Summer Intense Rainfall Events vs Winter Flooding

The UK has recently experienced two different types of flooding events. The first was coastal flooding caused by the low pressure of extra-tropical cyclone essentially pushing water from the sea into the rivers and along the coast. The second was caused by the prolonged rainfall associated with extra-tropical cyclones which have typically travelled across the Atlantic from the East coast of North America, picking up moisture which then falls as rain as the cyclones pass over land due to the presence of raised land. Both these events are more typical during the winter period (October – March) and last several days resulting in a lot of rain over this period. This leads to long periods of flooding (days) and typically over large areas, e.g. the whole of southern England.

The Flooding From Intense Rainfall (FFIR) programme is looking at the flooding that is caused by more short-lived events, such as convective storms that are more typical during the summer period (March – September), resulting in flooding that lasts hours and affecting a much smaller area. One of the most recent infamous examples of this was the flooding in Boscastle in August 2004 ( which saw 75 mm of rain over 2 hours resulting in the destruction of houses, businesses as well as a significant economic impact. The summer 2007 floods that affected the whole of the UK were also due to short-lived intense rainfall events over a much wider area.

The difficulties faced when predicting these events is that they have a much smaller scale, typically less than 10s of kms, compared to the winter events which can have scales of 100s to 1000 kms. The timing and location of the rainfall is particularly difficult to predict and have a big impact on how well a specific catchment can cope – large catchments, e.g. the Thames can cope with a short intense rainfall event much more easily than a small catchment, e.g. the Valency which runs through Boscastle. To predict these events with greater accuracy a number of questions need to be addressed: 1) what are the atmospheric conditions that lead to these rainfall events, 2) how does the hydrology of each catchment vary and thus their ability to cope with such events, and 3) how do the different catchments respond to different rainfall intensities. These are the research questions of FFIR which is looking at catchments in the UK and their responses to surface water and flash floods.

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