چكيده لاتين :
Introduction
The turbulent jets downstream of hydraulic structures, such as sluice gates, jet nozzles, spillways, or weirs,
may cause severe local scour. The scouring process is inherently complex due to the rapid change of turbulent
flow characteristics with sediment motion in the scour zone. Excessive scour is undesirable and may be detrimental to the foundations of the structures. Sluice gates are wood or metal sheets that slide vertically and act as
a portal to regulate the water passage within the irrigation channels, dams, and some wastewater control plants.
The operation of the gates in small channels is traditionally accomplished manually, while in modern channel
networks or relatively large channels, an electrically or hydraulically powered mechanism should be considered. The flow passing the gate forms a high-velocity jet with a relatively high erosive force that may develop
a scour hole in an erodible channel downstream of the gate. To reduce the destructive action of the jet and regulate the flowing fluid under the gate, a rigid and non-erodible apron is commonly constructed under and downstream of the gate. In this case, the scour forms downstream of the apron, and depending on its shape and
depth, it may threaten the stability of the gate foundation. In fact, the apron is generally a thin concrete layer
implemented on the canal bed sediments, and its stability depends strongly on the strength of the sediment particles against detachment. The interaction between the flow and the sediment particles may extend the scour
hole up to the downstream edge of the apron over a relatively short time. Afterwards, the sediment particles
beneath the apron detach and wash away over a longer duration, causing to undermine the apron. Depending
on the flow intensity, sediment size, and apron length, this phenomenon may occur in a few hours to several
months. A collapsed apron may interrupt the flow passage and threaten the whole structure stability.
Recently, several studies have been conducted so as to improve our understanding of the phenomena of
scour and sediment transport due to a horizontal jet issuing from below a sluice gate. The high-velocity jet issuing from below the gate produces shear stresses that exceed the critical shear stress for the incipient motion of
the bed material. Over a period of time, the scouring of the bed material causes the flow depth downstream
from the gate to increase, thereby resulting in a mechanism of reduced bed shear stress, which in turn reduces
the rate of scour. Though the flow field is simple, the scour mechanism is complex and dynamic.
2- Methodology
The dimensions and flow conditions of the numerical model were the same as Kells et all., 2001
laboratory model. The geometries of the numerical model were created using flow3D software, and
solutions were created out by VOF method. In this study, the effect of grain size on the dynamics of
local scour processes is discussed in the context of the erosion that takes place downstream from a
submerged sluice gate. Three gradations of non-cohesive bed material were used to study the scour
process for various tailwater depth and sluice openings. The sand gradations included three sizes of
uniformly graded. A total of 18 model was simulated, each for a period of 2 h. An equilibrium scour
condition was attained over this time period for all of the models, although a sense of similarity in the
bed profiles is observed in the region close to the sluice gate.
3- Results
The present results indicate that the depth and the area of scour are highly dependent on the bed grain
size, both increasing as the grain size is reduced. Moreover, the maximum scour depth increases with
increases in the tailwater depth.Finally, it was found that the location of the point of maximum scour
depth, as measured from the upstream end of the erodible sand bed, moved downstream with an increase in either the opening or tailwater depth and upstream with an increase in the grain size.
4- Discussion & Conclusions
The primary purpose of this study was to evaluate the effect of grain size and, to a lesser extent, of
grain size distribution on the dynamics of local scour below a sluice gate. In short, it was found that the
grain size has a significant influence on the extent of scour, which occurs, with more scour occurring
for smaller-sized material. As well, it was found that less scour occurred for a graded sand than a uniform one having a similar median grain size. Of particular interest, however, is that for any given grain
size, the greater is the tailwater depth, the greater is the depth, extent, and volume of scour. It appears
that the tailwater serves to slow the rate of jet expansion, thus increasing the length of bed exposed to
high velocity, hence high shear stress, conditions.
