On River-Aquifer Exchange Flow With Irregular and Semipervious Bank

River water flowing into and out of riparian aquifers driven by stage fluctuations during floods is critical for streamflow and biogeochemical cycling of hyporheic zones. Previous studies paid limited attention to the complex exchange flow between rivers and the adjacent unconfined aquifers through irregular and semipervious banks which are commonly seen in nature. This study proposed a new semi-analytical solution for describing the river-aquifer exchange flow by incorporating river stage hydrograph, and semipervious banks with irregular geometries. The solution was verified by comparing with the numerical solutions of the one-dimensional Boussinesq equation and two-dimensional variably saturated flow models. The comparisons indicated that the semi-analytical solution was robust to closely approximate the bank storage exchange flow through an irregular and semipervious bank. Moreover, an innovative dimensionless river bank index (BI) was proposed to quantitively identify the importance of riverbank geometry irregularity. BI = 0.4 and BI = 0.76 are the critical threshold values to determine whether realistic bank geometry should be incorporated or not. Specifically, when BI is less than 0.4, the conventionally vertical bank can be applied to substitute realistic irregular riverbanks; When BI is greater than 0.76, however, realistic bank geometries should be included and otherwise significant errors may occur, especially when BI is much greater than 0.76. The semi-analytical solution and BI have been successfully applied to the Hanford Reach of Columbia River, Washington, USA. The proposed solution and BI are useful for conceptual and diagnostic analysis of bank storage exchange flow and for understanding which factors and responses are important.