Stepwise adjustment of deltaic channels in response to human interventions and its hydrological implications for sustainable water managements in the Pearl River Delta, China

Human activities have exerted influential effects on the evolution of channels in river deltas throughout the world in recent decades. In this study, the adjustment processes of deltaic channels in the Pearl River Delta (PRD) in response to diverse human interventions were examined during the 1960s-2010s. Changes in the channel geometries (i.e., cross- and longitudinal sections and at-a-station hydraulic geometry) indicate that channel evolution in the PRD can be divided into three periods since the 1960s, corresponding to stepwise changes in the hydrological processes in the PRD. During the 1960s-1980s period, channel topography showed slight change, mainly resulting from the variations in the water and sediment inputs, and hence at-a-station hydraulic geometry also displayed insignificant change. Such a kind evolution indicates a negligible impact from human interventions and an insignificant change in the hydrological processes in the PRD. From the 1990s to the early 2000s, the deltaic channels were substantially deepened, mainly due to large-scale sand excavation occurred in the PRD. And changes in at-a-station hydraulic geometry reveal an increase in the changing rate of the velocity with the discharge but a decrease for water depth and channel area. In response to channel deepening, the water level decreased but the tidal range increased at the Makou and Sanshui stations during this period. Furthermore, uneven down-cutting of the riverbed also significantly altered the divided water and sediment ratios in the PRD. After the prohibition of sand excavation (i.e., mid-2000s), although sediment reduction caused by dam construction was favorable for the further incision of deltaic channels, the incision magnitude was considerably decreased. In addition, we observe a relatively slight change in at-a-station hydraulic geometry. These results indicate an adjustment stage for the deltaic channels during this period. Correspondingly, the divided water and sediment ratios showed insignificantly change in the PRD, as did the water level and the tidal range. The results obtained from this study may help to set scientific guidelines for sustainable water management strategies for flood control, embankment protection, water and sediment regulation and prevention of salt-water intrusion in the PRD and other river deltas worldwide that are subject to strong human interventions.