Environmental Engineering Reference
In-Depth Information
• The contribution from sediments or other residues along the streambed.
The pollution flux at a given location will be equal to the product of the measured flow
rate and the corresponding value of the pollutant constituent. During a specific rainfall
event, the pollutant constituents, associated with transported suspended solids, will vary
during the duration of the hydrograph. Therefore, it is advisable that the pollution fluxes
during such events would be calculated based on EMCs, which would reflect not only
storm water pollution constituents, but also those contributed from point sources
upstream and from riverbed sediments. For a given storm duration, the variation of the
pollutant constituent concentration would be due to the runoff quality mainly. The
contribution from the other two sources could be accepted as constants. A schematic
presentation of the formation of a pollution flux at a given location in a stream or river is
shown in Figure 5.5.
The total pollution flux during the rainfall event (PF
e
) could be calculated as the
product of the total volume of flow for the event duration and the EMC for the
corresponding constituent. During dry periods, the total flux variation would be due
mainly to the variation of the pollution load contributed from the point sources upstream
the location and from those present along the streambed.
An accurate estimation of seasonal and annual pollution fluxes would be equal to the
sum of all diurnal fluxes during the anticipated period of time. If a monitoring station is
located immediately before the confluence point of a river (stream) into a lake, then the
pollution fluxes evaluated would be equivalent to the pollution load contributed to the
lake. In cases of automatic monitoring stations, a considerable amount of data might be
available for accurate evaluation of actual pollution fluxes. However, in most cases of
regular monitoring, measurements are performed once or twice per month, which do not
allow an accurate estimation, and assumptions with respect to averaging available data
should be made, which might lead to errors in the estimated loads. In this respect the
remarks made in Chapter 4 and in section 2.3 of this chapter should be considered.
4.2
Estimating urban runoff volumes by the SA-SCS method
The evaluation of pollution loads and fluxes generated by runoff requires quantification
of rainfall excess. A simple procedure for such a type of estimation was described in
Chapter 4. In this section, a more detailed description of the SA-SCS method (Shulze et
al. 1993) has been done. In order to illustrate the different steps and procedures, the
methodology applied for the specific case of Kuwadzana, during this specific study will
be presented in more details.
The first step of the procedure was the evaluation of several key drainage area
characteristics. The first characteristic that was determined was the drainage area size,
based on the topographic series maps of Harare TR8025 and TR8027. The other
characteristics were the shape of the drainage basin for each
