Newsletter November 2023

Non-return valves play a key role in drainage design and the flood protection of buildings.

Whilst they have been used for many years, the growing demand for urban development, the impact of climate change and the strain on sewer infrastructures, have made the correct use of non-return valves more important than ever.

Our engineers at Civilistix have shared 4 essential tips to help ensure non-return valves perform as intended and effectively mitigate the risk of flooding from sewers:

1. Location: safeguarding against sewer and rainwater intrusion.

Non-return valves are typically installed to prevent sewer backup during flash floods. However, it is often overlooked that once the valve is shut to protect the system from sewer water, it hinders the outflow of rainwater from roofs and external areas which can then build up in the system.

Incorrect valve placement can result in water accumulation within low points of the local drainage network, generally in undesirable locations such as lightwells, internal gullies or even toilets or kitchens.

When choosing the location, it is critical to understand where the water will go once the valve is closed. This is particularly significant in older combined drainage systems where rainwater downpipes share drains with the foul drainage. To address this issue, consider installing an external gully lower than the finished floor level. This offers a safe route for excess water to exit, as stated in Part H of the Building Regulations.

2. Choose the right non-return valve for the level of protection needed.

There is a wide range of valves available, varying in price and design. Each product has different applications and the choice of product should align with the level of protection needed. The following need to be considered when deciding on which non-return valve to use – the level of risk, the installation requirements and how the valve will perform once installed. Flap push fit valves are less expensive and offers flexibility as they can be installed in the chamber inlets. They only need a small amount of flow to open. However, as they are closed until water drains down, there is a risk of blockage under heavy foul flows and they may not be suitable for high-pressure conditions. On the other hand, a cast iron floating ball system remains open by default and has a more robust closure device. This reduces the risk of a blockage. They are required to be installed within an accessible manhole but might face durability issues.

There is a wide range of NRV’s available, including products with a membrane that can be installed in the outlet pipe thereby protecting the entire system with one valve, or built-in valves in small chambers that can raise the pipe against the cover when water flows from the sewer. Whichever valve you use, ensure it is compliant with the BS EN 13564 standards and it provides the level of protection needed for your system. 

3. Don’t delay valve decisions at early design stages.

In construction, it is common to provide notes in drawings indicating the requirement for a non-return valve, leaving the selection and specification for the contractor to choose. This may lead to coordination problems and project delays, as the works required for

the valve may include the construction of a new chamber near foundations or in areas with limited accessibility. To speed up the construction, the contractor may choose an inadequate valve type and location thereby turning the protection measure into a liability. To streamline the process, specify the valve type and location during the early design stages. 

4. Special consideration for Basements and lower ground refurbishments (Low points).

It is usually an advantage when the basement can be connected to the sewer without a pump and can run under gravity. However, as mentioned earlier, once the valve is shut and the system is full of water, appliances at lower levels may lack the necessary pressure to drain against the accumulated water. Where there is a higher risk of flooding from sewers, the installation of a pump might be imposed in the planning consent conditions. If not, consider non-return valves with small built-in pumping systems and emergency closure mechanisms that can be operated remotely.

By following these four tips you can optimise the effectiveness of non-return valves in your drainage design, enhancing flood protection and safeguarding your building against sewer-related flooding events.

Civilistix Consulting Engineers were appointed to prepare a detailed drainage and external hardstanding engineering package for a proposed care home consisting of 60 beds along with associated external hardstanding area.

Our design utilised a full gravity approach to both surface and foul water. We kept the drainage system as shallow as possible whilst simultaneously preventing conflicts between drainage runs and ensuring surcharging during more severe storm events did not exceed cover levels.

Our SuDS strategy was integrated with the space we had available and utilised permeable paving to the central car parking area with a beneath geo-cellular storage tank releasing runoff at 5 l/s.

Further to the proposed permeable paving accommodating a portion of the attenuation storage provision, it also allowed level design to be more flexible removing the need to orchestrate falls allowing for improved access to less mobile personnel within the car home facility. It also meant positive drainage features such as gully’s and linear channels could be omitted, and savings made.

Geo-cellular storage tanks are a type of underground or surface stormwater management system used for the temporary storage, attenuation, and infiltration of rainwater or stormwater. They are designed to manage and control stormwater runoff, reduce the risk of flooding, and promote groundwater recharge. Geo-cellular storage tanks are typically used in urban and construction environments where space is limited and traditional above-ground retention or detention ponds are not feasible.

These storage tanks are made up of modular units or cells that are typically made from high-density polyethylene (HDPE) or other durable materials. These cells are assembled to create a matrix or grid structure that can be installed underground or on the surface. They are designed to support the weight of vehicles, making them suitable for installation under parking lots, roadways, or other paved surfaces.

The key functions and benefits of geo-cellular storage tanks include:

• Stormwater Retention: They temporarily store excess stormwater, reducing the peak flow rates during heavy rain events and preventing downstream flooding.
• Infiltration: Geo-cellular units can be designed to allow for the infiltration of stormwater into the ground, promoting groundwater recharge.
• Water Quality: They can be integrated with sedimentation and filtration systems to improve the quality of the stormwater before it is released into the environment.
• Space Efficiency: Geo-cellular units are space-efficient, making them suitable for areas with limited space for stormwater management.
• Versatility: These systems can be customized and designed to meet specific project requirements.
• Sustainable Stormwater Management: They help mitigate the adverse effects of urbanization on natural hydrology and reduce the strain on existing stormwater infrastructure.
• Load-Bearing Capacity: Geo-cellular units can support the weight of vehicles and provide structural stability when installed beneath paved surfaces.

Geo-cellular storage tanks are an important component of modern stormwater management practices, helping to address water runoff issues in urban areas and promote sustainable water management. They are often used in combination with other stormwater management techniques, such as permeable pavements, green roofs, and bioretention areas, to create comprehensive and sustainable stormwater management systems.