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SuDS & LLFA Requirements

In the UK, surface water drainage designs must be approved by the Lead Local Flood Authority (LLFA) or the relevant Water Company (if adopting under a Section 104 agreement). These authorities enforce the National Standards for Sustainable Drainage Systems (SuDS) to ensure developments are resilient against climate change and do not increase flood risk.

This platform provides the tools to demonstrate compliance with these rigorous standards.

1. Climate Change Allowances​

As global temperatures rise, rainfall intensity is predicted to increase. LLFAs require drainage networks to be designed to accommodate these future increases.

  • The standard requirement is to simulate the 100-year return period storm plus a Climate Change Allowance (CC).
  • Historically, a flat 30%30\% increase was applied (e.g., 100yr+30%100yr + 30\%).
  • Modern guidance (based on the latest Environment Agency data) often mandates localised allowances ranging from 35%35\% to 45%45\%, depending on the river basin district and the design life of the development.
  • In the Platform: You can define multiple Return Period tests in the Schedule Panel, seamlessly adding a percentage CC allowance to the base rainfall data.

2. Urban Creep​

Urban creep refers to the gradual increase in impermeable area over the lifetime of a development (e.g., homeowners building conservatories or paving over driveways).

  • LLFAs generally require a 10%10\% allowance for urban creep on all residential plots.
  • This means if a housing estate has 1.0ha1.0 ha of roof area, the model should test it as 1.1ha1.1 ha to ensure the attenuation structures will not fail 50 years in the future.
  • In the Platform: You can use the "Additional Catchment Area" field on a Node to explicitly apply urban creep values without overriding your base CAD-measured catchment areas.

3. No Flooding Criteria​

The standard performance criteria for a UK drainage network are strictly defined:

  • 1 in 1-year storm: No surcharging of the pipe network (water levels must remain below the pipe soffit).
  • 1 in 30-year storm: Surcharging is permitted, but absolutely no surface flooding can occur (water levels must remain below manhole cover levels).
  • 1 in 100-year + CC storm: Flooding is permitted only if it is safely retained on site within designated areas (e.g., car parks, dry swales) and does not pose a hazard to buildings or third-party land.

When reviewing the Schedule Panel in the platform, these criteria are instantly visible. The platform highlights surcharged nodes and flooded volumes, allowing you to rapidly adjust pipe diameters or add crate storage to bring the system into compliance.

4. The SuDS Management Train​

LLFAs expect designers to follow the SuDS hierarchy, prioritising source control and surface features over deep underground pipes:

  1. Prevention: Rainwater harvesting, green roofs.
  2. Source Control: Permeable paving, soakaways.
  3. Site Control: Swales, detention basins.
  4. Regional Control: Large retention ponds.

While the platform expertly models the hydraulics of pipes and underground crates, you can also model above-ground SuDS features. By assigning a "Pond" or "Swale" storage type to a node and defining its depth-area profile, the 1D routing engine will accurately simulate the filling, attenuation, and slow release of water from these sustainable green infrastructure assets.