Round
Elliptical
Box/Rectangular
These pipe shapes can work in an incredible variety of situations. Round being the simplest and widely used of all the shapes, boxes are used when flow is higher, and elliptical are used where there are additional issues such as vertical horizontal constraints. But even so, many of these pipe shapes can be used in lieu of one another. We need to be informed with the best information possible to make the most prudent choice.
Round pipe is the most used shape for all pipe materials for many reasons. It allows for the most easily reproducible shape as many of the machines used for the manufacturer of a reinforced concrete pipe are set up for round pipes. These machines can manufacture a range of size from 300mm to 3000mm.
Specifications as well as CSA A257 are accepted, at minimum, throughout all municipalities within Ontario. Individual municipalities may have additional requirements for any reinforced concrete pipe that will be installed. As part of the Canadian Precast Concrete Quality Assurance Program, a pipe will be removed from a production run and tested to ensure these minimums are met. These tests are required to be documented and reported to the governing agency to ensure that quality is consistent.
But a circle needs support on the outside! Therefore, when installing a round pipe, it is important to meet the provincial and local requirements to provide a proper bedding to support the pipe.
Elliptical pipes get to do something special that circular cannot, they can fit in places where there is not enough room for a circular pipe! If there is not enough cover above the pipe, then an equivalent sized horizontal elliptical pipe can be used to convey the water. What about horizontal clearance? If there is not enough room for a circular pipe, a vertical elliptical pipe can go in that space with the flowline being in the narrow part of the pipe.
Elliptical pipes are synonymous with the solution to utility conflicts. In many urban areas, utilities are ubiquitous. It is hard to develop an area without needing to consider the placement of other utilities. Elliptical pipes really shine where there is also an issue of low cover, and you must maintain a certain amount of flow. That is because these pipes have equivalent capacity when compared to circular pipes. Therefore, it is an easy task for either the engineer or the contractor to determine if an elliptical pipe is an acceptable alternative to RCP.
Now let’s think inside the box! Round, and elliptical, has it’s own benefits and the incredibly ubiquitous use in development. However, it is not a universal answer to all situations. Box Culverts allow for higher hydraulic capacities than other shapes of pipe. This allows for higher flow rates. The main benefit of box is the versatility of having a wide range of span and rise dimensions.
That is where a box culvert comes to center stage. Box Culverts can have less cover and the distribution slab is not a benefit. Culverts are easy to install and can be installed in parallel or several rows.
A unique benefit of a box culvert is the culvert can be used as a structure itself. This means that any lateral piping coming into the box or vertical attachment, such as a maintenance hole tee, can be installed directly into the box. This is true for large pipe also. This ease means that if the contractor installs the box culverts according to manufacturer, provincial and regional specifications, any branching lines or structures can be located and tied into the main alignment without having to worry about locating a junction box. This can save time in installation and save money for the client. One could even have a transition of a circular pipe into a box culvert.
Therefore, a theoretical design and development of a storm system could look as such: a main line of culverts that has transitions from a circular pipe to box culverts of varying sizing (as needed according to the hydraulic design), laterals and vertical structures connecting to the main line without a junction box, then going directly to the outfall.
When reviewing options, understanding the environment and the conduit hydraulic capacity is key. Many considerations have to be taken into account, including the land’s topography, water table, existing roads, utilities, and other considerations all play in the selection of pipe shapes.
Flow volume will differ from shape to shape, which will be another factor in determining if you need a specific sizing or shape and if you need multiple rows of structures to safely discharge the stormwater.
The most widely accepted formula for evaluating the hydraulic capacity of non-pressure sewers is the Manning formula:
This formula is:
Where:
Q = discharge in cubic meters per second
n = Manning’s roughness coefficient
A = cross-sectional area of flow in square meters
R = hydraulic radius in meters (equals the area of the flow divided by the wetted perimeter)
S = slope of pipe line in metre of vertical drop per metre of horizontal distance
Under any give flow conditions, the area A and the hydraulic radius R are constant for a particular size and shape of pipe. The hydraulic capacity of pipe on a give slope is primarily dependent on n, the roughness coefficient. Table 1 provides recommended values for roughness coefficient for concrete pipe.
These values are substantiated by extensive research and have been adopted for use.
Since the designer is usually concerned with selecting a conduit size for a given design flow and constant pipe slope, the Manning formula can be rewritten to compare equivalent flow capacity as:
By evaluating the hydraulic capacity for the various types and sizes of pipes available. Based on the Manning’s formula, the charts below have been calculated to provide a comparison of products, shapes and hydraulic capacities.
Note: columns showing the runoff coefficient represents the hydraulic capacity and is expressed in m3/s.
An example of how to select an equivalent based on the hydraulic capacity is provided below.
For 1800mm pipe with a roughness coefficient of n=0.013, the hydraulic capacity of the pipe is 119.9 m3/s. In order to meet the hydraulic capacity the 1800mm pipe, a 2305×1465 HE pipe which provides 117.5 m3/s would be an equivalent. When considering box culverts, a 2400×1200 box would suffice as it has a hydraulic capacity of 120 m3/s.
