Drainage Outlet

The Ministry of Transportation (MTO) has undertaken a study to define the outlet liability assessment factors involving highway rights of way in Ontario as part of the cost assessments and apportionment of municipal drain construction and maintenance under the Drainage Act. Design tools were developed to aid in the application of the new assessment process, and help to ensure that MTO highway rights of way are fairly assessed.

The cost assessment process builds on the process outlined in the 1988 paper entitled: "Drainage Assessment Revisited", by E.Dries and H.Todgham (Ref. 2 in the project report)

Based on the findings of the study, MTO requires that all assessment for municipal drains that include MTO lands follow the outlet liability assessment process as outlined below. MTO will not support assessments that do not follow this process.

Background of The Study:

The Drainage and Hydrology Group of the Highway Design Office undertook the study in 2004. A one-year consultation process was followed with the Ontario Land Drainage Committee and Ontario Drainage Engineers community prior to adopting the findings of the study.

The objectives of this study are to:

  1. determine how MTO highway rights of way (ROW) are currently being assessed for outlet liability;
  2. determine appropriate outlet liability assessment factors for MTO ROW; and
  3. provide a reference tool for Drainage Engineers to determine outlet liability assessment factors for MTO ROW.

For a review of the details of the study refer to the project report available online through the MTO Library Catalogue.

The Process for Calculating Outlet Liability

Outlet liability assessment factors are highly dependent on the soil type of the agricultural lands in the watershed, and the total width of the highway ROW. It was found that even in the most extreme condition, the equivalent agricultural hectares multiplication factor is less than 6 for highway lands in Ontario. The process for calculating the appropriate outlet liability assessment factors for MTO ROW is outlined below.

The approach for determining the outlet liability assessment factors is based on determining the equivalent hectares multiplication factors for MTO highway ROW compared to the adjacent agricultural land. This is done by calculating the percentage of the ROW that is developed then using the table or figure below to determine the outlet liability assessment factor for the applicable local runoff coefficient for adjacent agricultural land. The factors presented are for percentage ROW development ranging from 20% to 90% and runoff coefficients of adjacent agricultural land from 0.15 to 0.5. Developed portions include the area covered by asphalt, concrete, or granular material, whereas undeveloped areas include ditches, grassed medians, and vegetated roadsides.

Outlet Liability Assessment Factors

The table and chart below provide the design values for determining the equivalent land area multiplication factors for MTO highway rights of way.

Table: Outlet Liability Assessment Factors

Run-off Coefficient for Adjacent Agricultural Land

Note: C values refer to Background Agricultural run-off Coefficient for Watershed

% ROW Developed C = 0.15 C = 0.20 C = 0.30 C = 0.40 C = 0.50
20 4.0 3.0 2.0 2.0 1.5
30 4.0 3.0 2.5 2.0 1.5
40 4.5 3.5 2.5 2.0 1.5
50 4.5 3.5 2.5 2.0 1.5
60 5.0 4.0 2.5 2.0 1.5
70 5.5 4.0 2.5 2.0 1.5
80 5.5 4.0 3.0 2.0 1.5
90 6.0 4.5 3.0 2.0 2.0


Design Chart: Outlet Liability Assessment Factors

Design Chart: Outlet Liability Assessment Factors

Sample Applications for Calculating Outlet Liability Assessment Factors for Municipal Drains Assessments involving Ministry of Transportation Rights of Way

The following are two examples on the application of the process for calculating the outlet liability assessment factors.

Example 1:

This example involves a drain located in Southwestern Ontario where both Highway 401 and a municipal road are located within the drainage area. The developed width of Highway 401 is approximately 27 metres, while the MTO owns a total width of 91 metres, representing 30% ROW development. The municipal road has a developed width and total right of way of 10 and 20 metres, respectively, which translates to 50% ROW development. The soils in the watershed are classified as sandy loam and have a runoff coefficient of approximately 0.25 (based on table 1 in the project report).

Applying the proposed design values (from the table above), the equivalent hectares multiplication factors for these roads would be in the range of 2.5 to 3.0. This is due to the fact that although the width of the developed portion of the municipal road is approximately half that of Highway 401, the percentage development of the municipal ROW is 20% greater. Based on previous Engineers Reports reviewed for this study it would not be uncommon for a highway such as the 401 to be assessed using a factor in the 4-7 times agricultural range, whereas the municipal road would likely be assessed in the range of 2 to 4.5.

Example 2:

This example involves highway QEW in the Niagara area. In this area the QEW has a developed width of 35 m, or 64% of the 55 m MTO-owned ROW. The soils in the watershed have a runoff coefficient of 0.2. Based on this information the appropriate equivalent hectares multiplication factor is 4.0 (from table 3).

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