“Sustainable Development” is an increasingly important concept in most industries today, and one in which the water industry has been making rapid progress for over a decade. In countries around the world, including the USA, Europe, and Australia, Sustainable Urban Drainage (SUD) structures - permeable pavements, ponds, and the like - are reducing the volumes of direct urban runoff, and helping to resolve problems such as flooding and pollutant spills to rivers.
However, until recently, waste water modelling packages were not able to model the runoff and groundwater infiltration characteristics of SUD structures, with the result that modelling – an important tool in all waste water planning and operational analysis - could not properly evaluate these important development options. With the need for ever more accurate and detailed hydrological models, this is now being addressed, and in this article we describe an approach to modelling the three main hydrological processes that occur in SUD structures:
- highly attenuated runoff, as experienced with permeable pavements
- exfiltration - allowing water to be lost to the ground via structures such as soakaways and French drains
rainfall runoff lost to infiltration at a constant rate, which occurs with structures such as ponds and swales.
The approaches we describe are implemented within a commercial software package that is applied all over the world. Therefore they must be adaptable enough to be applied to a wide variety of conditions whilst at the same time being robust and simple enough to understand and apply within the constraints of commercial projects.
Sustainable Urban Drainage Structures
Water engineers are increasingly turning to SUD structures to harness and enhance natural characteristics of an urban catchment in order to achieve hydraulic or water quality objectives whilst minimising the disruption to and alteration of the existing environment. Almost all SUD structures provide an element of storage that is utilised during periods of rainfall. The retained flow is either gradually released back into the natural watercourse or sewerage system, or allowed to infiltrate into the surrounding ground. In addition to their storage function, SUD structures can be used to filter out pollutants.
These SUD solutions are used in preference to more traditional methods such as building large storage tanks. As well as having environmental benefits, SUD structures are often the lower cost investment option – the Technical Chief of Malmo, Sweden, Peter Stahre, said “it is always cheaper to use Best Management Practice (SUD structures) than traditional solutions”.