Impact of managed realignment design on estuarine water levels

Large realignment sites near the mouth of an estuary have the potential to raise water levels throughout the estuary. This increase is produced by the newly created intertidal area drawing additional water into the estuary on the flood tide. Although some of this additional water enters the realignment site, a proportion also bypasses the realignment site and continues upstream, thus increasing water levels in these areas of the estuary. The modelling results described in this paper suggest that an important determinant on estuary water levels is the rate at which the realignment site fills. Schemes that fill more slowly are predicted to have less impact than those that fill more rapidly, even if high water levels within the schemes are similar. For the schemes tested in this paper, the rate at which the site flooded appeared to depend less on breach dimensions and more on the width of the site that the flood waters had to cross in order to cause full inundation. It is proposed that those options with the shortest distances result in the flood tidal waters experiencing less frictional losses. These options showed a more rapid rise in water levels within the site, and produced a correspondingly larger increase in water levels within the estuary.