Behavior of a Small Pulsed River Plume in a Strong Tidal Cross-Flow in the Akashi Strait

Abstract

We carried out a field study of a small river plume emptying into Osaka Bay near the Akashi Strait in western Japan, to understand the fate of its urban-runoff-laden waters. As the river is weak compared to tidal currents, we find that the behavior of the plume is strongly influenced by phasing between tidal stage and currents (a westward-traveling progressive tidal wave). When the tidal stage rises, sea water floods up the river, and concentrated river effluent cannot be seen in Osaka Bay. During most of the river's ebb, a topographic eddy transports river effluent towards the energetic Akashi Strait, while strong vertical shear in the tidal flow mixes the effluent with seawater. However, there is a short interval of time during which the direction of tidal flow in the Strait changes direction and the magnitude of the current is weak. At this time, shear is weak enough to allow a stratified plume of concentrated river water to form, and this plume is driven offshore, and into Osaka Bay, by baroclinic circulation. A sewage outfall, which is located very close to the river mouth, is subjected to these same conditions and therefore exhibits similar behavior. Understanding the relation between tidal phase and plume behavior will be useful to Kobe City, as well as other cities in a similar environment, for minimizing the environmental effects of their wastewater and non-point-source runoff.

Estimation of far-field horizontal and vertical turbulent diffusion coefficients from the concentration field of a wastewater plume near the Akashi Strait

“…11, N reached a maximum value of approximately 0.06 Hz. Bricker et al [1] showed the vertical shear in horizontal velocity in this same region to be approximately 3 Hz under a similar flow regime. The resulting gradient Richardson number Ri = N 2 /(du/dz) 2 is 4 × 10 −4 , which indicates that stratification is much too weak to prevent shear-generated turbulence from forming throughout the water column.…”

“…At the northern edge of our survey area, within the mouth of the Fukuda river, this was seen as a weak southward surface flow and a weak northward flow near the bottom. This baroclinic flow is not addressed more in this paper, but is discussed in detail in Bricker et al [1].…”

“…Bricker et al [1] described the general behavior of this wastewater plume, in combination with effluent from the Fukuda river, a small nearby drainage, but could not distinguish these two water masses from each other. In the present paper, however, we do distinguish between these two water masses, by recording data with high spatial and temporal resolution, through the use of a tow-body CTD.…”

“…The study detailed in this paper investigates the horizontal and vertical eddy diffusivity coefficients governing the dispersion of a near-shore wastewater plume near the Akashi Strait, by monitoring the salinity and temperature fields emitted by a wastewater diffuser of known discharge rate. Bricker et al [1] qualitatively investigated the dynamics of this region, while the present paper is a quantitative investigation of the coefficients governing diffusion of scalars in the region.…”

Περιγραφή Και Προσομοίωση Συμπεριφοράς Πλουμίου Του Ποταμού Νέστου

“…Απηφ ζεκαίλεη ηα χδαηα ηνπ Νέζηνπ ζπκπηέδνληαη ζην ζηφκην θαηά ηε θάζε ηεο παιηξξνηαθήο πιήκκεο (αλφδνπ ηεο ζηάζκεο ησλ πδάησλ) θαη απειεπζεξψλνληαη θαηά ηελ θάζε ηε άκπσηεο (θαζφδνπ ηεο ζηάζκεο ησλ πδάησλ), πηζαλφλ κε ηελ κνξθή θιέβαο θαζψο ε κείσζε ηεο αιαηφηεηαο (έσο θαη 6,5 psu) είλαη κεγαιχηεξε απφ ηελ πεξίπησζε ηεο πξνζνκνίσζεο Run1 (Δηθφλα 7.10β). Παξφκνηα ζπκπεξηθνξά αλαθέξζεθε θαη ζε άιιεο κειέηεο πνπ ζρεηίδνληαη κε ηελ επίδξαζε ηεο παιίξξνηαο ζηα πινχκηα(de Ruijter et al 1998, Liu et al 2000, Bricker et al 2006. πσο φκσο αλαθέξνπλ νηMestres et al (2003) θαηUlses et al (2005), ην κηθξφ παιηξξνηαθφ εχξνο πνπ ραξαθηεξίδεη ηελ Μεζφγεην δελ πξνθαιεί ζνβαξέο κεηαβνιέο ζηελ εμάπισζε ηνπ πινπκίνπ.…”