The influence of ambient groundwater discharge on exchange zones induced by current-bedform interactions

We use numerical modeling to investigate the interactions between laminar flow in a water column, current-bedform induced flow in underlying permeable sediments, and discharge of deep groundwater to the water column. Groundwater discharge reduces the spatial extent of current-induced exchange zones within the sediments and may completely prevent the development of current-bedform induced flow at high groundwater discharge. Although exchange zone size is reduced by groundwater discharge, the reduction in exchange flux is not significant, suggesting reduced residence times of materials cycled through these smaller zones. Fluid exchange zones tend to be centered around the reattachment points of eddies that also mark the location of flow divides within the sediments; while deep groundwater is discharged preferentially along troughs. Exchange zone depth and the water column Reynolds number are functionally related through a Morgan-Mercer-Flodin model. The relationship between the current-induced flux through the sediments and the Reynolds number is described by low-order monotonic polynomial functions. (c) 2006 Elsevier B.V. All rights reserved.