Urban Drainage Case Study
Sustainable Urban Drainage Systems (otherwise known as SUDS) are drainage solutions designed to reduce the impact of surface water displacement on new developments. SUDS has been for some time a significant aspect for any landscaping project. SUDS show no sign of waning due to increasing levels of urbanisation and wholesale use of sealed surfaces including roofs, pavements and roads. Issues surrounding surface water flooding became a concern to Beaufort Landscapes on a recent design and build project we carried out on a garden in Surbiton, Greater London where the client sought an extension to their back garden patio area.
According to council records the site lies on top of river gravel. On closer inspection the sub-soil consisted of a deep layer of heavy clay which we would have to dig through to reach the gravel below. Only once we had reached the free draining gravel would we be able to install a soak away. Although a tried and tested approach, this solution would burden our client with materials, excavation and waste removal costs and ultimately exceed their budget for the project.
As a more cost effective option to the above we decided to run with a solution similar to that of a rain garden. In essence a rain garden is a shallow depression with absorbent, free draining materials encouraging natural absorption of water run-off. The area is then planted with specific vegetation that can withstand occasional temporary flooding i.e. Iris. To ensure this solution would be feasible in withstanding occasional storm water we carried out a percolation test which determined the absorption rate of the site. This test consisted of digging a hole to the depth of 12” and filling it with water several times in order to reach saturation point. We then noted the time it took for the water to be completely drained from the hole and applied this to a conversion chart. Based on the results we were confident that our solution would work.
Our design would adhere to the same basic principles as a ‘rain garden’ but instead of planting directly into the ground we used potted plants atop a layer of decorative aggregate. Not only did this provide an attractive solution, it also allowed the clients to plant their favourite bedding plants without risk of water logging. Based on the size of the patio, we used the following online calculator as a guide to determine the size required for our ‘rain garden’ raingardenalliance.org/right/calculator. Given the soil conditions, we decided the drainage area should be roughly 1/3 the area of the patio. To be on the safe side, we actually excavated a layer of clay to around 200mm and replaced this with MOT, sand, grit, gravel and finally a decorative aggregate in the form of Scottish Pebbles.
Days following completion of the garden heavy rainfall was witnessed in the Surbiton area and the client was pleased to see that our solution had proved a success.