The effective design of authentic infiltration drainage can be complicated and the challenges are further exacerbated in an existing urban environment as a retrofit solution. The “wetness” of the ground, type of soil and level of groundwater all will have a profound effect on the performance of infiltration drainage, to varying degrees given a set of climatic circumstances and up to and including zero infiltration. Infiltration testing and groundwater information will give a baseline to work from.
Even if a street located SuDS structure was unable to infiltrate, it would still provide source control attenuation and storage that would slow down the rate of runoff and impact the “time of concentration” from the subcatchment. Multiply this by hundreds of such features and the overall impact will be significant from the cumulative attenuation volume in the subcatchments.
Understanding exceedance is essential. All drainage and flood risk structures have an exceedance level; it’s just a matter of how high the bar is set through the design process. And that means that all street located SuDS structures will fail at some point; therefore understand the exceedance limitations and what will happen to the resultant overland flows. All of this can be hydraulically modelled, including the overland flow paths in 2D.
Blue Green Infrastructure (BGI) will be less effective when the soil is saturated but even then there will be a slowing down or attenuation compared with running off a “rapid response” surface like tarmac. This attenuation will help any grey infrastructure to cope with a storm when the ground is saturated. With the current Tunnel scheme, water must be pumped out rapidly so that it is recharged for the next storm.