london is vulnerable to flooding. a storm surge generated by low pressure in the atlantic ocean sometimes tracks eastwards past the north of scotland and may then be driven into the shallow waters of the north sea. the surge tide is funnelled down the north sea which narrows towards the english channel and the thames estuary. if the storm surge coincides with a spring tide, dangerously high water levels can occur in the thames estuary. this situation combined with downstream flows in the thames tells engineers when to operate the barriers.