In the last 10 years there has been a global shift in recharging the ground water of an urban area rather than removing the water after it rains (3). This comes in response to the frequency of extreme weather events and the demands for resilient cities. I’ve experienced this shift firsthand in Amsterdam. The municipality approached our studio to undertake low cost resilient street scape design, the most effective surface treat was to increase the permeability of the sites. Referee to the image below for an example an intervention technique.
Historically, water has been diverted straight to our oceans or river systems with little to no filtration, through a network of kerbs, pits, pipes and channels to prevent floods, but this doesn’t always work. In fact, it actually increases the effects of drought. Figure1 demonstrates the importance of permeable surfaces and how the cycle tends to work for forested areas vs Urban areas
As more research has become available, landscape architectures have learnt the importance of water sensitive design and the importance of recharging ground water on a local level. The following table demonstrates how each surface interacts with water after a down poor.
| Natural ground cover (nature reserves) | 10%-20% impervious surface (rural housing) | 35%-50% impervious surface (residential suburbs) | 75%-100% impervious surface (city environment) | |
| Shallow infiltration | 25% | 21% | 20% | 10%% |
| Deep infiltration | 25% | 21% | 15% | 5% |
| Run off | 10% | 20% | 30% | 55% |
| Evaporation | 40% | 38% | 35% | 30% |
(4)
It may sound simple, but water gives life to our flora and fauna, by keeping this water locally we are able to increase the biodiversity of that space. As a result of increasing that biodiversity a multitude of benefits come such as: Reduce local climate temperature, improved mental well-being (as a result of the greening), increase activity, increase in native habitats, reduced desertification and wind erosion and increased evaporation. Furthermore, if we compare the natural ground cover to the 75%-100% impervious surface we notice that runoff is heavily affected. A issue with increased run off is flooding, the best example of this was the floods in Houston Texas in September 2019
Water sensitive urban design is at the fore front of Rotterdam’s urban development in Holland. A great example of current project being undertaken to understand the effects our plants, soil and topography have on water infiltration is the Sponge Garden by De Urbanisten. This project focuses on the sponge effect, a process that moderates the effect of drought and flooding, it can absorb a lot of water quickly, hold it and gradually return the water to the subsoil. The project has a mix of different soil, plant and topographical combination to explore the most effectives solutions for the ever-growing technique.
The Sponge Garden by De Urbanisten (1)
As research continues to develop further in the topic, we look at current precedents to replicate and improve upon in professional practice. The Scarborough foreshore has a great example of plants that filter heavy metals and oils from the carpark.
As the climate continues to have more frequent extreme events. Interventions such as increasing water infiltration can be the difference between an entire town or city flooding or being able to keep their shops open.
References:
1) http://www.urbanisten.nl/wp/?portfolio=sponge-garden
2) https://www.melbournewater.com.au/sites/default/files/forested-urban-stormwater.jpg
4) https://stormwater.pca.state.mn.us/index.php?title=File:Urban_hydrology.jpg