The relationship of sediment transport rates and paths to sandbanks in a tidally dominated area off the coast of East Anglia, U.K.

Abstract

Sand transport measurements of bedload and suspended load in the Sizewell‐Dunwich Banks area, East Anglia have shown that the suspended mode is dominant. The depth‐integrated spring tidal residual is 5.66 g cm−1 sec−1, although the neap rate is only one‐fifth of this. The calculated bedload transport rates also vary, from 0.012 to 0.040 g cm−1 sec−1, correlating with changing meteorological conditions. In order to predict the bedload sediment circulation pattern from midwater current meter measurements, five sediment transport equations were calibrated, using fluorescent dyed sand. Yalin's relationship gave the best estimates. The bed shear stress was determined by extrapolating the velocity profile as a power law relationship, with an exponent equal to 0.1, from midwater down to 2 m and as a lognormal profile from 2 m to the sea‐bed. Roughness length values appropriate to the substrate were used. Although bedload transport residuals are mainly to the south, the banks trend northwards from the coast and have also elongated in this direction. This is thought to be in response to the dominance of the suspended sediment transport. It is suggested that a tidal residual eddy mechanism is responsible for the banks’maintenance, similar to the process operating in Start Bay, Devon. The well‐documented westward movement of the banks is likely to be related to wave processes.

Cartographic, Geophysical and Diver Surveys of the Medieval Town Site at Dunwich, Suffolk, England

“…Off shore, tidal and wave-generated currents have less energy and are therefore less likely to disperse the larger stonework, but instead scour and accretion of sand is known to create periods of exposure and burial at the site (Bacon, 1974). The preservation of subsurface Pleistocene or early Holocene river-channel sediments at the site has been demonstrated by the Institute of Oceanographic Studies (Lees, 1979;Lees, 1983;Gaffney et al, 2007). Knowledge of the position and dimensions of the former Dunwich river would be valuable for reconstructing the location of the harbour area and the northern limits of the town.…”

“…Some possible response in the sea‐floor to the presence of larger masonry blocks inducing scour around the sites (such as St Nicholas', Target 4) appears evident. Interactions between the tidal currents (locally attaining 1 ms −1 , Lees, 1983: 8) and the larger blocks will induce turbulence and vortex‐shedding resulting in the potential to create longitudinal scour. This mechanism results in local lowering of archaeological remains into the sea‐floor, which in time could increase the relative depth of deposition.…”

“…The results from the combined multibeam, side-scan and boomer surveys revealed the site of the town to be covered by an inner sandbank with depths of fine sediment up to 3.3 m, sloping down to a platform covered for the most part by sand up to 2.7 m deep. The composition of the sand can be estimated from IOS sediment grab samples (Lees, 1979), diver surveys undertaken by Bacon and others, and the project diver surveys. All report the fine sediments to be composed of browncoloured fine sands (0.12-0.17 mm, Lees, 1979).…”

Morphology and dynamics of large subtidal dunes in Bahia Blanca estuary, Argentina

“…However, in other works (Aliotta, 1987;Aliotta and Perillo, 1987;Waage, 2012) Several investigations (Harris, 1982;Lees, 1983;Twichell, 1983;Collins et al, 1995;Reynaud et al, 1999;Hennings et al, 2000) have shown that the dunes' asymmetry and geometry in cross-section have been widely used as indicators of net bedload sediment transport and hydrodynamic conditions. Following these concepts, the asymmetric shape and orientation of large and small-scale dunes in our study area clearly reveal the ebb flow dominance in the navigation channel.…”

Shoreline retreat and sediment release in response to accelerating sea level rise: Measuring and modelling cliffline dynamics on the Suffolk Coast, UK

“…Deposition of sediment released by cliff recession in nearshore and offshore regions might potentially lessen subsequent shoreline retreat, particularly in relatively shallow waters typifying the southern North Sea. In such areas, nearshore sandbanks are a major feature of the sea bed and these sandbanks are known to reduce both wave heights (Stansby et al, 2006;Horrillo-Caraballo and Reeve, 2008) and tidal current flows (Lees, 1983). For the coastal zone at the western margin of the southern North Sea (the coast of East Anglia) circulation of sediment within littoral cells provides a potential positive benefit to low-lying regions, where deposition of sediment can mitigate against potential flood threats from rising sea level (Dawson et al, 2009).…”