Progressive Vector Diagrams Research Articles

Bacterial Impacts of Ocean Outfalls: Legal Challenges

Simple analytical methods are used to help establish wastewater treatment permit conditions. However, a recent lawsuit alleged one such screening method was inadequate to show bacterial water quality standards would be protected shoreward of Honolulu's Honouliuli outfall. In response, a progressive vector diagram (PVD) and initial dilution plume model were combined to better estimate the frequency of pathogenic bacteria reaching beaches. For each period in a representative current meter data record a vector summation process was applied until the distance to a sensitive site was reached. Corresponding travel times were noted. The United States Environmental Protection Agency (EPA) PLUMES UM model and farfield algorithm were used to estimate dilutions. Sometimes dilution alone reduces concentrations below the state standard. Most importantly, however, the analysis shows an order of magnitude reduction in frequency of incidence at the site of concern compared to the simple current rose screening method. The PVD method also better estimates travel times, showing that exposure to bacteriologically lethal sunlight is common. The approach and subsequent independent work support the existing permit.

On Long-Term Net Flow over Great Bahama Bank

Abstract A 398-day time series of middepth current measurements is combined with available wind and bottom pressure measurements and historical salinity data to characterize long-term net flow patterns over Great Bahama Bank between the Tongue of the Ocean and Exuma Sound. The progressive vector diagram constructed from current measurements indicates a resultant flow of just over 2 cm s−1 toward 002°. Semidiurnal tides dominate the instantaneous current. A one-dimensional numerical model is used to investigate the local response to regional forcing. Results suggest that the long-term net flow is a response to the combined effect of wind-driven and density-driven flow toward the northwest, and a tide-induced residual current toward the east-southeast, aided by a quasi-steady barotropic pressure gradient toward the north-northeast. The slope of the sea surface is specified according to its ability to reproduce measurements. Under steady-state conditions, the model reproduces the middepth flow and indicates ...

On the Characteristics of Bottom Tidal Currents in Northeast Taiwan

A self-recording current metering subsurface mooring was deployd on the slope where the water depth is 270m, about 14 nautical miles SSE from the Peng-Chia-Yu islet northeast of Taiwan. Data shows that bottom tidal motions are very strong, and their periodicities are diurnal, semidiurnal and fourth diurnal. Semidiurnal tidal motions have the most energy, and act as the major component in the observed current field. Tidal current progressive vector diagram shows that the principal axis of current ellipse points NW-SE-Due to interaction with the sea floor, onshore tidal motions impinging on a sloping continental shelf will acquire a vertical motion which way contribute to the local upwelling.

Circulation and mixing processes and their effect on pollutant distribution in the western Arabian Gulf

Current measurements at four offshore locations in the western Arabian Gulf from 1986 to 1989 show periods of stagnation or low currents, and steady Shamal-(northwest storms) driven currents which have significance to the mixing and transport of pollutants in this area. A major effect of Shamal on the net circulation was observed in June 1986 at the Station CM3 surface where the net drift increased from 50 km during the first two weeks to about 100 km during the next two weeks. The drift direction during this period was 45° clockwise from Shamal wind direction. At CM4, the net drift was even reversed from southward to northward when Shamal winds changed to ‘Kaus’ winds (southeast storms) in August 1987. Progressive vector diagrams show that the residual current in the region controls the pollutant transport of the surface water at CM1 generally towards the south and southwest and the bottom water towards the southeast. In contrast to CM1, the net drift at CM2 showed smaller southeastward drift. However, the near-bottom flows at all stations were towards the southeast and southwest, except near-bottom at Station CM3, where eastward and northeastward residual advective transport is observed during the whole observation period. This unique northeastward near-bottom current has a special significance to the overall residual circulation of the Arabian Gulf. The strongest tidal current transport occurs at Station CM1, which is near the location of an amphidromic point for M2 harmonic tidal constituent. Movement of pollutants offshore over a tidal cycle would be generally toward the southeast during flood and toward the northwest during ebb tide. Large tidal mixing is observed in the region and the associated turbulent diffusion causes pollutants to be rapidly dispersed and thereby shows almost uniform distribution in the vertical. Complex tidal mixing and transport exist at Station CM1 where predominantly diurnal tidal elevations exist with mixed, mainly semi-diurnal tidal currents and vice versa (predominantly semi-diurnal tidal elevations exist with mixed mainly diurnal tidal currents) at CM3. An earlier study by John et al. 1 indicated that the formation of high-salinity water in the Gulf of Salwah may be one of the most important sources of the salinity-related density gradient proposed by Hunter 2 to drive circulation in the Arabian Gulf. However, this study shows that the net displacement of water towards the southeast was also due to the predominant northerly and northwesterly winds.

Characterization of deep-sea storms

To describe and quantify the physical forcing of sediment transport events at the HEBBLE site, velocity, stress and sediment concentration data were obtained during a year long deployment. Through the year, a variety of intense sediment transporting “events” were recorded. Careful analysis of these “events” reveals that only a small proportion of the peaks in suspended sediment concentration correspond to “local” erosion. Most sediment “events” were clouds of fine suspended material advected into the region. Analysis of progressive vector diagrams reveal that these advected clouds were bounded by regions with concentration gradients as high as 200 μg 1 −1 km −1. The high gradients suggest streams of sediment-bearing water, with sharp fronts of sediment concentration along their sidewalls, were veering across the region. The largest concentrations of suspended sediment were observed during local suspension events. The growth and decay of the sediment clouds during the “local” events did not exactly track the boundary shear stress. Rapid erosion occurred as the stress exceeded a critical shear stress. Maximum suspended concentrations occurred shortly after erosion began, indicating a source limited erosion process. The depositional period lasted for several weeks, whereas the erosion occurred in only a few days. Photographs of the bottom reveal depositional features (crag and tail dunes) which are aligned with the post-storm flow, not the erosional flow direction.

The response of the upper ocean to solar heating II: The wind‐driven current

AbstractThe current profile generated by a steady wind stress is disturbed by the diurnal variation of mixed layer depth forced by solar heating. Momentum diffused deep at night is abandoned to rotate inertially during the day when the mixed layer is shallow and then re‐entrained next night when it deepens. The resulting variation of current profile has been calculated with a one‐dimensional model in which power supply to turbulence determines the profile of eddy viscosity. The resulting variations of current velocity at fixed depths are so complicated that it is not surprising that current meter measurements have seldom yielded the classical Ekman solution. However, the progressive vector diagrams do exhibit an Ekman‐like response (albeit with superimposed inertial disturbances) suggesting that the model might be tested by tracking drifters designed to follow the flow at fixed depths. The inertial rotation of the current in the diurnal thermocline leads to a diurnal jet, the dynamical equivalent of the nocturnal jet in the atmospheric boundary layer over land. The role of inertial currents in deepening the mixed layer is clarified, leading to proposals for improving the turbulence parametrizations used in models of the upper ocean. The model predicts that the diurnal thermocline contains two layers of persistent vigorous turbulence separated by a thicker band of patchy turbulence in otherwise laminar flow.

Benthic Observations on the Madeira Abyssal Plain: Fronts

Analysis of data from a mooring with five vector-averaging current meters between 10 and 70 m above the bed of the Madeira Abyssal Plain reveals the existence of narrow regions with relatively large gradients of potential temperature, or “fronts.” The orientation and structure of the fronts is examined by combining the temperature and current data and plotting contours of equal potential temperature on the progressive vector diagrams, a procedure justified because of the known horizontal coherence of the currents and the relatively long time-scales of evolution of the benthic boundary layer. Sections through the fronts show that they are typically ∼300 m in width. They extend horizontally for at least 8 km. The temperature differences across the observed fronts are only 2–4 mdeg C. The frontal surfaces are tilted at ∼10 deg to the horizontal, the observed cold fronts being steeper and with isotherms more closely compacted in the lower levels, than warm fronts. These features possibly result from the straining of the temperature field by mesoscale motions as proposed by Armi and D'Asaro.

Determination of circulation and short period fluctuation in Ilha Grande Bay (RJ), Brazil

A mesosoale study was made of the Ilha Grande area. The local circulation described through -progressive vector diagrams showed a clochwise bottom circulation determined in June 1976, while in the upper 10 m the direction of the flow entering the Ilha Grande Bay was towards the center in the west and towards the Marambaia sandbank in the east side of the Bay. Short periods and amplitude fluctuations were evaluted using power spectral analysis, Fourier and Maximum Entropy Method, which showed that in the upper 10 m predominant periods decrease from 1.1h (A = 6.3cm sec-1) (position = 3C) to 1.0h (A = 7.4 cm sec-1) (position = 2D) and increase to 5.8h (A = 6.8 cm., sec-1) (position = ID), while at the bottom layer the predominant period increases from 0.4 h (A = 5.0 cm sec-1) (position = 3G) to 6.4h (A = 7.0 cm sec-1) (position = 2G) and to 4.4h (A = 7.9 cm sec-1) (position = 1G) . From the original data it has been possible to determine an intense pulsation between 30-70 cm sec-1 in the upper 10 m with about 1.0h period and 10-20 min duration in all the stations.