Q. Show the Limitations of Rigid bed models?

Due to vertical exaggeration, model slope automatically becomes steeper than in the prototype, this slope exaggeration being the same as the vertical exaggeration. When river has a very flat slope even such exaggerated slope in the model may be found to be insufficient to cause satisfactory bed load movement. The model is then made steeper than the slope obtained due to vertical exaggeration and it is called tilted. Complications arising out of this expendient like non-reproduction of backwater heights and lengths are required to be taken care of separately. Complications arise even due to vertical exaggeration alone. Scour holes are not correctly formed since side slope of scour hole in the model cannot become steeper as angle of repose of bed material can?t be very different. Weir coefficients become higher in the model since shape of weir crest is vertically exaggerated. Pier contraction effect becomes excessive in the model because the height of pier is exaggerated. Suitable corrective measures are introduced to overcome such difficulties and where this is not possible, allowance for divergence in the model from the proto type is made in interpreting model results.