Stormwater Management Requirements for Land Development Proposals

Checking the Capacity of the Highway Drainage System

The capacity of the highway drainage system should be checked as part of the receiving drainage system analysis. Complete the following tasks, depending on the highway drainage works that form part of the receiving drainage system.

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.


Checking the Design Capacity of the Highway Bridge or Culvert

Where the proposed land development will be discharging to a receiving drainage system that leads to a highway bridge or culvert, the design capacity of the existing highway bridge or culvert must be examined. The SWM report must present the following parameters calculated at the upstream face of highway bridge or culvert using the total catchment area which must include the proposed land development area:

  1. peak flow rate, headwater level and flow velocity (at the highway bridge or culvert outlet) at the design flow frequency with the proposed land development area at pre-development conditions;
  2. peak flow rate, headwater level and flow velocity (at the highway bridge or culvert outlet) at the design flow frequency with the proposed land development area at post-development conditions;
  3. peak flow rate and the corresponding headwater level calculated using the regulatory storm with the proposed land development area at pre-development conditions; and
  4. peak flow rate and the corresponding headwater level calculated using the regulatory storm with the proposed land development area at post-development conditions.

The SWM report must document if the capacity of the highway bridge or culvert will be exceeded because of stormwater runoff discharging from the proposed land development. Review the following conditions and report the results in the SWM report. If the capacity is exceeded, an impact should be identified in the SWM report. Proceed to Identifying Impacts to the Receiving Drainage System for further details.

Where the pre-development peak flow (calculated in a) is less than or equal to the design flow capacity; or the pre-development headwater level (calculated in a) is less than or equal to the allowable headwater level, the capacity of the highway bridge or culvert is not exceeded if:

  • the post-development peak flow rate (calculated in b) is less than the design flow capacity; and
  • the post-development headwater level (calculated in b) is less than the allowable headwater level; and
  • any relief flow over the highway bridge or culvert (calculated in d) is less than the allowable depth of flow on the highway surface; and
  • the post-development flow velocity (calculated in b) does not cause erosion or scour; and
  • fish passage (for highway culverts only), river ice, and debris flow are not affected (refer to chapter 5 of the "Drainage Management Manual" (MTO 1997): fish passage in culverts on page 66, river ice on page 78 and debris flow on page 93).

If a highway bridge or culvert has excess capacity that is to be used to accommodate stormwater runoff from the proposed land development, the portion of the excess flow capacity allocated to the proposed land development must be presented in the SWM report. A means of calculating capacity allocation can be done using the following relationship.

proposed land development area proposed land development area excess flow capacity x excess flow capacity
tributary area
That is, open bracket, proposed land development area, divided by tributary area, close bracket, times excess flow capacity

Such an allocation of excess flow capacity will not be approved by MTO if the excess flow capacity is required for future highway works; or if the excess flow capacity causes a drainage impact to the property of riparian landowners located upstream or downstream of the highway right-of-way.

Where the pre-development peak flow (calculated in a) is greater than the design flow capacity; or the pre-development headwater level (calculated in a) is greater than the allowable headwater level, the capacity of the highway bridge or culvert is not exceeded if:

  • the post-development peak flow rates (calculated in b and d) are less than or equal to the pre-development peak flow rates (calculated in a and c); and
  • the post-development headwater levels (calculated in b and d) are less than or equal to the pre-development headwater levels (calculated in a and c); and
  • the post-development flow velocity (calculated in b) is less than or equal to the pre-development flow velocity (calculated in a); and
  • fish passage (for highway culverts only), river ice, and debris flow are not affected (refer to chapter 5 of the "Drainage Management Manual" (MTO 1997): fish passage in culverts on page 66, river ice on page 78 and debris flow on page 93).

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Checking the Design Capacity of the Highway Storm Sewer or Roadside Ditch

Where the proposed land development will be discharging to a receiving drainage system that leads to a highway storm sewer or roadside ditch, the design capacity of the existing highway storm sewer or roadside ditch must be examined. The SWM report must present the following parameters calculated at the location where external stormwater runoff enters the highway storm sewer or roadside ditch, using the total catchment area which must include the proposed land development area:

  1. peak flow rate and the corresponding depth of flow and flow velocity calculated using the design flow frequency with the proposed land development area at pre-development conditions;
  2. peak flow rate and the corresponding depth of flow and flow velocity calculated using the design flow frequency with the proposed land development area at post-development conditions;
  3. peak flow rate and the corresponding depth of flow calculated using the regulatory storm and the proposed land development area at pre-development conditions; and
  4. peak flow rate and the corresponding depth of flow calculated using the regulatory storm and the proposed land development area at post-development conditions.

The SWM report must document if the capacity of the highway storm sewer or roadside ditch will be exceeded because of stormwater runoff discharging from the proposed land development. Review the following conditions and report the results in the SWM report. If the capacity is exceeded, the impacts should be identified in the SWM report. Proceed to Identifying Impacts to the Receiving Drainage System for further details.

Where the pre-development peak flow (calculated in a) is less than or equal to the design flow capacity, the capacity of the highway storm sewer or roadside ditch is not exceeded if:

  • the post-development peak flow rate (calculated in b) is less than the design flow capacity; and
  • freeboard requirements (for the highway roadside ditch only) are satisfied; and
  • the post-development depth of flow (calculated in b and d) is less than the allowable depth of flow; and
  • the post-development flow velocity (calculated in b) does not cause erosion or scour.

If a highway storm sewer or roadside ditch has excess capacity that is to be used to accommodate stormwater runoff from the proposed land development, the portion of the excess flow capacity allocated to the proposed land development must be presented in the SWM report. A means of calculating capacity allocation can be done using the following relationship.

proposed land development area proposed land development area excess flow capacity x excess flow capacity
tributary area
That is, open bracket, proposed land development area, divided by tributary area, close bracket., times excess flow capacity

Such an allocation of excess flow capacity will not be approved by MTO if the excess flow capacity is required for future highway works.

Where the pre-development peak flow (calculated in a) is greater than the design flow capacity, the capacity of the highway storm sewer is not exceeded if:

  • the post-development peak flow rate (calculated in b and d) is less than or equal to the pre-development peak flow rate (calculated in a and c); and
  • the post-development depth of flow along the highway surface (calculated in b and d) is less than or equal to the pre-development depth of flow along the highway surface (calculated in a and c); and
  • the post-development flow velocity (calculated in b) is less than or equal to the pre-development flow velocity (calculated in a).

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Checking the Design Capacity of the Highway Stormwater Management Detention Facility

Where the proposed land development will be discharging to a receiving drainage system that leads to a highway stormwater detention facility, the design capacity of the existing highway stormwater management detention facility must be examined. The SWM report must present the following parameters calculated using the tributary area, which must include the proposed land development area:

  1. peak flow rates calculated at the location where external stormwater runoff enters the highway stormwater detention facility using the design flow frequencies with the proposed land development area at pre-development conditions;
  2. peak flow rates calculated at the location where external stormwater runoff enters the highway stormwater detention facility using the design flow frequencies with the proposed land development area at post-development conditions;
  3. maximum storage volume used, headwater level, and peak flow rates and velocities (at the outlet of the highway stormwater detention facility) determined by routing the peak flows calculated in (a) through the design stage-storage-discharge relationship;
  4. maximum storage volume, headwater level, and peak flow rates and velocities (at the outlet of the highway stormwater detention facility) determined by routing the peak flows calculated in (b) through the design stage-storage-discharge relationship;
  5. peak flow rate and the corresponding depth of flow in the emergency spillway calculated using the regulatory storm with the proposed land development area at pre-development conditions; and
  6. peak flow rate and the corresponding depth of flow in the emergency spillway calculated using the regulatory storm with the proposed land development area at post-development conditions.

The SWM report must document if the capacity of the highway stormwater management detention facility will be exceeded because of stormwater runoff discharging from the proposed land development. Review the following conditions and report the results in the SWM report. If the capacity is exceeded, an impact should be identified in the SWM report. Proceed to Identifying Impacts to the Receiving Drainage System for further details.

Where the pre-development maximum storage volume used (calculated in c) is less than or equal to the allowable storage volume; or the maximum pre-development headwater level (calculated in c) is less than or equal to the allowable headwater level; or the maximum pre-development peak flow rate (calculated in c) is less than or equal to the design flow capacity, the capacity of the highway stormwater management detention facility is not exceeded if:

  • the post-development peak flow rates (calculated in d) are all less than the design flow capacity; and
  • the post-development headwater levels (calculated in d) are all less than the allowable headwater level; and
  • the post-development storage volumes (calculated in d) are all less than the maximum storage volume; and
  • the post-development depth in the emergency spillway (calculated in f) is contained within the emergency spillway; and
  • the post-development flow velocities (calculated in d) do not cause erosion or scour.

If a highway stormwater management detention facility has excess storage volume capacity that is to be used to accommodate stormwater runoff from the proposed land development, the portion of the excess storage volume capacity allocated to the proposed land development must be presented in the SWM report. A means of calculating capacity allocation can be done using the following relationship.

proposed land development area proposed land development area excess flow capacity x excess flow capacity
tributary area
That is, open bracket, proposed land development area, divided by tributary area, close bracket, times excess storage volume capacity

Such an allocation of excess storage volume capacity will not be approved by MTO if the excess storage volume capacity is required for future highway works.

Where the maximum pre-development storage volume used (calculated in c) is greater than the allowable storage volume; or the maximum pre-development headwater level (calculated in c) is greater than the allowable headwater level; or the maximum pre-development peak flow rate (calculated in c) is greater than the design flow capacity, the capacity of the highway stormwater management detention facility is not exceeded if:

  • the post-development peak flow rates (calculated in d and f) are less than or equal to the pre-development peak flow rates (calculated in c and e); and
  • the post-development headwater levels (calculated in d) are less than or equal to the pre-development headwater levels (calculated in c); and
  • the post-development storage volumes (calculated in d) are less than or equal to the pre-development storage volumes (calculated in c); and
  • the post-development depth in the emergency spillway (calculated in f) is less than or equal to pre-development depth in the emergency spillway (calculated in e); and
  • the post-development flow velocities (calculated in d) are less than or equal to the pre-development flow velocities (calculated in c).

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