Observed Effects of Geometric Distortion in Hydraulic Models

Geometric distortion in hydraulic models is a systematic change from geometric similitude involving the use of exaggerated depths or slopes. In many cases the advantages of distorted models more than compensate for their disadvantages; hence distorted models are commonly used in many countries. Questions of costs and benefits, size of prototype, laboratory facilities, time, and requirements of turbulent flow and adequate tractive force in the model may make geometric distortion necessary or desirable in hydraulic laboratory models. For both technical and economic reasons the degree of geometric distortion is usually greater for large than for small prototypes.

In this paper, studies of specific models are summarized and analyzed for effects of geometric distortion in the behavior of the models. Comparative data are presented. for an undistorted and a distorted model of the same prototype showing effects of slope distortion and depth distortion upon velocity conversion factors, from model to prototype. Data are presented for other models showing effects on bed formations of variations in bed materials (including light-weight materials), and slope and depth distortions. Selected studies of flood control and river navigation in the Mississippi River System are summarized and analyzed. Brief reference is also made to observations, pertinent to effects of geometric distortion, in many other hydraulic studies.

Analysis of the data showed that a lesser degree of distortion may be required in movable bed models if light-weight materials are used to simulate the stream bed. The analysis also outlined representative methods of compensating for distortion and effecting hydraulic similarity in specific details when over-all similarity cannot be expected.