Peak Water Level Response to Channel Deepening Depends on Interaction Between Tides and the River Flow

Abstract

The Pearl River Delta represents a tide‐influenced lowland delta where sand mining has deepened the distributary channels. This study sets out to investigate the impacts of channel deepening on peak water levels across the delta. Based on collected hydrological data before and after sediment mining and hydrodynamic modeling, three regions are identified where sand mining‐induced changes in peak water levels have a different underlying cause. In the river‐dominated regions, peak water levels have significantly decreased because of the sand excavation, and thus the associated flood risk was reduced. In a transition region, the sand mining practice has increased peak water levels. In the tide‐dominated coastal region, the changes in peak water levels follow the rise in sea level and respond to human activities such as land reclamation. To understand the underlying mechanisms, the changes of subtidal friction governing tidal dynamics before and during sand mining are explored, and a simple linear regression model is used to predict subtidal water levels governing peak flood levels. Results from the hydrodynamic model and the linear regression models quantify exactly how the direct effect of channel bed lowering, river‐tide interaction, tidal amplification, and sea level rise have contributed to peak water level change, which offers a new analysis framework that can be applied in other systems.

Peak Water Levels Rise Less Than Mean Sea Level in Tidal Channels Subject to Depth Convergence by Deepening

“…A similar effect was found for the Blyth estuary (Suffolk, eastern England) (French, 2008), and a simplified analysis of 36 estuaries worldwide confirms that this effect predominantly holds for relatively deep estuaries and deltaic channels (Leuven et al., 2019). This implies that the fate of shallow deltaic channels is probably very different, because they have not yet experienced the same degree of urbanization and deepening as for example, the RMD, the Pearl Delta (Bao et al., 2022) and the Yangtze Delta (Chen et al., 2016; Zhang et al., 2019).…”

“…Observations from individual estuaries have indicated that dredging can decrease channel roughness, increase tidal amplitudes, enhance depth convergence, decrease flood wave attenuation and reduce tidal dampening and storm surge dampening (Familkhalili et al., 2020; Muñoz et al., 2022; Talke et al., 2021; Talke & Jay, 2020). Capturing this behavior in modeling has proved challenging (Cai et al., 2012), particularly when estuaries are heavily anthropogenically influenced, and move away from classic, width converging estuary morphology, such as in the Rhine‐Meuse Delta (RMD) in The Netherlands (Cox, Lingbeek, et al., 2022) and the Pearl Delta in China (Bao et al., 2022). With channel deepening, the tidal range generally increases and storm surge waves also amplify, while high river discharge events generally exert a smaller control on peak water levels in the delta channels (Talke et al., 2021).…”

“…With channel deepening, the tidal range generally increases and storm surge waves also amplify, while high river discharge events generally exert a smaller control on peak water levels in the delta channels (Talke et al., 2021). Understanding and disentangling the effects of dredging from the effects of climate change is both important and challenging (Bao et al., 2022; Cox, Lingbeek, et al., 2022).…”

“…Many deltas are under pressure from changing boundary conditions and human influence, including sea‐level rise (SLR) (Giosan et al., 2014; Khojasteh et al., 2021; Leuven et al., 2019; Oppenheimer & Hinkel, 2019), land subsidence (Minderhoud et al., 2018; Syvitski et al., 2009) and dredging for reasons of navigation and sand mining (Bendixen et al., 2019; Cox, Dunn, et al., 2021; Hackney et al., 2020; van Dijk et al., 2021; Wu et al., 2016). The complex interplay between tides, storm surges and the river discharge causes that the effects of dredging and relative SLR on peak water levels can change across a delta plain (Bao et al., 2022). Although dredging and channel deepening tend to be local measures, typically focused on one navigation channel or one section of a channel, they may have an overarching effect on the hydrodynamics and associated peak water levels of the entire estuary (Talke & Jay, 2020).…”