Hydraulics of simple habitat structures

Habitat structures are built in rivers to provide feeding and resting areas for fish. At the present time, only rough guidelines are available for the design of these structures. This paper presents the results of a laboratory study on the flow and erosion around simple habitat structures. Hemispheres with diameter D of 74 and 130 mm were placed on smooth, rough as well as erodible beds and Froude number of the approaching flow was in the range of 0.074 to 0.6. The relative depth d/h where d is the depth of and h is the height of the body was found to be the important parameter and was varied from about 0.6 to 4.3. Four different regimes of flow were found, which were classified based on the relative depth. Downstream of the body, there was a recirculation region (closed wake) with a length of about 2D which was followed by an open turbulent wake. The structure of flow in this open wake was analyzed in two layers using the concept of the wall wake. In the plane of symmetry, the inner layer was analyzed using the law of the wall whereas the outer layer was analyzed using the wake equation of Schlichting. The variation of the velocity in the transverse direction was also analyzed using the concept of similar profiles. Further an empirical correlation was found for the velocity scale. The amplification of the bed shear stress near the body, especially for the rough bed was signiflcant. Some observations were also made on the nature of erosion around the hemisphere placed on erodible beds of two sand sizes of 1.11 and 2.1 mm. It was found that the pattern of erosion was different for the different flow regimes. The maximum equilibrium clear water scour depth occurred in front of hemispherical bodies and was approximately equal to 0.67 D.