The project has focused on the physical processes that influence or configure the shelf-slope matter and energy exchanges with particular emphasis on submarine canyons and gravity flows. The approach to the problem has been carried out combining field data analysis, laboratory experiments and theoretical analysis through both analytical and numerical modeling. Field data analysis has covered field observations within submarine canyons, dense water cascades down the slope and gravity current intrusions. Observations from an intensive experiment within a submarine canyon have revealed new aspects relative to a reach internal structure inside the canyon with a great spatial heterogeneity and temporal variability that have important implications on shelf/slope exchanges. Velocity fields and vertical matter fluxes are significatively and systematically higher in the dowcanyon wall compared with the upcanyon wall according to the main current flow. Systematic laboratory experiments have been conducted to better understand the influence of submarine canyon on the dynamics and structure of a frontal current induced by a source of mass and buoyancy above the sloping bottom in the rotating fluid. The basic frontal current structure near the canyon is disturbed by: i) the quasi-stationary cyclonic eddy above the canyon and ii) the system of anticyclonic eddies translating in the clockwise direction behind the canyon. The anticyclones are periodically formed just behind the canyon and move one by one around the cone due to the topographical beta-effect producing disturbances in the frontal current structure. These results help to interpret field observations where the appearance of multiple nepheloid layers over the continental slope behind some canyons can be related with the disturbances of the coastal current generated by the mentioned above anticyclonic eddies.

Gravity currents down a continental slope have been the other major topic in this project. Intense gravity flows has been observed and recorded inside a submarine canyon showing peaks of high concentration of particles and intense velocities near the bottom associated to active trawling activities along the wall at shallower depths. Also, the characteristics of dense water cascades through canyons and on smooth slopes, produced by intense cooling at the surface over continental shelves around the world, have revealed that they evolve dynamically as near-bottom gravity currents and/or as intermediate-depth intrusions. A detailed studied of the dynamics of gravity flows upon a slope, under both homogeneous and stratified ambient conditions, has been carried out through laboratory experiments and also through both theoretical and numerical modelling. In the case of homogeneous ambient conditions it has been observed three different regimes depending on basic non-dimensional parameters which have been characterised as laminar, wave and turbulent modes. Both analytical and numerical modelling have been specifically formulated to reproduce quasi-identical conditions to the laboratory experiments with results in good qualitative and quantitative agreement with observed laboratory effects.

In the case of gravity currents surrounded by stratified ambient conditions, the laboratory experiments have shown that the interaction of the downslope flow with the interface (pycnocline) in a two-layered fluid presents three possible types of development. The gravity current can (i) just ignore the interface and continue downslope as a gravity current, (ii) separate from the bottom and transform into an intrusion, and (iii) split into two parts, one moving further downslope, and another penetrating into the pycnocline. The enhancement of turbulent entrainment rate of the gravity current fluid to the ambient fluid was observed in the pycnocline region, therefore dramatically reducing the down slope mass transfer. Analytical mathematical predictions and numerical simulations have provided details on the dynamics of this intrusion process. An important feature when the gravity current intrudes and continues downslope is that the speed of propagation of both intrusion and downsloping flow is significantly higher than the speed of motion of the initial gravity current. Thus, the layering of the flow will influence on the amount of sediment, both deposited and carried further with the flow. Numerical simulations for the case of penetration of downslope gravity current into the linearly stratified fluid show, that the process of veering may have a pulsating (periodical) character. This kind of interaction may lead to the generation of step-like density structures and layered suspended matter field that may explain the local homogeneities in sediment size observed in regions where such currents exist. Other basics aspects as the influence of the initial conditions on the entrainment into the gravity flow, the turbulent mixing of the density flow development have been analyzed through a mathematical model. It has provided a simple solution that gives the boundaries and intensity of the mixing layer in a steady-state flow over an incline as well as the nonstationary structure of the density flow head.

Additionally matter transport in shelf areas has also been studied from a theoretical point of view. A modified model has been re-written and one analytical simplified version has been obtained for quick estimates of water transport taking into account the Ekman veering effect of the bottom boundary layer. A new sediment transport algorithm has been proposed to produce an automatic adjustment from deep water to a shallow water situation. Numerical experiments have been carried out for a range of parameters typical for the shelf areas and heads of canyons. Also, numerical experiments have been used to study the impact of gravity flows on the sediment re-suspension. The analysis of simulations have revealed the complicate structure of the frontal part and the current head where frontal divisions in the density and velocity fields do not coincide in space. The phenomenon, discovered through the field data analysis is reproduced in the numerical experiments and allows a new possible reason for bottom sediment re-suspension: dynamical effects appear in surrounding fluid before the arrival of the gravity current head.

In summary, the research undertaken in this project can be considered as basic research in the sense that we have focused on fundamental processes of the ocean dynamics. Our approach has been to understand and carry out a detailed description of the physics of some common but poorly explored phenomena in the shelf/slope ocean area. However, the emphasis has been put in how such processes (current-topography interaction, downsloping gravity currents) affect the matter and energy exchanges in the shelf/slope region. Thus, our results have a main potential impact on marine environmental studies because, a better knowledge of how water and sediment are mixed or exported from shelf areas to open ocean is crucial to understand the marine ecosystem structure and functioning, the spread of pollution through sediment transport, etc.

- Field Observations on submarine canyons:
- Laboratory experiments on frontal currents at the sloping bottom with and without a canyon in rotating fluid.
- R
_{l}is the frontal current width, R_{s}is the sloping bottom width, R_{d}is the baroclinic deformation radius, - U
_{s}is the topographic Rossby wave velocity, U_{l}the frontal current velocity, - h
_{e}is the Ekman layer depth, h_{0}the frontal current depth, h_{c}is the canyon depth, - R
_{c}is the canyon width, tg (β) is the canyon wall slope, tg (α) is the slope, - R
_{2}is the distance between the source and the canyon tip and Fr is the Froude number - Sediment transport dynamics:
A process-based sediment transport model is presented that combines the knowledge of sediment transport in rivers and estuaries with the understanding of physical processes specific for the shelf seas. Due to formation of bottom Ekman spiral, the near-bottom current and hence the sediment transport deviates from the direction of surface current.

Fig. 9. Comparison of the VVS with observations and traditional formulations. (From Shapiro, 2004a.) The model takes into account velocity veering induced by Ekman spiral and estimates both direction and rate of transport of suspended particulate matter (SPM) generated by a steady or slowly varying current, through suspension, relocation and deposition of sediment in a shallow sea. The model, called VVS, uses a combination of analytical and numerical methods: analytical integration, in the vertical, and numerical integration, in the horizontal. The model predicts also vertical erosion/deposition fluxes at the seabed. The deviation angles and other parameters of SPM transport are computed for a range of water depths 5-50 m, particle settling velocities 0.1 6 cm/s, and current speeds 0.4-1.2 m/s (Fig. 11). The model converges automatically to traditional engineering-style formulations, in extreme case of strong current in very shallow water, where velocity veering is of minor importance.

The velocity veering implies that the direction of the sediment flux is generally different from the direction of the surface flow. The model has been applied to compute the sand transport by currents in a shallow sea, taking into account the time lag between erosion and deposition events and the velocity veering within the sediment-laden (nepheloid) layer caused by the Coriolis force. A comparison of model results with field data, collected at Long Island Shelf, supports the relevance of Coriolis-induced veering of currents on the direction of the sediment flux (Fig. 12). It was also found that both the rates and, in particular, spatial distribution of the areas of erosion/deposition differ significantly compared with other more classical models.

Fig. 12: A) Vertical SPM FLux at the sea bed for a meandering jet, water depth 20 m (whiteline is the zero contour). B) Veering angle of the sediment transport relative to the surface current direction (degrees). (From Shapiro et al., 2004.) - Observations and Modelling of gravity flows:
Fig. 13. Left: Cascading sites around the world ocean. Right: Salinity (solid lines) and temperature distribution (shaded areas) in September 1970 in the northern Sea of Okhotsk.(From Ivanov et al., 2004.) A physical process which is very relevant for the matter exchanges in shelf/slope regions is the development of gravity currents down the continental slope due to several different physical mechanisms. An inventory of field evidences of cascades of dense water around the world ocean has been done allowing to quantify, compare and contrast the properties of water cascades (Fig. 13). Non-dimensionalised data from all climate zones fit well to a unique curve, which describes a relationship between the forcing of a cascade and its internal structure.

Dense water cascades evolve as near-bottom gravity currents and/or as intermediate-depth intrusions. A series of laboratory experiments with the gravity current over the slopping bottom in non-rotating fluid (both homogeneous and 2-layered) were fulfilled to investigate the dynamics, structure of the dense water bottom flow and its interaction with pycnocline. In the case of homogeneous fluid, different regimes of the gravity current were revealed and described in terms of basic non-dimensional parameters. The comparative analysis of numerical simulations and quantitative data from laboratory experiments carried out allowed to propose a formulae for an estimation of the speed of gravity current moving along sloping bottom:

U ~ 2 (q g' sin α)^{1/3},where q is the flow rate, g' the reduced gravity and α the bottom slope. This expression is derived from the balance between the skating force and the resistance of the shape of the gravity-current head. This relation allows simple estimation of possible speed of propagation of gravity currents along the slope in submarine canyons. Three types of flow structures were observed and classified: a laminar, a "wave" and a turbulent flow mode (Fig. 14). As expected laminar and turbulent flow modes are observed under the low and very high Re numbers . "Wave" mode regime depends on both the bottom slope and the Froude number; it was never observed for small bottom slopes (α < 5 degrees and not very high Froude number (Fr <1). For the turbulent flow regime (observed for α = 44 degrees, Re = 66 and Fr = 2.4) an intense entrainment of the surrounding fluid and significant increase of a volume of the current is observed. Change from "non-wave" to "wave" regime occurs in the reange 1< Fr <1.5 without a significant correlation with the value of Re number.

Fig. 14: Different flow regimes observed in the experiments: laminar (top), wave (middle), turbulent (bottom).

A numerical model was applied to describe the structure of a bottom density flow. It solves the full 2-D Navier-Stokes equations written in vorticity - stream function - density variables with boundary conditions at the slope, turbulence parameterization and suspended matter transport specifically implemented in this project. A new code was adjusted to study gravity flows interacting with high-density gradient layers and the capability of gravity currents to lift up and to carry bottom sediments. The results of the mathematical modelling of gravity currents were inter-compared with the laboratory experiments with good agreement.

In addition, an analytical model of influence of surface waves on the structure of downslope gravity current was developed. The structure and features of nonlinear stationary waves in the fluid stratified by both density and velocity were investigated using a special set of auxiliary functions and introducing an integro-differential operator allowing to reduce the dimensionality from 2D to 1D. Then, it was solved by an invented modification of the first Stokes method. A system of equations for successive approximations was obtained and solved for the 5 lowest approximations and a nonlinear dispersion relation for the waves moving up-stream and downstream was obtained. This method can be applied to solve other problems of theory of nonlinear waves in dispersive media, like propagation of long weakly-nonlinear waves and their envelopes at the surface of shear flow with linear mean velocity profile. The obtained nonlinear correction coefficients to the phase speed can be used for investigation of self-modulation phenomenon.

Numerical experiments has also been used to study their implications on the sediment resuspension induced by the pass of a gravity current and to compare with field data on a density intrusion in the coastal area. The analysis of simulations on gravity currents down the slope surrounded by a homogeneous ambient fluid allowed to reveal the complicate structure of the frontal part and the current head (Fig. 15).

Fig. 15. Top: distribution of density gradient (green iso-lines) along the streamline (blue lines). Characteristic scales h _{0}= 5 cm, u_{0}= 5 m/s, δρ_{0}= 0.003 g/m^{3}. Bottom: distribution of the velocity module (green) with the density field (in gray colour). Arrows mark the dynamic disturbances which arrive before the density front.

The most relevant is the fact that frontal divisions in the density and velocity fields do not coincide in space. The phenomenon discovered in field observations made in the Vistula Lagoon (the Baltic Sea) is well reproduced numerically. This allows a new possible reason for bottom sediment re-suspension: dynamical effects appear in surrounding fluid before the arrival of the gravity current head. A new criterion was defined to marking out the frontal zone in a density stratified flow: the module of the density derivative along the streamline.

The interaction of the downslope flow with an interface in a two-layered fluid can also exhibit 3 possible types of development. Both laboratory and numerical experiments have demonstrated, that the gravity current can (i) just "ignore" the interface and continue downslope motion as a gravity current; (ii) separate from the bottom and transform into intrusion , and (iii) split into two parts, one moving further downslope, and another penetrating the pycnocline (Fig. 16).

Fig. 16. An example of the numerical simulations of a intrusion of a gravity current into a pycnocline. Characteristic scales: h _{0}= 50 m, u_{0}= 2 m/s, δρ_{0}= 0.001 g/m^{3}(Re =104, Fr =0.34). (From Zatsepin et al., 2004)Fig. 17. Veering of the current in a linearly stratified ambient. Iso-pycnals are shown in thin lines, traser - in thick lines. An important feature for this last case is that the speed of propagation of both intrusion and downsloping flow is significantly higher than the speed of motion of the initial gravity current. Numerical simulations were also performed for the case of a downslope gravity current intruding into a linearly stratified fluid (Fig. 17). The results show, that the process of veering may have a pulsating (periodical) character. Additional experiments on gravity flows and its interaction with a density interface in a large flume (5m long) have been undertaken to complement the previously cited experiments(Fig. 18). Three series of laboratory experiments in large flume (5 m long) have been performed for different types of ambient stratification (glycerin, sugar and thermal stratification) with dimensionless flow parameters corresponding to experiments in the small flume. It is shown that the basic features of flow revealed in previous experiments (splitting of density flow at the pycnocline, roll wave generation, density flow head formation) are the same as in small scale experiments.

Fig. 18. Evolution of an intrusion (nepheloid layer) due to a gravity flow in stratified fluid in the large flume experiments.A mathematical model have been applied to simulate the evolution of a density current front before and after its interaction with the pycnocline. The velocities and intensities of bottom current heads and an intrusion along the pycnocline are calculated and compared with 2-D empirical calculations made in the small flume experiment and numerical simulations. New features of density flow in stratified surroundings such as the dependence of entrainment process and intrusion flow along the pycnocline on the flow conditions at the shelf break as well as the formation of an internal hydraulic jump at the pycnocline have been discovered in the large scale experiments. A simple mathematical model of the density flow development which predicts the shape and the velocity of the density flow head has been derived.

In this project we have focused in the field observations in two submarine canyons incising the Northwestern Spanish shelf in the Western Mediterranean Sea (fig. 1). In the Palamós submarine canyon, currents, particle fluxes and ecology were studied in a field experiment (fig. 2). Seven mooring arrays equipped with current meters and sediment traps were deployed along the main canyon axis, on the canyon walls and on the adjacent slope.

The current field observed during the most complete survey revealed a meandering pattern with cyclonic vorticity just upstream from and within the canyon. The associated vertical velocity ranged between 10 and 20 m/day and was constrained to the upper 300 m. This latter feature, together with other computations, suggests that during this survey the meander was not induced by the canyon but by some kind of instability of the mean flow. In the canyon, suspended sediment concentration, downward particle fluxes, chlorophyll and particulate C and N were significantly higher upcanyon from about 1200 m depth than offshore defining, along with the different hydrodynamics, two canyon domains: one from the canyon head to about 1200 m depth more affected by the canyon confinement and the other deeper than 1200 m depth more controlled by the mean flow and the shelf-slope front.

In addition, common gravity flows inside the canyon has been observed and recorded in both currentmeters and turbidimeters near the bottom during the whole summer season under very calm weather conditions. Peaks of high concentration of particles correlated to intense velocities near the bottom appears associated to active trawling activities along the wall at shallower depths (fig.3b). An important consequence of it is the contribution of such high energetic events to the shelf/slope matter fluxes within the canyon. These anthropogenic generated gravity currents account for an important fraction of the net horizontal flux in the canyon.

In the Foix canyon characteristics nepheloid layers attached to the bottom slope has been reported and observed (fig.4). This layers were also observed along the shelf slope upstream the canyon location. Cross slope transects of beam attenuation coefficient (BAC) showed that nepheloid layers are reinforced within the canyon and downstrean.

Systematic laboratory experiments on the influence of the canyons on the dynamics and structure of a frontal current induced by the source of mass and buoyancy above the sloping bottom in the rotating fluid have been conducted. The system of basic non-dimensional similarity parameters was determined on the base of the hydrological and hydrodynamic conditions in the Palamós canyon region.

Two sets of experimental runs with the same values of basic parameters were provided: one in the absence of canyon, another – in the presence of canyon. At the beginning of each experiment the
homogeneous fluid in the tank was spun up until it had reached the state of solid body rotation with angular velocity *Ω*, where *f* is the Coriolis parameter. Water of approximately the same density as of the ambient fluid (slightly denser, or lighter) was then accurately without any mixing injected with constant flux rate *Q* within the experimental run at the top of the cone through a source. The injected fluid was colored with thymol blue indicator to distinguish it from the ambient fluid.

In
both experimental cases (with and without canyon) soon after the beginning of the injection the axisymmetric anticyclonic frontal current was formed around the source. The width of this current (the distance between the source edge and the front of the colored fluid) was growing at the first stage of the experimental run. But in short time the downslope bottom Ekman layer flow was formed with the flux rate *Q _{e} = Q* at the front of the anticyclonic current (Fig. 6a). After that the position of the front did not change in time considerably, however, the downslope spreading of the colored fluid was observed in the bottom Ekman layer (Fig. 6b).

The strong influence of canyon on the structure and dynamics of frontal current and downslope Ekman layer flow was observed. While in the absence of the canyon the downslope Ekman layer flow was continuous from the top to the bottom of the cone (see figs. 6). In the presence of the canyon the velocity of fluid above the canyon was significantly reduced in comparison with the case without canyon. As a result the downslope Ekman layer flow velocity in the presence of canyon was smaller.

Considerable part of this flow was trapped by the canyon at its upstream wall and propagated downward as a jet along the axis of the canyon (Fig. 7a,b). At the downstream wall of the canyon the direction of the bottom Ekman layer flow was opposite: upward.

The described above experimental results were obtained for the case in which the density stratification is close to neutral. In the strongly stratified fluid the downslope bottom Ekman layer flow is considerably reduced or even blocked. We can appreciate the sloping of the frontal interface and the suppression of the Ekman boundary layer in fig. 8a. When the canyon is present the frontal current is strongly deflected offshore. The surface signature is clearly appreciated in fig. 8b. The animation also show the formation of eddy motion at the head of canyon.

A quantitative analysis of the similarity between the cone configuration and the real configuration of the Palamós canyon lead us to design a second much more "realistic" configuration (Figs. 9ab).

The nondimensional parameters for natural and laboratory conditions were stablished (see Table 1):

Parameters | Model | Palamós canyon |
---|---|---|

R _{l}/R _{s} | 0.2 | 0.5 |

R _{d}/R _{s} | 0 ~ 0.006 | 0.3 |

U _{s}/U _{l} | 0.35 ~ 1 | 2 |

h _{e}/h _{0} | 0.01-0.02 | 0.3 x 10^{-3} |

h _{c}/h _{0} | 0.5-1 | 0.7 ~ 4 |

R _{c}/R _{l} | 1 | 1 |

tg (α) / tg (β) | ∞ | 10 |

R _{l}/R _{2} | > 10 | > 10 |

Fr | 0.1 ~ 0.6 | 0.175 |

Table 1: Non dimensional Parameters |

Experiments reveal that a considerable part of the less dense frontal current is deflected by the canyon at its downstream wall in the downslope direction. At the upstream wall of the canyon the flow has the opposite (upward) direction. The upward and downward flows in the vicinity of the canyon are not equal to each other, the downward flow is much stronger and looks like an eddy-generating jet while the upward one is rather week and wide. This asymmetry may be explained as a result of the continental slope topographical beta-effect. The basic frontal current structure near the canyon is disturbed by: i) the quasi-stationary cyclonic eddy above the canyon and ii) the system of anticyclonic eddies translating in the clockwise direction behind the canyon.

The anticyclones are periodically formed just behind the canyon and move one by one around the cone due to the topographical beta-effect producing disturbances in the frontal current structure Fig. 10. However, the most strong shallow-deep basin exchange is produced by the cyclon-anticyclon eddy pair at the downstream canyon wall. The experimental data was processed and quantitative characteristics of canyon influence on the frontal current dynamics and structure were obtained. These results help to interpret the field observations in the Palamós and Foix canyons where the appearance of multiple nepheloid layers over the continental slope behind the canyons can be related with the disturbances of the coastal current generated by the mentioned above anticyclonic eddies.

- ICM rotating platform: A 1 m diameter, direct-drive rotating platform.

- LIH channel: It is a channel 5 m long , 0.6 m height and 0.4 m wide.

Movies from Experiments(click on the figures to see the movie)

Get the Flash Player to see this player. |

Get the Flash Player to see this player. | Get the Flash Player to see this player. |

Frontal current under baroclinic conditions without canyon (salinity difference ΔS: 3 ppt). (304 Mb in MPEG format) | Frontal current under baroclinic conditions in the presence of a submarine canyon (triangular shape and salinity difference ΔS: 3 ppt). (237 Mb in MPEG format) |

Get the Flash Player to see this player. | Get the Flash Player to see this player. |

Frontal current under barotropic (left) and baroclinic (right) conditions with and without canyon (6.8 Mb and 4.5 Mb in AVI format) |

Get the Flash Player to see this player. | Get the Flash Player to see this player. |

Gravity current interacing with a pycnocline in a two-layer system (2.3 Mb and 2.4 Mb in AVI format) |