Geology topics

J.R. Lacy

Publications and source records attributed to J.R. Lacy.

5 recordsLinked to original sources

USGS2011

An inexpensive instrument for measuring wave exposure and water velocity

Ocean waves drive a wide variety of nearshore physical processes, structuring entire ecosystems through their direct and indirect effects on the settlement, behavior, and survivorship of marine organisms. However, wave exposure remains difficult and expensive to measure. Here, we report on an inexpensive and easily constructed instrument for measuring wave-induced water velocities. The underwater relative swell kinetics instrument (URSKI) is a subsurface float tethered by a short (<1 m) line to the seafloor. Contained within the float is an accelerometer that records the tilt of the float in response to passing waves. During two field trials totaling 358 h, we confirmed the accuracy and precision of URSKI measurements through comparison to velocities measured by an in situ acoustic Doppler velocimeter and those predicted by a standard swell model, and we evaluated how the dimensions of the devices, its buoyancy, and sampling frequency can be modified for use in a variety of environments.

Limnology and Oceanography: Methods↗

USGS2005

Estimating hydrodynamic roughness in a wave-dominated environment with a high-resolution acoustic Doppler profiler

Hydrodynamic roughness is a critical parameter for characterizing bottom drag in boundary layers, and it varies both spatially and temporally due to variation in grain size, bedforms, and saltating sediment. In this paper we investigate temporal variability in hydrodynamic roughness using velocity profiles in the bottom boundary layer measured with a high-resolution acoustic Doppler profiler (PCADP). The data were collected on the ebb-tidal delta off Grays Harbor, Washington, in a mean water depth of 9 m. Significant wave height ranged from 0.5 to 3 m. Bottom roughness has rarely been determined from hydrodynamic measurements under conditions such as these, where energetic waves and medium-to-fine sand produce small bedforms. Friction velocity due to current u*c and apparent bottom roughness z0a were determined from the PCADP burst mean velocity profiles using the law of the wall. Bottom roughness kB was estimated by applying the Grant-Madsen model for wave-current interaction iteratively until the model u*c converged with values determined from the data. The resulting kB values ranged over 3 orders of magnitude (10-1 to 10-4 m) and varied inversely with wave orbital diameter. This range of kB influences predicted bottom shear stress considerably, suggesting that the use of time-varying bottom roughness could significantly improve the accuracy of sediment transport models. Bedform height was estimated from kB and is consistent with both ripple heights predicted by empirical models and bedforms in sonar images collected during the experiment. Copyright 2005 by the American Geophysical Union.

Journal of Geophysical Research C: Oceans↗

USGS2005

Hydratools, a MATLAB® based data processing package for Sontek Hydra data

The U.S. Geological Survey (USGS) has developed a set of MATLAB tools to process and convert data collected by Sontek Hydra instruments to netCDF, which is a format used by the USGS to process and archive oceanographic time-series data. The USGS makes high-resolution current measurements within 1.5 meters of the bottom. These data are used in combination with other instrument data from sediment transport studies to develop sediment transport models. Instrument manufacturers provide software which outputs unique binary data formats. Multiple data formats are cumbersome. The USGS solution is to translate data streams into a common data format: netCDF. The Hydratools toolbox is written to create netCDF format files following EPIC conventions, complete with embedded metadata. Data are accepted from both the ADV and the PCADP. The toolbox will detect and remove bad data, substitute other sources of heading and tilt measurements if necessary, apply ambiguity corrections, calculate statistics, return information about data quality, and organize metadata. Standardized processing and archiving makes these data more easily and routinely accessible locally and over the Internet. In addition, documentation of the techniques used in the toolbox provides a baseline reference for others utilizing the data.

Conference Paper↗

USGS2004

Accuracy of a pulse-coherent acoustic Doppler profiler in a wave-dominated flow

The accuracy of velocities measured by a pulse-coherent acoustic Doppler profiler (PCADP) in the bottom boundary layer of a wave-dominated inner-shelf environment is evaluated. The downward-looking PCADP measured velocities in eight 10-cm cells at 1 Hz. Velocities measured by the PCADP are compared to those measured by an acoustic Doppler velocimeter for wave orbital velocities up to 95 cm s-1 and currents up to 40 cm s-1. An algorithm for correcting ambiguity errors using the resolution velocities was developed. Instrument bias, measured as the average error in burst mean speed, is -0.4 cm s-1 (standard deviation = 0.8). The accuracy (root-mean-square error) of instantaneous velocities has a mean of 8.6 cm s-1 (standard deviation = 6.5) for eastward velocities (the predominant direction of waves), 6.5 cm s-1 (standard deviation = 4.4) for northward velocities, and 2.4 cm s-1 (standard deviation = 1.6) for vertical velocities. Both burst mean and root-mean-square errors are greater for bursts with ub ??? 50 cm s-1. Profiles of burst mean speeds from the bottom five cells were fit to logarithmic curves: 92% of bursts with mean speed ??? 5 cm s-1 have a correlation coefficient R2 > 0.96. In cells close to the transducer, instantaneous velocities are noisy, burst mean velocities are biased low, and bottom orbital velocities are biased high. With adequate blanking distances for both the profile and resolution velocities, the PCADP provides sufficient accuracy to measure velocities in the bottom boundary layer under moderately energetic inner-shelf conditions.

Journal of Atmospheric and Oceanic Technology↗

USGS2003

Interaction of lateral baroclinic forcing and turbulence in an estuary

Observations of density and velocity in a channel in northern San Francisco Bay show that the onset of vertical density stratification during flood tides is controlled by the balance between the cross-channel baroclinic pressure gradient and vertical mixing due to turbulence. Profiles of velocity, salinity, temperature, and suspended sediment concentration were measured in transects across Suisun Cutoff, in northern San Francisco Bay, on two days over the 12.5-hour tidal cycle. During flood tides an axial density front developed between fresher water flowing from the shallows of Grizzly Bay into the northern side of Suisun Cutoff and saltier water flowing up the channel. North of the front, transverse currents were driven by the lateral salinity gradient, with a top-to-bottom velocity difference greater than 30 cm/s. South of the front, the secondary circulation was weak, and along-channel velocities were greater than to the north. The gradient Richardson number shows that stratification was stable north of the front, while the water column was turbulently mixed south of the front. Time-series measurements of velocity and salinity demonstrate that the front develops during each tidal cycle. In estuaries, longitudinal dynamics predict less stratification during flood than ebb tides. These data show that stratification can develop during flood tides due to a lateral baroclinic pressure gradient in estuaries with complex bathymetry.

Journal of Geophysical Research C: Oceans↗