Response to a 1:100 Year Rainfall Event - City of Lethbridge 2002 Flooding Investigation
During a 65 h period in June, 2002 a total of 137 mm (5.4 in) of rain fell on the City of Lethbridge, representing a rainfall event greater than the City’s 1:100 y return frequency.
Overloaded sewers created surcharged conditions throughout portions of the system leading to sanitary sewer backups, and, in three locations, to sanitary sewer overflows (SSO).
A situation analysis of this event required the distribution of a flooding questionnaire to City residents that provided key insights into the performance of the system and aided the subsequent sewer performance evaluation.
The performance of the sanitary sewer system was analyzed though the use of the RT-SWMM hydrologic / hydraulic model, which is slightly modified from the USEPA’s SWMM to include additional model parameters to simulate dry weather flow sewage generation hydrographs for various land uses. Extraneous wet weather flows, or inflow and infiltration (I&I) into the sanitary sewer was simulated using SWMM’s Runoff Block. ARCview GIS was used to generate and present results from the modeling.
The sanitary sewage collection system model was calibrated to time-sensitive data that included Public Operations observations and measurements of manhole surcharging and overflows, wastewater treatment plant and lift station data, and results from the flooding questionnaire such as timing of sewage backups and basement flooding hydraulic grade lines.
The RT-SWMM model was used to simulate the performance of the existing sanitary collection system in terms of observed system behavior, customer complaints and claims in order to provide a basis and scale on which “at-risk of flooding” residences could be measured and compared to the City’s desired level of service - 99.5% protection from sewer backup flooding. The model was also used to quantify the benefit (or relief of at-risk residences) that the City’s planned sanitary system improvements would have provided to the existing system during this or a similar rainfall event. The impact of future City development was also considered in this analysis.
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