Helston is impacted by fluvial (river) flooding from the Cober but is also heavily influenced by Loe Bar, which bounds Loe Pool and causes a restriction to outflows to the sea.
The St Johns Road area of Helston has suffered from repeated floodin
- An 1894 report mentions that houses in Helston were submerged and furniture was seen to be floating about in the low-lying districts. There was serious flooding in many streets, the gas works were inundated and supply stopped.
- There was highway flooding in 1952 and the St Johns area was also flooded, including the Cattle Market and car park.
- In 1973 Penzance Road and St Johns Road area flooded. Manhole covers lifted. Porthleven Road near the cattle market was closed. In 1976 Various areas were affected by surface water runoff, including Porthleven Road, Castle Green, Monument Road, Godolphin Road, Cross Street and Lismore Cross. In 1979 the St Johns Road area flooded again. The River Cober temporarily exceeded the capacity of the outlet tunnel from Loe Pool causing the level to rise.
- In 1988 Penzance Road and St Johns Road area flooded when the River Cober channel capacity was exceeded.
- In 1992 houses flooded in Church Street during heavy rainfall. In June 1993 St Johns Road, Castle Green, Five Wells Lane and Cross Street flooded affecting about 50 properties. This was due to a significant meteorological event with very intense rainfall.
- In 2002 very heavy rainfall caused significant flooding across Helston due to high flows from the River Cober and the Five Wells Lane tributary, with blocked screens and high surface runoff. In 2004 the Cattle Market, Coronation Park, St John’s area, Cades Park, Manor Close, Loe Bar Road and Grange Road were all affected by heavy rainfall, river overtopping and surface water runoff.
- Properties flooded in Treneer in 2012 when a culvert was overwhelmed and in 2013 surface water runoff from agricultural land caused significant flooding in Prospidnick and Chyandal and also issues in Lowertown.
