A methodology to predict the pollutant loads in combined sewer flow
In the design of urban drainage systems, synthetic design storms are commonly used to predict the peak flow rate in sewer systems and such storms are usually based on local intensity-duration frequency curves or design storm profiles. To estimate the quality of storm flow, the UPM Manual (1994) has highlighted the development of detailed and sophisticated simulation models to estimate the pollutographs, that is, the temporal variation in the concentration of pollutants in urban drainage systems. The data requirements of these models are quite onerous, and as a consequences implified models like SIMPOL have been developed. This model predicts the BOD at 1 hour time intervals and is based on the representation of the sewer system by a series of tanks. This approach may be considered satisfactory for the prediction of accumulative pollution over an annual series of events but for the prediction of acute effects, for example, the first foul flush, the temporal variation in the concentration pollutants in sewer flow is required. There is a need therefore to describe the change in pollution over a much smaller time interval than that proposed in SIMPOL and this is particularly so when consideration is given to the comparison of the design and control options which may be proposed, for example, the real time control of storage tanks to retain the first flush of pollutants. The work outlined in this thesis presents an alternate simple methodology to estimate the pollutographs corresponding to a particular storm event. The work is based on the results of the measured pollutographs recorded on the WRc sewer quality archive (1987) from two catchments at Great Harwood and Clayton-le- Moors in the North West of England. The relationships for the shape of the pollutograph were obtained by the direct comparison of the observed pollutographs. The peak TSS concentrations were obtained by a detailed regression analysis of the observed peak TSS concentrations, the antecedent dry weather period and the hydrological parameters of maximum rainfall intensity, average rainfall intensity and storm duration. These parameters were then related to the shape of the pollutograph and the results of this methodology were shown to satisfactorily reproduce results for the catchments considered. For practical applications, the suggested procedure provides a methodology to calibrate the design pollutographs for any catchment from a limited number of monitored storm events and to utilise these together with time series storms to assist in the performancea ssessmenat nd selectiono f alternative design options. The work has the limitation that it is catchment specific but as more information for different catchments becomes available, it may be possible to establish standard pollutographs for application to a wide range of catchment conditions.