Carlisle flood defences

Carlisle flood defences

1. General

   • With towns flood defences breached
     • Council to find out what why current defences had been inadequate and what needed to be done to limit damage in future
   • The lower Eden strategic and planning appraisal report was prepared to provide an overview of flood management within the wider catchment
   • As part of the strategic and planning appraisal, however a number of generic option or flood management were investigated and discounted for environmental and economic reasons
     • Do Minimum
       • Comprises only maintenance work to ensure no further deterioration in existing structures
         • Thereby sustaining sustaining persent level of flood defences
       • low cost approach assumes over long term, increased flooding would have significant adverse impacts:
         • Human beings
         • economy
         • heritage sites and all types of land use on the floodplain
     • Upstream storage
       • would involve the creation of larger- scale upstream storage reservoirs
         • E.g construction of a larger dam across the river Eden and the excavation at the M6
           • A dam at the M6 would require existing motorway embankment to be rebuilt as a dam structure, requiring temporary diversion of the motorway during its construction
             • The resulting reservoir would effectively double the extent of the existing 100 year flood outline upstream of M6 inundating areas like Low Crosby, Warwick Bridge and potentially Hadrian's wall
               • Estimated construction costs would exceed £300mill- impacts to SSSI
     • Upstream managed realignment
       • Involves moving existing line of flood defence back from the river to provide a larger area of natural floodplain storage
       • Hydraulic modelling demonstrated that the floodplains in the lower catchment are currently almost fully utilised during flood events
         • Even during low return period flood events; and in high return period events, given the volume of water that passes down the Eden,, the benefits would be few

2. The Decision

   • Council had to balance need for effective flood defences with bearing costs and social and environmental impacts
   • Final appraisal determined that the do nothing/ minimal approach was not acceptable owing to the risk of property and lives.
   • The ambitious upstream storage option was ultimately rejected as it involved unsupportable economic and environmental costs without necessarily dealing with immediate flood risks to town
     • A decision was made to raise existing flood defences while at the same time effecting local realignment
       • This meant the setting back of existing defences on the River Petteril upstream of Butchery Bridge to create a larger storage area on the floodplain
         • These measures were identified as environmentally acceptable with the latter option providing more opportunity for environment enhancement
           • Therefore combined option was identified as the preferred option for eden and petrol flood risk area

3. Additional financial measures

   • In 2005 council agreed to allocate the £1.5 million govt grant as follows
   • survey of flood- affected homes
   • £325,000 for uninsured vulnerable properties to make good any flood damage
   • £325,000 for insured vulnerable properties to make good any flood damage