wave attenuation over mudflats depends on slope and sediment properties. In macrotidal northern European marshes, a 50 percent reduction in wave height within tens of meters of vegetated salt marsh has been observed. This study was designed to evaluate the role of mudflats and marshes in attenuating waves at a site in San Francisco Bay. In prehistoric times, the shoreline of San Francisco Bay was ringed with tidal wetlands, with mudflats at lower elevations and marshes above. Most of the marshes around the Bay emerged 2,000–4,000 years ago, after the rate of sea-level rise slowed to approximately 1 mm/year. Approximately 80 percent of the acreage of tidal marsh and 40 percent of the acreage of tidal mudflats in San Francisco Bay have been lost to filling and draining since 1800. Tidal wetlands are particularly susceptible to impacts from sea-level rise because the vegetation at each elevation is adapted to a specific tidal-inundation regime. The maintenance of suitable marsh-plain elevations depends on a supply of sediment that can keep up with the rate of sea-level rise. Sea-level rise, which according to recent projections may reach 75 to 190 cm by the year 2100, poses a significant threat to wetlands in San Francisco Bay, where landward migration is frequently impossible due to urbanization of the adjacent landscape. In this study, we collected data in Corte Madera Bay and Marsh to determine whether, and to what degree, waves are attenuated as they transit the Bay and, during high tides, the marsh. Corte Madera Bay was selected as a study site because of its exposure to wind waves, as well as its history of shoreline erosion and marsh restoration and monitoring. Data were collected in the winter of 2010, along a cross-shore transect extending from offshore of the subtidal mudflats into the tidal marsh. This study forms part of the Innovative Wetland Adaptation in the Lower Corte Madera Creek Watershed Project initiated by the Bay Conservation and Development Commission (BCDC) ( http://www.bcdc.ca.gov/planning/climate_change/WetlandAdapt.shtml ).

California↗

USGS2010

Wind-enhanced resuspension in the shallow waters of South San Francisco Bay: Mechanisms and potential implications for cohesive sediment transport

We investigated the driving forces of sediment dynamics at the shoals in South San Francisco Bay. Two stations were deployed along a line perpendicular to a 14 m deep channel, 1000 and 2000 m from the middle of the channel. Station depths were 2.59 and 2.19 m below mean lower low water, respectively. We used acoustic Doppler velocimeters for the simultaneous determination of current velocities, turbulence, sediment concentration and fluxes. Maximum current shear velocities were 0.015 m s −1 at the station further from the channel (closer to the shore) and 0.02 m s −1 at the station closer to the channel. Peak wave-induced shear velocities exceeded 0.015 m s −1 at both stations. Maximum sediment concentrations were around 30 g m −3 during calm periods (root mean square wave height <0.15 m). During wavy periods, sediment concentrations increased to 100 g m −3 and sediment fluxes were 5 times higher than in calm conditions (0.02 g m −2 s −1 versus >0.10 g m −2 s −1 ) at the station further from the channel 0.36 m above the bed. Closer to the channel, sediment concentrations and vertical fluxes due to wind wave resuspension were persistently lower (maximum concentrations around 50 g m −3 and maximum fluxes around 0.04 g m −2 s −1 ). Most resuspension events occurred during flood tides that followed wave events during low water. Although wave motions are able to resuspend sediment into the wave boundary layer at low tide, the observed large increases in sediment fluxes are due to the nonlinear interaction of wind waves and the tidal currents.

California↗

USGS2008

Evaluation of a Single-Beam Sonar System to Map Seagrass at Two Sites in Northern Puget Sound, Washington

Seagrass at two sites in northern Puget Sound, Possession Point and nearby Browns Bay, was mapped using both a single-beam sonar and underwater video camera. The acoustic and underwater video data were compared to evaluate the accuracy of acoustic estimates of seagrass cover. The accuracy of the acoustic method was calculated for three classifications of seagrass observed in underwater video: bare (no seagrass), patchy seagrass, and continuous seagrass. Acoustic and underwater video methods agreed in 92 percent and 74 percent of observations made in bare and continuous areas, respectively. However, in patchy seagrass, the agreement between acoustic and underwater video was poor (43 percent). The poor agreement between the two methods in areas with patchy seagrass is likely because the two instruments were not precisely colocated. The distribution of seagrass at the two sites differed both in overall percent vegetated and in the distribution of percent cover versus depth. On the basis of acoustic data, seagrass inhabited 0.29 km2 (19 percent of total area) at Possession Point and 0.043 km2 (5 percent of total area) at the Browns Bay study site. The depth distribution at the two sites was markedly different. Whereas the majority of seagrass at Possession Point occurred between -0.5 and -1.5 m MLLW, most seagrass at Browns Bay occurred at a greater depth, between -2.25 and -3.5 m MLLW. Further investigation of the anthropogenic and natural factors causing these differences in distribution is needed.

Scientific Investigations Report↗

USGS2007

Bed forms created by simulated waves and currents in a large flume

The morphology and evolution of bed forms created by combinations of waves and currents were investigated using an oscillating plate in a 4-m-wide flume. Current speed ranged from 0 to 30 cm/s, maximum oscillatory velocity ranged from 20 to 48 cm/s, oscillation period was 8 s (except for one run with 12 s period), and the median grain size was 0.27 mm. The angle between oscillations and current was 90°, 60°, or 45°. At the end of each run the sand bed was photographed and ripple dimensions were measured. Ripple wavelength was also determined from sonar images collected throughout the runs. Increasing the ratio of current to wave (i.e., oscillatory) velocity decreased ripple height and wavelength, in part because of the increased fluid excursion during the wave period. Increasing the ratio of current to waves, or decreasing the angle between current and waves, increased the three-dimensionality of bed forms. During the runs, ripple wavelength increased by a factor of about 2. The average number of wave periods for evolution of ripple wavelength to 90% of its final value was 184 for two-dimensional ripples starting from a flat bed. Bed form orientations at the end of each run were compared to four potential controlling factors: the directions of waves, current, maximum instantaneous bed shear stress, and maximum gross bed form normal transport (MGBNT). The directions of waves and of MGBNT were equally good predictors of bed form orientations, and were significantly better than the other two factors.

Journal of Geophysical Research C: Oceans↗

USGS2007

Seabed ripple morphology and surficial sediment size at the SAX04 experiments near Fort Walton Beach, Florida, fall 2004

Data presented in this report originates from measurements obtained off the Florida coast (fig. 1) as part of the Sediment Acoustics Experiment (SAX04) and Ripples Department Research Initiative (DRI) (Office of Naval Research (ONR), Critical Benthic Environmental Processes and Modeling, Long Range BAA 04-001, Sept. 10, 2003). The aim of this document is to present methods employed to extract data and the resulting measured ripple characteristics (ripple height, wavelength, and orientation) and seabed grain sizes. Application and analysis of the data with respect to hydro- and morphodynamics will be addressed in subsequent reports. Sediment transport in the coastal region is a complex process involving interactions between flow dynamics, sediments, and bedforms. Sediment type and bed geometry directly influence entrainment of sediments into suspension, and at sites where ripples occur (sand formations on the order of several cm high and less than two meter long wavelengths), the understanding of ripple dynamics is an essential component in improving sediment transport models. To gain a better understanding and ability to predict sediment transport, a field study was undertaken to investigate morphology, orientation, and dynamics of ripples on the seafloor. The data obtained from the field campaign also supports an on-going effort to study the effects of ripples on low grazing acoustic penetration into sandy marine sediments for the detection of objects, such as mines (Jackson and others, 2002).

Florida↗

USGS1996

Suspended-solids flux at a shallow-water site in south San Francisco Bay, California

Time series measurements of current velocity and suspended solids-concentration (SSC) made during December 1993 and March 1994 at a shallow-water site in South San Francisco Bay were used to estimate and compare suspended-solids flux during the two periods. In December, the average residual flux at the site was 2.88 g/m/s, to the northeast, whereas in March the average residual flux was four times greater, 12.2 g/m/s, and was directed to the southeast, the direction of flood tide. Residual flux was decomposed and the three components that accounted for most of the flux were analyzed: residual advective flux (Stokes drift flux ( u ' h ' c ), and dispersive flux, which is the tidal cycle correlation between velocity and SSC ( u ' h'c ). During both periods, the Stokes drift flux was to the north, and the dispersive flux was to the southeast. In December, these two components, with nearly opposite directions, had the greatest magnitudes, resulting in a lower total residual flux. In March, the residual advective flux was greater than the Stokes drift flux and was in the same direction as the dispersive flux because of a southeasterly residual current. Wind data indicate that the residual current in March was induced by persistent northwest winds. The southeasterly dispersive flux in March and December was due to generally higher SSC on flood than ebb tides. Increases in SSC frequently occurred at low water (before flood tides). Comparison of calculated bottom orbital velocities to SSC identified wind waves as a mechanism of resuspension.

Conference Paper↗

USGS1995

1994 Annual report: San Francisco Estuary regional monitoring program for trace substances

This is the second Annual Report of the Regional Monitoring Program for Trace Substances (RMP). It describes concentrations of pollutants in water, sediment, and tissue samples of oysters, mussels, and clams at 15 to 24 sampling locations for three discrete sampling events - during the wet season in February, in April during a period of declining Delta outflows, and during the dry season in August. The sampling stations are located throughout the Estuary and at the major tributaries including the mouths of the Guadalupe River and Coyote Creek in the extreme southern portions of the Estuary to the confluence of the Sacramento and San Joaquin Rivers (Figure 1). In 1994, water samples were also taken at two upstream locations at Rio Vista (Sacramento River) and Manteca (San Joaquin River) to determine river contaminant concentrations during a six-week period in spring.

California↗