Drainage Gradients and Falls

When it comes to creating efficient drainage systems, the concepts of drainage gradients and falls are fundamental.

These elements play a pivotal role in ensuring the smooth flow of water and wastewater through pipes, preventing blockages, leaks, and flooding. If you’re planning or maintaining a drainage system, understanding these principles will save you both time and money over the long run.

It is also crucial to adhere to building regulations and consult the relevant approved document, such as the Building Regulations 1991 – Drainage and Waste Disposal Approved Document, to ensure your drainage system meets all legal and safety requirements.

At BDS Drainage, we are passionate about helping businesses and homeowners create robust, compliant drainage solutions. This guide takes a comprehensive look at drainage gradients and falls, factors that influence drainage design, and how new technologies and regulations in 2025 will shape the industry.

What are drainage gradients?

Defining drainage gradient

A drainage gradient refers to the slope of your drainage pipe, measured over a specific distance. This slope ensures that water and waste can flow freely from one end to the other using gravity. Without the right gradient, water might pool in the pipe, leading to clogs and costly repairs. An adequate gradient ensures a sufficient vertical fall or vertical drop to keep water and solids moving efficiently through the system.

Maintaining an average gradient is essential for system efficiency, as it helps prevent solids from settling and reduces the risk of blockages.

A proper gradient is critical to avoid two common issues:

If the slope is too steep (e.g., steeper than 1 in 40), water flows faster than solids, leaving debris behind that eventually causes blockages.
If the slope is too shallow (e.g., less than 1 in 110), water lacks momentum to move debris, leading again to blockages.

How drainage gradient affects drainage

The drainage gradient adjusts how efficiently water flows through the pipes, including drainage pipes and foul water pipework. Steeper gradients are suitable for quickly draining stormwater, whereas shallower ones may work for smaller domestic systems typical of domestic applications and often involving sloping foul water pipes. Calculating the gradient correctly ensures your pipes work effectively, accommodating the nature of the surface water or foul water they handle.

Calculating drainage gradient

You can calculate the gradient of a drainage pipe using the above formula, which is essential for designing effective pipe schemes. The formula is:

Gradient = Fall divided by the given distance

For example, if you have a 0.6m fall over a 48-metre section of pipe, the calculation would be:

Gradient = 0.6 / 48 = 0.0125

This means the gradient is 1 in 80 (gradient 1 0.0125 80), which is a typical scenario in many pipe schemes and metre sections. Commencing calculations with a gradient of 1 in 80 is standard practice to ensure proper flow and avoid blockages. Using this method, you can determine the required pipe drops for effective drainage installation.

You can also calculate the fall if the gradient and given distance are known by rearranging the above formula. The same formula applies to various pipe schemes and can be used for different given distances to ensure correct installation and flow.

Gradient = Fall ÷ Distance

For instance, if a 48-metre pipe section drops by 0.6 metres:

Gradient = 0.6 ÷ 48 = 0.0125
Converted ratio = 1 ÷ 0.0125 = 1 in 80 (gradient written as 1 in 80)

This means the drainage pipe has a gradient of 1 in 80, which is ideal for most systems. This ratio ensures efficient flow without exacerbating risks of blockages or erosion.

Understanding drainage falls

Defining drainage fall

Drainage fall refers to the vertical distance (also known as vertical fall or vertical drop) by which a pipe or channel drops over its length. It ensures that gravity aids in moving water, preventing stagnant pools within the system.

How drainage fall affects drainage

A proper fall ensures that water continuously flows along the entire drainage path, providing adequate flow and maintaining adequate flow velocities. Insufficient falls, particularly in flat terrains, reduce water velocity, causing solids to slow and increasing the chance of sediment build-up and blockages. Conversely, excessive falls can cause high flow rates, leading to wear and erosion within the pipe.

How to calculate fall

The formula to calculate a drainage fall is:

Fall = Gradient × Distance

To calculate the fall, you multiply the pipe gradient by the distance. The pipe gradient is the ratio of the vertical drop (fall) to the horizontal distance. In other words, the gradient is the fall divided by the distance. This calculation helps ensure the correct slope for effective drainage.

For example, if the required gradient is 1:80 and the pipe run is 10 meters, the fall is:

Fall = 1/80 × 10 = 0.125 meters (125mm)

This result is the pipe fall for that section.

Understanding pipe gradient and how to calculate the fall is essential for proper installation and to comply with building regulations and the approved document. Proper pipe fall and gradient prevent blockages and ensure efficient flow in drainage systems.

Fall = Gradient x Distance

For example, for a 50-metre section of pipe (metre section) with a gradient written as 1 in 80, the fall over the given distance of 50 metres is calculated as follows:

Fall = 0.0125 × 50 = 0.625 metres (or 625 millimetres)

Get these calculations wrong, and you’re likely to face performance or compliance issues down the track. But don’t worry, at BDS Drainage, we simplify this complex process for our clients.

Key factors in drainage design

Proper drainage design is about more than just gradient and fall. There are critical factors to consider to ensure long-term success and efficiency:

When selecting pipe material and size, the internal diameter of pipes and the invert level are critical for accurate flow calculations and ensuring proper water movement. Invert levels must be carefully considered when connecting sections of pipe to maintain consistent flow and prevent blockages. The layout must also account for surface level slopes, as uneven or steep gradients may require special design features to ensure effective drainage.

For manhole and access chamber construction, materials such as precast concrete, precast concrete rings, and several precast concrete rings are commonly used due to their durability and ease of assembly. The on site construction process can be significantly sped up by using precast concrete rings, which allow for faster and safer installation, especially for deeper manholes.

Access to the drainage system is provided by access chambers, inspection chambers, and plastic access chambers, which are essential for maintenance tasks, system inspection, and cleaning. In domestic systems, domestic market plastic is often used for access chambers due to its suitability for residential applications. Cleaning rods are commonly used via access chambers to clear blockages and maintain system functionality.

Deeper manholes are equipped with step irons, assist access, and gain access features to ensure safe entry for maintenance and inspection. Cast iron lids, iron lids, and galvanised steel lids are specified for manholes and access chambers in areas where traffic crosses the drainage system, providing the necessary strength and durability.

By considering these factors—such as invert levels, internal diameter, material selection, access chamber types, maintenance tools, and the impact of surface level slopes—you can ensure a robust and efficient drainage system design.

Drainage pipe material and size

The choice of pipe material (PVC, concrete, etc.) and its diameter influences flow capacity and resistance to wear. The internal diameter and invert level are essential for accurate flow and gradient calculations, as they determine the effective area for water movement and the elevation at which water enters and exits the pipe. Bigger pipes or highly durable materials may be necessary for areas with heavy water flow or complex systems. When connecting pipes of different sizes, invert levels must be checked to ensure proper alignment and prevent flow disruptions.

Soil type

Compact soils like clay create more resistance and slower absorption, which may require steeper gradients. Sandy soils, however, drain more effectively and can use gentler slopes.

Climate and rainfall

Locations with heavy rainfall need systems designed with higher water capacity to prevent overflow. Drainage systems must also account for potential flooding during extreme weather events.

Common drainage problems and solutions

Even with proper gradients and falls, drainage systems can still face challenges. Here are some of the most common issues and how BDS Drainage tackles them:

Blockages

Caused by debris, grease, or inappropriate materials in the pipes. High-pressure jetting or mechanical rodding clears these effectively. In systems with steep pipe drops, drop manholes and back drop manholes are installed to manage the gradient and prevent solids from being left stranded, which helps in preventing blockages.

Leaks

Often caused by deteriorated or damaged pipes. Solutions include CCTV inspections and either patch lining or full-length re-lining.

Root intrusion

Tree roots can breach underground pipes, leading to blockages or structural damage. BDS employs root cutting technology and pipe repairs to sustainably resolve this issue.

The drainage gradients and falls 2025 guide

Looking ahead to 2025, the drainage industry is changing rapidly. Here’s what to expect:

New pipe schemes are being developed to ensure that the recommended fall and proper pipe gradient are maintained in all installations, aligning with updated standards and best practices.

New technologies

Artificial intelligence is now powering drainage analysis. Sensors installed in pipes can automatically monitor flow and predict blockages before they occur. Combining this with improved pipe materials means systems now perform more efficiently and last longer.

Updated regulations

Standards around gradients and pipe installation are becoming stricter to comply with environmental sustainability policies. For instance, rainwater management is focused on reusing water rather than releasing it into sewers. Staying compliant will help avoid penalties or system redesigns.