Chapter V – Roads, sandpits and forest infrastructures
Division III – Bridge, culverts, removable structures and rudimentary structures
§4. General provisions applicable to bridges or culverts

Section 101

The minimum discharge capacity that a culvert must possess is determined on the basis of the peak flow calculated using the method provided for in Schedule 6 for drainage basins having an area equal to or less than 60 km2 or in Schedule 7 for drainage basins having an area greater than 60 km2 and on the basis of the size of circular conduits provided for in Schedule 8. Conduits that are not circular, arches or bridges must have a discharge surface sufficient for a peak flow calculated using the method provided for in Schedule 6 or 7, as the case may be, and so that the water depth is at all times less than or equal to 85% of the vertical clearance of the structure.

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Objectives

To ensure the lifespan of a road, bridge or culvert
To ensure the free flow of water

Explanations

Culverts and bridges that are built, improved or repaired must have a discharge capacity large enough to evacuate peak flow. This is calculated using the method provided for in Schedule 6 for drainage basins having an area equal to or less than 60 km2 or using the method provided for in Schedule 7 for drainage basins having an area greater than 60 km². The diameter of the circular conduits must be calculated using the method provided for in Schedule 8, i.e. based on the flow rate, type of conduit entrance (projection, bevelled or straight) and burial, as well as the presence or not of outlets.

Additional information

Validation of peak flow calculation when the result of this calculation imposes the installation of a conduit with a diameter much larger than the average width of the watercourse

The methods for calculating peak flows in Schedules 6 and 7 are based on several assumptions, which may make them less suitable for some drainage basins. Sometimes significant differences can be observed between calculated peak flows and flows recorded in the field.

Where the results of flow calculations require the installation of conduits with diameters far greater than the average width of the watercourse observed in the field, it is possible, based on evidence indicating the water level reached by flooding in previous years or by establishing a relationship with basins measured on or near the same territory, to notify the management unit in charge Ce lien ouvre une nouvelle fenêtre prior to the installation of a conduit with a diameter less than that determined by the calculation. Supporting documentation submitted by the professional in charge must be submitted (including, but not limited to, calculation of peak flow, average watercourse width, free flow of fish to be insured or not, result of field surveys reflecting choice, identification of inconsistency in flow calculation, etc.).

In this situation, the diameter of the conduit should be consistent with the average width of the watercourse to be crossed. Under no circumstances should the conduit reduce the width of the watercourse by more than 20%, measured at the upper bank limit. The height of water in the conduit must be less than or equal to 85% of the clearance height after it is buried.

Installation of a culvert to cross a watercourse with a width of 600 mm or less

The rational method prescribed in Schedule 6 is used to establish the 10-year interval peak flow (Q₁₀) at the crossing point of the watercourse for drainage basins having an area equal to or less than 60 km². This method is based on several hypotheses, and as a result may not be well adapted to certain drainage basins. A difference, sometimes significant, may be observed between the peak flows calculated and the flows measured in small intermittent watercourses linked to small drainage basins. Indeed, sometimes significant differences can be observed between the calculated peak flows and the flows recorded in small rivers associated with small watersheds.

In this context, the procedures must be applied for watercourses with a width of 600 mm or less, mapped or not:

The culvert does not need to be laid out in a way that ensures the free flow of fish. As a result, there is no need to verify if any of the situations described in the first and second paragraphs of section 103 are present.
The culvert must be laid out in accordance with the following conditions:
1. the diameter of the conduit must be at least 450 mm;
2. the conduit must be installed following the natural slope of the watercourse and be buried under the watercourse bed to a depth equivalent to 10% of its diameter, without however exceeding 500 mm, regardless of its size;
3. the culvert may include one smooth wall conduit or two in the case of parallel conduits.
When the diameter of the conduit determined according to the calculated peak flow is greater than the width of the watercourse, the conduit installed must belong to the diameter class closest to the average width of the watercourse. The conduit must in no case reduce the width by more than 20%, measured at the upper limit of the banks.
When the diameter of the conduit determined according to the calculated peak flow is less than the width of the watercourse, the conduit installed cannot reduce the width of the watercourse by more than 50%, measured at the level of the upper edge of the banks.
It is acceptable not to calculate the peak flow rate of the watercourse before installing the culvert provided that the conduit installed is in the diameter class closest to the average width of the watercourse. The conduit must in no case reduce the width by more than 20%, measured at the upper limit of the banks. If, under the second paragraph of this section, the Minister requests the calculation of the peak flow rate of the watercourse, the reason which guided the choice not to make this calculation before the work must be provided to him within 48 hours.

How is the width of a watercourse measured?

The width of a watercourse is determined using the average of at least four representative measurements of the natural watercourse taken upstream and downstream from the crossing point. If signs of narrowing or broadening are present, the section concerned must be excluded (for example, a zone disturbed by a beaver dam). The width is measured at the upper limit of the banks, determined as indicated in the definition of “bank” or “shore”.

Every person authorized to build or improve a bridge or a culvert must, at the request of the Minister, give to the Minister within 48 hours of the request the calculations of the peak flow performed prior to the work. The foregoing also applies to a person who repairs a bridge or a culvert.

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Objective

To allow for regulatory compliance to be checked

Additional information

Installation of a culvert to cross a watercourse with a width of 600 mm or less

It is acceptable not to calculate the peak flow rate of the watercourse with a width of 600 mm or less before installing the culvert provided that the conduit installed is in the diameter class closest to the average width of the watercourse. The conduit must in no case reduce the width by more than 20%, measured at the upper limit of the banks. If the Minister requests the calculation of the peak flow rate of the watercourse, the reason which guided the choice not to make this calculation before the work must be provided to him within 48 hours.

Unmapped watercourse

It is difficult to calculate the peak flow of watercourses that are now shown on the ecoforest maps. In such a situation, it is not mandatory to complete the calculation before installing the culvert if its conduit belongs to the diameter class closest to the average width of the watercourse. The conduit must in no case reduce the width by more than 20%, measured at the upper limit of the banks.

How is the width of a watercourse measured?