Towards predicting the influence of freshwater abstractions on the hydrodynamics and sediment transport of a small, strongly tidal estuary: The Devonshire Avon

Management strategies for freshwater drainage basins often do not take into account the effects of human interventions (such as water abstractions, i.e. water extractions) on their receiving estuaries (e.g. EA, 2007; Wolanski et al., 2004). However, it is necessary to know how such interventions are likely to influence estuarine hydrodynamics and sediment transport processes in order to then assess important management aspects such as the likely effects to an estuary's ecology and sedimentation; for example, potential accretion and dredging requirements. Following Schuttelaars et al. (in this issue) we consider a tiered approach, but in which simplistic models (rather than analytical models) are used initially to determine whether modelling of greater complexity, effort and expense is required to assess the intervention. To this end, we have linked a section-averaged, along-estuary hydrodynamic and salt-balance model with a depth-averaged, across-estuary hydrodynamic model to simulate currents and tidal behaviour that can be used in a sediment-transport model to investigate, as a first step, intervention scenarios for shallow, strongly tidal estuaries quickly, easily and at low cost. The essential element is a sensitivity analysis in which simulations for before-and-after intervention scenarios are compared and percentage differences tabulated. The methodology is applied to the predicted consequences on the hydrodynamics and sediment transport in the Devonshire Avon Estuary, UK, of water abstractions due to an up-stream dam and reservoir. Although the freshwater abstractions generally are small, the results nevertheless indicate an increased potential for up-estuary transport and retention of sediment in the central and upper reaches of the estuary.