Environmental Engineering Reference
In-Depth Information
problems associated with mine discharges. Dissolved sulfates, together with chlorides,
are frequently present in mine runoff to an appreciable concentration. In general,
pollutant constituents depend on the geological formation of the site.
3.4
Pollution concentrations and runoff events
Given the nature of the generation of diffuse pollution, pollution concentrations vary
considerably depending on: the rainfall characteristics, runoff erosivity, land cover,
atmospheric depositions and other parameters. In addition, it is expected that a flush
effect would lead to higher concentrations of pollutants at the beginning of the storm. The
same applies to rainfall events at the beginning of a rain season. Many attempts have
been made to obtain more uniform values, which could be used in models for the
simulation of different scenarios and the prediction of undesirable effects. In general, it
has been agreed that pollution concentrations of runoff should be determined as event
orientated statistical values. This approach requires a considerable monitoring effort and
a large data set in order to determine a statistically significant result with respect to mean
pollutant concentrations of a given type of storm. Correlation between these values and
specific land use practices has not been shown in all cases, but a statistically significant
difference was found between urban areas and open spaces. Statistically significant
results to confirm flushing effect in storm sewers were not found for the climatic
conditions of USA (Novotny 2003). More information regarding the monitoring of runoff
pollution and the determination of the concentrations of pollutant constituents is
presented in Chapter 2.
4 METHODS OF SURFACE RUNOFF DETERMINATION
Diffuse pollution is closely related to the amount of rainfall and the portion of it, which
would reach water bodies in the form of runoff. It has been mentioned that diffuse
pollution of surface water bodies is associated mainly with the particulate material in
runoff and the soil erosive capacity. Therefore, the correct estimation of surface runoff
should be the basis of the estimation of pollution loads generated by surface runoff. It
should be noted that not every rainfall would generate runoff or should contribute in the
same way to diffuse pollution loads. Diffuse pollution abatement measures should focus
on typical storms of medium magnitude with relatively high frequency and low return
periods. These are the ones contributing the most significant pollution loads. However, in
engineering practice, of most importance has been the determination of the “design
storm”, which is applied to determine design flows of drainage systems and other flood
protection structures. Therefore, the analysis of the available rainfall data in the case of
the estimation of pollution from surface runoff requires specific attention and rainfall data
evaluation.
4.1
Rainfall excess and surface runoff
Surface runoff is that portion of the rainfall, which reaches natural or man-made drainage
canals, after it falls on the ground and travels to the point of consideration, and, which
