Slope curvature and pool-riffle interactions drive hierarchical hyporheic exchange: A predictive framework for river corridor management

Abstract

Riverbed topography exerts a primary control on hyporheic exchange, yet the combined influence of reach-scale slope curvature and pool-riffle morphology on subsurface flow organization remains poorly quantified. Using a high-resolution, idealized two-dimensional numerical model that couples turbulent river hydraulics with groundwater flow and solute transport, we systematically vary regional slope (S), longitudinal slope curvature (δ), and pool-riffle amplitude (a) and wavelength (λ) to isolate their independent and combined effects on hyporheic processes. We demonstrate that increasing reach-scale convexity—representing knickzone morphology common to mountain streams—generates vertically nested hyporheic flow cells that expand exchange zones up to an order of magnitude. Slope curvature amplifies penetration intensity and flow-path diversity until a threshold (δ ≈ 2–4), beyond which pool-riffle geometry becomes the dominant control. Pool-riffle aspect ratio (a/λ) governs exchange fluxes through a universal power-law scaling, while larger λ deepens flow paths and accelerates dilution, and larger a enhance flushing intensity. These multi-scale interactions produce non-Fickian residence time distributions and distinct mixing regimes. By explicitly linking reach-scale convexity to hyporheic exchange, this study provides a predictive framework for understanding how knickzone morphology and pool-riffle geometry jointly regulate solute retention, offering new guidance for river restoration and corridor management.