Flooding investigation results

On Friday July 24, 2009, a major rainfall dropped significant rain across the city. While localized records are not available for the west end, some pockets of the City received in excess of 100mm of rain. The heavy rain exceeded the system’s normal operating capacity and led to localized street flooding and sewer backups in 23 different clusters around the City, predominantly in the Stittsville and Kanata, areas which were built before current design standards were in place.

Municipal sewer systems

The sewer systems where basement flooding occurred share similar design characteristics:

  • There are separate sanitary and storm sewers (or ditches) in flooded areas.
  • Storm sewers are sized to handle small and fairly frequent rainfalls (occurring every 2-5 years).
  • An installed basement backwater valve on perimeter foundation drain is the main line of defence when storm sewers are overwhelmed.
  • Depending on the class of road, road culverts are sized to handle major rainfalls occurring every 5-100 years.
  • There are significant differences between previous and current design standards in the communities most affected by flooding, including:
    • Installation of inlet control devices on storm sewers;
    • Provisions for overland drainage for large/infrequent rainstorm events;
    • Installation of backwater valves on perimeter foundation drains; and
    • Inflow reduction methods at all locations where there is potential for surface runoff to enter the sanitary system via a maintenance hole cover.

What happened

  • Significant rainfall exceeded the sewer system’s normal operating capacity. The inflow of storm water into sanitary sewers exceeded the capacity of the system, leading to a high occurrence of basement flooding.
  • Many homes were flooded because of surcharging of the storm sewers. Flooded basements then drained into the local sanitary sewer system via floor drains, which then drained into the Stittsville trunk sanitary sewer. The inflow of water into the sanitary system caused basements to flood. Excess water from the local system then overtaxed the Hazeldean pumping station and surcharged the Stittsville trunk sewer. This contributed to sanitary flooding in areas close to the Stittsville trunk sewer, especially in the Glen Cairn area.
  • Communities affected by basement flooding were built using standards that do not include provisions to deal with very intense and infrequent rainstorms.
  • The City’s water and sewer infrastructure functioned as intended.

Next steps

  • The City has retained Greenland Engineering to undertake an independent review of the investigation. Greenland Engineering will lead an Expert Panel of government and industry planners who will complete a review of flooding causes and flood avoidance solutions.
  • Flood assessments and solutions to reduce flooding are being identified and evaluated.
  • An action plan will be developed for the implementation of flooding solutions.

Preventive actions – Immediate

Residents will be encouraged to participate in the Residential Protective Plumbing Program. Protective plumbing methods range from the installation of sump pumps and reconfiguration of footing drainage to the installation of approved full-port backwater valves connected to the main City sewer. Through the RPP Program, the City offers homeowners in flood prone areas the opportunity to have their protective plumbing requirements evaluated and to receive financial incentives to offset the cost of protective measures that prevent basement and cellar flooding.

Preventive actions – Short Term

  • A thorough analysis of potential remedial strategies will be undertaken.
  • Implementation will happen as designs are completed and capital budget allocations are made.

Preventive actions – Long Term

A Wet Weather Management Plan is underway to identify areas of the City where improvements can be made to limit flooding.

Future residential developments

  • The Third Party Review of the Carp River Restoration confirms no significant impacts on water levels from developments, such as Kanata West or Fernbank, are forecast.
  • Development can occur without increasing risk of flooding to existing communities if:
    • Developments are built in accordance with current City sewer design standards and not “grandfathered” using pre-amalgamation standards;
    • All storm water management controls (for example, inlet control devices and storm water management ponds) are in place as soon as sewers are constructed;
    • Sanitary and storm services are sealed until basements under construction are covered; and
    • Backwater valves and clean out caps for laterals are closed tightly when basements are completed.
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