Stormwater Management Requirements for Land Development Proposals

Mitigating Impacts to the Receiving 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 drainage system. Wherever stormwater runoff discharging from the proposed land development may impact the highway drainage system, impacts to the highway right-of-way should be assessed, and the capacity of the highway drainage systems must be checked.

However, MTO recognises that the property of riparian landowners located upstream or downstream of the highway right-of-way cannot be damaged by stormwater runoff discharging from the proposed land development. Even though this responsibility is within the mandate of the regulatory agencies, MTO may become liable if the stormwater runoff from the proposed land development is conveyed through a highway drainage system and damages any riparian property located upstream or downstream of the highway right-of-way.

During the analysis of the receiving drainage system, impacts were identified and it was determined that a method of mitigation is required by MTO to mitigate the impact. Impacts may be mitigated by:

The MTO Regional Highway Planning and Design Office or the local MTO District Office may alter requirements presented in this document, since an extensive SWM report is not always required. MTO may make provisions to accept a drainage impact analysis that has a lower level of detail associated with it, provided that the proponent submits a plan showing how stormwater runoff from the proposed land development will be conveyed to the receiving drainage system. The proponent must be able to demonstrate that drainage impacts to the highway right-of-way or upstream/downstream riparian landowners will not occur, and that the capacity of the highway drainage system will not be exceeded as a result of stormwater runoff discharging from the proposed land development.



Providing Stormwater Management Controls

If impacts have been identified such that stormwater management controls may be required to mitigate these impacts, the SWM report must provide documentation on the following areas.

Documenting the Need for Stormwater Management Quantity Control

The need for stormwater management controls originates from the need to mitigate impacts that the proposed development may cause to the receiving drainage system. The local municipality, conservation authority, MNR, MOE, and the DFO may also require stormwater management quantity control. These organisations should be consulted in conjunction with MTO.

When the need for stormwater management quantity control originates from a previous drainage study, refer to Reviewing Previous Drainage Studies.

Documenting MOE Requirements for Stormwater Management Quantity Control

Stormwater management quantity control requirements for land development proposals are stated in the document “Stormwater Management Planning and Design Manual” (MOE 2003).

Where MOE requirements are set in a previous drainage study, refer to Reviewing Previous Drainage Studies.

Documenting MTO Requirements for Stormwater Management Quantity Control

Where a stormwater management quantity control facility is to be located within the highway right-of-way, refer to pages 64 to 74 in Chapter 3 of the "Drainage Management Manual" (MTO 1997) for information on developing a stormwater management design. Design information on wet ponds/extended dry ponds (page 79) and dry ponds (page 100) are presented in chapter 4 of the DMM.

Where a stormwater management quantity control detention facility is to be located within the property proposed for development, methods documented in the “Stormwater Management Planning and Design Manual” (MOE 2003), are generally suitable, except for the following techniques.

  • Parking lot or roof top storage: the MTO does not recognise any benefit from the attenuation of stormwater runoff using parking lot or roof top storage, where the control is achieoved through an orifice device or a roof top control device. MTO's concern is that as the continued functioning of such a control device cannot be guaranteed. The MTO will consider an alternative form of control devices (e.g. a short segment of storm sewer, equal to the diameter of the required orifice that leads from a manhole to and is directly connected to the storm sewer system). In general it should be demonstrated that the failure of a storage facility will not result in unsusceptible impacts to the Highway Drainage System.
  • Grassed ditches and swales: The runoff hydrograph is attenuated due to the resistance offered by the grassed surface and some degree of quantity control can thereby be achieved. The long term viability, operation and maintenance of grassed swales and ditches will be a concern to MTO. MTO reserves the right to reject the benefits achieved from conveyance controls, where the long term viability, operation and maintenance cannot be reasonably guaranteed.
  • Infiltration facilities: the MTO does not accept the location of an infiltration facility, within a proposed land development area, where it may impact the structural integrity of the highway sub-grade. If the use of infiltration facilities was a requirement of a regulatory agency, and no other stormwater management control option could be utilised, the infiltration facility should be located where it can be demonstrated that there will be no impact to the highway sub-grade.
  • Roof leader disconnection and cisterns: in general, MTO does not accept the benefits from roof leader disconnection and cisterns if the continuing functioning and long tem reliability cannot be guaranteed.

Where MTO requirements are set in a previous drainage study, refer to Reviewing Previous Drainage Studies.

Selecting the Level of Water Quantity Control

The level to which peak flows will be reduced depends upon the level of mitigation that is required, and the degree to which MTO is exposed to risk. MTO reserves the right to impose a higher level of control upon the land development proposal (i.e. as compared to the requirements of other regulatory agencies). In such cases an MTO drainage representative should be contacted for clarification. Peak flows must be reduced to a level that will restore:

  • water surface elevations and/or flow velocities, at the reference points and range of frequencies specified in Table 7, to a level(s) that will satisfy the risk criteria; and/or
  • the capacity of the highway drainage system to a level(s) that will satisfy the risk criteria.

Where the level of control is set in a previous drainage, refer to Reviewing Previous Drainage Studies.

Documenting the Need for Stormwater Management Quality Control

The need for stormwater management quality control is based on the sensitivity of the receiving drainage system, and may be a requirement placed on the land development proposal by the regulatory agencies before any impact assessment has been completed for the proposed land development.

Where the need for stormwater management quality control originates from a previous drainage study, refer to Reviewing Previous Drainage Studies.

Documenting MOE Requirements for Stormwater Management Quality Control

Generally, in land development proposals, the MOE will determine the need for stormwater quality control. The “Stormwater Management Planning and Design Manual” (MOE 2003) provides general guidance on the planning and design of stormwater management quality control facilities.

Where MOE requirements are set in a previous drainage study, refer to Reviewing Previous Drainage Studies.

Documenting MTO Requirements for Stormwater Management Quality Control

Where a stormwater management quality control facility is to be located within the highway right-of-way, refer to pages 64 to 74 in Chapter 3 of the "Drainage Management Manual" (MTO 1997). Chapter 4 also presents design information on wet ponds/extended dry ponds (page 79) and dry ponds (page 100).

Where a stormwater management quality control detention facility is to be located within the property proposed for development, methods documented in the “Stormwater Management Planning and Design Manual” (MOE 2003), are generally suitable except for infiltration facilities.

Where MTO requirements are set in a previous drainage study, refer to Reviewing Previous Drainage Studies.

Selecting the Level of Water Quality Control

The level of control will depend on the sensitivity of the receiving drainage system.

MTO reserves the right to input into the level of control for water quality treatment imposed upon the land development proposal, when drainage from the proposed land development will be entering the highway surface drainage system and will be conveyed to the receiving drainage system. In such a case, the primary concern for MTO is with regards to the riparian rights of upstream or downstream landowners. If the MTO could become unduly exposed to legal action, MTO reserves the right to impose or increase, whichever is applicable, the level of control imposed upon the land development proposal. An MTO drainage representative should be contacted for clarification.

Where the level of control is set in a previous drainage study, refer to Reviewing Previous Drainage Studies.

Identifying Design Criteria for Stormwater Management Controls

Where a previous drainage study has been referenced for design criteria, refer to Reviewing Previous Drainage Studies.

Parking Lot or Roof Top Storage

  • The device used to achieve the parking lot or roof top storage.
  • The location and layout of the proposed parking lot or roof top storage locations.
  • The volume controlled and the corresponding water surface elevation.
  • Maintenance procedures/responsibilities: refer to Clarifying Operation and Maintenance Responsibilities.

Stormwater Management Detention Facilities

  • The location and layoutof the detention facility should be confined within the land development property boundaries. The MTO does not generally allow detention facilities to be located within the highway right-of-way; however where a mutual benefit is recognized, PHY Directive B014 provides guidance. The MTO Regional Highway Planning and Design Office must be contacted before making such a recommendation. The following sections should be reviewed.

Issues to consider when selecting the location and configuration are as follows.

  • Maintenance access should be sufficient to allow for the passage of equipment required for the dredging and removal of sediment.
  • Multiple storage facilities located in the same drainage basin will affect the timing of the hydrograph as it travels downstream. This could increase or decrease peak flows in downstream locations. Coordination of stormwater management detention facilities with other drainage structures, on a watershed or subwatershed basis, is a primary consideration.
  • The size of a detention facility is typically measured in terms of surface area and depth (refer to the "Drainage Management Manual" (MTO 1997), Chapter 4 page 79, and the “Stormwater Management Planning and Design Manual” (MOE 2003).
  • Inlet and outlet configuration: the design of the outflow control will determine the outlet flow rate and hence the detention time for the facility. The outlet may include devices such as weirs, orifice plates, perforated risers, or a combination of them.
  • A flow splitter may be needed to direct the stormwater runoff into the quality control facility. When the required storage volume has been captured, the flow splitter will divert the stormwater runoff to a quantity control facility or back to the receiving drainage system.
  • Emergency spillway location, type and capacity: an emergency spillway should be designed to pass the regulatory flood, without failure, under blocked outlet conditions. Reference should be made to the "Technical Guidelines for Flood Plain Management in Ontario" (MNR 1987) for design criteria related to potential loss of life from dam failure.
  • Maintenance access provisions should be included to ensure access to trash racks, and for removal of sediment. Access ramps should be designed to support maintenance equipment.
  • Special safety requirements: roadside safety for errant vehicles should be considered where detention facilities are located near a highway (consult the MTO Regional Highway Planning and Design Office for further details).
  • A minimum freeboard depth: as a guide use 0.3m.
  • Maintenance procedures/responsibilities: refer to Clarifying Operation and Maintenance Responsibilities.
  • Setbacks from highway: the Public Transportation and Highway Improvement Act mandates the MTO to enforce setback requirements, for structures constructed within certain distances from the highway right-of-way. The stormwater management detention facility is considered to begin at the berm toe of slope, which should generally be setback 14m from the highway property line. Contact the local MTO District Office or the MTO Regional Highway Planning and Design Office for more details.
  • Ownership: refer to Clarifying Operation and Maintenance Responsibilities.

Analyzing and Documenting Results of the Design

Where stormwater management quantity controls are provided, the approach used in the analysis of the receiving drainage system should be repeated with one change: the proposed stormwater control is added to the receiving drainage system. Re-calculate peak flows, water surface elevations, or flow velocities at the reference points and range of frequencies specified in Table 7. The SWM report should present results in a table that compares the results for the pre-development scenario to the results for the post-development scenario(s). The computational methodology used in the design of the stormwater management controls must be documented in the SWM report. The SWM report must also document that the identified impact has been mitigated by the proposed stormwater management controls by showing that peak flows have been reduced to a level to that will restore:

  • water surface elevations and/or flow velocities, at the reference points and range of frequencies specified in Table 7, to a level(s) that will satisfy the risk criteria; and/or
  • the capacity of the highway drainage system to a level(s) that will satisfy the risk criteria.

Where stormwater management quality controls are provided, the SWM report must document how MOE and/or MTO requirements were satisfied by the proposed design. The documentation as noted in the above section may also be required if there is a concern that the proposed controls will impact the receiving drainage system.

Where stormwater management quality and quality controls are provided, the SWM report must provide the documentation as noted in both of the sections noted above.

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Modifying the Receiving Drainage System

If modifications to the receiving drainage system, including the highway drainage system, may be required to mitigate the identified impacts, the SWM report must provide documentation on the following areas. Modifications to the receiving drainage system include erosion protection works.

Documenting the Need to Modify the Receiving Drainage System

Before the proposed modification can be approved, the SWM report must provide documentation on the areas listed below. This step should be completed at the earliest possible stage of design and should include the tasks presented below.

Where the need to modify the receiving drainage system was determined in a previous drainage study, refer to Reviewing Previous Drainage Studies.

Documenting the Requirements of the Organizations Identified as responsible for Operation and Maintenance

Whenever modifications to the receiving drainage system are proposed, the organization responsible for the operation and maintenance for the component that is to be modified, must be contacted to ensure that they approve the proposed modification.

For modifications to highway drainage works, the MTO Regional Highway Planning and Design Office must be contacted before making any recommendation to modify a highway drainage works. The MTO does not generally allow highway drainage works to be modified unless it can be demonstrated that alterations will be of benefit to the highway; however where a mutual benefit is recognised, Directive B014 provides guidance. The following sections should be reviewed.

Where the MTO Regional Highway Planning and Design Office has accepted the proposal to modify the highway drainage works, the SWM report must document the basis of the approval. Where the modification to the highway drainage system was proposed in a previous drainage study, refer to Reviewing Previous Drainage Studies.

For modifications to other components of the receiving drainage system (i.e. other than the highway drainage system), contact the local municipality, and/or the local conservation authority. Where the responsible organisation has accepted the proposal to modify the receiving drainage system, the SWM report must document the basis of the approval. Where a modification to another component of the receiving drainage system was proposed in a previous drainage study, refer to Reviewing Previous Drainage Studies.

Documenting Requirements of the Regulatory Agencies

Whenever a modification is proposed to any component of the receiving drainage system (including the highway drainage system), the regulatory agencies must be contacted to ensure that the proposed modification will comply with their respective mandates. Contact the local municipality, local conservation authority, the MNR, MOE, DFO, and in some cases the DTO.

In the case where the proposed modifications do not involve the highway drainage system, the MTO Regional Highway Planning and Design Office should be contacted to ensure that MTO drainage policy are not compromised by the proposed modifications.

Where the requirement of a regulatory agency is set in a previous drainage study, refer to Reviewing Previous Drainage Studies.

The SWM report must document all the regulatory agencies that were contacted, and the basis of the approval of the proposed modification.

Identifying the Receiving Drainage System Modification

The SWM report should present the component of the receiving drainage system that is to be modified along with a description of the proposed modification(s). Refer to Table 6 for relevant information about the modification that should be provided in the SWM report.

Where the modification was identified in a previous drainage study, refer to Reviewing Previous Drainage Studies.

Identifying Design Criteria used in the Receiving Drainage System Modification

Where modifications to the highway drainage system have been accepted by the MTO Regional Highway Planning and Design Office, design criteria for highway drainage works along with the design procedures and considerations presented in the "Drainage Management Manual" (MTO 1997), must be followed to the satisfaction of the MTO Regional Highway Planning and Design Office. The SWM report must document how the highway drainage works design criteria were satisfied.

Where modifications to other components of the receiving drainage system (i.e. other than the highway drainage system) have been accepted by the organisation responsible for its operation and maintenance, the design criteria of that organisation must be followed to their satisfaction. The SWM report must provide appropriate documentation.

Design criteria, drainage management policy, or the guidelines and manuals of other regulatory organisations should also be considered. The SWM report should document the process followed to contact other regulatory organisations and the criteria that were proposed as a result.

Where a previous drainage study has been referenced for design criteria, refer to Reviewing Previous Drainage Studies.

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.

Analyzing and Documenting the Results of the Modification

The approach used in the analysis of the receiving drainage system should be repeated with one change: the proposed modifications to the receiving drainage system must be included. Re-calculate peak flows, water surface elevations, or flow velocities at the reference points and range of frequencies specified in Table 7. The SWM report must present the results in a table that compares the results for the pre-development scenario to the results for the post-development scenario(s). The SWM report must also document that the identified impact has been mitigated by the proposed modifications by showing that:

  • water surface elevations and/or flow velocities, at the reference points and range of frequencies specified in Table 7, are restored to a level(s) that will satisfy the risk criteria; and/or
  • the capacity of the highway drainage system is restored to a level(s) that will satisfy the risk criteria.

The computational methodology used in the modification of the receiving drainage system must also be documented in the SWM report.

The SWM report must document how MTO requirements, and requirements of the other regulatory agencies were satisfied by the proposed modification.

For simple erosion protection works: if the proposed modification only involves simple erosion protection works such as lining material or rip-rap placement, the SWM 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 specified in Table 7. Where the erosion protection works are major, such as drop structures, the procedures noted above must be followed.

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