Performing the Hydraulic Design

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

This group of tasks uses all the information and data collected to perform the detailed hydraulic design of the bridge or culvert. The design will take into account the drainage issues identified and provide the design alternative that will address these issues.

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 bebased 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.

Develop the Design Criteria


This task will outline the hydrologic and hydraulic design criteria for the proposed crossing. These design criteria will have to be approved by the regional engineering office responsible for the project. The regional office should be contacted to determine the approval requirement. It is advisable to acquire the necessary approvals before completing the design of the project.

Documentation Requirements

This section of the report should include a comprehensive list of the design criteria that will govern all the alternatives being considered. These criteria will identify, but are not limited to the following:

  • The proposed location of the structure. This information should be supported with sketches, maps, photo mosaics and descriptions.
  • Special considerations addressing the following, if applicable (Refer to Chapter 5 of the MTO Drainage Management Manual for further details):
    • Crossing of inland lakes.
    • Tidal Crossings.
    • Consideration of wave action.
    • Physical modelling of bridge.
  • The return period for the design storm and the regulatory storm, as per Directive B-100 (PDF - 65 KB). This section should outline the hydrologic design criteria adopted and the underlying rationale for selecting these criteria for the proposed bridge crossing. This should be based on the class of road, present and future projections of traffic density and Ministry directives, policies and practices and the MTO Drainage Management Manual.
  • Any deviations from the minimum requirement set by Directive B-100 (PDF - 65 KB). The rationale for this deviation must be documented.
  • Highway profile and horizontal alignment.
  • The vertical clearance, as defined by the CHBDC. If the requirement of the CHBDC can not be met, a deviation report has to be prepared that will provide the rationale for this deviation/exception. There is a specific requirement for the information that is to be included in the deviation/exception report. Refer to the section MTO Approvals for the requirements for approving the design criteria for water crossing structures over waterways, contravening the CHBDC.
  • Freeboard requirements at the approach.
  • The number of footings, pier spacing and location of abutments.
  • The existing drainage problems that will be addressed by the proposed design criteria.
  • Restrictions based on other structures upstream or downstream.
  • Requirements for permanent erosion control measures.
  • Relief flow requirements.
  • Fish passage requirements.
  • Debris and ice flow requirements.
  • Navigation requirements.
  • Local stream modifications.
  • Any other external constraints.
  • Minor access routes under water crossings.

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The rationale for selection should be included for all the above criteria. Should any design criteria, drainage management policy, guideline or manual of a riparian landowners 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.

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Conduct the Hydrologic Analysis


In order to determine the size of the opening of the bridge or culvert it is essential to determine the flow rate that should be accommodated by the bridge or culvert structure. This flow may be accommodated through the structure as well as allowed to bypass the structure as relief flow. The appropriate means of accommodating the flow should be determined based on MTO Directive B-100, the drainage manual and the Canadian Highway Bridge Design Code and associated exemptions. The hydrologic analysis must be done for, but not limited to the following:

  • The design flood, as per Directive B-100 (PDF - 65 KB), for the design of the structure opening.
  • The check flood, as defined by Directive B-100 (PDF - 65 KB), for assessing the structural integrity of the crossing.
  • The regulatory flood, as defined by Directive B-100 and the local Conservation Authority, to assess the impacts of the proposed design on adjacent land, structure and relief flow on the roadway.
  • In most cases, the 2, 5, 10, 25, 50 and 100- year flood should be determined.

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Refer to the section Design Criteria for Highway Drainage Works for more detail.

Documentation Requirements

This section should discuss and document the design flood discharge arrived at, based on the design criteria adopted, and outline the method used in the analysis. Documenting the Computational Methodology (Table 8), below gives the information that should be covered in the documentation.

Recognizing that the techniques generally used to arrive at a design flood estimate are statistical in nature and the results may vary with additional data in later years, it is suggested that several methods should be used to verify the results (as outlined in the MTO Drainage Management Manual).

If a stream flow gauging station with a long period of record is available, Single Station Frequency Analysis should be used to derive the design flow rate. The most reliable stream gauge data available from the nearest stream gauging station should be used for Single Station Frequency Analysis. The flow rate data documented in the past by other agencies or in other reports should be used with caution and as a rough estimate only.

If there is no gauging station close to the crossing location or if data is not available, data from other gauging stations on the watercourse can be transposed tothe location of the crossing. If such data is not available, data from an adjacent watercourse can be transposed to the crossing location, as long as the two watersheds are hydrologically equivalent. In cases where transposition of the stream flow data is conducted, confirmation of the resulting stream flows, for the different return periods, should be done using other precipitation based hydrologic methods.

If single station frequency analysis is not feasible the use of other methods based on precipitation data must be used. In all cases, method based on precipitationdata will be required for the determination of the Regulatory flow rates.

It is ultimately the responsibility of the designer to arrive at the design discharge based on accepted engineering standards and practices, common sense and application of state-of-the-art techniques.

This section should document all the methods used in determining the flow rate and provide the underlying assumptions. Reference to any reports, technical papers, textbooks, and manuals should be provided.

Documenting the Computational Methodology (Table 8)

Computational Analysis Documentation in Hydrology Report
Flow Rate Calculation Method used, variables, and applicability limitations of the method must be documented. Also, assumptions made have to be stated.
Identifying Catchment Inputs The subsequent values of the different parameters, any method used to calculate these values, and justification for selecting these values must be documented. Also, the assumptions made have to be stated.
Selecting Precipitation Data The type of storm data used: single event (continuous, synthetic, historic, or IDF curves), meteorological station used, and storm duration time step (where applicable) must be documented. Also, the rationale for selecting all of the above and the assumptions made.

* If any of the methods used have not been included in the MTO Drainage Manual or reviewed in the MTO document "Evaluation of Drainage Software", the Hydrology Report should include a description of the method used and justification for the use of this method on highway projects in Ontario. Based on the location of the project, the applicability of the methods used for Canadian Shield and non-Shield areas should also be confirmed.

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Design the Bridge or Culvert Opening


The main factor controlling the level of impact of a water crossing structure on the watercourse is the size of the bridge or culvert opening. The smaller the opening the greater the backwater effect in the watercourse upstream of the crossing. At the same time, if the opening is made larger, the span of the structure would be greater and therefore, the cost higher.

The process of establishing the size of the opening starts by assuming an initial size. Once an initial size has been determined, hydraulic analysis will determine the backwater effect. If the backwater effect is unacceptable, according to the design criteria, a new opening configuration is used. Therefore, the process of assessing the proper size of the opening is an iterative one, which is then refined until all the design criteria are met.

There are a number of methods that can be used to determine the size of the opening (s) of the structure. Refer to the sections Bridge Analysis or Culvert Analysis for details of these methods.

Documentation Requirements

The Hydrology Report should provide a complete documentation of the different aspects of the design of the bridge or culvert structure.

For Bridges

The Hydrology Report should document the information listed below. For details on bridge analysis and methods of design refer to the section "Bridge Analysis" and Chapter 5 of the MTO Drainage Management Manual.

  • The analysis and results of the initial bridge opening size.
  • All input parameters. This information can be provided in a summary table in addition to documentation as part of the input parameters to the computer software model, if one was used in the analysis.
  • Upstream and downstream water level controls and water surface elevations.
  • Water surface profiles for the different design flow rates used in the analysis. This information to be provided in tables. Cross-section drawings and longitudinal profile drawing. The predevelopment water surface elevations should be documented along side those for the proposed alternatives.
  • Resulting soffit elevation and freeboard at the approaches supported with drawing and sketches indicating the cross section elevations of the structure and the natural watercourse.
  • Assessment of impact of the proposed design(s) on debris and ice accumulations as well as fish passage. Refer to the section "Design for Ice and Debris" for detailed information.
  • Details of flow controls, drops and the potential for development of hydraulic jumps. The details are to include location, water levels upstream and downstream and methods of addressing their impacts, physical and hydraulic.
  • The type of flow condition under the different flow rates whether, open channel flow, pressure flow or weir flow and whether critical, subcritical or supercritical flow.
  • The type of hydraulic analysis methods used whether hand calculation methods or computer software. The discussion in this section should include confirmation that the methods used are acceptable to MTO. Refer to the MTO Drainage Management Manual Chapters 3 and 5 and to the MTO Drainage Web document "Evaluation of Drainage Management Software" for details on acceptable methods. These references also provide the documentation requirements if the methods used have not been evaluated by MTO.
  • Calibration and verification of the water surface profile model used. This section should describe the calibration process undertaken and how the available data was used to calibrate and verify the model. Sensitivity of parameters should be identified. In some cases water surface profile models may have been developed. This should have been identified in the section on existing drainage studies. The results of these models may be used but must be verified. The verifications should ensure that the model reflect the present conditions, realistic future development scenarios and up-to-date water management policies.
  • Sketch of proposed structure(s) and roadway grades in plan and profile showing crown grade elevation, super structure, bent locations, limits and elevations of rip rap and any channel modifications.

If water surface profile software such as HEC-RAS or HEC-2 is used, refer to the section "Minimum documentation requirements"(web link*) in the MTO web document "Evaluation of Drainage Management Software" for details on the minimum information that should be in the Hydrology Report.

For Culverts

The Hydrology Report should document the information listed below. Any special considerations that require a special design should also be included and the resulting design feature documented. For details on culvert analysis and methods of design refer to the section "Culvert Analysis" and to Chapter 5 of the MTO Drainage Management Manual.

  • Culvert type, shape, size, material, skew and number of barrels - special consideration should be given to whether the culvert is an open footing or closed footing (box) culvert. Different vertical clearances are specified by the CHBDC for open footing vs. box culverts.
  • Culvert slope, inlet and outlet elevations and longitudinal profile, especially in cases where there are drops inside the culvert.
  • Tailwater elevation and associated analysis. The analysis is to be included in an appendix.
  • Type of inlet and outlet treatments including slope tapers, collars, wing-wall, etc. This information should indicate the rationale for selecting the proposed end treatments and an assessment of structural vulnerability to lifting, piping and other forces. If clay seals are required this should be indicated and their location identified in drawing and sketches. The inlet, outlet and other minor energy loss coefficients should also be documented.
  • The type of hydraulic analysis methods used (design tables or computer software). The discussion in this section should include confirmation that these method(s) are acceptable to MTO. Refer to the MTO Drainage Management Manual chapter 3 and 5, and the MTO Drainage Web document "Evaluation of Drainage Management Software" for details on acceptable methods and the documentation required if the methods used have not been evaluated by MTO.
  • Hydraulic analysis for all required flow rates.
  • Headwater elevations for inlet and outlet control conditions identifying the maximum allowable headwater elevation, as described in the design criteria.
  • Inlet and outlet velocity, as well as velocity within the culvert.
  • Performance curve for inlet, outlet control and the governing conditions.
  • Energy dissipaters, debris control measures and special erosion control measures upstream and downstream.
  • Details of protection measures for corrosion or abrasion inside the culvert, if applicable.

If water surface profile software such as CulverMaster is used, refer to the section "Minimum documentation requirements" in the MTO web document "Evaluation of Drainage Management Software" for details.

The High Water Level

The Design High Water Level (HWL) estimates should be documented with background information and calculation procedures(s) included in an appendix and briefly described in the body of the report. The Normal Water Levels should also be documented. The definition of these terms is as described in the CHBDC.

According to the CHBDC, there are a number of High Water Levels that need to be determined throughout the design process. These include the following:

  • HWL for Backwater Computations (CHBDC Section
  • HWL for Establishing Soffit Elevation (CHBDC Section
  • HWL for Establishing Approach Grade (CHBDC Section

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Analyse Scour and Determine the Depth of Footings


Scour is the lowering and/or widening of the streambed due to the erosive forces exerted by flowing water. Channel scour is an important consideration in the design of water crossings as it may undermine the foundations of the structure, possibly leading to its failure. There have been documented failures of structures as a result of scour. Channel scour design requirements are presented in the Drainage Management Manual Chapter 5, in a number of AASHTO publications and in the Canadian Highway Bridge Design Code (CHBDC), 2001.

The following facts should be kept in mind when performing the analysis.

  • For each case, various methods of calculating the depth of scour should be considered depending on the site characteristics.
  • The limitations of each method should be reviewed. If a particular method is not suited to the site conditions, it should not be used.
  • Scour depths resulting from any analysis should be compared with soil stratigraphy at that depth, including relative compaction, to verify that the initial assumptions of soil properties are valid.

Documentation Requirements

This task should identify the extent of scour, both local and natural, and the resulting decision on the type, depth and location of the bridge footings. The information to be documented should include but is not limited to the following:

  • The check flood used for the analysis of scour.
  • Input parameters to the analysis. This would include:
    • Stream width, depth and slope
    • Stream bed material
    • Constrictions in the channel opening
    • Obstructions in the channel opening
  • The method used in the analysis of local and natural scour. Refer to the section "The Potential for Scour" for more details on the analysis methods of local and natural scour.
  • The results of the analysis and the methods used to arrive at the final scour depth.
  • The type of footings being proposed and the proposed depth of footing for the abutments and piers.
  • Plot of estimated scour depths on profile view, for each of the design alternatives being considered.

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Design for Ice and Debris


The design of a crossing should be checked for the potential impact of ice and debris on the flow through the structure. Ice jams are usually formed during ice break-up and are caused by:

  • Constriction of flow
  • Obstruction of flow
  • Channel bend (Radius < 4 times the channel width)
  • Solid ice sheet downstream acting as an obstruction due to upstream flows experiencing earlier ice break-up (e.g. Rivers flowing north to James Bay or Hudson Bay).

There are two ice-related design aspects that need to be completed in order to determine the required soffit elevation and any training work needed to accommodate the ice flow. These tasks are:

  • Assess River Ice Conditions
  • Estimating Design High Ice

Documentation Requirements

Design ice flow conditions including high ice elevation and the effective thickness of ice floes related to the CHBDC should be estimated and documented based on anticipated discharges and stages.

Where possible, such assessments should be complemented or verified with field data such as ice scars on trees, banks, and historic information based on interviews with long term local residents in the area. This information is valuable for the design of piers and abutments.

If the stream has the potential for jamming due to debris or ice, the implications on the proposed waterway opening should be assessed and documented. This would include assessing the minimum span between piers, span configurations, minimum clearance between the HWL and the soffit. In those cases, historical information gathered from local residents or archives should be included in this discussion.

In situations where ice problems require the construction of ice control device such as weirs, the design of such devices should be provided and their impact on the flow in the watercourse under ice free conditions should be documented.

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Design to Convey the Regulatory Storm


A regulatory flood is a design flood specified by the Ontario Ministry of Natural Resources for floodplain management purposes. In Ontario, the regulatory flood can be one of the following:

  • Hurricane Hazel
  • The Timmins Storm, or
  • The 100-year flood.

Directive B-100 (PDF - 65 KB) identifies the regions where these regulatory floods apply.

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Water crossings should be designed to the design flood frequencies identified in Directive B-100. However, B-100 also states that, "If a drainage facility designed to the criteria specified would increase flooding of buildings or developable land during a regional flood, the facility shall be designed to the regional flood criteria unless otherwise approved. The overall benefit (tangible and intangible) of designing to the regional flood shall be commensurate with the additional cost of the facility, and the proposal should be discussed with the municipality and the landowners adversely affected".

Designing for the regulatory flood could be achieved either through providing for relief flow or by accommodating the regulatory flood through the structure. This will depend on the highway geometrics, the level of service, safety and other considerations. Designing for relief flow means allowing the flow to bypass the main waterway opening and pass over the approach grade or through one or more relief structures. It is generally preferred that the relief flow occur away from the water crossing structure. This would occur by placing the structure away from the road sag. Refer to the MTO Drainage Management Manual for details on locating the structure and relief flow.

Relief flow is beneficial in that it acts as a "safety valve" against bridge or culvert failure in the event of an extreme flood.

  • It is a means of reducing backwater during ice jams or extreme floods.
  • It reduces the cost of maintenance as it reduced the potential damage to the bridge or culvert.

Documentation Requirements

The hydraulic analysis providing the water levels under relief and non-relief flow conditions should be documented. The rationale for selecting the best approach should be presented.

If the relief flow option is selected, the path of the relief flow and the impact on adjacent lands and buildings must be documented in the report. The documentation must identify:

  • The major flow path presented in maps, schematics and text. The locations of all lands and structures affected by the major flow should be indicated.
  • Water levels at the structure, over the approach and along the major flow path.
  • The design of mitigating measures to reduce the impacts of the flow of water on the bridge or culvert and the approach grade.
  • In cases where flow relief structures are proposed in the approach embankment, the design of structures should also be documented.
  • Flood protection measures, if applicable.
  • Cost implication.

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