Characteristics of tail erosion downstream of non-prismatic stilling basins

This thesis highlights the following main conclusions:8.1.1 Stilling basins without sill1- The scour pattern is asymmetrical and the max. scour occurs either on the left or on the right from the longitudinal centerline of the channel downstream of SESB.2- The deposition and Max. deposition depth are also asymmetrical.3- The lower expansion ratio yields smaller values of the Max. scour and4- Statistical Prediction model (Eq.6.1) is developed and verified forcomputing the maximum scour depth.8.1.2 Stilling basins with sill1- The scour patterns downstream of sudden expanding SB are asymmetric even when using central sill of limited height at any specific position (except at the optimal positions of sill with optimal heights).2- The optimal position of the sill that minimizes the magnitude of the Max scour and that reduces the extent of scour processes is function of Froude number under the gate F(h expansion ratio e and relative height of sill hsiG for a particular movable soil based on (Eq. 6.3). Within the experimental condition of this study, the optimal position is at XsfL=O.2 to 0.3 or at XslG=7.5 to 11.25.3- The optimal height of sill that reduces the scour to very limited magnitude or to zero is a function of Froude number under the gate Fo,expansion ratio e, and rel~tive position of sill Xs/G based on (Eq. 6.3). Within the limitations of this study, a relative height of sill hg/G ~ 1.5 minimizes the Max. scour greatly and reduces greatly the extent of scour processes.4- A statistical prediction model (Eq 6.3) is proposed, calibrated, andverified to predict the Max. scour downstream sudden expanding SB.5- In the light of the results of this thesis, the existing sudden expanding SB should be protected from both sides and bed for distance of about one times the length of the SB at least when a central sill is used and for a distance 2.5 times the length of the stilling basin for the case of no sill.6- ANN may be considered as a good technique for predicting the Max.depth of scour downstream of sudden expanding stilling basin.8.2 RECOMMENDATIONS1- The prediction models, Eqs. (6.1) and (6.3) may be recommended tofmd the maximum depth of scour downstream SESB without and with centralsill respectively.2- The Artificial Neural Networks may be recommended to predict themaximum depth of scour downstream SESB.8.3 RECOMMENDATIONS FOR FUTURE WORK1- Long term scour should be considered for physical• models of suchstructures as the time shows major effect on the Max. scour and on the extentof the scour• and deposition process.2- It is recommended to conduct an extensive experimental study toinvestigate the effect of expansion ratio on the scour and deposition process as the length to the Max. scour, the Max. deposition height and its distance from the end of the basin as well as the width and length of the both scourhole and the hump.