Lessons from application of geoenvironmental policies regulating development

Tudor Botzan

In the proceedings of: GEO2011: 64th Canadian Geotechnical Conference, 14th Pan-American Conference on Soil Mechanics and Geotechnical Engineering, 5th Pan-American Conference on Teaching and Learning of Geotechnical Engineering

Session: Geoengineering for Development & Education

ABSTRACT: Toronto and Region Conservation Authority (TRCA) regulates development in areas prone to geoenvironmental hazards using a set of specific policy provisions. These provisions were formulated taking into account conditions occurring along the watershed valleys and stream corridors. Such conditions include: the size and geometry of valley slopes, evidence of erosion and the recession in time of the top of slope, as well as the setback of slope crest from proposed development. In reviewing the permit applications for development, TRCA staff is looking at the determination of the long-term stable top of slope based on guidelines provided by the Ontario Ministry of Natural Resources. The paper presents lessons extracted from this direct experience acquired in the permitting process.

RÉSUMÉ: L applique des provisions régulatrices concernant la construction dans les zones naturellement hasardeuse. Ces provisions ont été formulées en considérant les conditions rencontrés sur les vallées des rivières et dans les ravines. Les conditions regardent les formes de relief, la du pante et la construction proposée. En revoyant la documentation pour le permis de construire, le persle calcul pour la ligne stabile de la crête du pante basé sur la guidance fourni par le Ministère des Ressources Naturelles. La communication présente les leçons appris pendant le procès de revue des permis. 1 TY ROLE Under the Conservation Authorities Act, a permit is required for waterway alteration, grade change or construction in natural hazard areas. TRCA is one of the local environmental agencies managing such permits, through Ontario Regulation 166/06. The TRCA jurisdiction covers over 2400 square kilometers, including the City of Toronto as well as part of the Regional Municipalities of Peel, York and Durham (Figure 1). Figure 1. Toronto & Region Conservation Authority jurisdiction (TRCA IT/GIS Group) With the intensification and expansion of urbanization along valley lands and stream corridors, the risk of exposure to natural hazards has increased tremendously in the past few decades. In this light, the TRCA Planning neation of the developable limits in the proximity of stream corridor slopes and ravines. Using technical knowledge, the TRCA staff has developed policies and criteria to support the regulatory provisions. 1.1 Geoenvironmental Policies and Criteria Provisions regarding the geoenvironmental aspects and criteria are presented in several chapters of the TRCA policy document (TRCA, 1994). They reflect conditions such as occurrence, geometry and size of landforms in the proximity of proposed development. Some of those conditions trigger the need for a geotechnical stability assessment. Two of the criteria considered in this context are the height and the steepness of the valley or ravine slope. Of particular essence in determining the developable limits is the long-term stable top of slope (LTSTOS) line. The LTSTOS is the imaginary projection, over a 100-year span, of the existing top of slope (ETOS). Once the LTSTOS line is determined, developable limits are delineated by applying the 10-metre buffer policy provision (TRCA, 1994). A guideline issued by the Ontario Ministry of Natural Resources (OMNR, 2002) is utilized to determine the LTSTOS. Two types of allowance described in the OMNR Guideline are of interest to the TRCA review team: toe

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