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David H. Schoellhamer

Publications and source records attributed to David H. Schoellhamer.

At least 55 records · Page 3Linked to original sources
USGS2010

Measuring sediment accretion in early tidal marsh restoration

Sediment accretion is a critical indicator of initial progress in tidal marsh restoration. However, it is often difficult to measure early deposition rates, because the bottom surface is usually obscured under turbid, tidally-influenced waters. To accurately measure early sediment deposition in marshes, we developed an echosounder system consisting of a specialized acoustic profiler, differential global positioning system unit, and laptop computer mounted on a shallow-draft boat. We conducted a bathymetry at the Tubbs Setback tidal restoration site on San Pablo Bay, California, along north–south transects at 25-m intervals. Horizontal position was recorded within 1 m each second and water depth to 1 cm every 0.05 s. Bottom elevations were adjusted for tidal height with surveyed tide gages. We created detailed bathymetric maps (grid cell size: 12.5 m x 12.5 m) by interpolation with inverse distance weighting. During the third year after restoration, sediment accretion averaged 57.1 ± 1.1 cm and the estimated sediment gain was 132,900 m3. The mean difference between the elevations from the bathymetry system and the 9 sediment pins was 2.0 ± 1.0 cm. The mean difference of the intersection points of east–west and north–south survey transects was 2.1 ± 0.2 cm, which provided a measure of repeatability with changing water levels. Our echosounder system provided accurate and repeatable measurements of sediment accretion of a recently restored tidal wetland, and this system proved to be a viable tool for determining sediment deposition in marshes and assessing early restoration progress.

California↗
USGS2009

Hindcasting of decadal‐timescale estuarine bathymetric change with a tidal‐timescale model

Hindcasting decadal-timescale bathymetric change in estuaries is prone to error due to limited data for initial conditions, boundary forcing, and calibration; computational limitations further hinder efforts. We developed and calibrated a tidal-timescale model to bathymetric change in Suisun Bay, California, over the 1867–1887 period. A general, multiple-timescale calibration ensured robustness over all timescales; two input reduction methods, the morphological hydrograph and the morphological acceleration factor, were applied at the decadal timescale. The model was calibrated to net bathymetric change in the entire basin; average error for bathymetric change over individual depth ranges was 37%. On a model cell-by-cell basis, performance for spatial amplitude correlation was poor over the majority of the domain, though spatial phase correlation was better, with 61% of the domain correctly indicated as erosional or depositional. Poor agreement was likely caused by the specification of initial bed composition, which was unknown during the 1867–1887 period. Cross-sectional bathymetric change between channels and flats, driven primarily by wind wave resuspension, was modeled with higher skill than longitudinal change, which is driven in part by gravitational circulation. The accelerated response of depth may have prevented gravitational circulation from being represented properly. As performance criteria became more stringent in a spatial sense, the error of the model increased. While these methods are useful for estimating basin-scale sedimentation changes, they may not be suitable for predicting specific locations of erosion or deposition. They do, however, provide a foundation for realistic estuarine geomorphic modeling applications.

California↗
USGS2009

A Tidally Averaged Sediment-Transport Model for San Francisco Bay, California

A tidally averaged sediment-transport model of San Francisco Bay was incorporated into a tidally averaged salinity box model previously developed and calibrated using salinity, a conservative tracer (Uncles and Peterson, 1995; Knowles, 1996). The Bay is represented in the model by 50 segments composed of two layers: one representing the channel (>5-meter depth) and the other the shallows (0- to 5-meter depth). Calculations are made using a daily time step and simulations can be made on the decadal time scale. The sediment-transport model includes an erosion-deposition algorithm, a bed-sediment algorithm, and sediment boundary conditions. Erosion and deposition of bed sediments are calculated explicitly, and suspended sediment is transported by implicitly solving the advection-dispersion equation. The bed-sediment model simulates the increase in bed strength with depth, owing to consolidation of fine sediments that make up San Francisco Bay mud. The model is calibrated to either net sedimentation calculated from bathymetric-change data or measured suspended-sediment concentration. Specified boundary conditions are the tributary fluxes of suspended sediment and suspended-sediment concentration in the Pacific Ocean. Results of model calibration and validation show that the model simulates the trends in suspended-sediment concentration associated with tidal fluctuations, residual velocity, and wind stress well, although the spring neap tidal suspended-sediment concentration variability was consistently underestimated. Model validation also showed poor simulation of seasonal sediment pulses from the Sacramento-San Joaquin River Delta at Point San Pablo because the pulses enter the Bay over only a few days and the fate of the pulses is determined by intra-tidal deposition and resuspension that are not included in this tidally averaged model. The model was calibrated to net-basin sedimentation to calculate budgets of sediment and sediment-associated contaminants. While simulated net sedimentation in the four basins that comprise San Francisco Bay was correct, the simulations incorrectly eroded shallows while channels deposited because model surface-layer boxes span both shallows and channels, and neglect lateral variability of suspended-sediment concentration. Validation with recent (1983-2005) net sedimentation in South San Francisco Bay was poor, perhaps owing to poorly quantified sediment supply, and to invasive species that altered erosion and deposition processes. This demonstrates that deterministically predicting future sedimentation is difficult in this or any estuary for which boundary conditions are not stationary. The model would best be used as a tool for developing past and present sediment budgets, and for creating scenarios of future sedimentation that are compared to one another rather than considered a deterministic prediction.

Scientific Investigations Report↗
USGS2008

Dissolved Oxygen in Guadalupe Slough and Pond A3W, South San Francisco Bay, California, August and September 2007

Initial restoration of former salt evaporation ponds under the South Bay Salt Pond Restoration Project in San Francisco Bay included the changing of water-flow patterns and the monitoring of water quality of discharge waters from the ponds. Low dissolved oxygen (DO) concentrations became evident in discharge waters when the ponds first were opened in 2004. This was a concern, because of the potential for low-DO pond discharge to decrease the DO concentrations in the sloughs that receive water from the ponds. However, as of summer 2007, only limited point-measurements of DO concentrations had been made in the receiving sloughs adjacent to the discharge ponds. In this report, we describe two short studies aimed at understanding the natural variability of slough DO and the effect of pond discharge on the DO concentrations in the sloughs. Pond A3W (a discharge pond) and the adjacent Guadalupe Slough were instrumented in August and September 2007 to measure DO, temperature, conductivity, and pH. In addition, Mowry and Newark Sloughs were instrumented during the August study to document DO variability in nearby sloughs that were unaffected by pond discharge. The results showed that natural tidal variability in the slough appeared to dominate and control the slough DO concentrations. Water-quality parameters between Guadalupe Slough and Mowry and Newark Sloughs could not be directly compared because deployment locations were different distances from the bay. Pond-discharge water was identified in Guadalupe Slough using the deployed instruments, but, counter to the previous assumption, the pond discharge, at times, increased DO concentrations in the slough. The effects of altering the volume of pond discharge were overwhelmed by natural spring-neap tidal variability in the slough. This work represents a preliminary investigation by the U.S. Geological Survey of the effects of pond discharge on adjacent sloughs, and the results will be used in designing a comprehensive DO study to determine normal variability for this region.

Open-File Report↗
USGS2008

Water quality

Sustainable water policy in California will require maintaining or improving water quality. The Delta is an important source of drinking water for Californians, but sustaining a quality sufficient for human and agricultural consumption presents a number of problems and challenges to water managers. Similarly, poor environmental water quality is recognized as one of the influential stressors contributing to the ecological problems of the Delta (Bennett 2005; Kimmerer 2004).

California↗
USGS2007

Management of fluid mud in estuaries, bays, and lakes. II: Measurement, modeling, and management

Techniques for measurement, modeling, and management of fluid mud are available, but research is needed to improve them. Fluid mud can be difficult to detect, measure, or sample, which has led to new instruments and new ways of using existing instruments. Multifrequency acoustic fathometers sense neither density nor viscosity and are, therefore, unreliable in measuring fluid mud. Nuclear density probes, towed sleds, seismic, and drop probes equipped with density meters offer the potential for accurate measurements. Numerical modeling of fluid mud requires solving governing equations for flow velocity, density, pressure, salinity, water surface, plus sediment submodels. A number of such models exist in one-, two-, and three-dimensional form, but they rely on empirical relationships that require substantial site-specific validation to observations. Management of fluid mud techniques can be classified as those that accomplish: Source control, formation control, and removal. Nautical depth, a fourth category, defines the channel bottom as a specific fluid mud density or alternative parameter as safe for navigation. Source control includes watershed management measures to keep fine sediment out of waterways and in-water measures such as structures and traps. Formation control methods include streamlined channels and structures plus other measures to reduce flocculation and structures that train currents. Removal methods include the traditional dredging and transport of dredged material plus agitation that contributes to formation control and/or nautical depth. Conditioning of fluid mud by dredging and aerating offers the possibility of improved navigability. Two examples—the Atchafalaya Bar Channel and Savannah Harbor—illustrate the use of measurements and management of fluid mud.

Journal of Hydraulic Engineering↗
USGS2007

PONDCALC: A tool to estimate discharge from the Alviso Salt Ponds, California

Former commercial salt ponds in Alviso, California, now are operated by the U.S. Fish and Wildlife Service (USFWS) to provide habitat for birds. The USFWS has modified the operation of the ponds to prevent exceedingly high salinity. Ponds that were formerly hydraulically isolated from South San Francisco Bay and adjacent sloughs now are managed as flow-through ponds, and some are allowed to discharge to the Bay and sloughs. This discharge is allowed under a permit issued by the Regional Water Quality Control Board. As a requirement of the permit, the USFWS must estimate the amount of discharge from each discharge pond for the period May through November of each year. To facilitate the accurate estimation of pond discharge, a calculation methodology (hereafter referred to as 'calculator' or PONDCALC) for the discharging Alviso ponds has been developed as a Microsoft Excel file and is presented in this report. The presence of flap gates on one end of the discharge culverts, which allow only outflow from a pond, complicates the hydraulic analysis of flow through the culverts. The equation typically used for culvert flow contains an energy loss coefficient that had to be determined empirically using measured water discharge and head at the discharge structure of one of the ponds. A standard weir-flow equation is included in PONDCALC for discharge calculation in the ponds having weir box structures in addition to culverts. The resulting methodology is applicable only to the five Alviso ponds (A2W, A3W, A7, A14, and A16) that discharge to South San Francisco Bay or adjacent sloughs under the management practices for 2005.

California↗
USGS2007

Box Model of a Series of Salt Ponds, as Applied to the Alviso Salt Pond Complex, South San Francisco Bay, California

This report documents the development and application of a box model to simulate water level, salinity, and temperature of the Alviso Salt Pond Complex in South San Francisco Bay. These ponds were purchased for restoration in 2003 and currently are managed by the U.S. Fish and Wildlife Service to maintain existing wildlife habitat and prevent a build up of salt during the development of a long-term restoration plan. The model was developed for the purpose of aiding pond managers during the current interim management period to achieve these goals. A previously developed box model of a salt pond, SPOOM, which calculates daily pond volume and salinity, was reconfigured to simulate multiple connected ponds and a temperature subroutine was added. The updated model simulates rainfall, evaporation, water flowing between the ponds and the adjacent tidal slough network, and water flowing from one pond to the next by gravity and pumps. Theoretical and measured relations between discharge and corresponding differences in water level are used to simulate most flows between ponds and between ponds and sloughs. The principle of conservation of mass is used to calculate daily pond volume and salinity. The model configuration includes management actions specified in the Interim Stewardship Plan for the ponds. The temperature subroutine calculates hourly net heat transfer to or from a pond resulting in a rise or drop in pond temperature and daily average, minimum, and maximum pond temperatures are recorded. Simulated temperature was compared with hourly measured data from pond 3 of the Napa?Sonoma Salt Pond Complex and monthly measured data from pond A14 of the Alviso Salt-Pond Complex. Comparison showed good agreement of measured and simulated pond temperature on the daily and monthly time scales.

Scientific Investigations Report↗
USGS2006

Flow convergence caused by a salinity minimum in a tidal channel

Residence times of dissolved substances and sedimentation rates in tidal channels are affected by residual (tidally averaged) circulation patterns. One influence on these circulation patterns is the longitudinal density gradient. In most estuaries the longitudinal density gradient typically maintains a constant direction. However, a junction of tidal channels can create a local reversal (change in sign) of the density gradient. This can occur due to a difference in the phase of tidal currents in each channel. In San Francisco Bay, the phasing of the currents at the junction of Mare Island Strait and Carquinez Strait produces a local salinity minimum in Mare Island Strait. At the location of a local salinity minimum the longitudinal density gradient reverses direction. This paper presents four numerical models that were used to investigate the circulation caused by the salinity minimum: (1) A simple one-dimensional (1D) finite difference model demonstrates that a local salinity minimum is advected into Mare Island Strait from the junction with Carquinez Strait during flood tide. (2) A three-dimensional (3D) hydrodynamic finite element model is used to compute the tidally averaged circulation in a channel that contains a salinity minimum (a change in the sign of the longitudinal density gradient) and compares that to a channel that contains a longitudinal density gradient in a constant direction. The tidally averaged circulation produced by the salinity minimum is characterized by converging flow at the bed and diverging flow at the surface, whereas the circulation produced by the constant direction gradient is characterized by converging flow at the bed and downstream surface currents. These velocity fields are used to drive both a particle tracking and a sediment transport model. (3) A particle tracking model demonstrates a 30 percent increase in the residence time of neutrally buoyant particles transported through the salinity minimum, as compared to transport through a constant direction density gradient. (4) A sediment transport model demonstrates increased deposition at the near-bed null point of the salinity minimum, as compared to the constant direction gradient null point. These results are corroborated by historically noted large sedimentation rates and a local maximum of selenium accumulation in clams at the null point in Mare Island Strait.

California↗
USGS2005

Bay sediment budget: Sediment accounting 101

Comparison of a budget developed for 1955-1990 with a budget developed for 1995- 2002 showed decreasing sediment inflow and increased amounts leaving the Bay to upland disposal and sand mining, resulting in an increased rate of erosion of sediment from the Bay floor Finding a way to shift disposal from the Ocean back to the Bay could provide sediment for restoration projects and decrease dredging costs Increased erosion of the Bay is mobilizing legacy contaminants from the sediment bed Restoration projects could increase erosion and mobilization of legacy contaminants Sand mining, ignored in previous budgets, removes almost twice as much sediment from the Bay as dredging

California↗
USGS2004

Spatial and temporal variability of suspended-sediment concentrations in a shallow estuarine environment

Shallow subembayments respond differently than deep channels to physical forces acting in estuaries. The U.S. Geological Survey measured suspended-sediment concentrations at five locations in Honker Bay, a shallow subembayment of San Francisco Bay, and the adjacent channel to investigate the spatial and temporal differences between deep and shallow estuarine environments. During the first freshwater pulse of the wet season, the channel tended to transport suspended sediments through the system, whereas the shallow area acted as off-channel storage where deposition would likely occur. Following the freshwater pulse, suspended-sediment concentrations were greater in Honker Bay than in the adjacent deep channel, due to the larger supply of erodible sediment on the bed. However, the tidal variability of suspended-sediment concentrations in both Honker Bay and in the adjacent channel was greater after the freshwater pulse than before. During wind events, suspended-sediment concentrations in the channel were not affected; however, wind played a crucial role in the resuspension of sediments in the shallows. Despite wind-wave sediment resuspension in Honker Bay, tidally averaged suspended-sediment flux was controlled by the flood-dominated currents.

California↗
USGS2004

Trends in the sediment yield of the Sacramento River, California, 1957-2001

Human activities within a watershed, such as agriculture, urbanization, and dam building, may affect the sediment yield from the watershed. Because the equilibrium geomorphic form of an estuary is dependent in part on the sediment supply from the watershed, anthropogenic activities within the watershed have the potential to affect estuary geomorphology. The Sacramento River drains the northern half of California’s Central Valley and is the primary source of sediment to San Francisco Bay. In this paper, it is shown that the delivery of suspended-sediment from the Sacramento River to San Francisco Bay has decreased by about one-half during the period 1957 to 2001. Many factors may be contributing to the trend in sediment yield, including the depletion of erodible sediment from hydraulic mining in the late 1800s, trapping of sediment in reservoirs, riverbank protection, altered land-uses (such as agriculture, grazing, urbanization, and logging), and levees. This finding has implications for planned tidal wetland restoration activities around San Francisco Bay, where an adequate sediment supply will be needed to build subsided areas to elevations typical of tidal wetlands as well as to keep pace with projected sea-level rise. In a broader context, the study underscores the need to address anthropogenic impacts on watershed sediment yield when considering actions such as restoration within downstream depositional areas.

California↗
USGS2004

Salt-Pond Box Model (SPOOM) and Its Application to the Napa-Sonoma Salt Ponds, San Francisco Bay, California

A box model to simulate water volume and salinity of a salt pond has been developed by the U.S. Geological Survey to obtain water and salinity budgets. The model, SPOOM, uses the principle of conservation of mass to calculate daily pond volume and salinity and includes a salt crystallization and dissolution algorithm. Model inputs include precipitation, evaporation, infiltration, and water transfers. Salinity and water-surface-elevation data were collected monthly in the Napa-Sonoma Salt-Pond Complex from February 1999 through September 2001 and were used to calibrate and validate the model. The months when water transfers occurred were known but the magnitudes were unknown, so the magnitudes of water transfers were adjusted in the model to calibrate simulated pond volumes to measured pond volumes for three ponds. Modeled salinity was then compared with measured salinity, which remained a free parameter, in order to validate the model. Comparison showed good correlation between modeled and measured salinity. Deviations can be attributed to lack of water-transfer information. Water and salinity budgets obtained through modeling will be used to help interpret ecological data from the ponds. This model has been formulated to be applicable to the Napa-Sonoma salt ponds, but can be applied to other salt ponds.

Water-Resources Investigations Report↗
USGS2002

Effects of tidal current phase at the junction of two straits

Estuaries typically have a monotonic increase in salinity from freshwater at the head of the estuary to ocean water at the mouth, creating a consistent direction for the longitudinal baroclinic pressure gradient. However, Mare Island Strait in San Francisco Bay has a local salinity minimum created by the phasing of the currents at the junction of Mare Island and Carquinez Straits. The salinity minimum creates converging baroclinic pressure gradients in Mare Island Strait. Equipment was deployed at four stations in the straits for 6 months from September 1997 to March 1998 to measure tidal variability of velocity, conductivity, temperature, depth, and suspended sediment concentration. Analysis of the measured time series shows that on a tidal time scale in Mare Island Strait, the landward and seaward baroclinic pressure gradients in the local salinity minimum interact with the barotropic gradient, creating regions of enhanced shear in the water column during the flood and reduced shear during the ebb. On a tidally averaged time scale, baroclinic pressure gradients converge on the tidally averaged salinity minimum and drive a converging near-bed and diverging surface current circulation pattern, forming a "baroclinic convergence zone" in Mare Island Strait. Historically large sedimentation rates in this area are attributed to the convergence zone.

California↗
USGS2001

Hydrodynamic and suspended-solids concentration measurements in Suisun Bay, California, 1995

Sea level, current velocity, water temperature, salinity (computed from conductivity and temperature), and suspended-solids data collected in Suisun Bay, California, from May 30, 1995, through October 27, 1995, by the U.S. Geological Survey are documented in this report. Data were collected concurrently at 21 sites. Various parameters were measured at each site. Velocity-profile data were collected at 6 sites, single-point velocity measurements were made at 9 sites, salinity data were collected at 20 sites, and suspended-solids concentrations were measured at 10 sites. Sea-level and velocity data are presented in three forms; harmonic analysis results; time-series plots (sea level, current speed, and current direction versus time); and time-series plots of low-pass-filtered time series. Temperature, salinity, and suspended-solids data are presented as plots of raw and low-pass-filtered time series.The velocity and salinity data presented in this report document a period when the residual current patterns and salt field were transitioning from a freshwater-inflow-dominated condition towards a quasi steady-state summer condition when density-driven circulation and tidal nonlinearities became relatively more important as long-term transport mechanisms. Sacramento-San Joaquin River Delta outflow was high prior to and during this study, so the tidally averaged salinities were abnormally low for this time of year. For example, the tidally averaged salinities varied from 0-12 at Martinez, the western border of Suisun Bay, to a maximum of 2 at Mallard Island, the eastern border of Suisun Bay. Even though salinities increased overall in Suisun Bay during the study period, the near-bed residual currents primarily were directed seaward. Therefore, salinity intrusion through Suisun Bay towards the Delta primarily was accomplished in the absence of the tidally averaged, two-layer flow known as gravitational circulation where, by definition, the net currents are landward at the bed. The Folsom Dam spillway gate failure on July 17, 1995, was analyzed to determine the effect on the hydrodynamics of Suisun Bay. The peak flow of the American River reached roughly 1,000 cubic meters per second as a result of the failure, which is relatively small. This was roughly 15 percent of the approximate 7,000 cubic meters per second tidal flows that occur daily in Suisun Bay and was likely attenuated greatly. Based on analysis of tidally averaged near-bed salinity and depth-averaged currents after the failure, the effect was essentially nonexistent and is indistinguishable from the natural variability.

Water-Resources Investigations Report↗