1,721,050 research outputs found
Coastal wetlands at risk: learningfrom Venice and New Orleans
No full textCoastal regions have progressively become more vulnerable to intense hydrodynamic
and atmospheric events, thus raising important questions about their fate in the century
of global warming. A variety of natural and anthropogenic factors have contributed to
this fragility: eustacy, isostasy, soil compaction, reduced sediment supply and reduced
extension of natural defenses (barrier islands and coastal wetlands). With the aim to
emphasize the crucial role played by the intense human manipulation of the environment,
we provide a brief overview of the state of knowledge on this extremely complex
problem, moving from two cases of special importance: Venice and New Orleans. We
discuss similarities (causes of wetland degradation and related restoration problems)
as well as differences (different economical scales involved in the restoration projects,
different risk reduction ensured by wetland and natural defense restoration and, fi nally,
cultural relevance of the environment to be preserved) between these two regions
Temporal scour evolution at wood bundles under clear water condition
No full textIn the current scenario, there is a rapid degradation of the various water resources due to a host of climatic and anthropogenic factors. Natural river channels are particularly destabilized, leading to recurrent siltation and flood problems. Low head eco friendly hydraulic structures are a viable option to recreate the natural flowing conditions in a river which consequently leads to a stabilized channel morphology [1]. In this regard, low head structures such as log frames [2], block ramps [3], and wood b undles [4] are typically used to control the sediment load in rivers as well as to create localized scour holes in the stilling basin, also promoting biodiversity in the channel.
Among the above mentioned structure typology, wood bundles are placed both in straight and curved channels to provide perching sites for various invertebrates and to enhance bank safety. In channel curves, they redirect strong velocity gradients away from the outer bank towards the channel center thereby protecting the riverbank from failure due to excessive erosion. On the other hand, the scour holes in the stilling basin of wood bundles serve as an ideal spot for growth of various aquatic organisms includ ing fishes. The equilibrium scour characteristics associated to these structures have been recently investigated by [4], however the temporal evolution of the scour hole still poses a serious gap in knowledge. Therefore, this paper focuses on the temporal evolution of the scour hole at wood bundles in straight channels and presents some preliminary findings on the topic.
A set of experiments were performed on wood bundles for a large range of hydraulic conditions and structure geometry under clear water conditions. Several factors like discharge and tailwater, structure height, number of structures in series combination an d spacing between consecutive structures in series were varied. Data analysis showed that the scour depth evolution is considerably dependent on the inflow conditions which can be modelled by a novel densimetric Froude number. Moreover, the results also in dicate that the scour development is usually faster for higher densimetric Froude number. Therefore, this paper provides insights on the evolution of scour hole at wood bundles in various stages of its growth which can facilitate the design of such structu re typology in future.
To aid in comparison, Fig. 1a shows the flow characteristics and eddy formation at a wood bundle during the scour evolution process whereas Fig. 1b shows the equilibrium scour morphology for the same structure after test completio
Kinematic characteristics of blunt nosed chevrons in movable bed channels
No full textRivers are a crucial component of freshwater ecosystems. Nevertheless, they are increasingly threatened by anthropogenic activities, resulting in environmental degradation, and affecting the sediment transport capacity and channel stability. Active restoration of such ecosystems can be achieved by implementing low-head eco-friendly structures, e.g., cross vanes, J-hooks, W-weirs [1], block ramps [2] and, more recently investigated, wood bundles [3], and chevrons [4], which modify the sediment load and create low-velocity localized scour zones. In particular, blunt nosed chevrons are U-shaped structures facing upstream used to divert the flow and to promote scour processes in order to allow navigability in rivers. In addition, it has been found that such structures are also effective in enhancing the habitat for fish species, promoting biodiversity [5]. In this regard, different species of macroinvertebrates have been found in proximity of the rock body of chevron structures, providing nourishment for aquatic species. Despite the availability of guidelines for the design of blunt nosed chevrons for a long time ([6], [7]), information concerning the effects of such structures on the surrounding morphology is fragmented. Recently, [5] investigated the effects of blunt nosed chevrons on the equilibrium scour characteristics in a straight channel, while [4] extended the analysis to the temporal scour evolution for both straight and curved channels, as well as the equilibrium morphology in curved channels.
Despite the advancements in predicting the equilibrium morphology and its temporal evolution in proximity of blunt nosed chevrons, the characterization of the flow kinematics in proximity of such structures has not been fully assessed yet, especially in proximity of the generated scours. Therefore, the present study focuses on the velocity fields occurring around blunt nosed chevrons at selected vertical positions below the water surface, with particular focus at the maximum scour location.
To this end, a set of experimental tests were performed under clear water conditions for a range of hydraulic (i.e., water discharges between 0.008 and 0.016 m3/s; tailwater depths between 0.06 and 0.12 m) and geometrical configurations of blunt nosed chevrons inserted in a straight channel (i.e., different combinations of structure height, number of structures and spacing for series combination, position along the curved channel). Punctual velocity measurements at selected locations were performed by means of an Acoustic Doppler Velocimeter (ADV). Preliminary data analysis showed that flow velocities steeply decrease along the vertical depth at the location of maximum scour created downstream of the chevron structure, allowing favorable nursing conditions for aquatic species.
Finally, Figure 1 shows an exemplary configuration of two chevrons in series arrangement, under dynamic (left) and static (right) conditions
Quantifying the effect of valley confinements on the long-term evolution of meandering rivers
No full text
Un database di scenari di allagamento causati da brecce arginali come strumento per aumentare la resilienza dei territori di pianura
No full textNew formulation of the 2D steep-slope shallow water equations
No full text- A new formulation of the 2D shallow water equations on steep bottom slopes is presented.
- The water depth is measured vertically and the velocity components are assumed parallel to the local bed surface.
- The equations represent mass conservation and momentum balance along two directions locally parallel to the
bottom surface.
- The new equations are hyperbolic and reduce to the standard 2D shallow water equations for small bottom slopes
IMPLEMENTAZIONE DI CONDIZIONI AL CONTORNO INTERNE PER PONTI E MANUFATTI IN UN MODELLO 2D ALLE ACQUE BASSE
No full text- …
