Along-channel variability of Total Exchange Flow in a narrow, well-mixed estuary: influence of the M4 tide

Abstract. This study provides preliminary estimates of Total Exchange Flow (TEF) along the Guadalquivir River Estuary (Spain) at different cross-sections during low river flows. The analysis combines 3 years of observations from a real-time monitoring network with an analytical exchange flow scenario featuring a sectionally-homogeneous M2+M4 oscillating tidal flow and salinity. Exchange profiles and volume and salinity transports sorted by salinity classes are computed. The results indicate that bulk along-channel TEF estimates decrease upstream. The largest net incoming water volume transport, on the order of 300 m3 s−1, is attained at the lower part of the estuary, near where the largest salinity gradient is observed. This value is about 12-fold the normal river flow from the head dam at Alcalá del Río. Knudsen bulk quantities are consistent with the weakly-stratified character of the Guadalquivir estuary, whose mixing completeness is larger than 67 % at all cross-sections. The covariance between M2 salinity and current seems to play a more important role in exchange flow in the Guadalquivir estuary than the effects due to tidal asymmetry. Overall, the inclusion of the M4 improves TEF estimates in ∼ 10 % in the Guadalquivir estuary. A sensitivity analysis shows that in other estuaries and semi-enclosed basins the effects of the M4 could be even larger. The inclusion of the M4 constituent changes the exchange flow profile by salinity class, modifying the range of salinities of both outflows and inflows.