Drainage
Drainage
The hydrologic cycle encompasses all movements of water molecules through the atmosphere , hydrosphere, and groundwater zones. One key part is the drainage of surface waters and groundwater back to the ocean. Natural drainage systems are well-adjusted to their climate , vegetation, and geology. Hydrologists and fluvial geomorphologists have studied these systems in great detail; human disturbances are readily apparent. Three impacts are considered here: wetland drainage and irrigation ; dam construction and mining alterations; and urbanization.
Robert E. Horton identified many drainage characteristics during the 1930s and 1940s. Later researchers added other useful quantitative parameters. All of these show the predictable patterns found in natural drainage systems. Best known are stream order and drainage density. Although influenced by the detail of the maps used (commonly 1:24,000 or 1:62,500 topographic maps), stream order provides a simple system for ranking tributaries. The smallest ones are labeled first order, second-order streams have at least two first-order tributaries, third-order streams have at least two second-order tributaries, and so forth. The Mississippi River is ranked as a tenth- or twelfth-order stream, depending on the map scale used. Horton also developed a bifurcation ration between stream orders, which is generally around three; that is, on average there are three second-order streams for each third-order one.
Drainage density is a measure of channel length per unit area. This varies widely depending on the nature of the vegetation cover and soil . Badlands and deserts have high drainage densities compared to well-vegetated basins with permeable soils.
Stream channels are finely adjusted to their drainage requirements, and attain a state of quasi-equilibrium. Human activities disrupt this equilibrium, forcing channel adjustments. This disruption is so pervasive that essentially all channels within settled areas have been impacted.
The earliest human intrusions are wetland drainage and irrigation of arid lands for agriculture. Recent litigation has involved decisions by federal officials to deny irrigation water to farmers in order to maintain needed supplies for endangered aquatic organisms. Only recently has the tendency to drain wetlands been reversed. With the comeback of the beaver from near extinction , wetland proponents now have an efficient ally.
Many large rivers have been dammed, with direct impact on the downstream riparian environment .To counteract this, in 1996, an experimental flood was produced on the Colorado River through the Grand Canyon, with the desired goal of improving habitat for endangered species there. Even farm ponds, through their sheer numbers, are major drainage interrupters. Another strong impact has come from mining, through the creation of huge holes, massive sedimentation , or increased runoff .
Urbanization has had the greatest local impact on drainage. Channels are altered, permeable areas are covered with impervious materials, and storm drains deliver runoff more rapidly to the overwhelmed drainage network. The effect is to deliver much more water in substantial less time, a sure formula for flooding .
Tulsa, Oklahoma, is a case study in urban flooding and costly remediation .A computerized system now evaluates rain and stream-gage data in real time, and issues flood warning via sirens throughout the city. Construction is banned within the 100-year flood, and valuable land has been set aside for detention ponds so that runoff does not exceed natural conditions up to the level of a 100-year flood. Though costly, these steps have been essential in order to protect lives and reduce property damage.
Changes in the drainage network are prime indicators of human impact. If we are to work with nature rather than fight her, we must understand and reckon with the impacts of our development on this fragile and finely tuned system.
[Nathan Meleen ]
FURTHER READING
Cooke, R.U., and J.C. Doornkamp. Geomorphology in Environmental Management. London: Oxford University Press, 1974.
Leopold, L.B., M.G. Wolman, and J.P. Miller. Fluvial Processes in Geopmorphology. San Francisco: W.H. Freeman, 1964.
Strahler, A.N., and A.H. Strahler. Modern Physical Geography 2nd ed. New York: John Wiley & Sons, 1983.
drainage
1. The passage of water over and through the land surface, ultimately towards the sea. See DENDRITIC DRAINAGE; DERANGED DRAINAGE; DISCORDANT DRAINAGE; DRAINAGE DENSITY; DRAINAGE PATTERN; INCONSEQUENT DRAINAGE; and SUPERIMPOSED DRAINAGE.
2. Process of removing the gravitational water from soil, using artificial or natural conditions, such that freely moving water can drain, under gravity, through or off soil. See MOLE DRAIN; and TILE DRAIN.
drainage
1. The passage of water over and through the land surface, ultimately towards the sea. See dendritic drainage; deranged drainage; discordant drainage; drainage density; drainage pattern; inconsequent drainage; and superimposed drainage.
2. The process of removing the gravitational water from soil, using artificial or natural conditions, such that freely moving water can drain, under gravity, through or off soil. See mole drain and tile drain.
drainage
drain·age / ˈdrānij/ • n. the action or process of draining something: the pot must have holes in the base for good drainage | the drainage of wetlands. ∎ the means of removing surplus water or liquid waste; a system of drains.