Abstract
Two conceptual models have been proposed to predict the response of sediment-laden estuaries to reduced tidal propagation caused by tidal barrages. One argues that reduced tidal energy will increase inter-tidal sedimentation; the other suggests that reduced tidal propagation will trigger a period of erosion before accretion of inter-tidal resumes. This paper investigates the relevance of sills across UK estuaries and causeways and barriers across estuaries in the Bay of Fundy (Nova Scotia, Canada) as possible analogues. Particular attention is paid to the tidal power barrage at Annapolis Royal because it exhibits important features that provide the underpinning evidence for morphological responses within tidal power headponds.
The construction of the tidal power plant at Annapolis Royal involved retro-fitting a tidal turbine and re-establishment of tidal influences into a headpond which had been maintained at a relatively constant level for some 24 years previously. The sequence of events at Annapolis Royal shows that headponds behave like river reservoirs, with bank erosion where sediments are weakly consolidated. It also demonstrates how resumption of tidal propagation can accelerate bank erosion; a process that is closely linked to wind-driven wave energy and the establishment of a foreshore profile that is better fitted to this energy regime. This model is very important both in the context of new tidal energy barrages and also for projects to retro-fit turbines into existing causeways. It reinforces the evidence from the Eastern Schelde in The Netherlands where it has been found that reduced tidal propagation has had profound effects on foreshore evolution with serious loss of mudflats and former saltmarshes. Annapolis Royal not only highlights erosionary processes; it also provides initial indications of the timescales required for evolution of more stable inter-tidal profiles.
The evidence provided by these various examples has important implications for coastal managers. Firstly, they show how analogues can be used to develop a conceptual geomorphological model of possible responses to causeways, sills and barrages such as some of the options being considered for the Thames Estuary. In addition, they highlight issues that need to be taken into account when pre-project planning considers likely issues and long-term management implications.