Implementation of a 250-Year Stormwater Management Standard

In response to a major flooding event, and predicting the increasing occurrence of extreme-precipitation events due to climate change, in 2004 the City of Stratford, Ontario, created a City Wide Storm System Master Plan that prepared it for a 250-year storm event. Most other plans of this type only take into account the 1-in-100 year storm event, so Stratford’s plan is exceptionally ambitious. However, it may also be prescient. At the time it was written, climate projections were much less precise than they are now. Current projections indicate that once-in-a-century storms may soon occur five or even ten times in a century by the year 2100. Accordingly, the heightened protection offered by this plan will likely prove extremely beneficial in the decades to come.

Understanding and Assessing Impacts

Since about the 1970’s, Canadian cities have a adopted a two-tiered approach to handling stormwater. An underground system that conveys the precipitation from relatively frequent, low-intensity events and an above-ground conveyance system that handles water from less frequent but more extreme precipitation events. In most communities, this system is designed to be able to effectively convey water from a 100-year storm event. That is, the storm with the most rainfall that occurs in a 100-year span (statistically speaking). Even relatively common storm events, the 20-year or 50-year storm for example, can still cause modest disruption to daily life by inhibiting traffic and public transit due to ponding on city streets, among other minor complications. However, events that result in widespread damage and basement flooding should be relatively rare. However, the last few decades have shown that these events are more common than previously anticipated, and only promise to get worse as a result of global climate change. Some projections indicate that the previously 100-year storm event may occur five to ten times more frequently than in the past.

Identifying Actions

The 2004 City Wide Storm System Master Plan came about as a response to a major flooding event in 2002. Flooding caused by extreme rainfall in the area caused extensive basement in Stratford. The affected citizens felt that the city had been delinquent in its responsibility to protect its citizens and launched a class-action lawsuit. The citizens won the lawsuit and more than 800 homeowners were awarded damages of $7.7 million. Coincidentally, just prior to the 2002 flood, the City of Stratford had initiated a sanitary sewer master plan to analyze the sanitary collection system, identify problems, and suggest potential solutions. This plan indicated that the city would need to invest $35 million in priority sewer projects and another $16.5 million in strategic projects; furthermore, the Sanitary Sewer Master Plan indicated the need to conduct a stormwater management analysis. It is this analysis that would later culminate in the City Wide Storm System Master Plan.

Implementation

The 2004 City Wide Storm System Master Plan evaluated the performance of the existing storm system, reviewed and updated city drainage policies, and created a city-wide computer model. The plan also included a review of the city’s drainage policies and standards, the development of a system improvement strategy, the implementation of a sewer flow monitoring program, and the completion of a drainage system inventory. Further actions implemented as a result of this plan include assessments of storm sewer capacities, major drainage system flow, and ponding areas. As part of the development of the City Wide Storm System Master Plan, a study on Court Drain Subwatersheds was conducted and it was this specific work that lead to the new standard of using the 250-year storm as the baseline for flood preparedness. This required upgrading the existing infrastructure to be able to accommodate a roughly 15% increase in the manageable peak flows over the existing capacity. Stratford also established two incentive programs: one to replace old sanitary laterals in order to reduce inflow and infiltration into the sanitary system and another to assist with installing sump pumps for storm laterals to reduce the risk of surcharging storm mains.

Outcomes and Monitoring Progress

As of 2014, the City of Stratford had investing some $70 million in upgrading its systems in order to comply with this new standard. Accommodating surcharges from the system required the construction of stormwater management ponds, overland flow routes, and oversized trunk storm sewers. According to the case study, the City of Stratford is now a leader in basement flood prevention in Canada.

Resources


Understanding and Assessing Impacts

Since about the 1970’s, Canadian cities have a adopted a two-tiered approach to handling stormwater. An underground system that conveys the precipitation from relatively frequent, low-intensity events and an above-ground conveyance system that handles water from less frequent but more extreme precipitation events. In most communities, this system is designed to be able to effectively convey water from a 100-year storm event. That is, the storm with the most rainfall that occurs in a 100-year span (statistically speaking). Even relatively common storm events, the 20-year or 50-year storm for example, can still cause modest disruption to daily life by inhibiting traffic and public transit due to ponding on city streets, among other minor complications. However, events that result in widespread damage and basement flooding should be relatively rare. However, the last few decades have shown that these events are more common than previously anticipated, and only promise to get worse as a result of global climate change. Some projections indicate that the previously 100-year storm event may occur five to ten times more frequently than in the past.

Identifying Actions

The 2004 City Wide Storm System Master Plan came about as a response to a major flooding event in 2002. Flooding caused by extreme rainfall in the area caused extensive basement in Stratford. The affected citizens felt that the city had been delinquent in its responsibility to protect its citizens and launched a class-action lawsuit. The citizens won the lawsuit and more than 800 homeowners were awarded damages of $7.7 million. Coincidentally, just prior to the 2002 flood, the City of Stratford had initiated a sanitary sewer master plan to analyze the sanitary collection system, identify problems, and suggest potential solutions. This plan indicated that the city would need to invest $35 million in priority sewer projects and another $16.5 million in strategic projects; furthermore, the Sanitary Sewer Master Plan indicated the need to conduct a stormwater management analysis. It is this analysis that would later culminate in the City Wide Storm System Master Plan.

Implementation

The 2004 City Wide Storm System Master Plan evaluated the performance of the existing storm system, reviewed and updated city drainage policies, and created a city-wide computer model. The plan also included a review of the city’s drainage policies and standards, the development of a system improvement strategy, the implementation of a sewer flow monitoring program, and the completion of a drainage system inventory. Further actions implemented as a result of this plan include assessments of storm sewer capacities, major drainage system flow, and ponding areas. As part of the development of the City Wide Storm System Master Plan, a study on Court Drain Subwatersheds was conducted and it was this specific work that lead to the new standard of using the 250-year storm as the baseline for flood preparedness. This required upgrading the existing infrastructure to be able to accommodate a roughly 15% increase in the manageable peak flows over the existing capacity. Stratford also established two incentive programs: one to replace old sanitary laterals in order to reduce inflow and infiltration into the sanitary system and another to assist with installing sump pumps for storm laterals to reduce the risk of surcharging storm mains.

Outcomes and Monitoring Progress

As of 2014, the City of Stratford had investing some $70 million in upgrading its systems in order to comply with this new standard. Accommodating surcharges from the system required the construction of stormwater management ponds, overland flow routes, and oversized trunk storm sewers. According to the case study, the City of Stratford is now a leader in basement flood prevention in Canada.

Resources