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Geology topics

D. H. Schoellhamer

Publications and source records attributed to D. H. Schoellhamer.

At least 55 records · Page 3Linked to original sources
USGS2001

Singular spectrum analysis for time series with missing data

Geophysical time series often contain missing data, which prevents analysis with many signal processing and multivariate tools. A modification of singular spectrum analysis for time series with missing data is developed and successfully tested with synthetic and actual incomplete time series of suspended-sediment concentration from San Francisco Bay. This method also can be used to low pass filter incomplete time series.

Geophysical Research Letters↗
USGS2001

Combined use of remote sensing and continuous monitoring to analyse the variability of suspended-sediment concentrations in San Francisco Bay, California

Analysis of suspended-sediment concentration data in San Francisco Bay is complicated by spatial and temporal variability. In situ optical backscatterance sensors provide continuous suspended-sediment concentration data, but inaccessibility, vandalism, and cost limit the number of potential monitoring stations. Satellite imagery reveals the spatial distribution of surficial-suspended sediment concentrations in the Bay; however, temporal resolution is poor. Analysis of the in situ sensor data in conjunction with the satellite reflectance data shows the effects of physical processes on both the spatial and temporal distribution of suspended sediment in San Francisco Bay. Plumes can be created by large freshwater flows. Zones of high suspended-sediment concentrations in shallow subembayments are associated with wind-wave resuspension and the spring-neap cycle. Filaments of clear and turbid water are caused by different transport processes in deep channels, as opposed to adjacent shallow water.

Estuarine, Coastal and Shelf Science↗
USGS1996

Factors affecting suspended-solids concentrations in South San Francisco Bay, California

Measurements of suspended-solids concentration (SSC) were made at two depths at three sites in South San Francisco Bay (South Bay) to determine the factors that affect SSC. Twenty-eight segments of reliable and continuous SSC time series data longer than 14 days were collected from late 1991 or 1992 through September 1993. Spectral analysis and singular spectrum analysis were used to relate these data segments to time series of several potential forcing factors, including diurnal and semidiurnal tides, the spring-neap tidal cycle, wind shear, freshwater runoff, and longitudinal density differences. SSC is greatest during summer, when a landward wind shear is applied to South Bay by the afternoon sea breeze. About one half the variance of SSC is caused by the spring-neap cycle, and SSC lags the spring-neap cycle by about 2 days. Relatively short duration of slack water limits the duration of deposition of suspended solids and consolidation of newly deposited bed sediment during the tidal cycle, so suspended solids accumulate in the water column as a spring tide is approached and slowly deposit as a neap tide is approached. Perturbations in SSC caused by wind and local runoff from winter storms during the study period were usually much smaller than SSC variations caused by the spring-neap cycle. Variations of SSC at the study sites at tidal timescales are tidally forced, and nonlinear physical processes are significant. Advective transport dominates during spring tides, when water with higher SSC due to wind wave resuspension is advected to the main channel from shallow water, but, during neap tides, advective transport is less significant. The findings of this and other studies indicate that the tidally averaged transport of suspended solids responds to seasonal variations of wind shear in South Bay.

Journal of Geophysical Research C: Oceans↗
USGS1996

Anthropogenic sediment resuspension mechanisms in a shallow microtidal estuary

The mechanisms that resuspend bottom sediments in Hillsborough Bay, a shallow, microtidal, subtropical estuary in West-central Florida, were determined by analysing hydrodynamic and suspended-solids concentration data collected during several instrument deployments made in 1990 and 1991. Large vessels in a dredged ship channel can generate forced solitary long waves that cause large water velocities and sediment resuspension at the study sites. An experiment was conducted with a trawler that resuspended bottom sediments, and some of the resuspended sediments remained in suspension for at least 8 h. A secondary impact of vessel-generated long waves and trawling is that sediments that are resuspended and newly deposited are more susceptible to resuspension by tidal currents than undisturbed bottom sediments. Natural sediment resuspension by wind waves and tidal current is less frequent or of smaller magnitude than anthropogenic sediment resuspension. The annual mass of sediment resuspended by vessel-generated long waves is estimated to be one order of magnitude greater than the annual mass of sediment resuspended by wind waves generated by winter storms.

Estuarine, Coastal and Shelf Science↗
USGS1996

Summary of suspended-solids concentration data, San Francisco Bay, California, water year 1994

Suspended-solids concentration data were collected in San Francisco Bay during water year 1994. Optical backscatterance sensors and water samples were used to monitor suspended solids continuously at two sites in Suisun Bay, two sites in Central San Francisco Bay, and three sites in South San Francisco Bay. Sensors were positioned at two depths at each site. In addition, a shallow-water instrument package was deployed in South San Francisco Bay three times for periods of several weeks to measure suspended-solids concentration and water velocity. Water samples were collected periodically and were analyzed for concentrations of suspended solids. The results of the analyses were used to calibrate the electrical output of the optical backscatterance sensors. This report presents the data-collection methods used and summarizes the suspended-solids concentration data collected from October 1993 through September 1994. Calibration plots and edited data for each sensor also are presented.

California↗
USGS1996

Summary of suspended-solids concentration data, San Francisco Bay, California, Water Year 1995

Suspended-solids concentration data were collected in San Francisco Bay during water year 1995 (October 1, 1994September 30, 1995). Optical backscatterance sensors and water samples were used to monitor suspended solids continuously at two sites in Suisun Bay, two sites in Central San Francisco Bay, and three sites in South San Francisco Bay. Sensors were positioned at two depths at each site. Water samples were collected periodically and were analyzed for concentrations of suspended solids. The results of the analyses were used to calibrate the electrical output of the optical backscatterance sensors. This report presents the data-collection methods used and summarizes the suspended-solids concentration data collected from October 1994 through September 1995. Calibration plots and edited data for each sensor also are presented.

Open-File Report↗
USGS1995

Sediment resuspension mechanisms in Old Tampa Bay, Florida

The mechanisms that resuspend bottom sediments in Old Tampa Bay, a shallow, microtidal, subtropical estuary in west-central Florida, were determined by analysing data collected during several periods from 1988 to 1990. Hydrodynamic and suspended-solids concentration data were collected at a relatively deep (4 m) site where a permanent platform was built and at a relatively shallow (1·5 m) site where a submersible instrument package was deployed. Bottom sediments were non-cohesive silts and fine sands. The primary sediment resuspension mechanism at both sites was wind waves, which were generated by strong and sustained winds associated with winter storms and tropical storms. At the platform, waves were depth-transitional, and estimated bottom shear stresses were most sensitive to wave period and water depth. Concentrations of suspended solids at this site corresponded well with wave motion, and non-linear wave-current interaction was small. At the shallow-water site, concentrations of suspended solids were elevated during periods of strong north-easterly winds and large bottom orbital velocities. At both sites, wind direction was an important factor in determining the occurrence and magnitude of sediment resuspension. Resuspended sediments settled within several hours as storm intensity diminished. Winds and waves generated by thunderstorms were more transient than those generated by winter storms and tropical storms. Based on the data collected during this study, thunderstorms are less likely to resuspend bottom sediment than winter storms and tropical storms. Maximum tidal currents at the study sites are usually less than 15 cm s −1 and did not increase observed concentrations of suspended solids.

Florida↗
USGS1995

Summary of sediment resuspension monitoring activities, Old Tampa Bay and Hillsborough Bay, Florida, 1988-91

Sediment resuspension was studied in Old Tampa BaY, the northwestern subembayment of Tampa Bay, and in Hillsborough Bay, the northeastern subembayment of Tampa Bay located along the coast of west-central Florida. Electromagnetic current meters, optical backscatterance sensors, and water samples were used to collect sediment resuspension data at several study sites in the two bays. This report describes the data-collection methods and summaries sediment resuspension data collected intermittently from October 1988 through December 1990 in Old Tampa Bay and from March 1990 through September 1991 in Hillsborough Bay. Example data are presented for selected monitoring events.

Water-Resources Investigations Report↗
USGS1993

Biological interference of optical backscatterance sensors in Tampa Bay, Florida

Optical backscatterance (OBS, D&A Instruments, Inc. 1 ) sensors for measuring suspended-solids concentrations have been deployed in Tampa Bay to monitor resuspension of bottom sediments. This paper describes biological factors that affected the OBS sensors deployed in Tampa Bay and discusses deployment strategies that minimize biological interference. Phytoplankton may interfere with the OBS sensors when the suspended-solids concentration is near or below the sensor response threshold. Fish swimming in front of the OBS sensors caused spikes in the OBS sensor output, so the median average was more appropriate than the mean average. An algal slime on the OBS sensors caused excessive backscatterance that dominated the backscatterance from suspended material. Because of the fouling problem, deployments were limited to less than a week, and OBS sensors were cleaned daily, if possible. Calibration of OBS sensors with water samples collected from Tampa Bay was satisfactory when biological interference was not significant. When properly deployed, the OBS sensors can successfully monitor sediment resuspension in Tampa Bay and similar subtropical estuaries.

Marine Geology↗