Communication from the Cape Resilient Landscaping Forum
Sustainable urban drainage systems & earth shaping…
Cape Town receives 2-3 times more rainwater than its annual requirement, so why are we not using this and where does it go? It runs off roads, paved areas and roofs and literally goes down the drain into storm water pipes and out to sea.
While people are scrambling for rainwater harvesting tanks and installing boreholes, it seems we have forgotten the enormous value and benefits of infiltration: capturing water runoff and allowing it to seep into the ground. The soil is in fact our largest and least expensive ‘storage tank’, one that provides moisture to plants for weeks and even months after it has rained. This is the basis of SUDS – sustainable urban drainage systems that play a major role in water-sensitive city design. Furthermore trees and shrubs are like living pumps: moving water from the ground, providing moisture and cooling the air – nature’s effective and free air conditioning.
Rainwater harvesting can be done with simple earth shaping – swales, berms, depressions, and other landforms – that capture and absorb the rain, instead of letting it run away. Earth shaping serves to reduce erosion, and is a convenient passive form of irrigation that requires no tanks, mechanical pumps, valves or irrigation systems. It functions whenever rain falls, and water flows and is especially useful on mountain slopes and with ‘oily’ or hydrophobic soils. Earthworks take some effort to install but require very little maintenance. They can be used anywhere – in public landscapes, gardens, schools, street planting, parks and farms. So let’s not waste this free water resource and ensure we maximize the potential of available rainfall.
For an excellent practical guide on how to create successful earthworks for rainwater harvesting consult Brad Lancaster’s ‘ Rainwater harvesting for drylands and beyond’. His advice is to start with long and thoughtful observation, and plan a small and simple intervention in the highest point of the property. The purpose is to slow down and spread the flow of water so that it can seep into the soil. Another important principle is to always plan the overflow route and manage surplus water as a resource. You can use earth shaping, planting and mulch to create a living sponge(s) in the landscape.
In summary, the most logical, cost-effective strategy to maintain a garden or landscape requires the following 4 steps:-
- Harvest rainwater and runoff with appropriate earth shaping so that water infiltrates the soil.
- Set yourself a goal of zero storm water runoff: consider storing rainwater in tanks and treating it for use in the house.
- Consider treating and reusing your “grey” water in the landscape.
- Take note that water extracted from boreholes is a valuable but finite resource that is at risk of being depleted by an ever-growing number of users. Consider borehole water as a last resort to be used under exceptional circumstances e.g. for food gardens.
If you love gardening there is no need for big capital outlays, no waiting for plastic tanks, no fossil fuels required – just time honoured farmer’s logic. Rain is the highest quality source of water for irrigation and all you need is careful observation, some planning and a spade to utilize it. So grab a spade, and start small and simple!
– Text by Marijke Honig
Example of sustainable urban drainage systems for a parking area: storm water runs from the paved surfaces directly into the ground via a central swale between car parking bays. The infiltration of water into the soil allows for plants and trees to flourish.