2008 Morpeth Flood - Physical Causes

Physical Causes

The River Wansbeck Valley is narrow and steep and as a consequence has exaggerated amounts of surface runoff. Based on three storm events, a reliable flow gauge located upstream of Morpeth at the confluence of the Wansbeck and Font showed that 56% of rain fall is converted into surface runoff. Because the soil was already saturated as a result of the wet summer, the effect of surface runoff was greatly enhanced. Furthermore, increased urbanisation since the 1960s in Morpeth meant that most water falling on the town would have drained directly to the river channel. Other tests investigating the catchment lag time (time lapse between the midpoint of storm rainfall and peak river level) indicate that the Wansbeck has a LAG time of only 8 hours. This means that any water falling in the catchment area would have been rapidly converted into channel flow by surface runoff and to a lesser extent by throughflow. This is due to the steepness of the valley and the soil composition.