Environmental Engineering Reference
In-Depth Information
quantities as well. Lamb (1985) points out rightfully that many water quality problems
and their solutions can be influenced radically by the volumes and flow rates that are
available, their variability, rates of renewals and losses, and the quantity of water for
intended future use. The variability of runoff water quality is strongly related to the
volumes of runoff generated, flow rates and the general pattern of rainfall events. Thus in
practice, both water quality and quantity evaluation and assessments should form the
basis for managerial decisions and policy formulation.
It has been widely recognized that urban runoff pollution can lead to a degradation of
aquatic life and endanger public health. The magnitude and nature of these effects vary
by region, depending on the type and concentration of pollutants in storm water, rainfall
characteristics, land use practice and other factors. In the USA, it has been found that:
“the size of the non-point source pollution, at least equals, if not exceeds, the total
pollutant loadings contributed by all point sources” (AFS 2000). Sometimes, urban storm
water runoff may contribute potential pollutants in quantities even higher than those of
untreated sanitary sewage from the same community, especially the first flush It was
reported that storm water contains more suspended solids than raw sanitary sewage, with
BOD, phosphates and total nitrogen in the range of 6-11% of the raw sewage values
(Lamb 1985). It is thus obvious that the contribution of storm water pollution to receiving
waters cannot be ignored.
In African conditions, many sewage treatment facilities have not met the required
standards, thus becoming a serious point pollution source. Under such conditions, the
pollution loads generated by runoff might be ignored. However, information presented in
Chapter 3 shows that pollution from urban drainage could be substantial and should be
considered in the management process.
One factor, which characterizes urban runoff water quality, is its high variability. The
concentrations of pollutant constituents may vary in magnitude by 10:1 or more in a
single storm, from area to area and from storm to storm, and could be of the magnitude of
the effluent from a secondary treatment facility (AWRC 1981). Therefore, there is a need
for event orientated monitoring approaches and for the consideration of the seasonal
variability with respect to the monitoring and regulation of urban storm water.
2.3
Quantifying runoff by a simple procedure
The rainfall excess or net rain, which is of concern for the determination of diffuse
pollution loads, depends on many factors, which could be summarized briefly as follows:
• Soil and surface cover characteristics - Other factors being equal, a soil with low
perviousness willproduce a higher proportion of net rain than a soil that allows water
to infiltrate readily into the ground. There is a greater volume of rainfall excess
generation in urban areas due to a large percentage of impervious covers. The
presence of vegetation decreases the amount of excess runoff by intercepting rainfall
and increasing evaporation losses.
• Topography - Flat land areas produce less runoff excess than steep slopes for similar
surface conditionsand rainfall patterns.
• Rainfall characteristics (precipitation, intensity and duration of the rainfall event) -
Rainfall excess ishigher with increased precipitation intensity because water
accumulates more rapidly on the ground surface, filling depression storage and giving
