الفهرس 
ACKNOWLEDGMENT
MECHANISM OF LOCAL SCOUR BEHIND HYDRAULIC SRUCTURESOnly 14 pages are availabe for public view
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Abstract
iWeirs may be used for measuring canal discharges, decreasing water slope in canals and the distribution of water to canals for irrigation. To empty the water of canal upstream the weir during the maintenance or constructing new water structures and escaping the silt accumulated to downstream, a bottom pipes are installed in weir body near canal bed. Recently, with increasing the demands of water to downstream, Irrigation Engineers operating the bottom
pipes to pass a part of flow rate through these in addition to over weir flow. Most of previous studies concentrate on estimating discharge coefficient of combined flow. However little information are available on scour behind this combined flow. So, the objectives of present research are to experimentally >investigate the influence of using bottom pipes for passing flow rate synchronically with that falling over weir crest on: 1) The dimensions of scour hole downstream to be protected against the harmful of scour. 2) The estimation of minimum solid bed length required to prevent scour behind the weir. The experimental program was performed in the Hydraulic Laboratory of Civil Engineering Department, Assiut University. The experiments were conducted in a rectangular flume having 300 mm wide and 500 mm deep. The flume has a working length 17.5m, which is made transparent for flow observation. The study was performed in two categories. Each category was for weir model without working pipes and weir model with two synchronic working bottom pipes with weirs over-flow. In the first category, solid bed length behind the weir model was chosen to be 600 mm and the analysis were based on scour hole dimensions and its location behind the model, while the second category was for estimating the minimum solid bed length behind the weir model to prevent scour. Top surface slope of the weir was chosen to be 5:1 and 10:1for both studied categories. The model of the weir was followedii by a solid apron following by sand basin. The mean diameter of sand was kept constant at 0.502mm. The channel bed slope was kept constant at 0.0001, while the discharge and flow depth downstream were varied to be in the range of Froude number in Egyptian canals. The findings from the present study may have practical applications when it is required to increase the passing flow rate from the upstream to downstream of the weir without affecting the safety of weirs and canal embankments and may be drawn as follows: 1) The dimensions of scour hole behind tested weir models are dependent on the head difference between upstream and downstream of the weirs, the
downstream water depth, the downstream Froude number and the total< discharge pass on weir crest synchronically with that through bottom pipes. 2) Using bottom pipes for passing flow synchronically with that falls over
weir crest helps in reducing scour dimensions behind the weir, as well as shortened the minimum required length of solid bed to prevent scour.
3) The required minimum length of solid apron behind the weir to prevent
scour (Ls(min)) is greater than the summation of an arbitrary solid bed length and the length of scour hole formed downstream of it (L+ls). 4) The predicted lengths of scour behind the solid weirs with top surface slope 10:1 are greater than those for 5:1 especially for high discharges and high head differences. 5) Empirical formulas in a power function were developed in dimensionless forms for estimating the protected scour dimensions of scour hole and the minimum length required to prevent scour. These formulas are valid in the range of the experimental parameters mentioned in the experimental works and appeared on the figures.6) from the comparative analysis with previous studies, it is found that from present study and estimated by using velocity distribution approach to protect the structure against danger of scour hole.