What is increased infiltration capacity: Increased infiltration capacity is one principle of Water Sensitive Urban Design (WSUD) whereby water is retained onsite to infiltrate into the ground or be reused at a later time. Increased infiltration capacity mechanisms include: permeable pavements, porous pavement, infiltration trenches, filter strips, infiltration basins, bioretention basins and swales. These mechanisms are excellent for a whole range of reasons including protecting natural storm water drainage systems, increasing natural habitat and ecosystems, increasing amenity and access to open land, protecting water quality, and minimising the cost of drainage infrastructure.
Increased infiltration capacity mechanisms however, are generally not considered an effective flood risk management measure. Why? These mechanisms are usually designed to store water and then slowly release water through ground infiltration or use. The storage and infiltration capacity of most of these systems are usually small relative to flood producing rainfall events. For example 10mm of rain over a 500 square metre property (assuming that 5mm of rain is initially soaked up by the ground) would produce 2.5 cubic metres or 2500 L of water that an infiltration mechanism would have to process. During a flood, rainfall would typically exceed the storage and infiltration capacity of most systems very early, limiting their ability to reduce down-stream flood flows. It is true however, increased infiltration capacity mechanisms generally retard some floodwater particularly in small flood events which will reduce the peak flow downstream but in general increased infiltration capacity mechanisms should not be considered a flood risk management measure.