Drainage flow rate calculation: Why it matters

Knowing the drainage flow rate capacity of channels is arguably the most important aspect to consider when installing both surface water drainage and foul water drainage systems.

Choosing undersized drainage channels can create blockages, causing water to seep into the building’s walls, structure, and the surrounding environment. And no one wants to deal with water seepage and flooding.

How can we calculate drainage flow rate accurately? What are the variables to consider when planning how to calculate drainage flow rate for a specific project? Let’s find out.

Variables for calculating surface water drainage flow rate

The most important variable when choosing a drainage channel is the amount of surface water that it will have to accommodate. More specifically, the volume of rainwater that needs to be drained is the top priority to consider: How much rainwater passes through the channel in a given period of time? What is the slope of the terrain where it will be installed? How important is the size and grid of the channel drain cover?

How to calculate flow rate: Volume of rainwater to remove

The volume of rainwater that passes through the channel varies according to location and expected rainfall. Among the variables to keep in mind for a correct calculation of the expected volume of water are:

  • The area, in square meters, where the outflow of water is expected. This is calculated based on the points with higher precipitation (under gutters, for example).
  • The runoff coefficient. This figure is calculated on an annual basis and takes into account the permeability of the surfaces involved in the drainage system. For example, agricultural soil and meadows have a relatively lower coefficient (around 0.10 - 0.15) as they absorb water more easily than asphalt, which has poor permeability (0.85 - 1.00).
  • Precipitation intensity expressed in millimeters per hour. The expected rainfall intensity is determined by observing historical data.

Using these inputs, the formula to predict the peak runoff is called the Rational method:





V = Maximum rate of runoff in l/sec

B = runoff coefficient

lcr = maximum rainfall intensity, expressed in mm/h

A = area of the drainage surface in m2

The channel drain slope

Once the maximum rate of runoff is known, we need to consider the slope at which the channel drain will be installed in order to choose a suitable channel. The greater the slope, the greater the flow rate.

However, keep in mind that there is an absolute minimum slope for drainage channel installation, which is at least 0.5%.

Drainage channel sizes

Drainage channel sizes are directly linked to the maximum rate of runoff and to the slope for drainage channel installation. It goes without saying that it is advisable to lay a channel with a greater hydraulic capacity than the expected rainfall intensity in order to avoid flooding and other complications from poor water disposal.

Drainage products come with technical data sheets which help you understand the drainage channel size most suitable for your drainage system on the basis of rate of runoff and slope.

Drainage channel grates and covers

To allow pedestrians, cyclists, and vehicles to pass over drainage channels, high quality grates and covers are required. In addition to protecting the channels, they also reduce the risk of blockages from debris.

There is an abundance of choice of channel drain grates. Each type of grating and cover offers different benefits depending on their typical use. Factors such as the type of traffic, the expected weight, the mechanical and chemical stresses it will experience (as required by the European Standard UNI EN 1433:2008), and the outflow requirements play a part in selecting the appropriate grate. 


We covered the variables to take into account when choosing a drainage channel and provided the basics for surface water flow rate calculation. Keeping these variables in mind, here are a few common mistakes when choosing channels for a drainage system:

  • Choosing drainage channels that are the wrong size. Channels that are too small can cause overflowing, channels that are too big will take up unnecessary space.
  • Installing the channel at the wrong slope, which causes inefficient water flow and may result in stagnation of water.
  • Not keeping in mind surrounding soil composition.
  • Incorrectly placing the channels.

When in doubt, consult a skilled installer when designing and installing a drainage channel system. All Dakota drainage products come with technical data sheets that provide guidelines on installation.

Want more info? Read our basic guide on how to install channel drains.

Dakota Group is a building materials supplier that serves primarily the EMEA region. As a leading European supplier of building materials, we offer a range of drainage supplies that work together within highly performant systems. We have established ourselves as a reliable drainage supplier, with over four thousand customers having chosen our products.

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