A better approach to rainwater drainage

by | Jul 12, 2023

Commercial partnership: Capcon

How Capcon used siphonic technology in a recent Dublin project to reduce raw materials by 70 per cent and overall rainwater costs by 35 per cent

In a recent major construction project in Dublin, the team at Capcon demonstrated yet again the significant benefits of early engagement with a rainwater drainage design specialist.

The savings across pipework meterage, raw materials, installation days, and overall costs were considerable.

By redesigning the conventional gravity drainage system to an optimised hybrid solution of siphonic and gravity drainage, we reduced the total cost of the rainwater package from €306,000 to €199,000, saving the client over 35 per cent or €100,000 in the cost of the rainwater drainage.

The traditional approach: Need for improvement

Conventionally, the design and coordination of the rainwater drainage system are often deferred to the main contractors, who then engage specialist sub-contractors to finalise the rainwater drainage design after primary building services have been coordinated.

While this “late engagement” method may be adequate for smaller projects, larger or more complex structures will likely encounter several challenges with this approach:

  • Coordination Issues: With the main building services already designed, there’s an increased risk of coordination problems.
  • Design Limitations: The flexibility to design the most efficient drainage system for a building’s specific requirements is significantly reduced. Late engagement means the rainwater is often shoehorned in or rerouted to fit in.
  • Increased Costs and Inefficiency: More pipework means more raw materials and extended installation times.
  • Uncertainty: Late engagement of rainwater specialists means uncertainty around the cost and scope of the rainwater drainage system often with knock-on risks to other services.

Siphonic drainage: The game-changer

Capcon Engineering are rainwater specialists – with particular expertise in siphonic drainage as well as traditional gravity drainage.

Essentially, siphonic drainage uses a vertically-induced vacuum to siphon rainwater from the roof of a building at high-velocity.

Siphonic drainage has a number of key features that distinguish it from traditional gravity drainage systems.

Key features of siphonic systems

  1. Full-bore flow: Siphonic systems uniquely feature an anti-vortex baffle plate at the roof outlet. This plate stops air from getting in, allowing the water to fill the entire capacity of the pipe – a phenomenon known as “full bore” flow. As a result, the pipe diameters in siphonic systems are usually about half the size of those in gravity systems, where two-thirds of the pipe’s volume needs to be air to help move the water.
  2. Zero-gradient runs: Unlike traditional gravity systems that require a downward slope to function, siphonic systems operate at zero-gradient. This makes siphonic drainage systems far easier to coordinate with other services in congested environments, achieving routes that would not be possible with a traditional gravity system.
  3. Long horizontal runs: The unique design of siphonic systems allows for long horizontal runs of pipe which are not possible for conventional gravity systems requiring downward slopes.
  4. Significantly reduced number of downpipes: Because siphonic systems can run horizontally for such long distances it significantly reduces, even eliminates the number of internal downpipes required. Multiple roof outlets can be combined and routed to single downpipe, which can be located in virtually any strategic location.
  5. Internal underground drainage is often eliminated: The long horizontal, zero gradient runs and the ability to combine multiple outlets to a single, strategically located downpipe means the requirement for internal underground drainage and manholes is often eliminated.
  6. Self-cleansing: Once a siphonic system is “primed” – which means it has enough water in it to start the siphoning process, usually about 10-15 per cent of its design capacity – the water flows through at such a high speed that it effectively flushes out any debris that might have slipped past the leaf guard and anti-vortex plate.

Advantages of siphonic systems for buildings

A siphonic or hybrid siphonic gravity solution offers several significant advantages for design of buildings:

  • Efficient, flexible and space saving: The flexibility of a siphonic system with zero-gradient, long horizontal runs and small diameter pipework can be utilised to solve complex routing issues, save valuable space and significantly reduce, if not eliminate internal downpipes, in-ground drainage and internal manholes. Siphonic systems are inherently versatile and can be utilised alongside traditional gravity pipework to form a hybrid solution catering to a building unique requirements such as green roofs, blue roofs, podiums, harvesting and attenuation requirements.
  • Efficient design is cost-effective and environmentally conscious: With early engagement an engineered siphonic drainage system bespoke to a building specific requirements will be highly efficient, typically with smaller diameter pipework and less meterage. In turn, less raw material and installation time are required, significantly reducing overall project costs. Using and transporting fewer raw materials, reducing civil drainage and installation time results in a more sustainable project with a reduced carbon footprint.
  • Reduced risk: Siphonic systems are far more equipped to cater for the extreme weather events than gravity systems. Firstly, all siphonic systems are designed with an additional 10 per cent security as per EN and BS standards. Secondly siphonic systems are high performing and capable of draining a roof far quicker than a gravity system. Thirdly, siphonic outlets can drain up to 65 litres per second – if an outlet is blocked for any reason, the other outlets on the system have spare capacity to drain the full volume of rainwater from the roof.

The Dublin project: A typical example of re-engineering for value

The project in question is a mixed-use development in Dublin’s city centre, consisting of three blocks and a network of terraces.

Capcon was provided with a traditional gravity design by the architects that used a network of 2400 meters of gravity pipework to drain the roofs and podiums.

With no harvesting tank or attenuation tank on-site, all systems had to be routed to one side of the building.

Whilst it was possible to drain all three blocks and podiums with a gravity system, our rainwater experts were able to devise a highly efficient hybrid system of siphonic and gravity drainage that was not only best-in-class but also provided substantial savings.

Quantifiable benefits

The implementation of a hybrid system resulted in significant reductions:

  • reduction of pipework from 2400 metres down to 1500 metres: 37 per cent saving
  • reduction of raw materials from 4.4 tonnes to 1.3 tonnes: 70 per cent saving
  • reduction of installation time from 110 to just 67 days: 40 per cent saving
  • reduction of project costs from €306,000 down to €199,000: 35 per cent saving

We also provided an end-to-end solution, including the design, supply, and installation of gravity drainage to the podium and along the building expansion joints.

This comprehensive approach streamlined the construction process, preventing potential complications and offering the construction partner a seamless rainwater drainage solution.

The Capcon team

Empowering construction professionals

The Dublin project underscores the practical benefits and substantial cost savings that can arise from early specialist engagement and the strategic implementation of siphonic drainage.

Gaining an excellent understanding of siphonic drainage and its problem-solving capabilities can have tangible benefits for all construction professionals.

The right knowledge can positively influence design decisions, leading to more efficient, cost-effective, and environmentally friendly outcomes in large-scale and high-tech construction projects.

With this in mind, Capcon has developed a 45-minute, Engineers Ireland Approved CPD “An Introduction to Siphonic Drainage”.

This course provides an in-depth understanding of siphonic drainage and its applications and is delivered in-house or online. You and your team can acquire essential knowledge of siphonic drainage, enhancing your capacity to utilise this approach in future projects for improved efficiency and cost-effectiveness.

At Capcon, we believe in the power of sharing knowledge and enhancing industry understanding as we continue to contribute towards a future of sustainable, efficient and best-in-class construction.

Capcon Engineering are Rainwater Specialists with special expertise in siphonic drainage solutions.

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