Technical Guide - River and Stream Systems: Flooding Hazard Limit and Technical Bulletin - Special Flooding Hazard Conditions in River and Stream Systems

Protecting People and Property: Ontario’s Flooding Strategy commits the province to updating existing technical guidance used to support implementation of the natural hazard policies outlined within the Provincial Planning Statement (PPS 2024). Most existing technical guidance provided by the Ministry of Natural Resources (MNR) was created between 1996 and 2002 and does not reflect updates in science, technology, and the use of mitigative measures since that time. The MNR is taking a multi-staged approach to updating its guidance.

The purpose of this posting is to seek feedback on a proposed updated technical guide and new companion technical bulletin that would update and replace MNR ’s Technical Guide - River & Stream Systems: Flooding Hazard Limit (2002), other than appendices. They include:

1. Technical Guide - River and Stream Systems: Flooding Hazard Limit; and
2. Technical Bulletin - Special Flooding Hazard Conditions in River and Stream Systems  

Proposed updated guidelines are intended to provide a consistent approach to how flooding hazards are identified across the province and help to mitigate risks to people, property, and communities from flooding. 

Proposed Updated Guidelines

1.  Technical Guide - River and Stream Systems: Flooding Hazard Limit

The current Technical Guide - “River & Stream Systems: Flooding Hazard Limit” (2002) is dated. It includes references to obsolete modelling software, and to a lesser extent, standard practices. 

Proposed updates to the technical guide focus on the sections associated with hydrology and hydraulics, to account for advances in technology and methodologies. 

With regards to hydrologic modelling, proposed updates include recommended technologies and modelling protocols, practices and platforms for informing water level calculations and flood line delineation, addressing the types of hydrologic models, model selection and calibration. 

With regards to hydraulic modelling, proposed updates include improving the suitability of hydraulic analyses for estimating water levels throughout a wide range of rivers and river characteristics within Ontario. New guidance to support choosing a hydraulic modelling technique, and direction around the use and applicability of 2-dimensional modelling consistent with Ontario’s policies, including guidance and standards for use and interpretation is provided. Guidance and requirements related to hydraulic model calibration, testing and sensitivity analysis are also included, as is additional guidance on evaluating uncertainty in model parameters and the resulting impacts on model simulations and the associated range of error in modelled results. 

Proposed updates have been developed by the province with input and feedback from a multi-partner flood mapping technical team, consisting of staff from provincial and federal government agencies, municipalities, and conservation authorities. 

2. Technical Bulletin – Special Flooding Hazard Conditions in River and Stream Systems

As an overall principle for flood management, the MNR prioritizes the use of non-structural and land use planning measures as its preferred approach to manage flood risks.

Specifically, the MNR’s current Technical Guide – River and Stream Systems: Flooding Hazard Limit (2002) does not support using structural measures such as dikes and flood walls as permanent forms of flood protection. Such structures have historically been opposed based on concerns that such measures can increase upstream flood levels and downstream flows and velocities, can be overtopped, and often inspire a false sense of security for additional development in hazardous areas. In addition, structural measures can be associated with high costs during construction, and in perpetuity, and can thereby add to the existing municipal infrastructure deficit. As a result, the current technical guide does not provide methodologies for considering structural measures in modelling and mapping flooding hazards. 

As part of his review, Ontario’s Special Advisor on Flooding identified two types of structural measures that are increasingly being used in Ontario to mitigate risks posed by flooding:

• regional flood control facilities are being used to provide for a reduction in flood flows when establishing the limits of the Regulatory floodplain; and
• flood protection landforms are being used as a form of permanent flood protection.

This new technical bulletin builds on the Special Advisor on Flooding’s findings and proposes recommendations related to the use of structural measures in flood mitigation and mapping. It is a companion guidance document to the Technical Guide – River and Stream Systems: Flooding Hazard Limit, and provides updated guidance on:

• when and how flood mitigation structures are to be considered in flood hazard mapping, including dams, dykes, berms, flood walls and flood protection landforms.
• how other special flood hazard scenarios should be modelling and mapped.

Regulatory impact analysis

The Ministry does not anticipate any direct environmental or social consequences or costs with the technical guidance updates. 

The proposal, which provides updated technical guidance on hydrologic and hydraulic methodologies used to identify areas along rivers and streams that maybe subject to flooding, is not expected to increase the costs of work for municipalities.  The updates within are meant to be in line with existing best practices, which are currently being used by many practitioners.