Mitigating Impacts to the Drainage System

Before proceeding with the tasks in this grouping, the following tasks should have been completed:

MTO is primarily concerned with impacts to the highway infrastructure and the watercourse in the vicinity of the water crossing. However, MTO recognises that the property of riparian landowners located upstream or downstream of the crossing cannot be damaged as a result of the construction of the water crossing

Accordingly, the drainage practitioner will complete the following tasks in the group:

The MTO Regional Structural Section or Bridge Office may alter requirements presented in this document. The drainage practitioner must demonstrate that adverse drainage impacts to the highway right-of-way and upstream/downstream riparian landowners will not occur. The design of the crossing should be based on runoff conditions anticipated 20 years from the time of design, taking full account of present and probable future municipal controls over increases of runoff from new development.

Designing Erosion Protection Measures


With the introduction of a water crossing structure, modifications to the watercourse, may be required to accommodate the structure and to address the possible impact due to the changes to the watercourse.

The average flow velocity for a given cross-section area provides the conveyance available for the passage of a flood. However, its magnitude also reflects the potential of the flow to cause erosion and scour. It should be noted that, depending on a given flow situation, the local prevailing velocities may be much greater than the average calculated value, especially in the areas of local acceleration such as on the outside of river bends.

Documentation Requirements

The following information should be included:

  • Assessment of the erosion potential upstream, downstream, and through the structure.
  • Type of erosion protection method proposed.
  • The analysis conducted to determine the suitability of the erosion protection methods used.
  • Engineering drawings, schematics and illustrations showing plans and cross sections describing the design, location and layout of these measures. These should include dimensions, material type, anchoring and support of the material used.

Where modifications to a watercourse need to be accepted by, or subject to requirements of an operating authority or agency, the design criteria should be set in consultation with the authority or agency. The Hydrology Report must document all requirements and identify how they were accommodated. Where there are conflicts between MTO requirements and those of the other organisation, discussions should be undertaken to reach a resolution. The resolution to any conflict(s) should be documented.

Requirements set based on drainage management and environmental policies, guidelines and manuals of other regulatory agencies should be adhered to and documented. Should any design criteria, drainage management policy, guideline or manual of a regulatory agency conflict with a design criteria, drainage management policy, guideline or manual of MTO, or vice versa, a meeting between the parties may be warranted to resolve the conflict.

Where design criteria have been adopted based on a previous drainage study, these criteria should be identified. Refer to the section "Review Previous Drainage Studies" for more details.

For simple erosion protection works: If the proposed modification only involves simple erosion protection works such as lining material or rip-rap placement, the Hydrology Report need only document how the proposed method will provide the necessary erosion protection for the flow velocities at the reference points and range of frequencies.

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Design Fish Habitat Protection Measures


Fish habitat protection measures are a requirement placed by DFO or MNR/Conservation Authority on their behalf. They are intended to accommodate the passage of fish through a water crossing structure and to address the impact of changes to the channel in the vicinity of the structure. Habitat protection measures are commonly used in culverts however, some are applicable to bridges.

Flow velocities, occurring during the migration periods must be evaluated for the proposed structure. If velocities exceed those that will allow fish species to migrate upstream, then fish habitat protection measures must be considered.

Fish habitat protection measures may be installed to allow fish passage, and create or enhance habitat - this will mitigate loss of habitat due to encroachment into the stream or channel realignment. Habitat protection measures include:

  • Pools
  • Clusters
  • Riffles
  • Deflectors
  • Sills
  • Groins
  • Weirs

Refer to Chapter 5 Page 66 of the MTO Drainage Management Manual for more details.

Documentation Requirements

The Hydrology Report should document the requirements of the regulatory agencies and provide a listing of the alternative measures considered to address these requirements.

Once a preferred and agreed upon option has been identified, the rationale for the selection of that option should be documented. An option may include a combination of protection measures. At this stage the hydraulic impact of introducing these habitat protection measures should be completed and the analysis documented. The impacts could include but are not limited to the following:

  • Increased backwater
  • Increased erosion upstream and/or downstream
  • Changes in the flow regime and associated increased potential for occurrence of a hydraulic jump.

The increases in backwater effect and/or erosion should be mitigated either through redesign of the opening or by reducing the head loss through the structure. Thesummary of this analysis should be included.

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Address Impacts on Lands and Structures


A watercourse will flood the surrounding lands when the flow rate exceeds the capacity of the watercourse and the floodplain. The construction of a water crossing may aggravate flooding problems and could be the cause of increased impacts on surrounding lands and structures. Directive B-100, identifies the process to be followed to identify how the regulatory flood is to be accommodated in the design of a water crossing. Refer to Directive B-100 (PDF - 65 KB) for details. Consultation with the conservation authority, the municipality and effected landowners should be undertaken to ensure the proposed alternatives are acceptable.

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Documentation Requirements

The hydrology report should identify the lands and structures that will be impacted by the extreme flow events (100-year and regulatory), with and without the water crossing structure in place. This should include the following information for each structure:

  • Location.
  • Elevation.
  • A measure of the importance of the structure (social and economic value).
  • Extent of impact for each extreme event.
  • Means of protecting the structures and mitigating the impacts.

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