Environmental Engineering Reference
In-Depth Information
4.1
Urban storm water characteristics
When discussing problems of diffuse pollution in urban areas, it is necessary to make a
differentiation between the water quality and characteristics of storm water runoff
collected and transported by urban drainage channels and the one contributed directly to
natural rivers and streams. Drainage channels are usually lined man-made structures,
which are expected to be maintained and cleaned regularly. Thus the storm water quality
in such channels directly reflects the corresponding quality of the drained area and its
land use patterns. Natural streams represent the combined effect of the storm water,
groundwater recharge, all upstream tributaries and point sources of pollution, as well as
the accumulated pollution from sediments and eventually, solid waste depositions. This
differentiation is important in terms of data interpretation and the unit pollution loads (per
unit area or per capita) calculation, and a careful characterization of the study area should
be done during the data analysis process. It has been discussed that discharges from storm
water canals could not be classified as purely diffuse pollution sources, as they have a
well-defined location, especially in cases of large conveying structures. In such cases, the
diffuse character of the pollution is related to the random character of the qualitative and
quantitative aspect of the pollution, rather than to its dispersed location. However, it must
be remembered that storm water drainage structures have numerous discharges along the
system at different locations, thus it should be regarded as a completely diffuse source of
pollution at the level of the whole catchment basin.
The results with respect to storm water quality collected and transported by drainage
channels in Harare, Zimbabwe represent an inconsistent data set of observations made
during the period 1995-2003 (Chapters 3 and 4). Despite the fact that the measurements
were done by different laboratories and information with respect to flow rates were not
available for the vast majority of the cases, results show a trend of pollution with respect
to phosphate and ammonia at the two major channels, exceeding significantly the
prescribed limits at the Coventry Rd channel and in the CBD. Pollution with respect to
metals concentration was found in respect to Pb mainly.
Storm water runoff from roads and highways is considered as a major source of
diffuse pollution but has not been studied in details in the case studies presented.
Indirectly, it has been included as a contributing element to the general characteristic of
urban drainage. It should be noted that in general, the vast majority of the countries in the
region do not have high traffic loads of the streets and roads, as in developed countries;
correspondingly, no high pollution levels could be expected. Exceptions, which need
special attention, are large highways and urban centers, mainly in the Republic of South
Africa.
Previous chapters emphasize the need for event-orientated monitoring programs,
which is most applicable for urban storm water. It could be recommended that research
projects, orientated specifically towards storm water quality assessment, could help
significantly to fill the gaps in existing monitoring programs and to obtain the necessary
data base for a reliable determination of unit pollution loads, which could be incorporated
in the integrated system analysis. Also, it is highly recommendable to include a proper
technical backup as part of such projects, with the provision of automatic monitoring
stations, collecting data with respect to flow rates and qualitative parameters, as well as a
more extended list of measured parameters. Such data, linked to rainfall characteristics
for the respective observations could form a sound base for the determination of typical
