Abstract. Assessing the impacts of both natural (e.g., tidal forcing from the ocean) and human-induced changes (e.g., dredging for navigation, land reclamation) on estuarine morphology is particularly important for the protection and management of the estuarine environment. In this study, a novel analytical approach is proposed for the assessment of estuarine morphological evolution in terms of tidally averaged depth on the basis of the observed water levels along the estuary. The key lies in deriving a relationship between wave celerity and tidal damping or amplification. For given observed water levels at two gauging stations, it is possible to have a first estimation of both wave celerity (distance divided by tidal travelling time) and tidal damping or amplification rate (tidal range difference divided by distance), which can then be used to predict the morphological changes via an inverse analytical model for tidal hydrodynamics. The proposed method is applied to the Lingdingyang Bay of the Pearl River Estuary, located on the southern coast of China, to analyse the historical development of the tidal hydrodynamics and morphological evolution. The analytical results show surprisingly good correspondence with observed water depth and volume in this system. The merit of the proposed method is that it provides a simple approach for understanding the decadal evolution of the estuarine morphology through the use of observed water levels, which are usually available and can be easily measured.

Abstract. Observations indicate that the fortnightly fluctuations in mean water level increase in amplitude along the lower half of a tide-dominated estuary (The Guadiana estuary) with negligible river discharge but remain constant upstream. Analytical solutions reproducing the semi-diurnal wave propagation shows that this pattern results from reflection effects at the estuary head. The phase difference between velocity and elevation increases from the mouth to the head (where the wave has a standing nature) as the high and low water levels get progressively closer to slack water. Thus, the tidal (flood-ebb) asymmetry in discharge is reduced in the upstream direction. It becomes negligible along the upper estuary half, as the mean sea level remains constant despite increased friction due to wave shoaling. Observations of a flat mean water level along a significant portion of an upper estuary, easier to obtain than the phase difference, can therefore indicate significant reflection of the propagating semi-diurnal wave at the head. Details of the analytical model shows that changes in the mean depth or length of semi-arid estuaries, in particular for macrotidal locations, affect the fortnightly tide amplitude, and thus the upstream mass transport and inundation regime. This has significant potential impacts on the estuarine environment.

<p>The morphodynamics of tidal channels can be considerably altered by intensive human activities, such as the land reclamation and channel dredging. Morphological alterations of Lingdingyang Bay (LDB) in the Pearl River Delta (RPD) have received great attention, while the changes in Shiziyang (SZY) tidal channel connecting LDB with Humen Outlet is less known. In this study, bathymetric maps of the Shiziyang tidal channel in the PRD at different periods were collected to investigate the geomorphologic pattern by means of a digital elevation model (DEM). It was shown that the water depth increased with the decreasing width convergence length, indicating intensified channel deepening and narrowing, while tidal channel bed was drastically cut down due to port construction and channel dredging in 1959, 1975 and 2004, respectively. Consequently, the SZY eroded at a rate of 0.164 m/yr during the study period, with a new pattern of eastward-erosion and westward-deposition. In addition, erosion in the downstream reach (0.271 m/yr) was significantly faster than that in the upstream reach (0.006 m/yr). The EOF results showed that the impact of natural factor, channel dredging and land reclamation on bathymetric changes accounted for 48.48%, 20.54% and 11.57%, respectively. Furthermore, the spring-neap variability of tidal wave celerity and tidal amplification in SZY both increased after human interventions and there is a clear transition pattern and stronger tidal hydrodynamics according to the results of 1D analytical hydrodynamics model. Based on the model results, the decadal evolution of SZY morphology can be well reproduced by the observing water levels. The results obtained from this study can be particularly helpful for quantifying the potential impacts due to human interventions, which is useful for setting future conservation planning strategies in estuarine zones.</p>

<p>Due to the intensive human activities, the tidal hydrodynamics of “Bay-Outlet-Tidal Channel” system experienced considerable spatiotemporal change owing to the morphological alterations. The outlets in the Pearl River delta (PRD), especially the connecting outlet between Lingdingyang Bay and Shiziyang Channel, are small-scale dynamic structures that have a special morphodynamic feature, playing an essential role in energy transport and transformation. In this study, bathymetric maps of the “Bay-Outlet-Tidal Channel” system in 1965, 1974, 1989, 2009, and 2015 were collected to investigate the geomorphologic structure by means of a digital elevation model (DEM). It was shown that the water depth increased with the decreasing convergence width, indicating the channel deepening and narrowing. The tidal hydrodynamics, especially focusing on the tidal amplitude and velocity amplitude, were explored using a 2D numerical model in the system of “Lingdingyang Bay-Humen outlet-Tidal Channel” considering dramatic anthropogenic effects. In addition, the 1D analytical model was used to reproduce the historical development of tidal hydrodynamics and its underlying mechanism. The results show that the relationship of tidal amplitude and velocity amplitude was dramatically affected by the morphological alterations, with the deepening playing a much more important role. It was shown that the tidal amplitude was increased by 0.0393 m, while a decrease by 0.0432 m/s for the velocity amplitude. Owing to the Outlet in the “Bay-Outlet-Tidal Channel” system, the channel networks and outer bay in the PRD were kept relatively stable, with the tidal energy decay rate of tidal channel remaining approximately constant (70%). These results quantifying the impacts of estuarine morphology on tidal hydrodynamics can provide scientific guidelines for sustainable water resources management in the PRD and other estuaries that are subject to intensive human interventions, especially regarding morphological alterations.</p>