Q. What do you mean by Rigid bed models?
Models with rigid boundary and geometrically similar form are used in studies connected with outlet works, stilling basins, spillways etc. Conditions of Froude Number equality, equality of horizontal and vertical scales and adoption of some flow formula enable working out of the scales of the model. The scale of boundary friction in this case works out to be such that model is required to be made smoother than the prototype.
Rigid boundary models are often constructed for determination of flood levels in rivers and flood routing studies. If geometrically similar model is constructed, depth, velocity and Reynolds Number in the model becomes too small whereas discharge required becomes too big. These difficulties can be surmounted if depths are exaggerated in the model. Such models are, therefore, termed vertically exaggerated models. Since long lengths of rivers are normally required to be reproduced in such models, availability of space usually puts constraint on horizontal scaling. Vertical exaggeration results in model roughness being required more than in the prototype which is often achieved by embedding artificial roughness elements. This puts another constraint on scaling. Exaggeration of depths in relation to horizontal dimensions is generally high in such models.