The drainage systems of today’s urban areas face two major challenges: flooding and water pollution. Without careful planning, new developments can significantly exacerbate these issues. At Harban Redwood, our structural engineers play a key role in designing tailored drainage solutions for each project’s specific needs. Using our expert understanding of sustainable drainage systems (SuDS), we help to manage surface water in built-up areas which lack natural drainage, helping to mitigate flood risks and improve water quality.
Dealing with Drainage Issues
Many of the key challenges facing modern drainage systems are closely linked to broader global environmental concerns. As the impacts of climate change become more pronounced, the risk of flooding continues to rise. At the same time, ongoing urban expansion into natural floodplains places additional strain on existing drainage infrastructure.
Moreover, as global pollution levels increase, drainage systems must be designed to effectively manage and filter contaminants. Contrary to common perception, rainwater often contains pollutants, and in some cases, improper wastewater management by utility providers results in sewage entering natural watercourses.
Necessity of Proper Drainage Systems
Effective drainage systems are essential for managing both foul and stormwater, safeguarding public health and protecting the environment.
Wastewater from sources such as toilets, kitchens, and bathrooms is directed to treatment facilities before being safely returned to natural water bodies and eventually abstracted for potable use. However, inadequate drainage can lead to water source contamination,and increase the risk of waterborne diseases.
In urban areas, impermeable surfaces like roads and pavements prevent rainwater from naturally infiltrating the ground, heightening the risk of flooding. This is particularly problematic in the UK, where approximately 70% of the sewer network combines foul and stormwater in the same pipes.
During heavy rainfall, these systems are often overwhelmed, leading to sewer overflows and pollution. While separate drainage systems can reduce pollution and overflow risks and allow for smaller treatment facilities, they require more space and are more expensive to install.
A practical and sustainable approach combines piped wastewater systems with Sustainable Drainage Systems (SuDS), which help manage stormwater closer to its source and reduce pressure on conventional infrastructure.
Flood Zones
The Environment Agency (EA) regulates flood Zones in the UK depending on the risks from a variety of sources, including rivers, seas, surface water runoff, and reservoirs.
Flood Zone 1 – These areas exhibit the lowest flood risk and generally the fewest restrictions for development. However, sites over 1 hectare must get a Flood Risk Assessment if they are within a Critical Drainage Area or have changed to a more vulnerable usage category.
Flood Zone 2 – With a medium risk of flooding, development in these areas requires an FRA.
Flood Zone 3 – These are high flood risk areas, and are divided further by local authorities into Zones 3a and 3b, the latter being functional floodplains with the strictest development controls.
Storm Water Drainage
Effective management of stormwater drainage systems can reduce the volume and flow rate of local watercourses, thereby preventing flooding. Implementing SuDS can also help mitigate water pollution. The criteria for a sustainable design vary depending on how ground infiltration or discharge into water bodies can be implemented based on location, runoff quantity, and quality.
SuDS
Below are some key components commonly used in SuDS, each contributing to reducing flood risk, improving water quality, and enhancing biodiversity.
Common Types of SuDS Components
Examples of SuDS components include swales, permeable paving, detention basins, and green roofs to manage water runoff from collection to discharge. Here are some of the most popular:
Rainwater Harvesting
Rainwater harvesting involves the collection and storage of rainwater from roofs and impermeable surfaces in tanks. The water can then be treated if necessary, depending on its intended use. This is a sustainable and climate-resilient solution that conserves water, reduces surface water runoff, and lessens drainage system burdens. However, its feasibility depends on demand, available roof area, rainfall patterns, and installation and maintenance costs.
Green Roofs
Vegetation on roofs provides a natural method of managing rainwater, and reducing runoff during hot, dry periods. However, they tend to be less effective in wet, winter months.
When accessible or visible, green roofs also offer amenity value and have high ecological benefits, supporting biodiversity in urban areas. Their feasibility depends on structural capacity and is generally best suited to flat or gently sloped roofs.
Blue Roofs
Blue roofs contain vegetation and a water storage layer designed to hold rainwater temporarily. They allow no discharge during light rain, but during medium and heavy rainfall, the water is slowly released in a controlled manner to prevent flooding and reduce pressure on drainage systems. They can only be used where the roof structure can support higher loads.
Infiltration Systems
Infiltration systems help manage runoff by allowing the water to seep into the soil. Where these systems are implemented, it must be ensured that the ground has sufficient absorbency.
The system requires sufficient space to maintain a clearance of at least 1 m from the groundwater level and the bottom of the drainage system. To prevent flooding or water damage, it must be at least 5 m. away from the structure.
Foul Water Drainage
When making a foul drainage assessment, a structural engineer must consider the property’s vicinity to a sewer, the need for non-main drainage, and the size of the development. A system will carry foul water drainage to a public sewer, a private sewer, a septic tank with a wastewater treatment system, or a cesspool (if allowed).
Effective planning and early-stage feasibility surveys for foul water drainage and flood risk assessments are crucial for minimising flooding and ensuring the longevity and resilience of urban developments.
If you require a Flood Risk Assessment, SuDS planning, or drainage system design, the expert engineers at Harban Redwood are ready to provide tailored, professional support. The earlier you prepare for your construction or renovation with an early-stage feasibility survey, the safer your property will be in the long term.
