Measurement of Recent Intertidal Sediment Transport in Morecambe Bay using the Waterline Method

Abstract

An illustration of how sediment transport may be measured over a large dynamic intertidal area at reasonable cost and spatiotemporal resolution using the waterline method employing remote sensing and hydrodynamic modelling is presented. Areas and volumes of accretion and erosion were measured in the study area of Morecambe Bay in north-west England over the period 1992–97. Tidal asymmetry is considered to be the dominant agent of sediment movement in the bay, with waves being of secondary importance. Waterlines (i.e. land-sea boundaries) were extracted from 31 synthetic aperture radar images of the bay covering a substantial fraction of the tidal range. Heights were attached to the waterlines using water elevations from nested hydrodynamic tide-surge models. Interpolation in space and time was carried out to produce a continuous spatiotemporal height map of the intertidal zone. Height maps with a spatial resolution of about 50m and height accuracy of about 40cm were constructed over an area of about 350km2. Strong temporal decorrelation in the bay limited the height accuracy achievable. Differences between height maps for 1992–94 and 1995–97 allowed sediment volume changes to be extracted. The intertidal region showed a loss of 16·1±4·5×106m3over the period, almost all the change being due to a significant decrease between mean sea level and the low water mark. Changes were also measured in three of the main river estuaries within the bay, the Leven, Kent and Lune. These included movement of the Leven north-east by about 2km cutting into Cartmel Wharf, accretion seaward of Grange-over-Sands and changes in the low water channels at the mouth of the Lune. These changes could be used to validate sediment transport models of the bay.