Scouring downstream of sediment- carrying free over fall water jet

Scouring downstream of sediment- carrying free over fall water jetEhsan Ghasemi1 and Maoud Ghodsian*21- Ex. M.Sc. Student, Faculty of Civil and Environmental Engineering, Tarbiat Modares University, Tehran-Iran.2- Prof., Faculty of Civil and Environmental Engineering and Water Engineering Research Iistitute, Tarbiat Modares University, Tehran, Iran*ghods@modares.ac.irExtended 

Prediction of scour and characteristics of scour hole due to out flow from hydraulic structures is important in hydraulic engineering. The dimensions of scour hole is influenced by different parameters including: flow discharge, drop height of flow, tailwater depth, sediment size, sediment load and time of scouring. Almost all the previous studies have focused on the scour hole characteristics downstream of free over fall clear water jets. Since water jets are not always clear and may carry sediment, especially during flood condition, and the effect of sediment load on the scour characteristics are not well studied. Therefore in this study, scour downstream of a free over fall clear and sediment-carrying water jet are studied experimentally. The main purpose of this study are to investigate the effects of sediment load, sediment size and discharge of the free falling jet on the scour depth and the longitudinal scour length. Also temporal variation of the sour depth and longitudinal scour length was also addressed. New dimensionless equations for scour depth and longitudinal scour length were obtained.

Experiments were conducted in a rectangular channel of 0.6 m width, 12 m length. The water was pumped from a sump to the channel at the end of which a free falling jet was formed. A rectangular free-overfall jet of 0.21 m width was established at the last 1 m length of the channel. Scour was simulated in a rectangular box of 1.5 m width and 2.51 m length located downstream of channel. Measurement of discharge was done by using a calibrated sharp crested triangular weir with apex angle of 90 degree. The depths of flow and longitudinal bed profiles were measured by using a digital pint gauge with ±0.01 mm theoretical accuracy. Temporal measurement of longitudinal bed profiles were also done by using the same digital point gauge. The bottom of the rectangular box was covered by a uniform sand layer of 0.45 mm thickness. Experiments were conducted for four different discharges 4.27, 7.48, 11.78 and 17.3 L/s and two sediment sizes (d50= 0.6 mm and 1.2 mm). Experiments were conducted with clear water free falling water jet and sediment caring free falling water jet. In experiments with sediment caring free falling jet, the dry sediment with constant rate was added to the water jet. Four values of sediment load 0.25, 0.5, 1 and 2 Kg/min were used. The added sediment to the jet was of the same size as the bed material size (d50= 0.6 mm and 1.2 mm). Time variations of scour depth and scour hole length were also studied.

Based on the result, by increasing the sediment load, the values of maximum depth of scour and length of scour hole decreased. The rate of scour reduction depend on the amount of sediment load in the water jet, water discharge and scouring time By increasing the duration of experiments, the increasing effects of densimetric Froude number Frd and jet discharge in the longitudinal bed profiles reduces. The decreasing trend of sediment load on the maximum scour depth is more pronounced in experiments with lower duration. In higher discharges, the reducing effect of the sediment load on the maximum depth of scour reduces. The reducing effect of sediment load on the longitudinal scour length is reduced with higher jet discharge. The reducing effect of sediment load on longitudinal scour length is enhanced in experiments with lower duration. Effects of dimensionless parameters on the scour depth and scour length were addressed. By increasing the densimetric Froude number the relative scour depth and relative longitudinal length of scour increases. By using the dimensional analysis, dimensionless equations for estimation the longitudinal scour profile, scour depth and scour length are obtained.

In this experimental study, the scour depths and the longitudinal scour lengths were compared in the clear water and sediment caring free falling water jets. It was found that by increasing the sediment load, the values of maximum depth of scour and length of scour decreases. The rate of scour reduction depend on the amount of sediment load in the water jet, water discharge and scouring time. New equations for estimation the longitudinal scour profile, scour depth and scour length are obtained.Keywords: free over fall jet, sediment carrying jet, scouring, dimensional analysis, densimetric Froude number, and longitudinal profile.