Urban Runoff
Sean Kelly, Darren Kidwell, and Gretchen Lehrer
Effects on Urban vs. Rural and Surfaces
Definition and Causes
Urban runoff is defined as a stream flow or the sum of surface runoff and subsurface runoff. Surface runoff occurs when the surface storage and the soil become saturated, infiltration ceases and subsequent rainfall becomes surface runoff. Subsurface runoff is rainwater that infiltrates the surface and flows much more slowly on its way to a stream than surface runoff (Horner et. al. 1994).
Rainfall and the soil conditions are the direct causes of urban runoff. Rainfall can take one of several routes once it reaches the earth’s surface. Rainwater can be absorbed by the soil on the land surface, intercepted by vegetation, directly impounded in many different surface features from small depressions to large lakes and oceans, or infiltrated through the surface and subsurface soils into the groundwater. Another route taken by falling precipitation is runoff. Soil characteristics in a watershed have a direct effect on the rainfall-runoff process and these include soil layer thickness, permeability, infiltration rate, and the degree of moisture in the soil before the rain event. The greater the permeability of the soil, or the ability to infiltrate rainfall to its lower strata, the less remaining to become runoff (Horner et. al. 1994).
Causes and Effects of Runoff on Urban Land Surfaces vs.
Rural Land Surfaces
Surface runoff occurs relatively rapidly in the urban watershed, since storage and infiltration capacity have been reduced to practically zero. Much of the surface consists of impervious materials such as concrete or asphalt (Nix 1994). Structures which add large amounts of impermeable areas to the watershed in general increase slopes and considerably diminish the water storage capability. The increased volumes and flow rates of runoff produced by urban watersheds have a number of harmful effects including flooding and stream erosion (Nix 1994).
The vegetative cover found naturally in rural areas directly effects the rainfall-runoff process and is an important measure in many runoff estimation techniques. Vegetation characteristics include various types, canopies, and densities, the extent of coverage, the degree of residue or natural litter at the base, and the degree of surface roughness. The water flow velocity of runoff over a smooth, impermeable surface such as a road or parking lot is about ten times faster than over a vegetated surface. Urbanization alters the hydrologic regime of surface waters by changing the way water cycles through a drainage basin. In a natural setting, precipitation is intercepted or delayed by the forest canopy and ground cover. Vegetation, depressions on the land, and soils provide extensive storage capacity for precipitation. Water exceeding this capacity travels via shallow subsurface flow and groundwater and eventually discharges gradually to surface water bodies. In a forested undisturbed watershed, direct surface runoff occurs rarely or not at all because precipitation intensities do not exceed soil infiltration rates (Horner et. al. 1994).
(Horner et. al. 1994) Comparisons of Developed vs. Undeveloped Runoff Conditions
Quality and Quantity of Urban Runoff
The water quality of urban runoff varies with its source and location. In urban runoff, most pollutants occur as solids or are associated with soil or other natural particulates. This condition differs among the specific pollutants. These pollutants come from many different sources. These sources include: transportation, industrial activities, decaying vegetation, soil erosion, animals, fertilizer/pesticide application, leaking sanitary sewers, direct connections of sanitary sewers to storm sewers, poorly operating septic systems, illegal disposal of oils, paints, etc. to the storm sewer system, accidental spills, leaking underground storage tanks, leachate from landfills, and leakage from hazardous waste sites (Nix 1994). The top 50 percent of pollutants found in Nationwide Urban Runoff Program samples are, lead, zinc, copper, chromium, and arsenic. Besides these pollutants, other water quality characteristics affect the behavior and fate of materials in water. The characteristics include temperature, pH, dissolved oxygen, alkalinity, hardness, and conductivity (Horner et. al. 1994).
Estimates of the quantity of runoff are determined first by evaluating several key drainage area characteristics. The first characteristic is the drainage area size, which is determined using topographic maps. Other characteristics are the shape of the drainage basin and its various slopes. The steeper the surfaces and channels of the basin the faster runoff can drain to its outlet. An elongated drainage area with more gradual slopes will result in slower runoff and drainage. An elongated drainage area with a longer distance from its upper reaches to its outlet may have a longer response time than a rounded one of equal size and therefore, a lower peak runoff rate (Horner et.al. 1994).
Soil characteristics in a watershed have a direct effect on the rainfall runoff process and they are included in most runoff estimating techniques. These characteristics include soil layer thickness, permeability or infiltration rate, and the degree of moisture in the soil before the rain event. The greater the soil permeability, the ability to infiltrate rainfall to its lower strata, the less remains to become runoff (Horner et.al. 1994).
The quantity of runoff can be estimated using one of several different methods. The most common methods include steady state, unsteady state, and computer models. Steady-state methods use uniform rainfall intensities, soil infiltration rates, and representative watershed response times. Steady-state conditions are a reasonably accurate way to estimate peak runoff rates from high to moderate frequency storms in small watersheds with relatively short response times. Unsteady state methods allow rainfall intensity, soil infiltration, and watershed response time to vary with time. These methods can more accurately compute runoff characteristics from rainfalls of varying intensities in single or even multiple storm events. Computer models can analyze rainfall-runoff process for a series of storm events over an extended time period from several years to several decades. These models account for changes in watershed factors and parameters during the time between storms as well as during storms. Factors that must be considered are temperature, relative humidity, surface evaporation and evapotranspiration, and groundwater levels and movement. All of these factors may significantly affect runoff response to the next rainfall in the analysis (Horner et. al. 1994).
Laws and Regulations
From 1978 through 1983, the EPA conducted a comprehensive study of urban runoff called the Nationwide Urban Runoff Program (NURP). This study provided a better understanding of the nature of urban pollutants from various urban land uses. This study focused primarily on monitoring runoff from residential, commercial, and industrial land and clearly presents information on the magnitude and variety of pollutants encountered in the urban environment (Horner et. al. 1994).
The 1987 amendments to the Clean Water Act contain provisions that significantly increase efforts to address water quality in urban runoff (Horner et. al. 1994).
On November 16, 1990, the EPA issued permit application requirements for storm water discharges associated with industrial activities (Horner et. al 1994).
The best way to avoid pollution from urban runoff is education. Education should be targeted to specific audiences, such as contractors and developers who build on the land surface destroying the natural flow of rainwater into the ground or other natural water source. If they are more educated then urban runoff pollution can be controlled and possibly prevented.
Runoff Control
Tanks
Coorsâ Brewing Co.
Runoff control represents various practices designed to keep water from contacting bare soil and/or controlling its velocity. Runoff control includes drains for surface and subsurface water, dikes, and channels placed along slopes to interrupt and divert runoff and roughness created on the surface to reduce velocity. A temporary pipe slope drain is effective in preventing runoff erosion on a slope from a higher elevation. Up slope runoff needs to be collected and directed into the drain and then discharged in a controlled way to prevent erosion at the slope bottom (Horner et. al. 1994).
An urban watershed not only has modified stream channels created by the increased peaks and volumes of runoff, but also has a series of total artificial closed channels, which are created when sewerage is installed under a city. Most urban areas collect runoff in gutters and sewers and discharge it at various points to receiving waters (Lazaro 1990).
Proposed Methods of Control
Reasons for further development:
Four proposed methods for control:
Faculty Advisor: Naraine Persaud, npers@vt.edu
Copyright © 1998 Naraine Persaud
Last Modified: January 1, 1999
