Inland Flood Risk Mapping and Modelling, Hillsborough River Basin, Prince Edward Island

In 2014, AMEC Environment and Infrastructure undertook a flood risk assessment for the Province of Prince Edward Island’s (PE) Department of Transportation and Infrastructure Renewal (DTIR) to identify infrastructure vulnerable to risks of flooding and to recommend adaptation measures for the Hillsborough River Drainage Basin (the Basin). With approximately 350 km2 drainage area, the Hillsborough River watershed is the largest on the island, equal to one-fifteenth of the total island land mass. With the exception of the City of Charlottetown and the Town of Stratford, the Basin is a predominantly rural area with farming as the dominant economic activity. There have been numerous flooding events in PE in the past decade resulting from both storm surge and heavy rain. This project aims to provide provincial officials with information on how to conduct long-term adaptation planning and make the necessary changes to rural plans and infrastructure programs to address climate change adaptation in a rural environment. This report provides an overview of the development of projected intensity-duration-frequency curves (IDF curves) for the Basin. The objective of the work is to develop IDF curves that reflect the changes in the characteristics of precipitation that might be caused by projected changes in climate for three time periods: 2025, 2055 and 2085. By adopting appropriate climate change adaptation strategies, the provincial government and the local governments in the Hillsborough River Basin can mitigate the impacts of more frequent and higher intensity precipitation events on storm water management systems and potential flooding of lands serviced by this infrastructure; and sea level rise and storm surge on existing built form and future development.

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Understanding and Assessing Impacts

With approximately 350 km2 drainage area, the Hillsborough River watershed is the largest on Prince Edward Island, equal to one-fifteenth of the total island land mass. There have been numerous flooding events in PE in the past decade resulting from both storm surge and heavy rain. A major storm surge and flooding event occurred on January 21, 2000 which caused the maximum water level in Charlottetown to be exceeded by almost 40 centimetres. As the storm passed through, massive chunks of ice piled up against the shore and the sea rushed in, flooding street and buildings. Hurricane Juan of September 2003 also caused significant flooding and damages. This case study report provides an overview of the development of projected intensity-duration-frequency curves (IDF curves) for the Basin. The objective of the work is to develop IDF curves that reflect the changes in the characteristics of precipitation that might be caused by projected changes in climate. Three time frames were adopted for projections: 2025, 2055 and 2085. This report documents the methodology and findings from each of the tasks. To obtain projected IDF curves, the precipitation intensities in the historical IDF curves were adjusted to reflect projected changes in climate using a statistical modeling technique that is described in detail in the report. Three data sets were used in this work: the local extreme precipitation event data; local monthly climate data; and future projected data from the Global Climate Models.

The results of the assessment show that the Basin is subject to a series of impacts, such as, flash floods resulting from overland flow due to intense precipitation events; and permanent high water levels due to relative sea level rise. Up to 75% of the main stem of the Hillsborough River is subject to tidal influences, and susceptible to flooding resulting from the combined effect of high tides and storm surges. The projected sea level rise and land subsidence in the future may cause an increase in flooding water levels in the flood prone areas.

Identifying Actions

The scope of work comprised five tasks as follows:

  • Task 1: Physical Survey;
  • Task 2: Climatic Analysis and Heavy Rainfall Projections;
  • Task 3: Flood Risk Mapping and Modeling;
  • Task 4: Infrastructure at Risk Assessment; and
  • Task 5: General Governance Assessment.

As a result of this work, six main adaptation recommendations were developed:

  1. Local governments and the Government of PE develop a policy framework for adapting to and mitigating the effects of climatic change in the Hillsborough River Basin.
  2. The flood protection standards be reviewed and upgraded.
  3. A detailed review of all applicable by-laws and planning instruments governing flood protection be conducted in light of the findings from this study.
  4. Local governments and the Government of PE develop a strategy and policy framework for addressing current and potential future developments in the flood prone areas.
  5. Local governments and the Government of PE collaboratively develop a warning and Emergency Response Plan for a potential high flooding event in the Hillsborough River Basin.
  6. A technical bulletin be prepared that summarizes the design information presented in this report and made available to the engineers and planners engaged in civil and municipal design, to ensure future developments meet adequate standards.

Implementation

Section 7 of the report describes a range of specific recommended adaptation measures from a governance perspective to non-structural strategies. Examples include but aren’t limited to: use of information technology tools such as GIS, flood models, warning systems, and mapping; flood plain mapping, hydrologic and engineering studies, insurance, information and education; flood insurance and tax adjustments; disaster preparedness and response; and post–flood recovery assistance. By adopting appropriate climate change adaptation strategies, the provincial government and the local governments in the Hillsborough River Basin can mitigate the impact of: more frequent and higher intensity precipitation events on storm water management systems and potential flooding of lands serviced by this infrastructure; and sea level rise and storm surge on existing built form and future development. A list of general considerations was also provided, which included, for example: Ensure that floodplain management goals are consistent with community-wide planning and development goals and guidelines; Review the municipal and rural plans to develop policies specifically addressing climate change adaptation (and mitigation); and Educate the citizenry about potential future flooding and climate change adaptation processes. The report recommends that provincial government and local governments in the Basin consider a multi-tiered approach comprising administrative measures, civil works programs and public communication. This would involve developing and adopting a comprehensive climate change adaptation strategy that integrates planning instruments, development controls and emergency response measures.

Resources

Link to Full Case Study