Environmental Engineering Reference
In-Depth Information
relatively low, and in addition they are representative of the wet season of 2002-2003
only. Additional data will be needed to establish a more reliable relationship.
The data set distribution plot regarding actual measured COD values (Fig. 4.3b) shows
that the 95 percentile value is 218 mg/l, the 50-percentile value is 108 mg/l, and the
average value of 121 mg/l is close to the 60 percentile of the entire data set. It shows a
relatively normal distribution. However, it indicates that if the average values are used for
comparison with regulatory instruments, as it is the current practice, it actually means
that in 40% of the measured observations the prescribed limit has been exceeded. With
respect to Zn, the 95-percentile value is 0.07 mg/l, but the 88-percentile value is 0.04
mg/l, with an average value of the data set equal to 0.035 mg/l, which shows that it
represents more accurately the water quality during the period of observations. The need
for statistical interpretation of the data sets and the link with corresponding regulatory
instruments is discussed in more detail in Chapter 12.
It should be noted that the equations derived do not eliminate the need for a regular
water quality monitoring and its importance should not be underestimated. Similar
equations, derived for different parameters and sites, could be regarded as valuable tools
for planning purposes and the evaluation of the impacts of different scenarios, e.g. if the
improved streets pavements reduces the amount of TSS in runoff by 20%, this could lead
to a reduction of the COD concentrations by 10%. It is expected that in the future, larger
data sets and more reliable monitoring practice would allow for the derivation of more
accurate relationships, which could be used with a higher level of confidence during the
planning stage and also, could form the basis of the design methods for diffuse pollution
control and abatement.
3.5
Single storm event
The single storm event characteristics were examined at SP1, representing the most
densely populated drainage area with a high percentage of impervious surfaces.
Measurements were taken on 26
th
February 2003, during the peak of the wet season,
where intensive storm events were recorded diurnally, thus representing typical wet
weather conditions. Results are shown in Figure 4.4
The storm event peak concentrations of TSS, COD and TP (Fig.4.4a, b) were detected
one hour after the start of the rainfall, corresponding to the peak flow rates in the channel,
while TDS concentrations did not vary considerably. The same applies to nitrates (mean
value = 0.61 mg/l) and ammonia (mean value = 0.21 mg/l). This fact confirms that
organic pollution and to some extent, phosphorous, are bounded to solid particles, carried
with the flow during peak flow conditions, while soluble elements like ammonia, nitrate
and TDS have more or less constant concentrations during the whole storm duration. This
implies
