Cannop Ponds - Flood Risk Modelling Results
Introduction to Flood Risk on the Cannop Ponds Project
Flooding in the Cannop Brook catchment usually occurs as a result of heavy rainfall.
Flooding could also occur if Upper or Lower Cannop dams were to fail.
If the dams were to fail during heavy rainfall, the amount of flooding would be greater than that due to the rainfall alone.
The severity of flood risk is determined by the likelihood of water entering built properties, the number of properties impacted and the depth to which those properties will be flooded.
The Environment Agency produces publicly available mapping for flood risk; visit their website.
Flood risk arising from potential failure of the reservoirs
Existing issues with the condition and design of Lower and Upper Cannop reservoirs mean that, whilst the likelihood of a dam failure is low, the risk of failure is greater than would be the case if both dams were in good condition and met current reservoir design standards.
In spring 2023, four options for the Future of Cannop Ponds were presented to the community. The implementation of any of the four will either remove the risk of flooding due to a dam failure altogether, or reduce this to an acceptable level.
Flood risk arising from heavy rain
Upper and Lower Cannop Ponds both store water during periods of heavy rain and in doing so reduce the peak volume of water that passes through the downstream communities of Parkend, Whitecroft and Lydney.
Each of the four options for the future of Cannop Ponds will impact the flow of water downstream in different ways. To evaluate and compare the impact of the options, a digital computer model of the Cannop catchment was developed.
This model makes use of industry standard computer software and ground level survey information to predict how changes to the ponds may impact on downstream flood risk. The model covers the catchment upstream from Parkend village, and only assesses the flood risk to Parkend.
The impact of the ponds on flood levels decreases as you move further downstream from Cannop Ponds. Below Parkend this impact eventually becomes negligible, as the effects of water joining the watercourse below the ponds become more significant for flood risk than the water flowing through the ponds themselves.
The model analysed each option, and the current arrangements, for several rainfall scenarios, up to a 1 in 1000 year storm. This is a storm of significant size which has a 0.1% chance of happening every year.
Relative Performance of the Four Options
Option One (Replace Spillway and Dam Upgrade)
The upgraded dams and spillways closely match the current storm water attenuation characteristics of the reservoirs. Consequently, the model shows little change, with no adverse impact on flood risk in Parkend.
Option Two (Maximise Storm Water Attenuation)
This option increases storm water storage in Lower Cannop pond, by lowering the normal water level. The model demonstrates an overall reduction in flood risk in Parkend.
Option Three (Cascade of Ponds)
This option provides some storm water storage by introducing smaller ponds, in lieu of the larger reservoirs. Removing the larger reservoirs eliminates the risk of flooding due to a dam breach. The modelling indicates a very small impact on downstream flood risk, which is driven by its slightly smaller storage capacity. If taken forwards this option would be optimised to improve storage and ensure there is no adverse impact on flooding.
Option Four (Naturalise Cannop Brook)
This option restores Cannop Brook to a watercourse, similar to how it would have been prior to construction of the reservoirs. The flood modelling results show an increase in flood risk in Parkend, resulting from the loss of the storm water attenuation benefit currently provided by the Ponds.
The modelling work demonstrates that Option 1 is neutral and Option 2 delivers a downstream flood risk benefit. Option 3 is also anticipated to be neutral, once the existing design is optimised. Option 4 doesn’t meet the project objectives of maintaining or improving downstream flood risk.