Effects of Upward Seepage on Scour Depth Downstream of the Free Falling Jets

In the present study the effect of upward seepage on scour depth downstream of the vertical jet such as grade control structure has been experimentally investigated. To reach such goal two series of tests without and with the presence of upward seepage using three different sediment sizes, three different flow discharges and three different tailwater depths were carried out at the hydraulic laboratory of Water Research Organization.  For the second series of tests five different upward seepage discharges ranged from 1.5 to10.5 liters per second were applied.  The resulting data showed that by applying upward seepage, the scour depth decreased. The amount of decrease was found to be dependent on the sediment size, tailwater depth and upward seepage discharge. The rate of decrease for higher seepage discharge, lower tailwater depth and small particle size was high. The present results were compared with the results of previous research on horizontal jet without horizontal apron and was found in good agreement due to flow pattern similarity while our results were in disagreement with the results of previous research on horizontal jet with horizontal apron. To find the reason, three components of flow velocities inside the scour hole were measured using a Magnetic Doppler velocity meter for both cases of with and without upward seepage. The plots of the vertical flow velocity component showed that when the upward seepage was applied, the down flow velocity near the bed decreased up to 47%. The decrease of this velocity component could cause the lift force to decrease which eventually could reduce the scour depth. Relations for scour depth prediction under the effect of upward seepage were presented.