The September 2008 floods on the River Coquet, Northumberland: Characteristics and immediate impact on river morphology

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Northumberland and its rivers were hard hit by an intense precipitation event on 6th-7th September 2008 which delivered over one month's worth of rainfall (ranging in places from 80 mm to150 mm) within a 24 hour period. The discharge response from Northumberland's eastward flowing rivers was rapid and spectacular. The River Wansbeck burst its banks affecting over 1000 residents in the town of Morpeth and further north, the River Coquet cut off the market town of Rothbury resulting in further damage and evacuation of residents from inundated properties. The September 2008 floods were the largest to affect the Wansbeck and Coquet river systems in historic time with all previous high water marks being exceeded. Further to the north, the River Breamish and River Till were also heavily impacted. While calculation of the flood flow is relatively easy for urbanised sections with good gauging facilities, upper reaches of catchments often are less heavily monitored. Yet, what happens in the upper sectors of catchments during high precipitation events has direct implications for downstream communities. We are focusing on the River Coquet due to the large amount of pre-flood baseline data already obtained by the PI, which will allow the effects of the 2008 floods to be more clearly defined. The river rises at Coquet Head on the Scottish border, has a catchment area of 636 sq. km and runs eastwards for a length of 62 km from source to mouth, draning the southern flanks of the Cheviot Hills. It is an excellent example of a fast flowing upland river on base-rich rocks, and along its length, is joined by a number of major tributaries including the River Alwin and the Ridlees, Usway, Holystone and Wreigh Burns before reaching the town of Rothbury. Downstream of Rothbury, the river is joined by other inflow tributaries before discharging into the North Sea at Amble. We will be focusing on the reach above Rothbury (with a sub-catchment area of 225 sq. km) as this marks a piedmont region where the River Coquet is notably dynamic, actively re-working its floodplain. We are applying under the NERC Urgency scheme to quantify flood impacts in this section of the catchment as the proposed field-based research clearly meets the Scheme's criteria - it is an unexpected and transient scientific opportunity where scientists need to respond rapidly to changing events. Here, fieldwork needs to be undertaken as soon as possible to survey transient flood wash limit evidence and to map channel morphology before the onset of subsequent floods, which may mask the signature of the September 2008 flood. Flood wash limits consisting of erosional trimlines, silt deposition extent and debris trash lines are all inherently transient as they are subject to removal during the flood clean-up as well as by natural processes including decomposition, actions of water currents and grazing. Despite the lack of direct flow gauging, standard hydraulic techniques (collectively termed the Slope-Area Method) exist, and will be used to reconstruct flood discharge from flood water slopes (reconstructed from debris wash limits demarcating the highest stage or flow depth), channel cross section area and flow velocities (reconstructed using hydraulic information and knowledge of channel roughness).