Extreme high water level (EHWL) dynamics in the tidal reach of the Pearl River: Coupling between terrestrial runoff and channel geomorphic changes

Abstract

A flood is one of the most severe natural disasters and its threat is increasing under global climate and population change. This leads to a worldwide concern on the long-term evolution of extreme high water level (EHWL), especially in the tidal reaches of high social-economic value and complex river-tide interactions, an important quantitative index of flood intensity. Here, the EHWL dynamics in the tidal reach of the Pearl River under the couplings of runoff variations and channel geomorphic changes are examined, based on hydrological data of > 50 years. The results showed an overall decrease in the EHWL of all grades (from 0.1 to 5 %) from 1966 to 2016, and the decrease is more significant in the upstream (0.5–3.7 m in magnitude) than the downstream sub-reach (−0.3 to 0.2 m in magnitude). An improved tidal harmonic analysis model, R-TIDE, is introduced to detect the potential flood-tide interactions and their transformations. The upstream EHWL consisted mainly of the fluvial component and while it should have increased as the flood runoff intensified after 2000, it in fact reversed due to channel geomorphic changes. The role of river runoff on the downstream EHWLs is less, given a co-dominance of fluvial and tidal components and an embedding of storm surge. In the lower sub-reach, the geomorphic changes-induced decrease of the downstream EHWLs is balanced by amplified tides. Further quantification of the relative contribution of these two terms indicates a dominant role of channel geomorphic changes (∼ 68/15 % in the upstream/downstream sub-reach) on the observed decrease of the EHWLs. These findings provide new insights into EHWL dynamics and help cope with future flood disasters.