Topological and dynamic complexity of the Pearl River Delta and its responses to human intervention

Abstract

River networks of deltas are complex systems, responsible for the transport of water, sediment and nutrients between land and sea. Topological complexity, dynamic complexity and flux vulnerability are three characteristics to describe the material transport within a deltaic river network, and they reflect the channel number and connectivity, flux distribution, and sensitivity of flux to external disturbances, respectively. Here, graph theory was applied to estimate the three characteristics of the Pearl River Delta (PRD) in 1977 and 2008, when the human-induced uneven riverbed downcutting occurred on a large scale. By identifying the channels through which water flows from the apex to each outlet, the eight subnetworks corresponding to the eight outlets were distinguished. Although the topological complexity changes little during this period, the dynamic complexity and vulnerability have great adjustments. The dynamic complexity greatly decreases in the subnetworks that have larger riverbed downcutting comparing to their adjacent subnetworks. We further reveal the positive relation between the decrease in dynamic complexity and human-induced riverbed downcutting under the same topological complexity. Moreover, the local vulnerabilities of channels with relatively high riverbed downcutting increase, leading to the emergence of “high vulnerability channels”. These findings suggest that the human-induced uneven riverbed downcutting is the main reason for the adjustments of dynamic complexity and vulnerability. The results obtained from this study may provide scientific guidance for the further development and utilization in the PRD and other deltaic river systems worldwide subject to intensive human intervention.