Spatial and temporal intensification of lateral hyporheic flux in narrowing intra-meander zones

Meander bends in alluvial rivers morphologically evolve towards meander cut-off with narrowing intra-meander necks, and this should steepen hydraulic gradients and intensify intra-meander hyporheic flux. This research used dye tracking and head loss measurements in a 1:500 planimetrically scaled laboratory river table to quantify the spatial and temporal intensification of intra-meander flux rates at two evolution ages. The younger meander bend, M1, had a sinuosity of 2.3, a river neck width of 0.39cm, and 0.6% river slope, and the older meander bend, M3, had a sinuosity of 5.2, a river neck width of 0.12cm, and 0.5% river slope. Flux into and out of the meander bend was estimated along the normalized curvilinear distance s*, with the meander neck at s*=0.1 and s*=0.9, the meander centroid at s*=0.37 and s*=0.63, and the apex at s*=0.5. Between the meander centroid and neck, we documented a 60% spatial intensification for M1 and a 90% spatial intensification for M3. Between M1 and M3, we documented a 135% temporal intensification at the neck and a 100% intensification at the centroid. Our empirical spatial and temporal intensification rates involving the M1 and the M3 scenario were one to three times lower than theoretical rates derived from a river evolution model with equivalent M1 and M3 planimetry. Overestimation by the theoretical model was attributed to exaggerated head loss caused by the model neglecting groundwater contributions to river stage. Hyporheic exchange provides critical ecosystem services, and its spatial and temporal variation with meander evolution should be considered in river management.