Distorted model and its importance

A distorted model is a scale model in which the scale ratios are not the same in all directions, i.e. the horizontal scale $L_r$ differs from the vertical scale $h_r$. Geometric similarity is therefore not complete. The distortion is $h_r/L_r$ (greater than 1).

Importance (why it is used)

1. Space and cost: for large and flat prototypes such as rivers, harbours, estuaries and tidal basins, an undistorted model would be either too big or so shallow that the depth would be a few millimetres.
2. Avoids scale effects: a larger vertical scale gives enough depth to keep the flow turbulent, avoiding surface tension and viscosity effects, and keeps the Reynolds number high.
3. Measurable quantities: depths, velocities and slopes become large enough to measure accurately.
4. Movable beds: it gives enough slope and tractive force to study scour, silting and sediment transport.
5. Better visualisation of flow patterns, waves and currents.

The disadvantages are that the results need correction factors and that velocity distributions, bed forces and wave patterns are not exactly reproduced.