Tide transformation in the Guadalquivir estuary (SW Spain) and process‐based zonation

Abstract

This study analyzes tide transformation in the Guadalquivir estuary (SW Spain). When fresh water discharges are less than 40 m3/s, the estuary is tidally‐dominated (flood‐dominated) and well mixed. Under such conditions, the estuary can be divided into three stretches, each characterized by a different tide propagation process. In the first stretch of 25 km, the dominant process is diffusion. In the next stretch, approximately over 35 km length, convergence and friction processes are in balance. At the head of the estuary, in the last stretch, the tidal motion is partially standing because of tidal reflection on the Alcalá del Río dam, located 110 km upstream from the estuary mouth. The reflection coefficient R varies with the frequency; for diurnal constituents its magnitude ∣RD∣ is 0.25; this value increases in the case of semi‐diurnal (∣RS∣ ≈ 0.40), and quarter‐diurnal constituents (∣RQ∣ ≈ 0.65), and reaches its minimum at the sixth‐diurnal components (∣RX∣ ≈ 0.10). The tidal reflection can generate residual currents that have consequences in the bed morphology. Furthermore, when the fresh water discharges are greater than 400 m3/s, the estuary is fluvially‐dominated and the water level can be calculated as the linear superposition of tide and river contributions. However, superposition arguments do not hold for currents at any point in the estuary.