Estuary

Summary

An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world. Most existing estuaries formed during the Holocene epoch with the flooding of river-eroded or glacially scoured valleys when the sea level began to rise about 10,000–12,000 years ago. Estuaries are typically classified according to their geomorphological features or to water-circulation patterns. They can have many different names, such as bays, harbors, l

Official source

https://en.wikipedia.org/wiki/Estuary

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Effects of entrance conditions on tidal hydrodynamics in idealized prismatic estuaries under sea level rise

Steve Hottinger

Understanding tidal hydrodynamics response to sea level rise (SLR) in estuaries is essential to predict future potential issues such as shoreline erosion, more frequent and intense flooding, coastal wetland threatening, and their economic and ecological associated losses. By going beyond the classic static or “bathtub” approach, this study uses the RMA-2 modelling software to capture the full energy dynamic resulting from SLR and changes in entrance conditions. Indeed, entrance restrictions are seen as a key factor controlling tidal propagation within the entire system and only little knowledge exists on their effects on the flow under SLR. An innovative numerical approach using idealized prismatic estuaries is undertook to fill this gap. More than 150 simulation cases are performed, varying the channels geometry, restrictions dimensions and tidal forcing to capture most of existing prismatic estuarine dynamics. The resulting tidal flow is described using tidal ranges (TR), but also flow velocity, tidal asymmetry, wave types, tidal prism, etc. to find out potential interesting trends. In short channels, tidal reflection is found to be the dominant process influencing tidal propagation while in long channels the flow is mostly controlled by bottom friction. TR are amplified up to 10% per metre SLR, exacerbating the future flooding problematics linked to a higher mean sea level. However, strong TR attenuation is observed when entrance restrictions are implemented, leading to potential insights on SLR effects mitigation using narrower inlets. SLR acts on the flow velocities distribution and on the tidal asymmetry by increasing medium velocities and decreasing flood dominance. Both will impact sediment transport, leading to stronger coastal erosion. This increases therefore the pressure on sensitive ecosystem already struggling to stay in pace with SLR. SLR can bring the channels closer or further from their natural period of sway. This change in tidal resonance can lead to modifications of the flow parameters behaviour. When channels become more resonant with SLR, amplification of the above phenomena are usually seen, while when they are brought away from resonance, attenuation or even reverse effects can be observed. All these issues are not captured by the bathtub approach and arise from changes in tidal dynamics. Furthermore, instead of focusing on specific estuaries as most of the previous studies, this project is using an idealized approach allowing a global evaluation of the future tidal hydrodynamics. Understanding tidal response to sea level rise in estuaries is crucial to develop efficient coastal management plans able to mitigate this threat on estuarine ecosystems and populated areas. In particular, over 130 port cities having a population higher than 1 million people are concerned by SLR issues in estuaries.

2019

Estuarine environments, as dynamic low-lying transition zones between rivers and the open sea, are vulnerable to sea level rise (SLR). To evaluate the potential impacts of SLR on estuarine responses, it is necessary to examine the altered tidal dynamics, including changes in tidal amplification, dampening, reflection (resonance), and deformation. Moving beyond commonly used static approaches, this study uses a large ensemble of idealised estuarine hydrodynamic models to analyse changes in tidal range, tidal prism, phase lag, tidal current velocity, and tidal asymmetry of restricted estuaries of varying size, entrance configuration and tidal forcing as well as three SLR scenarios. For the first time in estuarine SLR studies, data analysis and clustering approaches were employed to determine the key variables governing estuarine hydrodynamics under SLR. The results indicate that the hydrodynamics of restricted estuaries examined in this study are primarily governed by tidal forcing at the entrance and the estuarine length. In addition, SLR increases the average water depth and alters the nodal point location in a seaward direction, thereby significantly affecting tidal wave propagation patterns and reducing the degree of flood domination. In estuaries with restricted entrances, tidal range diminishes drastically in the restricted zone by 20–60%, and the maximum tidal current velocity is higher in the restricted part but lower in the upstream part compared to unrestricted estuaries. For estuary types examined, inundation extent under SLR will likely be greater than the extent expected through simply adjusting existing water levels upwards. The findings also underline that an engineered entrance restriction could regulate tidal wave propagation within an estuary to offset SLR induced tidal range amplification. However, this may pose additional management challenges.

2020

Monthly to decadal evolution of sandy beaches in the Pittwater estuary

Vincent Henri De Staercke

Even if the total length of shorelines in low-wave environments like bays, sounds, lagoons and estuaries greatly exceed that of open-ocean coasts, the morphodynamics of such systems are poorly understood. Only few studies developed models to predict changes in these environments and these systems are often managed with the same policies as open ocean beaches. The Pittwater estuary is located in the north of Sydney, it is a drowned valley estuary with a well developed ood-tidal delta. The present study investigates the morphodynamics of four beaches in the northern part of this estuary, at two dierent timescales. The long term stability of these beaches have been examined using historical aerial photographs that allowto determine the position of the shoreline for the last decades. At the monthly to seasonal timescale, the morphological changes have been studied from beach surveys that are perform every one or two months since May 2016. Given that the survey campaign began just before a major storm that occurred in June 2016, this data set allow to study the post-storm recovery of these sandy estuarine beaches. The results of the present study showthat the beaches within the Pittwater estuary are evolving dierently according to parameters like distance from the inlet, exposure to ocean generated waves or beach shape. Examination of the aerial photographs suggest that two out of the four studied beaches are experiencing changes visible over several decades. Concerning the monthly to seasonal changes, detailed analysis of beach surveys indicated that sandy estuarine beaches can be strongly eroded during a storm event and that post-storm recovery can take several years up to decades. The comparison with two beaches located along the open coast in the Sydney area conclude that estuarine beaches have a much slower recovery rate.

2018

Effects of entrance conditions on tidal hydrodynamics in idealized prismatic estuaries under sea level rise

Steve Hottinger

2019