A "state of the art" report on the subject of local scour around cylindrical piers is given here. After a description of the scouring process, a critical review of literature on model and field data is presented, and the empirical data are compared with theoretical considerations. The final result is a set of design suggestions together with possibilities for protection against scour.Sommaire Ce rapport donne le point des connaissances dans le domaine de l'érosion locale en autour des piles cylindriqucs: analyse du processus d'érosion, étude critique des résultats expérimentaux modèlc et nature disponibles dans Ia littérature, confrontation des résultats aux schémas theoriques. En conclusion est proposée une loi pour la prevision de la profondeur d'affouillement, ainsi que des dispositifs de protection.

This paper investigates the variation of the vegetative roughness coefficient with the depth of flow. A horsehair mattress is used in the experimental study to simulate the vegetation on the watercourses. Test results reveal that the roughness coefficient reduces with increasing depth under the unsubmerged condition. However, when fully submerged, the vegetative roughness coefficient tends to increase at low depths but then decrease to an asymptotic constant as the water level continues to rise. A simplified model based on force equilibrium is developed to evaluate the drag coefficient of the vegetal element; Manning's equation is then employed to convert the drag coefficient into the roughness coefficient. The data of this study are compared with those of selected previous laboratory and field tests. The results show a consistent trend of variation for the drag coefficient versus the Reynolds number. This trend can be represented by a vegetative characteristic number k. Given information such as the bed slope, the height of vegetation, and k, one can apply the proposed model to predict the roughness coefficient corresponding to different flow depths.

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