WIS 4-26-01 leak tightness standard explained

Surface water entering foul sewer networks through manhole covers is often overlooked, but the latest Water Industry Specification shows how significant that inflow can be. WIS 4-26-01 was published in August 2024 to provide a reference test method for leak tightness of covers used on foul sewers, helping specifiers and asset owners compare products and select the right level of protection for the application.

Source: Water UK Standards Board, WIS 4-26-01: Specification for leak tightness of covers for manholes and inspection chambers, Issue 1, August 2024.

WIS 4-26-01 is a UK Water Industry Specification for the leak-tightness of covers for manholes and inspection chambers. It is a supplementary requirement for covers that already comply with BS EN 124, and it is intended for use on foul sewers. The test applies to covers, with or without additional components such as seal plates, with a clear opening up to and including 1000 mm.

In simple terms, the standard provides the industry with a consistent way to measure how much water can pass through a cover when submerged. That matters because even limited ponding above a foul sewer manhole can introduce unnecessary clean water into the wastewater network.

WIS 4-26-01 states that a single 600 x 600 mm double triangular cover, manufactured within EN 124 tolerances, was found capable of admitting 16 litres per second under 150 mm depth of surface water. The document says this is equivalent to the design peak foul flow from 350 houses.

The same foreword goes on to state that, when prising slots and potential open keyways are included, inflow rose to 22 litres per second, equivalent to the design peak foul flow from 475 houses. It also notes that at only 5 mm immersion depth, a 600 x 600 mm double triangular cover within EN 124 tolerances was still found to admit more than 3 litres per second.

For context, the standard also notes that many small foul-only pumping stations operate at around 11 litres per second capacity. In practical terms, that means inflow through a submerged cover can be enough to place a significant load on foul-only infrastructure.

Where surface water enters a foul-only network, that water still has to be pumped and treated as wastewater. WIS 4-26-01 highlights that submergence of even a small number of covers can overwhelm the capacity of pipelines, pumping stations or treatment works. It also states that consequences can include foul flooding, environmental contamination, and increased pumping and treatment costs.

That is why leak tightness should be seen as a measurable performance issue, not simply a product feature.

WIS 4-26-01 assigns covers a leak tightness classification from Class 0 to Class 3, based on their maximum measured inflow rate, expressed as litres per square metre per 30 minutes of submergence. The maximum permissible inflow rates in the standard are:

This gives specifiers and network operators a common basis for selecting the level of sealing performance needed for each location.

WIS 4-26-01 was developed to help designers, specifiers and procurement teams choose covers that are appropriate to the sewerage system under consideration and the risk of submergence at that location. The foreword also notes one water company’s view that the cost of assessing each foul sewer cover location individually can outweigh the cost of selecting Class 3 products more widely across the foul network.

For us, that reinforces a simple point. Where foul networks are at risk from infiltration, leak tightness needs to be considered at cover level as part of the wider asset strategy.

WIS 4-26-01 gives the industry a clearer framework for understanding and comparing cover leak tightness. The figures in the specification show just how much water can enter a foul network through a standard cover under relatively shallow ponding.

That makes leak tightness an important consideration for foul sewer applications, particularly where network capacity, pumping performance and unnecessary treatment of clean water are concerns.

A standard 600 x 600 mm cover within EN 124 tolerances can admit:

• 16 litres per second at 150 mm ponding depth
• 22 litres per second when prising slots and potential open keyways are included
• More than 3 litres per second at just 5 mm immersion depth

WIS 4-26-01 provides a recognised method for testing and classifying that performance, giving the industry a stronger basis for specifying leak-tight solutions on foul sewers.

WIS 4-26-01 highlights the scale of surface water inflow that can occur through standard manhole covers. In response, we offer solutions to help control infiltration at cover level.

Our SealAble retrofittable seal plates provide a flexible approach to sealing. Made to work with compatible SealAble manhole frames from our Highway and Unite ranges, these ductile iron plates can be installed after the frame is in place. This allows network operators to introduce sealing where it is needed, upgrade performance over time, or retrofit plates as network conditions change.

We also offer flood and odour control plates. These are factory-fitted to the base of the frame and remain in place throughout the products service life, providing consistent sealing performance without the need for later installation.

Together, these options allow specifiers and operators to choose between flexible, retrofittable sealing and permanent, integrated solutions, depending on the needs of the network