Evolution of reversal of the lowest low waters in a tidal river network

Abstract The lowest low waters usually occur during spring tides in tide-dominated coastal environments. However, they could also occur during neaps, rather than springs, at more landward stations, and this phenomenon is known as the reversal of the lowest low waters (RLLW). The RLLW greatly depends on subtidal and daily oscillations in the water level, the evolutions of which along a channel are of considerable importance in navigation safety and freshwater intake. Observations collected at an inland hydrological station (Shilong) in the networks of the East River, China, show that the lowest low water level previously occurred during neaps, but now occurs during springs. To elucidate the causes of this evolution, T_TIDE was used to extract constituents from water level records collected over the past fifty years at five stations in this river network system, including astronomical constituents, overtides, and fortnightly tides. The results show that the astronomical constituents were significantly amplified in the upstream stations, while the fortnightly tide Msf decreased slightly in the long term. Physically, channel deepening caused by excessive sand excavation has increased the water depth and reduced the effective drag in the friction term of the momentum balance. The reduced frictional effect, reflected by the residual water level slope, is responsible for the significant increase in the astronomical tidal amplitudes. However, they were not strongly influenced by long-term variations in river runoff and sea-level rise. The amplitude ratio between Msf and S2 was used to analyze the spatiotemporal evolution of RLLW. This ratio at Shilong continuously decreased over time, and the location where the ratio was 1 moved upstream, indicating that the RLLW has traveled farther inland in recent decades. The evolution of the RLLW in the East River network implies that the tidal-dominant zone has enlarged in recent decades.