Urban Hydrology
The provision and disposal of water for urban populations is as old as the development of cities themselves. Unrestrained development has made the task ever more difficult and hydrologists and civil engineers face constant pressures to stay on top of the challenges of providing sufficient supplies and adequate treatment/disposal of effluents.
Urban areas have long existed as centres of trade, culture and government, with large-scale population migration away from rural areas to towns and cities increasing rapidly in the last century. Pressures on housing and services have brought specific problems to be solved, including high-rise buildings, mass-transport systems, dormitory and satellite towns, and out-of-town commercial parks. The impacts of such urbanization on catchment hydrology vary according to the local environment, development history and economic constraints on meeting water needs. These needs include water supply for domestic and industrial use, a waste water disposal system, a drainage system to control groundwater and remove local flood water, plus a flood defence system to protect against inundation from external areas. The effects of such developments need to be assessed to see how they interact with the ‘natural’ processes of rainfall, groundwater and drainage that are themselves modified by urbanization.
Content Table
Municipal Water Use
The volume of public water use depends on the size of an urban population and the services and utilities provided such as the extent of pipe networks for supply and sewerage. Much depends on climate conditions. Water withdrawal from fresh water resources is up to 10 times greater in Europe and North America than in some part of Africa and Asia: specific urban per capita water withdrawal is 500-1000 liters per day in many industrialized countries and only 100-150 liters per day in small cities with a large stock of individual buildings not fully provided with a centralized system. Actual consumption of water is usually not above 5-10% of total water intake. The modern trend in the development of public water supply all over the world is the construction in both large and small cities of effective centralized water supply and sewerage systems, connecting together an even greater number of buildings and populated areas. In future, however, the specific per capita water withdrawal is expected to increase, while consumption per se, expressed as a percentage of water intake, will decrease considerably.
Urban Water Quality
Imported, or piped water supplies are usually used by consumers in a rhythmic daily pattern, then collected in a waste water treatment system, treated as appropriate, and discharged back to the native river system. Thus the incoming water volume is artificially increased, chemically altered and diurnally varied. If the external water source comes from another catchment, flows downstream of the urban area will be increased. Urban runoff rates will often be four or five times greater than original rural rates, as well as being heavily contaminated by surface runoff contaminants. If the water supply is through groundwater abstraction, problems of saline intrusion may be endemic and might also lead to subsidence. Land-use change, particularly urban development, has a profound effect on the hydrology of river basins. It is only in recent years that such catchment-wide effects are now covered by legislation in Europe in the form of the Water Framework Directive. To be effective, such management control issues require extensive data acquisition on water volume and quality so that improvement and mitigation of hazards can be attempted.
Drainage
Increased paved and roof areas will increase local runoff and require improvements to convey floodwater rapidly to the residual urban river network. This rapid convergence through the drainage system brings greater risk of flash flooding, particularly when downstream capacity is restricted by culvert dimensions or partially taken up by groundwater infiltration. In older city centers, surface runoff and waste water drainage are usually provided by ‘combined’ sewer system taking smaller runoff events to the sewage works but with overflow facilities so that higher flows (and untreated sewage) can switch directly to the river network. With growing populations and increased per capita water use, many such overflows now work almost daily due to diurnal waste water patterns. Newer developments generally have separate sewers so that surface runoff is drained directly to local rivers or for the irrigation of open spaces (parks, sports grounds), special features (fountains, ponds) street and car washing and fire fighting. Quality regulations usually apply. At present, re-use of urban wastewater for industrial applications is somewhat limited but has great potential through advanced procedures in treatment processes.
Flood Risk
The highly impervious nature of urban areas means that high runoff rates often occur with intense summer rainfall, when the available ‘dilution’ in natural water courses is low due to rainfall being retained in dried soil profiles. A range of engineering techniques is used to reduce their effect on receiving waters. These include underground storm tanks and oversize sewers with restricted outlets, flow diversions, surface storage ponds and washlands. There is growing interest in controlling runoff ‘at source’ by increasing local infiltration, delaying runoff with porous pavements, gravel drains or wetlands and diverting surface water to non-portable use. The UK-based technique known as SuDS (Sustainable urban Drainage Systems) is one such example. Thus the potential impact of urbanization on flood runoff at large may be much reduced.
Figure 1. Stormwater rapidly funneled from upstream developed areas (Janczyn 2011)
Further Reading
HEC Model for Urban Hydrology Model
Hydrologic Prediction for Urban Watersheds with the Distributed Hydrology-Soil-Vegetation Model
Hydrology for Urban Land Planning - A Gudebook on the Hydrologic Effects of Urban Land Use
Urban Hydrology...Keeping an eye on what is important
Investigation of Soil Conservation Service Urban Hydrology Techniques
Urban Hydrology for Small Watershed
Related Articles
- Hydrology
- Snow Hydrology
- Groundwater
- Flood Risk Management Plans
- Flood control and disaster management
- Sustainable Urban Drainage System
- Stormwater
- Surface Runoff
- Roof Runoff
- Urban Runoff
- Surface Water Hydrology
References
This article was originally written by Celia Kirby
EC Water Framework: 2000/60/EC: Council of Europe, 2000.
Robinson, M. et al., 2000. Land use change. In: The Hydrology of the UK (Ed. M.C.Acreman). Routledge. 30-54.
Adams, B. et al., 2000. Groundwater. In: The Hydrology of the UK (Ed. M.C.Acreman). Routledge.
150-179.
Butler, D. 2004. Urban water – future trends and issues. In: Proc. British Hydrological Society Int. Conf. Hydrology: Science and practice for the 21st century (Eds. B. Webb et al.). Vol. II, 233-242.
Shiklomanov, I.A., 1998. World Water Resources: A new appraisal and assessment for the 21st century. United Nations, Paris.
Anon. 1998. Water in the 21st Century. World Water Council.
G. Janczyn. 2011. San Diego Stormwater Management: Public Comment Period is Almost Over.http://groksurf.com/2011/07/22/san-diego-stormwater-management-public-comment-period-is-almost-over/ (Accessed on January 22, 2012)
