Hurricane papers Hurricane Hazel and Extreme Rainfall In Southern Ontario[PDF] Cumming Cockburn Limited November 2000 ICLR Research Paper Series – No. 9
This case study was commissioned by the Institute for Catastrophic Loss Reduction to estimate the losses resulting from an extreme rainfall event similar to Hurricane Hazel affecting the area from Burlington to Ajax/Whitby in southern Ontario. Cumming Cockburn Limited focused on direct physical flood and water damage losses primarily to residential structures. Direct physical losses to infrastructure, crops, vehicles (plus indirect economic, human suffering, e.g., injury and fatality losses caused by flooding and wind) were not included in the study.
The October 1954 Hurricane Hazel tropical rainstorm dumped over 280 mm. (a total volume of 3.71 cubic kilometers) of rain in a 48-hour period and caused 81 deaths. It was the highest level of precipitation recorded in a 12-hour period anywhere in Ontario up to that time. The rainstorm, centred over the Humber River Watershed, affected a 30,000-square-kilometer area. It is unlikely, but possible, that a similar large rainstorm events could occur again at any time.
A survey of the key personnel at Conservation Authorities (Toronto & Region, Central Lake Ontario, Credit Valley and Halton Region) and Municipalities (Burlington, Milton, Oakville, Mississauga, Etobicoke, North York, Toronto, Scarborough, Pickering, Ajax and Whitby) in the study area was undertaken to obtain background information to estimate the potential for general water damage to basements and flood damage to structures located in flood hazard areas along streams and rivers.
The estimated total river related flood losses which might be associated with a Hurricane Hazel-type event moving through the study area was $240 million. It was suggested that this potential loss was significantly lower as a result of the excellent floodplain management adopted and enforced over the last 30 – 40 years by Conservation Authorities in order to prevent development and increased flood losses in these high-hazard areas.
In general, the survey revealed that the municipalities with a lower original design capacity, direct basement connections to storm and/or combined sewer systems had a higher potential for the number of structures to be affected by flooding during a severe tropical storm like Hurricane Hazel. The analysis indicated potential widespread basement flooding could exceed $400 million. The total flood and water damage loss in the study area could therefore exceed $640 million (not including other direct and indirect losses.
The potential for direct physical flood and water damage losses has grown in direct proportion to population growth. There is no reason to think that, without some intervention, that these losses will not continue to increase in the future as population continues to grow. The increased concentration of the population in urban centres will also lead to higher losses in the future. Losses will increase as the given the higher value of homes and household possessions. For example, in recent years an increased amount of high-value possessions are kept in basement recreation areas (televisions, computers, electronic entertainment centres, etc.), resulting in a significant increase in the potential for damages due to basement water damage. Also, as the infrastructure of urban centres continues to age, blockages can occur in the storm and sanitary sewers.
The proportional increase in potential flood damages along rivers and streams is likely somewhat less today than in 1954. This can be attributed directly to the excellent program for flood-proofing or prohibiting the re-building of flood-damaged structures in flood hazard areas, and to zoning regulations preventing new development in flood prone areas.
Significant flood events similar to Hurricane Hazel will recur. For example, Hurricane Floyd in September 1999 might be regarded as a “near miss”. Other large events will continue to occur, exceeding existing system design capacities on a random basis (i.e., similar to the 1989 Harrow Storm – 450mm of rain, etc.).
On the other hand, the legacy of Hurricane Hazel has significantly reduced the potential for riverine-related flood damages in the study area and across Ontario. Since 1954, the implementation of flood plain management policies restricting development or re-building of flood damaged structures in flood hazard areas and flood proofing programs have helped reduce direct physical flood losses. These policies should continue to be enforced, and serious attention should be given to adopting similar zoning policies in other municipalities across Canada in order to reduce flood losses.
With respect to municipalities, a given level of design for the drainage system (sanitary or storm) always has an associated risk of exceedance, which can be estimated over a selected time period (e.g., over the lifetime of the infrastructure). The risk of system failure varies with design level. The design criteria have been found to vary with time and location in the study area. The selection and application of consistent design criteria would provide a common level of risk for the insurance industry. Some relevant elements of the design criteria include:
♦ Adoption of best management practices for storm water management. ♦ Similar design levels of protection (e.g., 5-year for the minor system). ♦ Adoption of consistent technical and calculation design procedures, which should include consideration of the potential impact of climate change.
Current problem areas could be addressed by implementing programs to improve the storm water management capacity of the existing infrastructure by:
♦ Replacing combined sewers with separate sanitary and storm sewer systems. ♦ Upgrading existing drainage systems (e.g., storage systems, inlet restrictions and retrofitting with backwater valves, etc.) ♦ Eliminating weeping tile connections to storm sewers. ♦ Discharging roof leaders to the ground surface instead of to the sewer system. ♦ Increasing regular maintenance and inspection programs to avoid drainage system blockages; especially in older developments.
Information on flood and water damage losses should be assembled annually in a consistent format by a single agency. The information will help to confirm flood and water damage losses by type of severe precipitation, level and intensity of precipitation and other event characteristics. This information base will assist in the identification of problem areas and the prioritization of long-term flood damage mitigation programs.
Additional funding should be provided to Municipalities, Conservation Authorities and other key agencies to increase staffing and to help undertake flood damage mitigation programs such as relief sewer systems, updates to floodplain mapping, land acquisition in flood hazard areas, and flood control facilities, etc.