Interactions between river flow and seepage flow

Abstract

Many previous studies have been carried on the interactions between river flow and the seepage flow in the environmental and biological point of view. Even though the interactions between river flow and seepage flow is recognized as an important process in rivers, previous literature hardly touches on the stability or the limitations for the interactions. Since these interactions are occurred frequently at least in mountainous regions, the river flow cannot be well treated as a lined cannel flow. Understanding the stability of these interactions among river flow and the seepage flow would be advantages for several research areas; including river environmental engineering, ecological and biological studies. The subsurface layer below the river is known as the “hyporheic zone” and it can be defined as a saturated band of sediment that surrounds river flow and forms a linkage between the river and the aquifer. The zone facilitates to have bidirectional interactions as up-welling interactions and downwelling interactions. The origin of these interactions is due to the pressure and velocity differences between the two layers. The large velocity difference between the river flow layer and the seepage flow layer causes the instability of the flows. Due to this flow instability, a reciprocating flow motion is generated between the hyporheic layer and the above. In addition flow obstructions create an upstream high-pressure zone and a downstream low-pressure zone, resulting in hyporheic circulation under the object. The stability of these hyporheic interactions is analyzed using the linear stability analysis technique. Linear stability analysis technique is used to understand the stability of the natural phenomenon by many researchers. Navier-Stokes equations and Brinkman-Forchheimer equations are used in order to formulate the river flow and seepage flow interactions respectively. The open channel flow in river is analyzed using the mixing length turbulent model and spectral collocation method incorporated with the Chebyshev polynomials are used to perform the numerical solution of the perturbed equations. Stability diagrams are discussed with several slopes of the layers against the dimensionless particle diameter and wave number. It has been understood that the range for the occurrence of instability region increases with the slope of the combined river and seepage layers. However it is important to recognize another instability region which occurs even in the range of small dimensionless particle diameter with relatively high wave numbers. Several experiments are carried out, in order to understand the hyporheic interactions. Seepage layer is modeled using a Hele-Shaw which is a longitudinal parallel plate model. Methylene blue is used as the tracer to understand the hyporheic interactions and the experiment is conducted for two slopes as 0.1% and 0.2%. It can be concluded that the dimensionless dominant wave numbers have an effect on the combined channel slope and the Froude number of the river flow. In addition, it can be concluded that the residence time of hyporheic interactions are increased with the height of the river layer. Rough comparison between the theoretical analysis and the experimental observations is carried out. It can be concluded that the same tendency in the theoretical analysis and the experimental observations from the comparison figures