description: Bristol's Northern Stormwater Interceptor, a massive stormwater drain which acts as a flood prevention measure for Bristol, completed in the mid 90s.
Building of the Northern Stormwater Interceptor (NSI) started in 1962 to relieve flooding over many parts of the city. The Severn Estuary has the highest tidal range in the country and the Avon is tidal right up to the city. Because of these high tidal ranges the level of the Avon can, during spring tides, be higher than the low-lying parts of the city.
When there is the rare occurrence of storm surges in the Atlantic coupled with high spring tides then the tidal level can be further increased by several feet. This combination has resulted in serious flooding in the past.
Another problem is surface water run-off, due to the rapid growth of the city, roofs, roads and paved areas replacing open fields meant an increase in surface run-off and during periods of high rain fall this was causing Bristol's existing sewers and rivers to struggle with this surge of water.
The River Avon's catchment area is around 857 square miles and the Frome's is around 68 square miles. The NSI was completed in 1968 and successfully relieved the pressure on these two rivers. The NSI is around 5 miles long and dumps it's load back in to the River Avon.
This map shows the rough route of the two major sewer networks in Bristol, the Northern Stormwater Interceptor is in light blue, the dark blue pipe ways are foul water (sewers). The map also shows the location of the Blackrock Pumping Station where the drains are emptied out in to the River Avon. This map doesn't include the extension made to the Northern Foulwater Interceptor which were built as part of the final phase of the Bristol drain project in 1994. The tunnels now spread in to North West Bristol as far as Chipping Sodbury.
web links: Sur-Urban.com - some excellent photos from the brave explorers (external link) Sur-Urban.com - a second set of photos (external link) Free The Frome - a bit of history about the drain (external link) Wessex Water - plans to extend the tunnel under Bristol (external link)