Data collected by our project partner, Landcare Research, shows that the quality and volume of stormwater leaving the rain garden is a vast improvement on what flowed in to it.

On 12 August a brief deluge resulted in 50,000 litres of rain water entering the garden at a rate of up to 95 litres per second, but the garden acted as a sponge, soaking up the water and releasing it into the stormwater system at a rate of half a litre a second.

Similarly, the garden prevented a huge amount of sediment from entering the stormwater system and flowing into our streams and the sea.

More than 12 kilograms of suspended sediment (clay, sand and small gravel) entered the rain garden, making the water look almost black. By comparison, the water leaving the garden was crystal clear and contained less than 150 grams.

Landcare Research scientist, Matthew Taylor, says the rain garden is also producing some unforeseen benefits.

“On 11 August we recorded a high ammonium and phosphorus reading and an extremely high alkaline reading of pH 12 indicating that something like detergent had been spilt on the road.

By the time that water had filtered through the rain garden the readings were back to normal, preventing a potential pollution incident,” he says. Above: Black, sediment laden water flowing into the rain garden is ‘captured’ in a weir for measuring and testing.