Agenda item

(1)       Mr Tant, the Flood Risk Management Officer began his presentation by explaining that “SUDS” stood for “Sustainable Drainage Systems”.  The “U” no longer stood for “Urban.”

(2)       SUDS were designed to mimic a natural process for managing water runoff, thereby minimising discharge rates and volume whilst providing better water quality.

(3)       The main benefit of SUDS was that they avoided the use of conventional gravity sewers to discharge water, which could lead to flooding.  This could happen if the sewer system was extended whilst the existing sewers were not upgraded.  A particular concern was that if the sewers discharged into a combined sewer, any flooding would also involve effluent.  

(4)       Many combined sewers had been laid under roads in the late 19^th  and early 20^th Centuries. They may have been built to cater for 1 in 30 year storms, discharging into rivers if an event exceeded this design capacity.

(5)       Mr Tant then described the various forms of SUDS Techniques.  An important source control technique involved green roofs. These contained a thin layer of soil which was able to absorb rainfall (which evaporated at a later stage). A further benefit of this technique was that it provided better insulation for the property.

(6)       Rainwater harvesting involved storing roof water in tanks for use as garden water and lavatory flushing.  This water could not, however be used for washing or drinking.

(7)       Swale filter grids were vegetated surface features that drained water evenly off impermeable areas such as road surfaces. This water could be stored or else released to slowly infiltrate the ground.

(8)       Permeable paving (whether brick work with gaps between them or a gravel surface) enabled water to filter through to a storage point below the surface from where it could discharge to a local water course or sewer or infiltrate the ground below.

(9)       Infiltration techniques enabled water to be collected and disposed of by using the natural properties of local soil.  Features of these techniques were soakaways, detention basins (which dried out), detention ponds (which could support wildlife and discharged slowly after the rain had stopped falling). Another form of infiltration technique was underground storage.  This involved collecting water in large perforated pipes or in stormcells.  It was essential to ensure that the water collection process was properly filtered as the danger was that the system could become blocked through the accumulation of silt.

(10)     Surface water was sometimes drained from houses into a soakway. This was not an appropriate technique for clay surfaces. 

(11)     Mrs Rook asked why the drainage systems were sometimes vulnerable to 1 in 1 year storms. Mr Tant replied that at the time that a sewer had originally been laid, it might well have been designed to cope with a 1 in 25 year flood event.  As the town expanded, the system’s capacity became steadily reduced to the point where it could cope with only a 1 in 5 year event or less. 

(12)     Mr Tant also said that a number of properties drained directly onto the road as a result of their front gardens being concreted over.  People actually needed planning (and probably drainage) permission to do this, but this legal provision was one that most people were unaware of.

(13)     Mr Tant concluded his presentation by identifying urban creep and increased urban density rather than climate change as the main reason for urban flooding.

(14)     RESOLVED that the report be noted and that Mr Tant be thanked for his presentation.