EXPERIMENTAL STUDY ON BED SCOUR AND ITS PROTECTION BEHIND STANDING WAVE WEIRS CONSIDERING COMBINED FLOW OVER WEIR CRESTAND BOTTOM PIPES

Abstract

Weirs may be used for measuring of canals discharge, decreasing water slope in canals and distribution of water to canals for irrigation. To empty the water of upstream canal 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, a part of flow is passing through the bottom pipes 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 downstream of this combined flow. So, the objectives of this research are to experimentally investigate the influence of using bottom openings for passing flow with that falling over weir crest on the geometry of scour hole downstream and on the estimation of minimum solid bed length to prevent this scour. The experimental program was performed in a rectangular flume 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 on scouring. In the first category, solid bed length behind the weir model was chosen to be 600 mm and the analysis are based on scour hole dimensions and its location behind the model, while the second category is for estimating the minimum solid bed length behind the weir model to prevent scour. Top surface slope of the weir is chosen to be 5:1 and 10:1for both studied categories. The model of the weir is followed by a solid apron following by sand basin. The mean diameter of sand is kept constant at 0.502mm. The channel bed slope is kept constant at 0.0001, while the discharge and flow depth downstream were varied to cover the required range of Froude number in Egyptian canals. The study revealed that: Using bottom pipes for passing flow with that falls over weir crest helps in reducing scour dimensions behind the weir, as well as reducing the required length of solid bed which considered bed protection. It also found that the required minimum length of solid apron behind the weir to prevent scour is greater than the summation of an arbitrary solid length and the length of scour hole formed downstream of it. Moreover the results showed that the predicted length of scour behind the weir with top surface slope 10:1 is greater than that for 5:1 especially for solid weir with low discharges and low head differences. Empirical formulas in a power function are developed in dimensionless forms for estimating the protected length against scour and the scour dimensions. On the other hand, the comparative analysis with previous studies revealed that the minimum solid bed length behind the weir to prevent scour is greater than the estimated one by using a velocity distribution approach.