The surface water drainage design has been agreed at the planning stage, careful consideration has gone into each aspect, the detailed modelling has shown the flood risk will not increase to others and the site is safe, the application has been approved and conditions have been incorporated into the consent.
However it can still go wrong when construction begins. Control devices can be missing, attenuation volumes are too small, outfalls can be set at the wrong levels, permeable surfaces change. How do we ensure the design that has been agreed, and modelled is the design that is constructed, that the system will work as designed. Often ,to late, issues arise years down the line which are normally caused by poor construction and missing features. To get issues rectified once a problem has occurred is often very costly and difficult, developers have moved on and the residents need to foot the bill. Often these errors have occurred due to a misinterpretation of the drawings on site and miscommunication between the design engineers and the contractors, sometimes the contractors have not fully understood the importance of locating features exactly where they are on the drawings, sometime a decision has been made to change materials without everyone fully understanding the importance of surfaces, what ever the reasons behind the changes and errors it is often easier and quicker to get them resolved as early in the construction process as possible. The question is how do you go about checking the implementation of the SuDS system without going to site every week.
To help reduce these risks in Woking a condition is now incorporated on to consents that requires developers to submit a verification report prior to first occupation. This report needs to be detailed enough to demonstrate the location, scale and levels of each aspect. The report needs to include formation level photos, site photos and photos of each structure and control mechanism. Since the implementation of the condition on consents Very few errors have occurred in the construction of the drainage system and where errors have occurred they have been picked up easily and rectified quickly saving both the council and the developer time and money.