Hydrodynamic Modelling of Estuarine Flood Defence Realignment as an Adaptive Management Response to Sea-Level Rise

Abstract

FRENCH, J.R., 2008. Hydrodynamic modelling of estuarine flood defence realignment as an adaptive management response to sea-level rise. A numerical hydrodynamic model is used to investigate the sensitivity of a morphologically complex and heavily ''engineered'' mesotidal estuary to idealized sea-level rise scenarios and to evaluate the appropriateness of managed realignment as an adaptive response to sea-level rise. The hydrodynamic regime of the Blyth estuary (Suffolk, eastern England) is governed by a distinctive morphology that has resulted from the abandonment of a reclaimed midestuary tidal floodplain in the 1920s and 1940s. Hypsometric characteristics (extensive intertidal area and constricted inlet dimensions) make the outer estuary potentially sensitive to sea-level rise. Model results indicate that a sea-level rise of 0.3 m (as a best estimate scenario for 2050) could increase peak tidal current velocities and discharges by up to 20% and 28%, respectively. Extensive areas of potential tidal floodplain remain protected by embankments that will require upgrading to cope with sea-level rise. Realignment (or ''managed retreat'') of these defences can reduce local flood defence costs by eliminating unsustainable seawall but needs to be evaluated in the light of wider impacts.Modelling of hypothetical realignment scenarios shows that restoration of tidal exchange to the largest flood compartments could have an immediate effect on outer estuary hydrodynamics that is larger than worst case scenarios for half a century of accelerated sea-level rise (peak velocity and discharge increased by up to 35% and 32%, respectively). More generally, incompatibilities are apparent between flood defence and habitat restoration objectives, such that the appropriateness and feasibility of large-scale flood defence realignment could be questionable in estuarine contexts.

Spatial and Temporal Variability in Tidal Range: Evidence, Causes, and Effects

“…French [24] is a noteworthy counter-example since that study modified the bathymetric change based both on SLR projections and on historic sedimentation rates. Similarly, Passeri et al [62] use probabilistic methods to evolve the coastline and dune heights to accompany their adopted SLR scenarios.…”

Critical perspectives on the evaluation and optimization of complex numerical models of estuary hydrodynamics and sediment dynamics

“…Crucially, the process of validation is considered to provide a reasonable basis for ensuring confidence in the ability of the model to predict or provide understanding of estuary system behaviour in situations where no data are available (Hammersmark et al, 2005;French, 2008). As in other kinds of complex system modelling (Trucano et al, 2006), a distinction is made between the requirement to estimate model parameters and thereby demonstrate the replicative validity of the model (in the sense of Gaas, 1983), and the need to assess the predictive ability, or the extent to which it can reproduce real system behaviour when confronted with information not used at the estimation stage.…”

“…Given adequate resources, remote sensing can be used to evaluate modelled spatial distributions of suspended sediment (van Wijngaarden, 1999;Mason and Garg, 2001) and data assimilation techniques have been used to improve the prediction of bathymetric evolution (Scott and Mason, 2007). More typically, the problem is made more tractable through the use of multiple instrument deployments within the model domain (Sutherland et al, 2004;French, 2008).…”

“…One hypothesis emerging from this work is that the contemporary sedimentary regime of the estuary is characterized not by progressive infi lling with marine muds as occurred over much of the late Holocene (Brew et al, 1992), but by a fi ner balance between tidal sedimentation and wave-driven erosion within the enlarged mid-estuary intertidal zone. This hypothesis has been investigated further through the analysis of suspended sediment concentration, tidal and meteorological time-series , as well as through numerical hydrodynamic models (French, 2008).…”

“…A detailed account of the hydrodynamic model implementation is not relevant to this paper and the reader is referred to French (2008). In brief, the Telemac2D depth-averaged fi nite element scheme (Hervouet, 2002) is used in conjunction with an unstructured mesh of approximately 22 000 elements ( Figure 1c).…”