Environmental Engineering Reference
In-Depth Information
It has been mentioned that EMCs represent more adequately the actual runoff quality.
However, the EMCs from different storms with respect to the same type of land use
practice may vary considerably due to the different characteristics of the storm event. A
statistical analysis of data collected during a study in USA, found no correlation between
unit pollution loads and typical urban land uses (Novotny 2003), which was explained
with the high variability of EMCs for different rainfall events. For this reason, when
estimating annual or seasonal loads, a site mean concentration (SMC) should be obtained.
It is the arithmetic mean value of the EMCs measured at one site. Commonly, a large
variation is observed when comparing SMCs from different sites, with specific land use
patterns, and this gives reason for the determination of site-specific unit pollution loads.
Various approaches and methods for pollution loads estimation exist; the differences
in most cases are based on the different methods for runoff estimation applied. For larger
catchments, it is advisable to apply computer models. For smaller catchment areas and
preliminary surveys, the simple procedure, described above, could be applied.
In general, pollution loads are linked to the total runoff volume generated, the storm
duration and intensity. Marshall (1997) concluded that the shorter and more intense
storms have the largest impact on receiving waters. The frequency of pollution carrying
runoff events is greatly increased in developed watersheds with higher imperviousness.
On the other hand, diffuse pollution loads from pervious lands (croplands, woodlands,
urban lawns and parks) occur only during very large storms with lower frequency. The
time since the last storm event is also important. First rains produce the most
concentration of pollutants, a phenomenon known as the “first flush” effect and is
dependent on the street cleaning practice and the public's attitude to waste disposal.
Considering the rainfall pattern in the region, it is highly advisable to convey specific
research for the determination of SMSs. It would be expected that a statistically-
significant difference could be found between EMCs, determined during the beginning of
the rain season, and those determined during typical wet season conditions, characterized
by high frequency rainfall events.
2.5
Pollution abatement measures
Measures to reduce pollution from diffuse sources in urban areas, referred also as Best
Management Practice (BMP), could be classified as structural and non-structural
measures. Structural measures usually involve engineering structures, which help to
detain and reduce diffuse pollution by different treatment methods before it is discharged
into natural water bodies. Measures concerning urban planning development and the
reduction of the percentage of impervious surfaces in general, improved practice of
street/ road design and construction, introduction of environmental buffer zones and
others, could also be classified as structural. Some examples were discussed in Chapter 2.
In cases of storms with a high erosive potential, the application of catch basin inserts
could be a viable structural measure to reduce and alleviate the diffuse pollution
associated with particulate material (Lau et.al. 2001), but such practice would require
proper maintenance and regular cleaning.
Non-structural measures are related to the “soft” part of the management process and
include such measures as the introduction of proper management policies and strategies,
improvement of the regulatory basis, involvement of all stakeholders, community
