Survey on sewerage operation/management planning for flooding (II)

Under current design standard, sewers are designed to drain stormwater generated up to 10 year return period of storms. This implies sewer flooding could occur from rainfall exceeding a 10 year return period. 5, 10, 20 and 30 year return period of storm intensities were calculated for 22 locations (cities) of meterological stations over nation and compared to recorded rainfall intensities for last 30 years. The comparison resulted in numbers of year maximum rainfall intensities exceeded each return period. Using questionnaire survey for the incidences of flooding since 1980' of previous paper (Survey on sewerage operation/management planning for flooding (I)), actual rainfall records on date of flooding events were analyzed to demonstrate number of flooding events caused by exceedance of sewer capacity. For last 30 years, more than 6 years of year maximum rainfall intensity (20%) were larger than 10 year return period of storm in 4 cities of 22 used for first analysis. The number of rainfall records that exceeded 10 year return period was 50 of 260 actual flooding events investigated from survey.