Impacts of Climate Change on Stormwater Management: Stormwater Management Plan Update

In 2010, CBCL Limited conducted an assessment of the effects of increasing rainfall intensities and the impact of these projected changes on the Town of Stratford’s Stormwater Management Plan. Stratford is a town located in Queens County, Prince Edward Island. Situated immediately southeast of the city of Charlottetown, the town is the third-largest municipality in the province and has experienced population growth in recent decades since improvements were made to the Hillsborough River Bridge connecting the town with Charlottetown. In 2003 a Draft Stormwater Management Plan was submitted by CBCL Limited for consideration by the Town of Stratford. At a meeting on October 29th 2009, the Town indicated that it had considered the alternatives presented in the 2003 report and expressed a desire to adopt a stormwater management plan consistent with a goal of sustainable development. Then in 2010, CBCL Limited conducted this study to assess the effects of climate change on rainfall intensity and the impact of these projected changes on the Stormwater Management Plan. The results of the assessment of potential changes in rainfall resulting from climate change indicates that rainfall intensities will likely increase. Predictions indicate that the maximum increases in rainfall intensities could be expected to occur in the next 10 to 40 years. It is expected that higher rainfall intensities will result in higher peak runoff flows in the drainage systems from existing development and future development of currently undeveloped land as well as redevelopment of existing developed lands. Recommendations for improving the Stormwater Management Plan to accommodate climate change are provided, such as, but not limited to: built infrastructure upgrades/modifications, flood warning and flood preparedness systems, and flood risk mapping.

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Understanding and Assessing Impacts

In 2003 a Draft Stormwater Management Plan was submitted by CBCL Limited for consideration by the Town of Stratford. At a meeting on October 29th 2009, the Town indicated that it had considered the alternatives presented in the 2003 report and expressed a desire to adopt a stormwater management plan consistent with a goal of sustainable development. Then in 2010, CBCL Limited conducted this study to assess the effects of climate change on rainfall intensity and the impact of these projected changes on the Stormwater Management Plan. The overall plan for the study included the following main tasks:

  1. Import the USEPA SWMM5 model developed in the original study in 2003 and updated in the 2010 Study into the Stormnet computer model owned by the Province;
  2. Re‐run the updated models for a range of design rainfall events based on predicted increases in peak intensities related to climate change; and
  3. Use this data to update the figures and tables from the 2010 study report that are associated with the Town’s selected approach to stormwater management and are impacted by changes in rainfall intensity.

The study assessed the impacts of changing climatic conditions for 3 future periods centred on the years: 2020, 2050 and 2080 and for return periods of 10, 25, 50 and 100 years. The Climate Change Coordinator for the department reviewed the data prepared by Environment Canada and with assistance from the Canadian Climate Change Scenarios Network (CCCSN www.cccsn.ca), performed model validation for several global Climate Change models. Observed climatic data for the period of 1961‐1990 was input to 30 models and the resulting average rainfall was compared to the records at the Charlottetown Airport station. The assessment of potential changes in rainfall resulting from climate change indicates that rainfall intensities will likely increase. Whether intensities of all durations will increase to the same degree, the magnitude of the increases and the timing of the increases are uncertain. Please refer to the assessment report for a completed detailed description of the methodology used to design and rainfall intensities. It is expected that higher rainfall intensities will result in higher peak runoff flows in the drainage systems from existing development and future development of currently undeveloped land as well as redevelopment of existing developed lands.

Identifying Actions

Using the results of the rainfall intensity projections, a recommended approach in the stormwater management plan for addressing the impacts of climate change is recommended. Recommendations are provided in three key areas:

  1. Existing drainage systems (minor drainage systems and major drainage systems)
  2. Design rainfall events
  3. New developments

For example, for existing systems, the Town should develop a prioritized list of modifications to existing structures in each watershed. The report recommends that a new set of design rainfall events should be adopted by the Town as the basis of future stormwater management. And new development in the Town should not increase peak flows in the major drainage systems.

Implementation

The report provides recommendations for the implementation of measures for existing drainage systems. For example, for major drainage systems, it is recommended that the Town completes a master drainage plan and a flood delineation assessment for each watershed in the Town, starting with the watershed (s) where the potential risk of flood damage is greatest. An identification of the flood limits generated by the design rainfall event with a 1 in 100 year return period on Town Land Use Mapping is also recommended. As discussed in section 2.2.1 of the report, predictions indicate that the maximum increases in rainfall intensities could be expected to occur in the next 10 to 40 years. In order to be sure that the upgrades are in place in time for the occurrence of these maximum flows, construction should be completed in the next 10 years.

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