Urban runoff

Urbanization causes fundamental changes in the local hydrologic cycle , mainly increased speed of water movement through the system, and degraded water quality . They are expressed through reduced groundwater recharge, faster and higher storm runoff , and factors that affect aquatic ecosystems, particularly sediment , dissolved solids , and temperature. The resultant problems have encouraged municipalities to reduce negative impacts through storm water management.

Important research on these issues was spearheaded by the U.S. Geological Survey (USGS) during the early phases of the current environmental revolution. A sampling of titles used for the USGS Circular 601 series indicates the scope of these efforts: Urban Sprawl and Flooding in Southern California (C601-B); Flood Hazard Mapping in Metropolitan Chicago (C601-C); Water as an Urban Resource and Nuisance (C601-D); Sediment Problems in Urban Areas (C601-E); and Extent and Development of Urban Flood Plains (C601-J). Also relevant are Washington D.C.'s Vanishing Springs and Waterways (C752), and Urbanization and Its Effects on the Temperature of the Streams on Long Island, New York (USGS Professional Paper 627-D).

Most of these works cite a 1968 publication written by Luna Leopold, Hydrology for Urban Land Planning. Describing a research frontier, Leopold anticipated many of the concerns currently embodied in stormwater management efforts. He identified four separate but interrelated effects of land use changes associated with urban runoff : 1) changes in peak flow characteristics, 2) changes in total runoff, 3) changes in water quality, and 4) aesthetic or amenities issues.

Urbanization transforms the physical environment in a number of ways that affect runoff. Initially construction strips off the vegetation cover, which results in significantly increased erosion . Local comparisons with farms and woodlands show that sediment from construction and highway sites may increase 20,000–40,000 times. Furthermore, as slope angles steepen, the erosion rate increases even faster; and above a 10% slope (10 ft [3 m] rise in 100 ft [30 m]) no restraints remain to hold back this sediment. On a larger scale, a Maryland study comparing relatively unurbanized and urbanized basins found a fourfold erosion increase.

Pavement and rooftops cover many permeable areas so that runoff occurs at a greater rate. Storm sewers are built which by their very nature speed up runoff. Estimates range from a two- to six-fold increase in runoff amounts from fully urbanized areas. Even more important, however, is that peak discharge (the key element in flooding ) is higher and comes more quickly. This makes flash floods more likely, and increases the frequency of runoff events that exceed bank capacity.

This increased flooding causes much channel erosion and altered geometry, as the channels struggle to reach a new equilibrium. Less water infiltrates into groundwater reservoirs, which diminishes the base flow (the seepage of groundwater into humid-region channels) and causes springs to dry up. The urban heat island effect, combined with reduced baseflow, exposes aquatic ecosystems to higher heat stress and reduced flow in summer, and to colder temperatures in winter. Summer is especially difficult, as rainwater flows from hot streets into channels exposed to direct sunlight. As a consequence, sustaining life becomes more difficult, and sensitive organisms must adapt or die.

Water quality changes increase cultural eutrophication (water pollution caused by excessive plant nutrients), as fertilizer and pesticide residues wash from lawns and gardens to join the oil, rubber , pet manure, brake lining dust, and other degrading elements comprising urban drainage . When these are combined with the effluent from industrial waste and treated sewage, especially heavy metals , the riverine environment becomes a health concern.

Under arid conditions, the damage from mudflows may be increased because the land that once absorbed the sliding mud is now covered with streets and buildings. This situation can be seen in southern California, where hillside development has increased the threat of landslides.

Increasing attention is now being given to stormwater management around the United States. For example, Seattle, Washington has focused on water quality, whereas Tulsa, Oklahoma is primarily concerned with flooding. Both cities have responded to serious local problems.

By the 1950s, Seattle's Lake Washington and the adjacent water of Puget Sound had become so polluted that beaches had to be closed. Sewage treatment plants around Lake Washington and raw sewage dumping in Puget Sound were the fundamental causes. The residents of the area voted to form a regional water-quality authority to deal with the problems. As a result, all sewage treatment was removed from Lake Washington, and dumping of raw sewage into Puget Sound was halted. The environmental result was dramatic. As nitrate and phosphate levels dropped sharply, cultural eutrophication in these waters was greatly reduced, and beaches were reopened. The regional agency continues to seek better ways to improve the water quality of stormwater runoff.

Tulsa's efforts are a response to a series of devastating floods. Between 1970 and 1985, Tulsa led the country in numbers of federal flood declarations; these floods caused 17 deaths and $300 million in damages. Conditions in Tulsa are such that 6.3 in (16 cm) of rain within six hours is sufficient to produce what hydrologists call a 100-year flood. In May 1984, 10 in (25 cm) of rain fell during a seven-hour period, which was a rainfall frequency of about 200 years.

A leader in flood control, Tulsa has taken a multifaceted approach to stormwater management. During the 1970s, then Congressman James R. Jones sponsored legislation to buy out and tear down houses in the severely flood-prone Mingo Creek Basin; some had qualified seven times for flood disaster aid, with payments far exceeding market value. In a joint venture, Tulsa and the Army Corps of Engineers have spent $60 million and $100 million respectively to channelize Mingo Creek in its lower reaches and create detention ponds in the middle portions of the basin. Expected benefits include a $26.9 million reduction in annual flood damage. Continuing flood control efforts have incorporated the use of a state-of-the-art computerized flood warning system.

Recreational possibilities are being exploited in Tulsa's overall stormwater management program. Access roads are being used as running tracks; athletic fields double as detention reservoirs during flood conditions. Since existing parks have been modified to meet the flood program needs, the facilities have benefitted from the improvements, and the water authority leaves the maintenance to the parks department. Using existing parks also eliminates the need to buy out houses and disrupt existing neighborhoods.

Although the Tulsa plan has been successful, more heavily developed floodplains require different measures. Often land costs are so high that it may be cheaper to spend money on flood-proofing or tearing down existing buildings. In any event, urban runoff is an expensive problem to solve.

See also Heavy metals and heavy metal poisoning; Industrial waste treatment; Land-use control; Water treatment

[Nathan H. Meleen ]

RESOURCES

BOOKS

Howard, A. D., and I. Remson. Geology in Environmental Planning. New York: McGraw-Hill, 1978.

Mrowka, J. P. "Man's Impact on Stream Regimen and Quality." In Perspectives on Environment, edited by I. R. Manners and M. W. Mikesell. Washington, DC: Association of American Geographers, 1974.

OTHER

Leopold, L. B. Hydrology for Urban Land Planning: A Guidebook on the Hydrologic Effects of Urban Land Use. USGS Circular 554. Washington, DC: U.S. Government Printing Office, 1968.