Preliminary experimental tests of local erosion at river piers with a particular shape

In the present study experimental tests concerning scour phenomena have been carried out on a particular shape of the already effective circular piers in order to determine if any improvement is verified in their shape coefficient (with a corresponding decrease in scour). The circular pier foundation has been shaped as a truncated cone, partially or totally sunk into the mobile bed. In the experimental tests a preliminary comparison has been effected between scour at simple circular pier and non uniform piers with two different values of bevel angle β for the truncated cone foundation. Experimental results have showed that, for both values, the scour depth has been lower than that identified in experimental tests conducted on circular piers of the same diameter but without the truncated cone foundation. The reduction in scour detected in the tests with the truncated cone base can be attributed to the modification of the complex distribution of velocity within the scour, which is characterized by components that are vertical and transverse to the flow direction, with fluid involved in vortex movements (horseshoe-vortex and wake-vortex). The pressure field deriving from the presence of the pier results in gradients that tend to concentrate whirling thread and then release them as dissipating vortexes that essentially cause localized erosion. On passing the pier, the wake-vortex oscillates over time towards the sides: the non-uniform distribution of velocity then gives rise to vertical components. The truncated cone foundation spatially modifies the whirling phenomena of the horseshoe-vortex (especially the counter-vortex) which in this case partly occur on solid surfaces; moreover the truncated cone accompanies the fluid threads into the area where the horseshoe-vortex is shed: the different pattern of the hydrodynamic phenomenon on the one hand leads to a reduction in scour depth while, on the other, it creates a limited extension in the surface subject to erosion . In actual fact, the modification of vortex phenomena is evidenced by the different position for the onset of erosion. Hager and Oliveto (2005) pointed out that scour occurs around the pier in an area about 70° from the pier axis and then extends on the whole front section where the horseshoe-vortex is located. In the experimental tests conducted with the truncated cone foundations it was observed that scour originates in an area close to an angle from the channel axis ranging between 70° and 90° for the various geometries before extending all over the pier front. The early tests of comparison and the results obtained with the particular foundation shape of the studied pier (which can also be adapted to more complex foundation geometries, i.e. like the ones reported in the Manual of the State of Florida) show a favourable shape coefficient and suggest that the test schedule should be extended by including a larger number of diameters and varying the dynamic conditions of the stream flow, the height and angle β of the truncated cone base, and the duration of the tests.