RES undertook the CSO assessments for Northumbrian Water’s Technical Support Team within 12 drainage areas. For each CSO a report was produced documenting model build, the hydraulic analysis completed and to confirm that no significant detriment to the existing sewerage network and to individual property flood risk will occur.  The report included potential model limitations and risks and further recommendations for model improvements.

For each CSO existing sewer models were upgraded or for smaller rural villages new models were constructed and historically verified to Northumbrian Water’s current modelling guidance.  Each model included a 2D mesh to allow for the overland flow routes and property risk assessments to be undertaken using the DREAM2D software developed by RES for Northumbrian Water.  Each CSO plus associated manholes were surveyed to confirm current layout. 

The hydraulic analysis involved assessing the baseline scenario against CSO weir levels raised by 50%, 75% and abandoning the CSO.  The impact on flood volumes at individual nodes, individual property flood risk and also impact on spill frequency at downstream CSOs was then assessed.  

A flood risk scoring approach was adopted to quantify flood risk at individual properties within the local area. This assessment involved the analysis of overland flow paths using a 2D mesh generated using InfoWorks CS, as well as sewer surcharge levels predicted by the hydraulic model. The total impact depth of flooding contributing to a single property was assessed in accordance with Northumbrian Water’s standard flood risk matrix.