The Great Stour Project - River and Coastal Flooding

Annual Rainfall

Within the Kentish Stour catchment the average annual rainfall during the period 1961-1990 was 714mm with an effective rainfall of 266mm. Annual rainfall totals vary throughout the catchment. The Dover area receives the highest average annual rainfall for the catchment at more than 800mm, and the Thanet area receives the least, with less than 600mm. This is broadly in line with the pattern of relief with the highest rainfall totals recorded on the North Downs.

Rainfall is unevenly distributed throughout the year with 56% of precipitation received in the winter half year, from October to March.

November is usually the wettest month when the frequent passage of depressions from the Atlantic may bring extended periods of frontal rainfall. From December to February, the area occasionally comes under the influence of continental blocking anticyclones which may persist for several days. Whereas continental air streams produce lower precipitation totals, they can bring bitterly cold weather and blizzards.

March to July are the driest months of the year with an average of less than 50mms precipitation. In the summer half year, depressions follow a more northerly track resulting in a comparatively low incidence of rainfall in the south east. In the late summer, however, humidity and high temperatures often bring heavy convectional storms, accompanied by thunder and lightning. The intense rainfall that they bring can result in localised flooding where the intensity exceeds local infiltration capacities, notably in urban areas.

Surface drainage is strongly influenced by the permeable character of the catchment’s geology, most of the area being underlain by chalk, with Lower Greensand outcropping in the headwaters of the Great Stour. These aquifers provide natural reservoirs replenished by winter rainfall and supporting a predominantly spring fed river upstream. There are few surface streams and they all display a high degree of seasonal regulation; the effect of aquifer storage. This is reflected in the relatively small winter-to-summer flow ratio compared with a clay catchment.