the impacts of watershed urbanization on flood risk · 2015-05-26 · scenario 3: full build -out =...

The Impacts of Watershed Urbanization on Flood Risk Ryan Ness Toronto and Region Conservation Authority November 14, 2012

Game Plan

1. Review of Ontario stormwater and floodplain management policy

2. Policy limitations, conflicts and disconnects

3. Analysis: Humber River Watershed (Ron)

4. Moving forward at TRCA

TRCA Jurisdiction: Size: 3500 km2 Population: 4 million

Floodplain Management and Stormwater Quantity Management

Policy in Ontario

Ontario Floodplain Management Policy

“Development will generally be directed to areas outside of … hazardous lands adjacent to river and stream systems which are impacted by flooding and erosion hazards” Provincial Policy Statement on Land Use Planning and Development (2005)

Regulatory Events

Hurricane Hazel

Flood Hazard Mapping

Stormwater Quantity Management “Policy”

“Generally, accepted criteria are that maximum peak (site) flow rates must not exceed pre-development values for storms with return periods ranging from 2 to 100 years” Ontario Stormwater Management Planning and Design Manual Ministry of Environment, 2003

Stormwater Quantity Design Approach

• “2- to 100-year post-to-pre”

• Control to existing conditions or unit criteria

• Synthetic design event-based

• Local analysis

• Cumulative, watershed scale performance?

The solution (?)

Policy Limitations, Disconnects and Conflicts

Humber River Watershed Development Scenarios Scenario 1: Existing (2002) = 26% Urban Scenario 2: Approved OP = 36% Urban Scenario 3: Full Build-Out = 49% Urban

Peak flows increase even with detention

April 10-20 1992 - Main Humber Upstream of Woodbridge

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4/11/92 4/12/92 4/13/92 4/14/92 4/15/92

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RainfallScenario 1

Scenario 2Scenario 5

1986 2009

Regulatory floodplain expands

Limitations

• Stormwater quantity control is not a legal or policy requirement

• Detention facility design is highly simplified

– Synthetic design events

– Single event analysis

– Cumulative performance generally not assessed

Disconnects

• Different agency responsibilities

– Floodplain management: Conservation Authorities

– Stormwater management: Municipalities

• Stormwater quantity control typically limited to 100-year vs. much larger regulatory flood events

• Stormwater management design rarely examines cumulative effects on watershed flood flows

Conflict

“Stormwater management facilities may not be used to provide any reduction in flood flows” River and Stream Systems: Flooding Hazard Limit Ontario Ministry of Natural Resources (2002)

Implications

• Upstream development can increase real flood risk downstream for the regulatory event and for lesser events

• Expanded regulatory floodplains restrict downstream development and redevelopment potential

• Stormwater management facilities cannot be recognized in the calculation of regulatory flood flows and floodlines

• No policy-compatible solution is currently available in Ontario

Case Study: Humber River Watershed

• ~900 km2, 30% urban land uses

• Anticipated future development to near 50% urban

• Preliminary modelling suggested significant flood flow impacts from new development , regulatory and otherwise

• Pending approvals for major development

• What to do?

– Step 1: Assess the extent of the problem

– Step 2: Determine the form of a stormwater detention solution

Ron’s Presentation …

Moving Forward (continued)

Moving forward at TRCA

1. Proactive analysis of potential impacts in developing watersheds

2. Assess stormwater detention requirements

3. Review of alternative management options

4. Provincial policy harmonization

5. Developing updated technical requirements

3. Alternative Management Options

• Tableland stormwater detention facilities

• Flood remedial works

• Land acquisition

• Online control structures

4. Provincial Policy Updates

• Firm policy requiring stormwater management for flood risk mitigation

• Allowing SWM facilities (tableland or online) to be reflected in flood hazard delineation

• Assign agency responsibility for oversight and review of all aspects of flood hazard management including SWM

• Clear definition of tolerances for flood risk increase in affected downstream communities

5. Updated Technical Requirements

• Stormwater management design procedures that consider “real world” conditions and cumulative effects

• Design standards for stormwater management facilities that provide control for regulatory flood flows

• Standards for hydrologic modelling standards

Take-aways

• Expanding urban development can increase the regulatory flood risk in existing communities

• Conventional stormwater management does not prevent these impacts (and it may not be effective for smaller flows either)

• Current provincial policy does not prevent these impacts (in fact it eliminates the best option)

• Addressing the problem requires:

– New, proactive approaches to modelling and impact assessment

– Supportive provincial policy that connects floodplain management and stormwater management

– Big, big detention ponds?

Thanks to …

• AMEC

– Ron Scheckenberger

– Aaron Farrell

– Matt Senior

– Vahid Taleban

• TRCA

– Michael Heralall

– Shahzad Khan

– Fabio Tonto

Ryan Ness Manager, Research and Development Toronto and Region Conservation Authority rness@trca.on.ca

Thank You

PPS

• 3.1.6 Further to policy 3.1.5, and except as prohibited in policies 3.1.2 and 3.1.4, development and site alteration may be permitted in those portions of hazardous lands and hazardous sites where the effects and risk to public safety are minor so as to be managed or mitigated in accordance with provincial standards, as determined by the demonstration and achievement of all of the following: – development and site alteration is carried out in accordance with

floodproofing standards, protection works standards, and access standards;

– vehicles and people have a way of safely entering and exiting the area during times of flooding, erosion and other emergencies;

– new hazards are not created and existing hazards are not aggravated; and

– no adverse environmental impacts will result.