Total exclusion barrages as sea-level rise mitigators: The geomorphological trade-offs for new installations

Sea-level rise presents a looming problem for estuarine management and barrages of various types are part of the tool-kit of possible mitigation strategies. Evidence from existing installations shows that the morphological impacts of total exclusion barrages (TEBs) can be severe and few if any meet their design goals. We applied a previously developed regime model (FORM) to four existing TEBs (Moncton and Windsor in Canada, Tees Barrage in the UK and Le Châtelier in France), and demonstrated how it effectively modelled regimes before and after barrage installation. The model demonstrated a downstream non-linear 'bathymetric migration' as a new regime is established with the barrage at the head of the remnant estuary. Linear regression revealed a high correlation (R² ∼ 0.85) with a trend-line slope > 0.9 (when y intercept was forced to zero) between widths of the before and after regime states for the simplest of the systems (the Petitcodiac); whereas the Tees was still in transition. Analyses of the other two systems was mostly qualitative but were consistent with the results from the Petitcodiac. The model predicted similar adverse morphological change for each of the barrage scenarios which was confirmed by the observed changes. Although all would act to mitigate sea-level rise in the headponds, tidal range increases in the remnant estuary would exacerbate sea-level rise. We conclude that results do not justify any of the TEBs suggested for various sites in the Tamar River estuary in Tasmania, Australia. The objective of mitigation of both sea-level rise and excessive silt accretion must be carefully evaluated against adverse tidal, morphological and environmental consequences.