Welcome to Week 3 of our series exploring Defra’s new National SuDS Standards.
This week we’re looking at SuDS Standard 3: Managing Extreme Rainfall & Flooding.
Your drainage system works perfectly, until it doesn’t. What happens when the 1-in-100-year storm hits?
Standard 3 ensures drainage systems handle extreme rainfall up to 1-in-100-year events (plus climate change) and that exceedance routes safely direct water away from buildings.
Why This Matters:
Extreme rainfall events pose the highest risks to people, property, and infrastructure. Without adequate allowance for climate change uplift, developments face potential internal flooding, insurance challenges, and liability exposure.
Development in flood risk areas requires particular attention.
Planning authorities increasingly require detailed exceedance routing, showing safe overland pathways that protect buildings and critical infrastructure. Poor exceedance design is a common reason for technical objections.
Real Examples:
• Designing overland flow routes along streets and open spaces.
• Providing raised building thresholds to prevent internal flooding.
• Design SuDS features that can manage the design storm.
What You Need to Know:
Undertake detailed hydraulic modelling that includes climate change uplifts, blockage scenarios (especially for infiltration features), and identify safe overland flow paths. Finished floor levels and street gradients must work together to manage flood risk.
Standard 3 includes a tight restriction on allowable flow rates and orifice sizes and requires the use of the latest FEH methods when designing.
Factor in the cost and space requirements for overground attenuation storage, safe flood routes and raised thresholds. Ignoring exceedance early can lead to regrading of whole streets and increased construction costs late in the programme. Poor level design can trap water against buildings or in basements, leading to insurance challenges and planning objections.
Come back next Monday as we move on to Standard 4: Water Quality.
Click here for more information on our solutions to sustainable drainage and how we can help you.