DESIGN OF STORMWATER INFILTRATION FOR REDUCTION OF COMBINED SEWER OVERFLOW (CSO)

Stormwater infiltration is a significant tool for combined sewer overf low abatement because it involves a decrease in the impervious area co nnected to the sewer system. When allowing the infiltration structures to overflow into the existing sewer it is shown that for a required r eduction in CSO-volume there exists an unambiguous relation between th e infiltration structure volume and the size of impervious area connec ted to infiltration. Further, the presence of an optimal solution mini mizing the total trench volume is pointed out. For a Danish sewer syst em with a travel time of 30 min and an interceptor capacity of 0.2 mum /s the optimal solution for reducing the CSO-volumes by 40 percent inv olves connecting 65 percent of the impervious area to infiltration tre nches with a total storage volume of 3.6 mm. This corresponds to desig ning the trenches according to an exceedence return period of 0.04 yrs compared to the commonly applied design return periods of 2 to 10 yrs .