Culvert Design

Culverts should be located and aligned as closely as possible to the natural flow path to minimize disturbances to historical flow patterns, which can eventually result in erosion and/ or siltation.

Maximum Headwater

The maximum headwater for the design storm flow for culverts greater than 1000 mm diameter shall be 1.5 times the culvert height. The maximum headwater for culverts with a height of 1m or less shall be 1.5m if adjacent properties are not adversely affected. If the design flow exceeds 15 m3/s in an urban area, the maximum headwater shall not exceed the height of the culvert for an ultimate condition.

Flow Velocities

The culvert entrance will be affected by the approach velocity in the upstream ditch or channel. The approach velocity can be calculated assuming normal depth (Manning’s Equation) using the total design flow. In reality, this is a conservative estimate since the total design flow would not arrive at the culvert entrance until some ponding has already occurred. However, since the integrity of the roadway is at stake, this increased factor of safety may be warranted.

Flow velocities inside the culvert should also be evaluated. A non-erosive (velocity less than 4.5 m/s), self-cleaning (velocity greater than 0.6 m/s) is the target. If flow velocities are too low, suspended sediment will settle. Over time, this deposition will gradually reduce the hydraulic capacity of the culvert if not properly maintained. Alternatively, excessively high exit velocities may lead to erosion and eventually jeopardizing the integrity of the culvert and roadway.

Culvert Size, Geometry, Material, Strength

Culverts are available in many different shapes, sizes, and materials. Common culvert shapes include circular, arch, horizontal ellipse, vertical ellipse, and rectangular (box). Culverts sizes can vary from 300 mm to 2850 mm. Culvert materials include but are not limited to: concrete, metal, masonry, asbestos cement, vitrified clay pipe, and plastic. Specifically, reinforced concrete, metal, and plastic are most common in designs today.

Preliminary Design

At an early conceptual stage of a project, required roadway crossings can be identified by overlaying roadway alignments onto existing topography. Preliminary culvert designs can be started based on the assumptions and taking the steps outlined below:

Step 1: Assume inlet control which is acceptable for most roadway crossings.

Step 2: Assume worst case design flow, possibly an ultimate, full build-out design condition.

Step 3: Determine available headwater, measuring vertical distance from ground at the proposed culvert inlet to the finished grade elevation of the roadway shoulder (assuming that no adjacent ground point is lower).

Step 4: Assuming circular concrete pipe, minimum diameter of 300 mm and maximum diameter based on available headwater depth, Dmax = 0.67 x HW

Step 5: If either (i) a larger diameter is needed to satisfy the headwater constraint, or (ii) the diameter is greater than 1800 mm, then use multiple barrels, dividing the total flow evenly between the barrels.

Step 6: Check exit velocity. For velocities greater than 1.5 m/s, some level of erosion protection will be needed.

Once an initial assessment has been conducted, a more detailed design can continue that will consider precise location and alignment, design flow, culvert(s) size and material, entrance structure layout, outlet structure layout, erosion protection, headwall, wingwall and possible emergency overflow path.