Sand waves and sediment transport around the end of a tidal sand bank



Two detailed surveys have been made of the north-western end of the Haisborough Sand off the Norfolk coast using echosounder, 3-5 kHz reflection profiler and side-scan sonar. Asymmetrical sand waves indicate north-westerly sand movement on the southern side and south-easterly sand movement on the steeper northern side of the bank. Secondary, superimposed megaripples, which are probably better indicators of sediment movement, give evidence of a cross-bank component. Between the north-westerly and south-easterly facing sand waves on the tip of the bank there is a zone of symmetrical sand waves. These are usually taken to indicate zero net transport, but in this case the oblique orientation of megaripples in their troughs indicates transport parallel to the sand wave crests. This suggests the route by which sand travels around the end of the bank to form a roughly closed circulation. Sediment textural parameters support the notion that sand is winnowed from the foot of the bank on both sides and is transported to the middle with an overall net transport from the south to the north. Analysis of charts dating back to 1886 shows that the bank is stable within the error limits of position fixing, though that could allow more than 0-25 km shift to the north east in 100 years to pass undetected. A box model is drawn up for the estimated sediment fluxes around the end of the bank, and implications for residence times and circulation rates are drawn from it.