CHANGE OF WATER LEVEL AND ELECTRIC CONDUCTIVITY USING MONITORING DATA AT COMBINED SEWER PIPES IN TSURUMI RIVER BASIN

Abstract

 Urban flood damage caused by heavy rainfall during storms has been increased and therefore the effective use of existing flood control facilities is important. However, we have limited understanding of how the sewer systems, which design is based on the rational formula, would work against the heavy rainfall which is not uniform in space and time. Furthermore, the environmental pollution due to the untreated sewer overflow from the combined sewer systems during rainfall events is a serious social concern, but it is difficult to understand the behavior of the pollutant materials since it is practically difficult to monitor them.

 In this study, we installed the water level and electric conductivity (EC) sensors in three combined sewer trunk pipes near the pump stations in the Tsurumi river basin. From the monitored water level, we found that the overflow to the storm-water storage pipes occurred when the intensity of rainfall exceeds approximately 22-27 mm/h. From the relationship between water level and rainfall intensity, the equation of linear regression was estimated with the coefficient of determinations more than 0.85. This equation allows us to presume the occurrence of overflow to the storage pipes with given rainfall intensity. In comparison with the average dry weather wastewater (EC: 500-800 μS/cm), it was clear that the monitored EC values of less than 200 μS/cm was due to the dilution by storm water in combined sewer systems, but the dilution level differed among the sub-catchments. Comparisons of two rainfall events with the different antecedent dry weather period showed that EC data is useful for estimating the contribution of the first flush on the pollutant load from the time series of EC variation linked with the water level rise in the trunk pipes.