What defences does the river Lavant have against flooding?
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Lavant has been the subject of numerous flooding reports over many years and records go back to the 19th Century. When it comes to the chalk on the South Downs, this is likely to cause groundwater levels to increase rapidly over several tens of metres. This results in mass flooding. Groundwater protection measures have been put in place in parts of the area since the 1920s and are extensive.
These include controls on land use near groundwater boreholes; known as source protection zones, and a comprehensive aquifer protection policy. This has been based on assessments of the vulnerability of the aquifer to contamination across Sussex.
The river therefore responds to changes in groundwater levels throughout the year; it frequently runs dry during the summer months when groundwater levels within the chalk catchment are low. Flooding happens after heavy rainfall when surface water runoff cannot be absorbed in heavily saturated ground.
There has also been a River Lavant flood alleviation scheme put in place since 2012. It’s designed to reduce the risk of flooding in Chichester and the surrounding areas, all while not increasing risk in other places. It works by transferring flood water from the River Lavant at Chichester out to the sea at Pagham Harbour.
It was built by the Environment Agency, and has two phases to respond to rising water levels in the River Lavant. The first phase involves lowering water levels in Church Farm Pit and East Pit, to the east of Chichester, by adjusting sluice gates. This creates additional capacity to store flood water that is diverted into the pits.
The second phase includes moving a bit of the water from the River Lavant into the pits. Once the pits are full, flood water will then flow down the route of the alleviation scheme to outfall structures. These are located at Pagham Harbour.
This scheme has been carefully designed, which ensures that no property next to the Forebridge and Pagham Rifes is at any increased risk of flooding as a result of the operation of the scheme.