Responses Of River-Tide-Salt Dynamics Characteristics To The Topography Changes Induced By Human Interventions In The Pearl River Delta,China

The Pearl River Delta(PRD)is one of the most complex delta systems in the world.It has become an area with highly adjusted river-tide-salt dynamics characteristics and a sensitive and vulnerable area of water security,integrating economic construction center,high-intensity human interventions and overuse of water resources.Hence,it is of great significance to study the changes in topography driven by human interventions and associated responses of river-tide-salt dynamics characteristics,which helps to lay a foundation for the adaptability evaluation of typical treatment projects under the changing conditions in estuaries,and provide scientific guidelines for the sustainable development of the PRD.This dissertation reviewed the human interventions in recent decades in the Pearl River Basin(PRB)and their impacts on the riverbed evolution in channel networks and the coastline changes in estuaries.Further,the distribution of residual water level-tidal range(RWL-TR)in the PRD and associated responses to the human interventions were examined.In addition,a quasi-three-dimensional river-tide-salt dynamics model covering the whole PRD based on the EFDC was established to quantitatively evaluate the influence of riverbed down-cutting and coastline changes induced by human interventions on the characteristics of saltwater intrusion during dry seasons.The main content of this dissertation can be summarized as follows:(1)In recent decades,dramatic changes occurred in the topography of the PRD.From1950s to 2010s,human interventions such as dam constructions and conservations of soil and water in the PRB led that the annual sediment load reduced by more than two thirds,and thus the riverbed turned into erosion from weak siltation in 1970s.Combined with the influence of human interventions such as sand excavation and channel dredging,70%-90%of the channel riverbed in the PRD has been cut down seriously since 1980s:the area of typical cross-section was expanded by 25%-45%,the thalweg of main river channel was cut down by 3-10 m on average.Besides,due to the influence of land reclamation,the estuaries squeezed towards the sea obviously with the extension of 1-15 km,and more than 600 km~2of water area has shrunk in the estuaries of the PRD.(2)The characteristics of river-tide dynamics in the channel networks of the PRD present significant seasonal changes.The larger the upstream water discharge is,the stronger the river dynamics is in the channel networks.Hence,there is a higher RWL and a greater RWL slope,causing a more significant inhibition effect on the tide dynamics,thus the TR is lower.It’s observed that the RWL decreases rapidly from the upstream to the estuarine mouth,and its slope is in the flood period>flood season>dry season,whereas the TR presents a decreasing gradient from the estuarine mouth to the upstream of channel networks,and the decay rate of TR is in the extreme flood period>flood season>dry season.According to the spatial distribution difference of the RWL-TR,the river-tide dynamics characteristics of the PRD can be divided into four systems:1)the apex of Huangmaohai Bay(HB)and 2)the apex of Lingdingyang Bay(LB),which are obviously controlled by the tide with low values of RWL and high values of TR;3)the central part of the West and North River networks and 4)the upper reaches of the East River,which are mainly controlled by the river discharge with high values of RWL and low values of TR.(3)Responses of river-tide characteristics in the channel networks to the riverbed changes showed significant temporal and spatial differences.Due to riverbed down-cutting,the RWL and its slope decreased significantly,but the TR increased obviously.The larger the upstream water discharge was,the greater reductions of RWL and its slope were,hence the sensitivity of the responses of RWL-TR distribution characteristics to human interventions was vastly different in different seasons.During the extreme flood period and flood season,the changes of RWL were extraordinary sensitive to riverbed down-cutting,the reduction of RWL reached to 1.5-6.0 m under the same water discharge and there was a clear linear positive relationship between the RWL reduction and the depth of riverbed down-cutting.However,the RWL in the dry season remained very low and relatively stable values,which was not sensitive to the riverbed down-cutting.In addition,the location was closer to the estuarine mouth,and the RWL was more significant controlled by the sea level,and it is less sensitive to riverbed down-cutting,leading that the RWL at upstream decreased more than that at downstream.Therefore,the RWL slope of the PRD decreased by 10%-50%,meaning that human interventions greatly weakened the resistance of riverbed to the propagation of tidal waves in the PRD.Consequently,the tide dynamics of channel networks got enormously enhanced and the TR increased.(4)The Modaomen Estuary(ME),as a typical river-dominated estuary,and the LB,as a tide-dominated estuary,showed different responses to the coastline changes and riverbed evolusion.The ME was controlled by gravity circulation,and the land reclamation weakened its tide dynamics and decreased landward salt transport,but the riverbed down-cutting induced by human interventions enhanced tide dynamics and gravity circulation,which was advantageous to landward salt transport.The overall effects of human interventions have increased the TR and salinity in the ME,and further caused the longitudinal salinity gradient to be halved and augmented the salt intrusion length.Consequently,the ME has changed from partial stratification to high stratification.The LB was controlled by tidal dispersion,the land reclamation strengthened its funnel-shaped coastline,so the tidal energy accumulated at the new estuarine mouths extended to the sea,their flood currents and tide dynamics got enhanced.As a result,the TR and salinity near the new estuarine mouths increased.In addition,the continuous extension of estuarine mouths and narrowed coastline occurred in the LB were helpful to intensify the vertical salinity stratification.(5)The coastline changes and riverbed down-cutting induced by local human interventions in the estuaries of the PRD played a decisive role in the evolution of salt transport characteristics in the ME and LB during dry seasons.The upstream riverbed down-cutting and the rise of sea level played a minor role and their impacts were only one tenth of those induced by local human interventions in estuaries.