La Houille Blanche
Number 6, Octobre 1964
|Page(s)||707 - 718|
|Published online||24 March 2010|
Méthodes d'investigation sur modèles réduits des différents problèmes d'hydraulique côtière
Methods of investigating coastal hydraulics problems on scale models
Ingénieur à la SOGREAH.
This article gives a brief review of the possibilities and the range of applicability of the scale model as a means of hydraulic research into changes undergone by a sea coast. Considering only the action of waves on a sea shore, some of the conditions for its representation in correct similitude on the model are as follows:__ 1) Reproduction of the main aspects of wave propagation__i.e. refraction, diffraction, reflection__which requires an undistorted model; 2) Reproduction of the mode of wave generation, for waves from the open sea generally do not produce the same beach profiles as local wind waves; 3) Reproduction of the effect of waves on the beach material. It is very difficult to choose a suitable model material as Froude similitude is seldom achieved; not only the grain size of the material, but especially its density must be carefully determined if it is to show the same "potential transportability" as the prototype material. The model and prototype materials should, in theory, be in similitude with regard to the following properties:__ (i) Grain acceleration from rest and sinking velocity; (ii) Sediment discharge for a given wave size; (iii) Respective proportions of bed and suspended sediment loads. These conditions cannot generally be reconciled, except in very special cases where a very local effect is investigated to a comparatively large scale. The choice of a bed material and the corresponding model scale distortion and wave reproduction scales thus finally depends on the type of problem to be solved. This may also apply to the mode of wave generation. In view of this difficulty, an attempt has been made to classify such problems, as follows:__ Investigation of problems associated with sediment drift along a coast and changes in the shore line. The all-important requirement here is to reproduce the sediment drift as affected by wave obliquity, most of which takes place within the area between the breaking waves and the top edge of their wash. In this case, even though wind waves may sometimes be involved, paddle generating equipment may be adequate if rapidly adjustable for direction so as to enable the energy spectrum resulting from the various wave directions during the year to be reproduced. This type of representation provides a perfectly acceptable means of studying the development of a coastal spit, the silting-up of a harbour, methods of keeping a lagoon in open communication with the sea, etc. Though these models are usually slightly distorted and involve a certain amount of compromising, they can nevertheless be made to supply some very interesting indications. It is difficult, however, to use them for really accurate quantitative interpretation of the effects of the requisite structures and the determination of their dimensions. Investigation of the development of a beach cross-section and erosion due to head-on wave attack. This type of problem necessitates a more precise reproduction of wave action throughout the developing cross-section and, especially, of sediment drift out to sea. It is difficult to reproduce these effects with a paddle type generator, for the outward current along the sea bed associated with real-life wind waves is not reproduced On the model. Wind generating facilities may prove necessary in such cases; a 600 square meter tank featuring a set of thirty-four 2 H.P. fans has been built at SO.-GR. E.A.H. to investigate a protection scheme for a beach subjected to wind waves at various sea level conditions. The adjustment of such a model is obviously a very delicate matter; an essential prerequisite is the reproduction of developments observed in real life, with the waves and water levels which are likely to have caused them. The authors have found by experience that such effects can be very satisfactorily reproduced with such wind wave generating equipment, which also enables very fiat beaches to be obtained on the model with very little scale distortion. Conclusion. An experimental hydraulic model study can contribute some very interesting indications for problems associated with the effect of wind waves on coasts and beaches; such information provides very useful guide lines for coastal development and protection work, despite the difficulty of reproducing to a correct scale the effects of the various factors associated with wave type and propagation upon the shore. Though a model with slight scale distortion and conventional wave generating equipment is generally adequate to study the effects of oblique wave attack upon a sea shore, a study of shore erosion under practically head-on wave attack may, to be really accurate, also require means of simulating wind wave formation in order to reliably reproduce the action of such waves, or to produce the requisite "seaward" drift of the model shore material.
© Société Hydrotechnique de France, 1964