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You are here : eLibrary : IAHR World Congress Proceedings : 32nd Congress - Venice (2007) : THEME A: Engineering and Management of Fresh-water Systems : A field experiment to remove the forcing element from a riffle-pool
A field experiment to remove the forcing element from a riffle-pool
Author : Bruce MacVicar1 and André Roy
The mechanics that lead to pool formation and maintenance in gravel-bed rivers is an area of ongoing debate. A number of mechanisms have been identified that may play a role, including reversal of near-bed velocities, lateral flow convergence, and the generation of vortices as a result of fluid shear. This article follows from MacVicar [2006], where an intense flow measurement program was carried out in a forced riffle-pool at a range of flood stages including the bankfull flow. In that study we used mean flow and turbulence intensity profiles to document the occurrence of non-uniform flow in the pool. Specifically we found deceleration in the pool-head and acceleration in the pool tail. This was important for the near-bed hydraulic environment because sediments in the pool tail were subject to low mean velocities but high turbulent fluctuations, whereas the near-bed hydraulic environment in the pool-tail was characterized by very fast mean velocities. In the current study we wished to examine the effect of removing the forcing element from the pool with the objective of working towards a general model of riffle-pool hydrodynamics. The tree that forced the pool location was therefore cut and removed. The evolution of pool topography was then followed using repeated surveys and the hydrodynamics were measured during a moderate flood event. Flow measurements show that there are many similarities between flow dynamics before and after the removal of the tree. Similarities include the occurrence of flow convergence and divergence in the pool-head and pool-tail, respectively, as well as the perturbation to mean velocity and turbulence intensity profiles that is characteristic of flow deceleration and acceleration. These results are especially relevant to the debate on the mechanics of pool maintenance because, while subsequent flood events resulted in the significant alteration of the shape of the pool, they did not change the overall dimensions of the pool. This suggests that the forcing element is not necessary for pool maintenance. A hydrodynamic model of riffle-pools based on the perturbation of the lateral and vertical distribution of flow parameters may thus be a general model that is sufficient to account for pool maintenance in both free and forced pools.
File Size : 647,359 bytes
File Type : Adobe Acrobat Document
Chapter : IAHR World Congress Proceedings
Category : 32nd Congress - Venice (2007)
Article : THEME A: Engineering and Management of Fresh-water Systems
Date Published : 01/07/2007
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