Effect of unsaturated flow on groundwater-river interactions induced by flood event in riparian zone

•Larger κD leads to smaller head changes and more water exchange of aquifer-river.•Unsaturated zones have significant impacts on flood signal propagation in aquifers.•Traditional unconfined solutions underestimate heads and overestimate discharges. A coupled unsaturated–saturated model for groundwater flow in a riparian zone is established to evaluate the impacts of unsaturated zone on groundwater flow and aquifer-river interactions during a flood event. The semi-analytical solutions for hydraulic head and discharge are obtained by the Laplace-Fourier transform and verified with COMSOL Multiphysics. The results show that the groundwater flow induced by a flood wave is significantly influenced by the dimensionless constitutive exponent κD in the moisture characteristic curve: the larger the value of κD, the stronger the water storage capacity of unsaturated zones, leading to smaller fluctuations of the hydraulic head and higher water amount exchanging between an aquifer and river. The present solution approaches to the solution for an unconfined aquifer when κD=100, and approaches to the solution of confined aquifers when κD=0.01. A thicker unsaturated zone causes less hydraulic head change and slower head recession after a flood wave passes. The traditional solution for an unconfined aquifer tends to underestimate the hydraulic head and overestimate the water exchange between the aquifer and river because it neglects the water retention of the unsaturated zone. The solution is applied to field data, and it improves the prediction of hydraulic heads over a previous solution.