Roughness effects on flow in karst conduits

Karst aquifers are formed by intricate conduit networks where variations in geometry significantly influence groundwater flow and transport. At the conduit scale, pronounced wall roughness (k/D∼10 ^-1), variable cross-sections, and tortuous centrelines challenge classical hydraulic descriptions based on smooth or idealised pipes.

In this work, we investigate flow dynamics in real karst conduits using numerical simulations over a wide range of Reynolds numbers (Re=1-10⁴). The objectives are to quantify key geometrical and hydraulic descriptors, including effective cross-sectional areas, conduit centrelines, velocity distributions, and friction factors. Roughness characterisation is performed through spectral analysis of the conduit walls using Fourier-based techniques.

The conduit geometries are directly obtained from high-resolution scans of natural karst formations and retain their full geometric complexity. Flow simulations are carried out without geometric simplification, allowing wall-induced disturbances and roughness effects to be fully resolved.

The results highlight strong deviations from classical smooth-conduit behaviour, with geometry-driven heterogeneity significantly affecting velocity fields and friction losses across all flow regimes. These findings show that the dominant contribution to flow dispersion arises from large-scale roughness, allowing simplified conduit representations that preserve these features to yield hydraulic predictions comparable to those obtained in fully resolved geometries.