1 in 100 year rain event comes to Renfrew Close Rain Gardens

The Renfrew Close Rain Gardens in Beckton, East London, are designed to hold water from large rain events preventing it from entering the local storm drain system and overloading it. Their performance has been monitored by the SRI for the last two years for Newham Council and the Environment Agency. During that time, the basins have not experienced any rain events large enough to really put them to the test. So, to see how they would perform under extreme conditions, we simulated a 1 in 100 year rain event, the scale of rain event that would only be expected once every 100 years.

The Renfrew Close rain garden system is made up of four rain basins which were built as a demonstration project to showcase the potential of community- scale, retrofit sustainable drainage system (SuDS) schemes. Prior to installation, most surface water received by the site following rainfall was removed through a combined sewer system. This resulted in an unsustainable solution where rainwater caused unnecessary pressure on the sewer network.

1) Site map showing the rain garden set up. The installed basins are in blue, and diverted water flows to the basins via the routes shown with red arrows. Background image source © Google Maps.

The rain gardens have provided a more sustainable solution. Rainwater from a catchment area of 915 m2 is now diverted into the rain gardens instead of the sewer. Water is diverted from downpipes on the buildings along simple paved channels and vegetated swales into the four rain garden basins. The basins are sized to accommodate rainfall events of varying magnitudes.

Rain gardens provide wider benefits beyond rainwater management. The Renfrew Close Rain Gardens also provide beautiful, playful, productive and biodiverse greenspaces, that make room for people and nature.

2) Rain basin in full bloom at Renfrew Close © Stuart Connop

The rain gardens have been monitored for performance over the past two years, but very few large rain events have been observed on site. This is, in part, because the last two years have had an annual rainfall lower than the long term average.

Rainfall simulation provides a unique opportunity to mimic the conditions of extreme rainfall events and test the rain gardens to their designed capacities. Like most rain gardens, the main benchmark of success is for stormwater to be held in the rain basin and then infiltrate away within 24 hours without any water overflowing into the combined sewer system.

Basin 2 was tested first. Six thousand litres of water was poured in via a water tanker at regular intervals over a period of one hour (Video). This mimicked the volume of water that would flow in from the basin’s catchment during a 1 in 100 year rainfall event. The basin did not display any surface water pooling until around 3000 litres of water had been poured into it. Instead, the first 3000 litres was absorbed by a combination of the vegetation and infiltration into the substrate. The pooled water reached a peak of 16 cm deep after the final 1000 litres of water had been added. Following the cessation of the rain simulation, all of the ‘rainfall’ had infiltrated away just 26 minutes after the simulation had ended.

3) Basin 2 being filled with water during the rainfall simulation © Jack Clough

4) Pressure sensor data from the base of Basin 2 showing increasing pressure as the basin filled following the first 3000 litres and then a reduction in pressure back to the baseline shortly after infilling was completed.

Basin 4 was also tested with 6000 litres of water in one hour. Due to its larger catchment area this simulated a 1 in 10 year rainfall event. Similarly to Basin 2, this basin did not display any surface water pooling until 3000 litres of water had been poured into it.  This pooled water reached a peak of 4 cm deep after the final 1000 litres of water had been added.  Following the cessation of the simulation,  the water took just 11 minutes to infiltrate away.

5) Basin 4 rainfall simulation in progress. The water tanker directs water into the rain garden basin via a hose © Jack Clough

6) Pressure sensor data from the base of Basin 4 showing increasing pressure as the basin filled following the first 3000 litres and then a reduction in pressure back to the baseline shortly after infilling was completed.

Of particular interest from this simulation was that both basins were nowhere near being ‘full’ (at capacity) after the rainfall simulation. This demonstrated a much greater infiltration rate and capacity for holding large rain events than previously thought during the design phase of the rain gardens. This was true both for the 1 in 10 year test on basin 4 and the 1 in 100 year test on Basin 2.

In the event that the capacity of the basins is exceeded, and overflow system exists that diverts any excess storm water to the combined sewer system. For the duration of the monitoring, a V-notch weir was attached to the overflow from Basin 4. Data from this was checked after the rain simulation to investigate whether any water left the rain gardens system during the rainfall simulation. The data confirmed that no water overflowed from Basin 4 and left the site.

The rainfall simulation at Renfrew Close showed quite clearly that the rain basins were performing better than intended and designed. Such simulation events represent a valuable tool for providing feedback to local authorities that newly implemented SuDS systems are performing as designed (or better). They also represent an excellent mechanism for engaging the local community in the renovations occurring on their estates and reassuring them as to how they would function during extreme rain events. This can be important for promoting community buy-in for broader roll-out of such schemes through retrofitting.

By Jack Clough