The hydrodynamics of simulated patches of the Mediterranean seagrass Posidonia oceanica were studied in laboratory flume experiments in which the height of the pronated canopy was always greater than half the total water depth. The effects of variations in speed (from 0.08 m s Ϫ1 to 0.24 m s Ϫ1) and patch configuration on the hydrodynamics were investigated. Significant speeds penetrated the patches to approximately half their height. Reducing speed did not change the flow patterns observed, except to weaken and blur them. Flow encountering a single patch formed a turbulent wake at the height of the top of the canopy. Within this wake, the vertical shear stress decreased monotonically downstream, but the Reynolds stress increased initially and then decayed. When a second patch was positioned within the region where Reynolds stress increased (referred to as the ''06 patch''), the wake center penetrated it, causing average turbulent velocities with horizontal components 3.3 times higher and vertical components 4.2 times higher than in the upstream patch. When this patch was positioned where the Reynolds stress decayed (referred to as the ''14 patch''), the wake center rose above it. Nevertheless, the turbulence in the 14 patch had horizontal components 12% higher and vertical components 22% higher on average than in the 06 patch because its upstream end was closer to the Reynolds stress maximum. Thus the ratio of the patch separation to the length of wake in which the Reynolds stress increased was identified as central to quantifying the turbulence within the downstream patch. The increased turbulence is likely to be important in determining sedimentary and ecological patch characteristics by increasing retention of particulates in suspension and thus reducing depositional rates of, for example, larvae, nutrients, and dead organic matter.

Analyses of results from laboratory flume experiments are presented in which flow within gaps in canopies of flexible, submerged aquatic vegetation simulations is investigated. The aims of the work are (a) to identify the different flow regimes that may be found within such gaps, using Morris' classical definitions of skimming flow, wake interference flow and isolated roughness flow as a template, (b) to determine the parameter space in which those flow regimes are most consistently delineated, and (c) to provide quantitative measurements of the loci of each flow regime within that parameter space for these experiments. The sedimentary and biological implications of each flow regime are also discussed. The results show that five flow regimes may be identified, expanding on Morris' original set of three. The five are: (i) skimming flow; (ii) recirculation flow; (iii) boundary layer recovery; (iv) canopy through-flow; and (v) isolated roughness flow, the last being assumed to occur in some cases though it is not directly observed in these experiments. A Reynolds number based on the canopy overflow speed and the gap depth, and the gap aspect ratio are found to be the key parameters that determine these flow regimes, though a Froude number is found to be important for determining bed shear stress, and the length of leaves overhanging the gap from the upstream canopy is found to be important in determining the location of flow recirculation cells within the gap.

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This paper reviews current understanding of vegetated flows in fluvial channels. First, the physical aspects of this understanding are considered, starting with flow resistance estimation, where the methods used in practice are, to a certain extent, disconnected from theoretical mechanical understanding. Theoretical, semi-empirical and empirical approaches aiming to connect them are considered. Although progress has been made, these approaches have not yet been able to provide theoretically robust, logistically amenable methods that can be put into practice. This is because of the variety and complexity of fluvial channel vegetation, which make it difficult both to generalise theoretical approaches and to make empirical approaches logistically tractable. Mean flow and turbulence structure in fluvial vegetation, and their influence on particulate and solute transport are then reviewed, noting that progress in these areas has been less problematic. This is explained as an issue of scale. Studies of flow structures and transport are relevant to scales to which hydrodynamic instruments and concepts are best suited, whereas flow resistance studies are primarily relevant at reach scale, not the vegetation patch scale at which they are usually studied. Therefore, there is a need to find ways of parameterising vegetative flow resistance at reach scales. This requires understanding of how vegetation is spatially distributed at this scale. To obtain this the primary controls on vegetation distribution are then reviewed and its relationships to hydrodynamics, water quality, hydro-morphology and habitat structure considered. It is concluded that this scale-appropriate, inter-disciplinary approach is likely to be the best way of improving flow resistance measurement, whereas work at smaller scales on the hydrodynamic structure of vegetated flows can continue to provide new insights into hydrodynamic influences on ecology and solute and sediment budgets.

Satellite image sequences (covering periods of a few days throughout the annual cycle) of the waters off southern Iberia have been analyzed in conjunction with concurrent surface wind speed data from coastal stations. Qualitative analysis reveals a large degree of temporal and spatial variability in the thermal signature of the sea surface over periods of both a few days and several months. During the summer, a cool seasurface temperature signature extends from the western Iberian coast around Cape St. Vincent and eastward as far as Faro. At the same time, a warm signature originating on the Iberian coast between Faro and Cadiz extends into the Strait of Gibraltar. These two features are shown to sometimes adopt more westerly positions, and the strait experiences regions of cool thermal signature originating at its southern side. During winter, the surface flow into the Mediterranean through the Strait of Gibraltar is anomalously warm and appears to come from the interior of the Gulf of Cadiz. Quantitative measurements show that temporal variability over timescales of a few days at individual sites is maximum in midsummer. Spatial thermal variability over the whole region is found to peak toward the end of the summer. Statistical analyses of the data reveal the coupling between the surface wind field in the Gulf of Cadiz and the surface thermal pattern (especially during the summer). Wind‐induced, across‐stream upwelling in the Strait of Gibraltar, although dynamically subordinate to tidal and density‐driven processes, is shown to occasionally dominate the surface thermal signature.