From Hayle Pump

Save Our Sand - w e n e e d y o u r s u p p o r t 

Does dredging affect the long-term sustainability of Hayle Beach? 

The beach in front of Harvey’s Towans has recently seen beach levels reduce dramatically. The reduction in beach level is a direct result of the sand budget. Sand budget is affected by the amount of sand being supplied or removed. The only method of removal of sediment from this sediment cell is via dredging activities. Dredging creates a deeper, narrow, navigable channel which increases the flood tide velocities in the estuary. This dredged channel causes the flood tide velocities to increase. It acts similar to a hose pipe with a thin jet nozzle on the end. The velocity of the water remains the same through the hose but when the water encounters the nozzle, there is a build up behind which forces the water through the nozzle faster. This effect is what is causing the sand from the beach and dune system to be scoured out and drawn through into the harbour. 

This is causing a sediment transport sub-cell which is active to the east of the mouth of the estuary to be drawn towards the harbour, resulting in the sustained transport of sediment into this area from the beach and dunes, causing erosion. This sub-cell acts almost independently to the St. Ives Bay sediment cell. 

Maintenance dredging has occurred in the Hayle Harbour area over many years, to maintain a navigable channel for fishermen and other boat users.

Up to 30,000 tonnes, of dredged material has been removed annually since 1973. This equates to a loss of depth of 110mm a year from the Hayle Beach area alone, or 5mm a year spread over the St. Ives Bay coastal cell. This could lead to the reduction of beach height on Hayle  Beach by over 1m in a decade (or almost 4m since 1973), if no material was being supplied from the dunes in the area. In 2010, some 16,200 tonnes of sediment was removed from Hayle Harbour in just 4 months (January-April). ING did cease dredging at the beginning of April, a few weeks before the last dredging l icense had expired. 

This was the highest amount of sand ever removed in Hayle’s dredging history in a quarter. In May 2010, the new harbour master (Mark Capon), put a halt to any further dredging until he had completed a fully comprehensive review into the methods available to keep the channel navigable.

Since dredging finished the beach height has steadily reduced during the calmer weather. In February 2011 a very high spring tide was accompanied by a storm surge. As the beach height had been lowered the sea was able to penetrate right up to the dunes, undermining them. This erosion still continues on a daily basis as the beach has not yet stabilised. Next issue I will discuss what can be done to save our beach, our main form of coastal defence.

Anne-Marie Rance, Bsc (hons) Geosciences.

The full scientific paper can be viewed at http://www.sos-hayle. org.uk/downloads-information/