What Are SUDS?

Sustainable Urban Drainage Systems (SUDS) are techniques for managing stormwater in cities by mimicking natural drainage. Instead of rapidly piping water away, SUDS slow, store, infiltrate, and treat rainwater close to where it falls.

This approach reduces flood risk, improves water quality, enhances biodiversity, and creates more attractive, liveable urban spaces.

Why Traditional Drainage Is Failing

Conventional pipe-based drainage struggles in modern, dense cities:

• Higher runoff volumes from impervious surfaces (concrete, asphalt, roofs)
• Combined sewer overflows (CSOs) that discharge polluted water during storms
• Urban heat islands due to lack of vegetation and open water
• Reduced groundwater recharge because sealed surfaces block infiltration

Climate change, with more intense and frequent storms, further overloads ageing drainage networks.

Types of SUDS Techniques

SUDS combine multiple measures into an integrated "management train" from source to regional scale.

Source Control

• Green roofs: Vegetated roofs that absorb rainfall, reduce runoff, and add insulation
• Rainwater harvesting: Collecting roof runoff for non-potable uses (irrigation, toilet flushing)
• Permeable pavements: Surfaces that let water pass into a storage layer below

Conveyance

• Swales: Shallow, vegetated channels that convey and filter runoff
• Filter strips: Gently sloping vegetated zones that receive sheet flow from hard surfaces

Treatment and Storage

• Bioretention areas / rain gardens: Planted depressions with engineered soils that filter and treat runoff
• Constructed wetlands: Shallow, vegetated basins that treat water via natural physical and biological processes
• Detention basins: Areas that temporarily store runoff and release it slowly
• Infiltration trenches and soakaways: Subsurface gravel-filled or modular structures that store and infiltrate water

Benefits of SUDS

1. Flood risk reduction: Lower peak flows and reduced urban flooding
2. Water quality improvement: Removal of sediments, metals, hydrocarbons, and nutrients
3. Groundwater recharge: Infiltration-based systems help replenish aquifers
4. Biodiversity enhancement: New habitats for insects, birds, and amphibians
5. Urban cooling: Vegetation and water features mitigate heat islands
6. Aesthetic and recreational value: Greener, more pleasant public spaces
7. Climate resilience: Flexible management of both heavy rain and drought

Design Principles: The Management Train

Effective SUDS design follows a hierarchy:

1. Prevention: Reduce runoff through water-efficient design and good practice
2. Source control: Manage rainfall where it lands (e.g. green roofs, permeable paving)
3. Site control: Manage water within the development (e.g. swales, rain gardens)
4. Regional control: Manage remaining flows at larger scale (e.g. basins, wetlands)

Each stage reduces volume and improves quality before passing water downstream.

Regulatory Context in Spain and Catalonia

• The Catalan Water Agency (ACA) promotes SUDS in new developments
• Municipal planning ordinances (e.g. Barcelona, Sabadell, Terrassa) increasingly require SUDS in new builds and major refurbishments
• EU directives (Water Framework Directive, Floods Directive) encourage natural water retention and sustainable drainage

Raindrop's SUDS Services

Raindrop offers end-to-end SUDS consultancy and design:

• Feasibility studies: Site assessment (soils, groundwater, constraints, regulations)
• Hydraulic modeling: Simulation of runoff, flow paths, and SUDS performance under various storms
• Detailed design: Engineering drawings, specifications, and construction support for all SUDS types
• Maintenance planning: Long-term maintenance and monitoring strategies
• Integration with conventional systems: Hybrid solutions combining SUDS with traditional drainage

Raindrop supports developers, municipalities, and architects in delivering SUDS that are technically robust, visually appealing, and environmentally beneficial.