Urban Storm Water Management

Urban storm water management is concerned with the draining of urban areas during and following precipitation as well as dry weather flows. Historically, urban storm water management has focused on managing flood risk through the transit of precipitation within drainage systems, with either combined (sewage and surface water, e.g., the United Kingdom) or separate (surface waters only, e.g., Australia) systems. The aim of separate systems has been to channel the water rapidly, and often invisibly, to the nearest watercourse, while combined systems use the runoff to flush the system, reducing the likelihood of blockages but often requiring water treatment prior to release. Early examples of urban storm water management exist from the Minoean (e.g., Crete) and Greek (e.g., Pirene, Turkey; see first photo) civilizations. One of the longest-maintained systems is the Cloaca maxima in Ancient Rome, which provides an early example of urban storm water management, with drains collecting rainfall, draining marshlands, and transporting effluent to the river Tiber. London and Paris developed extensive sewer systems in the mid 19th century to remove the bad odors, believed at that time to be the cause of cholera. These systems were indeed responsible for significantly reducing the incidence of cholera, although this was because they led to a significant reduction in the contamination of local drinking waters. The expansion of urban areas during the past century has seen many new challenges within urban water management. As the proportion of catchments covered with impervious surfaces has increased, the quantity and quality of the runoff have changed. These challenges have been particularly acute in the rapidly urbanizing parts of the developing world.

Surface water flows are relatively unpredictable and exceedingly difficult to control, as the source—precipitation—is inherently stochastic. The amount of water discharged into drains varies depending on the duration and intensity of precipitation and the extent to which it can infiltrate into pervious surfaces.

Sewers fail when the infrastructure fails, such as when a blockage occurs or when the sheer volume of precipitation entering them exceeds their design capacity. The design of a piped infrastructure capable of providing drainage for all precipitation magnitudes is unfeasible on economic grounds; as a result, there is always the risk of system failure due to incapacity. Flooding risk is normally expressed as a statistical measure (probability ratio), based on a return frequency, such as a 1-in-100-year storm event.

Varying precipitation levels mean that combined sewers have to convey varying quantities of wastewater at varying concentrations. When [Page 2991]these systems are overloaded during heavy precipitation events, in an attempt to reduce the pressure on the system, untreated waste may be discharged into the nearest watercourse via a combined sewer overflow (CSO), with an obviously negative impact on the environment. On rare occasions when CSOs do not function properly, wastewater can circumvent the constraints of the system, potentially flooding streets, homes, and businesses.

Developers and water system managers are increasingly under pressure from environmental agencies and local authorities to implement approaches to mitigate the risk of flooding. The management of urban drainage in a sustainable manner has increased in importance in recent years. In North America, these approaches are often considered as best management practices (BMPs); in Europe, they are known as sustainable drainage systems (SuDS); and in Australasia, they are called water-sensitive urban design (WSUD). The aim of these systems is to reduce the impact of storm water runoff on receiving rivers, as storm water runoff adversely affects the flow regimes, causing river hydrographs to peak sooner, often at higher levels; the runoff may also wash contaminants into the watercourses, such as organic litter, soil/dust, salt, metals, fertilizers, pesticides, chemicals, oils, hydrocarbons, and a range of other substances that may accumulate on surfaces. The approaches applied depend on the specific issues presented at each site: the duration and intensity of precipitation events; the proportional area covered by impervious surfaces; catchment relief; the drainage system, both natural and anthropogenic, and the level of integration between the two; and land use, with urban areas often including residential, retail, and industrial developments and car parks.

...