Investigation of flow and local scour co-evolution around porous engineered logjams situated in a gravel bed river

Authors: Papanicolaou, Athanasios; MEROOK, ADAM - Rjh Consultants, Inc; WYSSMANN, MICAH - University Of Missouri; TSAKIRIS, ACHILLEAS - Northwest Hydraulic Consultants; BASNET, KESHAV - Orise Fellow

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 12/10/2023
Publication Date: 6/25/2024
Citation: Papanicolaou, A.N., Merook, A., Wyssmann, M.A., Tsakiris, A., Basnet, K. 2024. Investigation of flow and local scour co-evolution around porous engineered logjams situated in a gravel bed river [abstract]. Meeting Abstract.

Interpretive Summary:

Technical Abstract: Engineered logjams (ELJs) are permeable structures that are made of natural (timbers) and built (concrete dolos) materials to create favorable stream habitat conditions and the same time provide bank protection. Flow modification near ELJs situated in a gravel bed river can affect erosion rate and occurrence of scour conditions, as well as evolution of scour hole with flow. Flow modification near ELJs can affect entrainment rates and occurrence of scour conditions, as well as the geometry of the evolving scour hole. The degree and the rate at which permeable structures like ELJs affect near bed flow and bed scour volume in their vicinity have not been studied, especially, in gravel bed rivers with non-uniform size distribution, where sorting and interlocking of grains occur. Moreover, continuous measurements of scour volume in concert with flow estimations are lacking in the literature where most of the focus has been on the maximum scour depth estimation. The main goal of this study is to develop a decay function for relating flow and scour co-evolution to be used for scour estimation around ELJs situated in a gravel bed. Scour depth and water surface velocity near the ELJs are co-evolving variables that were correlated through unique monitoring using both the advanced sensor sonar technology coupled with point gage (as a verification tool) and the large-scale image velocimetry technique coupled with acoustic doppler velocimeter to ensure flow measurements at the surface and within the flow depth. The findings showed that the scour hole in the vicinity of an experimental ELJ obtained a conical shape. Flow field features and velocity magnitudes around an ELJ structure found to be evolving as scour evolves and both asymptotically approach an equilibrium value. Magnitude of the velocity decay near the ELJ toe observed to be decreased by 23% and 11% for clear water scour test and live bed scour test, respectively, from the flat bed condition before scouring starts to the equilibrium condition of scour. For both the scour tests, mean surface velocity decay at the ELJ toe is correlated with the increase in scour depth. For clear water scour, the time to reach equilibrium found to be similar for both flow velocity and scour depth. In live bed scour, however, the flow velocity is observed to approach an equilibrium value about 0.67 times later than the equilibrium scour depth. The decay function for relating flow and scour co-evolution is developed, by utilizing these co-evolution data, as a tool that can be used for scour depth estimation around porous ELJ situated in a gravel bed and then compared with literature experiments. Total amount of shear stress decay observed to be less for experiments at porous structures in a non-uniform gravel bed than at impermeable structures in a sand bed due to the combined effects of structure porosity and sediment particle interlocking. Further, multiple scour formulas were tested for predicting the maximum scour depth at both scour conditions and found to be overestimated at the porous ELJ in a gravel bed by 100% or more. Enhanced versions of formulas were then developed, as an additional tool, based on the effects of ELJ structure porosity and sediment non-uniformity on scour and found to results for predicting experimental maximum scour depths within 36% accuracy.