Sea-level-rise-induced threats depend on the size of tide-influenced estuaries worldwide

Abstract

The effects of sea-level rise on the future morphological functioning of estuaries are largely unknown because tidal amplitudes will change due to combined deepening of the estuary mouth and shifting amphidromic points at sea. Fluvial sediment supply is also globally decreasing, which hampers infilling necessary to maintain elevation relative to sea level. Here we model 36 estuaries worldwide with varying sizes, shapes and hydrodynamic characteristics, and find that small shallow estuaries and large deep estuaries respond in opposite ways to sea-level rise. Large estuaries are threatened by sediment starvation and therefore loss of intertidal area, particularly if tidal amplitude decreases at the mouth. In contrast, small estuaries face enhanced flood risks and are more sensitive to tidal amplification on sea-level-rise-induced deepening. Estuary widening can partly mitigate adverse effects. In large estuaries, expanded intertidal areas increase tidal prism and available erodible sediment for adaptation, whereas it slightly reduces tidal amplification in small estuaries.

Sensitivity of Flood Hazard and Damage to Modelling Approaches

“…The changes in total water level at the coastline presented here are within the range of the Intergovernmental Panel on Climate Change (IPCC) global projections of SLR, which range from 28 to 98 cm by 2100 [86] and the UKCP18 projections in Cardiff range from 0.27 to 1.31 m by 2100 [87]. Even a small change in sea-level rise will have a large impact on flood hazard as tidal amplitudes and estuarine morphologies change [88], and compound flood pathways may be exacerbated [89]. As the baseline on which storm surges and waves act is raised, wave energy propagates further shoreward exposing communities and infrastructure to the increased probability of defence failure [90,91].…”

“…Extreme events can cause initial damage from flooding, but lack of drainage, potentially exacerbated by subsequent events, can cause knock-on effects to impact economic sectors in relation to closure of ports and harbours or disruption of transport of goods, and critical services (e.g., power, water, and communications), essential for public safety. However, the response of estuarine areas to sea-level rise threats may be dependent on their size, as large estuaries may experience increased channel volume due to sediment starvation, and subsequently flood hazard may be reduced [88]. Flood hazard assessments should, of course, be site-specific and targeted at understanding local or regional conditions under changing climate conditions, which can provide information on impacts in local areas of high flood risk.…”

Coastal wetlands can be saved from sea level rise by recreating past tidal regimes

“…Current literature suggests that a major global environmental crisis in coastal ecosystems is underway, due to the loss of intertidal ecosystems, but offers limited practical solutions to prevent or mitigate the impacts 12 , 14 , 25 . The four most common options for managing the impacts of SLR on intertidal ecosystems are 26 – 29 : status quo (maintaining existing management strategy); retreat landwards (horizontal migration); sediment supply (vertical accretion) and protection/defence measures.…”

A morphological investigation of marine transgression in estuaries

“…A key assumption in many of the morphological models that have sought to investigate the impact of sea level rise is the assumption that the estuary will seek an equilibrium morphology (Leuven et al, 2019;van Goor et al, 2003). Given that the forcing conditions are continually changing over a range of time scales, it is difficult to demonstrate that equilibrium exists in other than a dynamic sense of defining a goal which the system is continuously chasing (Zhou et al, 2017).…”

“…Given that the forcing conditions are continually changing over a range of time scales, it is difficult to demonstrate that equilibrium exists in other than a dynamic sense of defining a goal which the system is continuously chasing (Zhou et al, 2017). Considering the system response over Holocene timescales provides one means of demonstrating that this is a plausible assumption (Leuven et al, 2019;Townend et al, 2007); For example, the stratigraphic work of Rees et al (2000) and observed changes in morphology of an estuary over a much shorter time scale (150 years) were shown to be consistent (Townend et al, 2007). That estuary systems are also capable of tracking relatively short-term signals such as the lunar nodal tide (Wang & Townend, 2012) further supports the idea that at least some systems are tracking an equilibrium.…”