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

Abstract 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.

[1]  Ian Townend,et al.  Estuarine flooding and managed retreat , 2002, Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences.

[2]  D. Reed.  Physical Contexts for Saltmarsh Conservation , 2001 .

[3]  J. Pethick,et al.  Estuarine and Tidal Wetland Restoration in the United Kingdom: Policy Versus Practice , 2002 .

[4]  John D. Boon,et al.  On basin hyposmetry and the morphodynamic response of coastal inlet systems , 1981 .

[5]  T. H. Roberts.  Habitat Value of Man-Made Coastal Marshes in Florida , 1991 .

[6]  R. Leafe,et al.  On the loss of saltmarshes in south‐east England and the relationship with Nereis diversicolor , 2004 .

[7]  Richard J. S. Whitehouse,et al.  Investigation using simple mathematical models of the effect of tidal currents and waves on the profile shape of intertidal mudflats , 2000 .

[8]  N. Pontee.  Designing sustainable estuarine intertidal habitats , 2003 .

[9]  Philip B. Williams,et al.  Salt Marsh Restoration Experience in San Francisco Bay , 2001 .

[10]  A. Elliott,et al.  Suspended sediment concentrations in the Tamar estuary , 2003 .

[11]  J. Dronkers,et al.  Tidal asymmetry and estuarine morphology , 1986 .

[12]  Anthony J. Davy,et al.  Drainage and Elevation as Factors in the Restoration of Salt Marsh in Britain , 2002 .

[13]  Patrick Meire,et al.  Modelling long-term tidal marsh growth under changing tidal conditions and suspended sediment concentrations, Scheldt estuary, Belgium , 2003 .

[14]  C. Watson,et al.  Sedimentation–erosion dynamics of abandoned reclamations: the role of waves and tides , 2000 .

[15]  J. Motyka,et al.  Coastal management: Mapping of littoral cells , 1993 .

[16]  H. Fischer.  Mixing in Inland and Coastal Waters , 1979 .

[17]  T. Benson,et al.  Morphodynamics and sediment flux in the Blyth estuary, Suffolk, UK: conceptual modelling and high resolution monitoring. , 2005 .

[18]  David G. Aubrey,et al.  A study of non-linear tidal propagation in shallow inlet/estuarine systems Part II: Theory☆ , 1985 .

[19]  Andrea Defina,et al.  Two‐dimensional shallow flow equations for partially dry areas , 2000 .

[20]  R. G. Hughes.  Saltmarsh erosion and management of saltmarsh restoration; the effects of infaunal invertebrates , 1999 .

[21]  P. Winn,et al.  Planning for the rising tides: the Humber Estuary Shoreline Management Plan. , 2003, The Science of the total environment.

[22]  A. M. Dixon,et al.  Habitat Creation Opportunities for Landward Coastal Re‐alignment: Essex Case Studies , 1998 .

[23]  S. Hull,et al.  Banked realignment: a case study from the Humber Estuary, UK , 2006 .

[24]  Michael A. Kolessar,et al.  Stability of Coastal Inlets , 1958 .

[25]  Comparison of natural and man-made salt marshes in Galveston Bay Complex, Texas , 1984, Wetlands.

[26]  A. Grant,et al.  Managed realignment in the UK – the first 5 years of colonization by birds , 2004 .

[27]  N. J. Cooper,et al.  The use of ‘managed retreat’ in coastal engineering , 2003 .

[28]  J. W. Elder.  The dispersion of marked fluid in turbulent shear flow , 1959, Journal of Fluid Mechanics.