Mapping the Unresolved Tidal Resource in Estuaries

Abstract

Estuaries are ideal locations for extracting tidal energy and yet the global resource appears poorly mapped. Estuaries typically have high tidal ranges and strong tidal currents, due to amplification processes; and this resource is juxtaposed to industrial/residential areas with electricity demand [1]. For example, 22 of the 32 largest cities in the world are adjacent to estuaries [2], and UK estuaries collectively worth over £5.5b to UK economy with >1/6^th of the population and ~1b tonnes of cargo traded at their ports [3].

Mapping the tidal energy resource is challenging due to the sensitivity of ocean-model simulated currents to the model resolution [4]. Both tidal amplitude and currents are heavily modified in estuaries [5]. Whilst many estuary-specific resource modelling studies have been achieved (e.g. [6]), no large-scale future tidal energy resource mapping project has been undertaken – likely due to computational cost (e.g. [4]). It is therefore unrealistic to hydrodynamically model all global estuarine systems, to resolve current and future changes to the tidal energy resource; instead we aim to apply a simplified analytical technique that could be calibrated and validated by the new NASA SWOT satellite mission (https://swot.jpl.nasa.gov/), as well as citizen science. Estuarine tidal dynamics have been observed to change rapidly due to changes in river-flow climatology and bathymetry (e,g. dredging); see [7]. Indeed, climate change driven impacts to tidal dynamics (sea-level rise and altered riverine climatology), alongside anthropogenic driven morphodynamical changes and interactions with future tidal dynamics, could increase the future estuary tidal resource [1].

Three physical processes drive mean tidal amplification excluding over-tides): (1) Funnelling - concentration of the tidal energy flux with reducing width; (2) Resonance - when estuary length-scales are close to the natural period of the basin; and (3) Shoaling (reduction in the propagation speed of the tidal wave resulting in an increase in amplitude). Each of these three processes are analytical solved, and a 1D analytical model applied to demonstrate the resource. Sea-Level Rise (SLR) scenarios are included to show SLR can modify estuarine tidal dynamics through both estuarine geometry (e.g. increasing resonance) and boundary conditions (global mean sea-level rise increasing boundary conditions). A normalised result of the analytical approach is shown in Figure 1, which demonstrates a computationally efficient analytical solution that includes future changes to tidal dynamics.

Our research will apply this analytical solution to the Bristol Channel, a region soon-to-be heavily instrumented as part of the “Cal-Val phase of the NASA SWOT project (https://projects.noc.ac.uk/swot-uk/swot-uk-bristol-channel). Finally, we will develop a technique to bring together digital observations from citizens and sensor networks (see https://digitalenvironment.org/), to validate our simplified approach and resolve the tidal energy resource.