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Storage Sizing Defra NSTS S2

Interception Storage Calculator

Standard 2: first 5 mm interception storage compliance for the design rainfall depth.

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Site Information
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Interception Assessment
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Compliance Summary
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Project Information

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Recorded for project context. NSTS Standard 2 interception is a fixed first 5 mm of every rainfall event and is not scaled by climate-change allowances (which affect event magnitude/frequency, not the interception depth).

Drainage Sub-Catchments

Add all drainage areas and their associated SuDS components. Each row represents a sub-catchment contributing to interception storage.

Name/Ref Total Area (m²) Impermeable Area (m²) SuDS Type SuDS Area (m²) Infiltration Rate (m/s) Gradient Action

Site Summary

Total Site Area
0 m²
Total Impermeable Area
0 m²
Total SuDS Area
0 m²
Coverage Ratio
0%

Interception Requirements (2025 National Standard S2)

All impermeable surfaces must manage the first 5mm of rainfall through SuDS interception features. The required interception volume for each sub-catchment is calculated as: Impermeable Area × 0.005m

Sub-Catchment Assessment

Sub-Catchment Required Vol. (m³) Component Type Compliance Mechanism Ratio Status

Seasonal Compliance Requirements

2025 National Standard S2 requires different interception thresholds for seasonal conditions:

Summer (May-Oct)
80%

of rainfall events intercepted

Winter (Nov-Apr)
50%

of rainfall events intercepted

Design Note: Deemed compliant components automatically satisfy these seasonal thresholds. Non-deemed components require detailed design verification to demonstrate compliance with seasonal percentages.

Deemed Compliance Reference

Components marked as "DEEMED COMPLIANT" automatically satisfy 2025 National Standard S2 Requirements 2.6-2.7. Others require detailed demonstration.

Component Type Deemed Compliant Criteria
Green Roof (unlined) Impermeable area ≤ green roof area (drains own roof)
Permeable Paving (unlined) Contributing impermeable area ≤ paving area
Bioretention Impermeable area ≤ 5× vegetated area; OR ≤ 25× if infiltration rate > 1×10⁻⁶ m/s
Swale (unlined) Gradient <1:100 AND impermeable area ≤ 5× swale area
Detention Basin (unlined) Impermeable area ≤ 5× basin base area
Infiltration Trench Deemed compliant for roads; size per BRE DG365
Rainwater Harvesting Designed per BS EN 16941-1:2024
Soakaway Sized per BRE DG365
Permeable Paving (lined) NOT deemed compliant: requires calculation
Attenuation Pond NOT deemed to deliver interception

Overall Compliance Status

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Design Status
PENDING

Add drainage areas to assess compliance

Interception Volume Summary

Total Required Volume
0 m³
Deemed Compliant Provision
0 m³
Non-Deemed Provision
0 m³
Sub-Catchments Status
0/0

Component Breakdown by Type

Component Type Count Total Area (m²) Deemed Compliant Requires Design Verification

Assumptions and Limitations

  • Interception volume calculated as impermeable area × 5mm (0.005m) depth
  • Deemed compliance criteria based on 2025 National Standard S2 Requirements 2.6-2.7
  • Infiltration rate thresholds apply only to bioretention systems
  • Swale gradient deemed compliance requires <1:100 slope
  • Seasonal percentages (summer 80%, winter 50%) are planning thresholds
  • Design verification required for non-deemed components and pre-treatment arrangements
  • This assessment does not substitute for detailed hydraulic design or site-specific ground investigation
  • Local authority requirements and site constraints may override deemed compliance

Export Your Assessment

Generate a professional PDF report of your S2 interception compliance assessment, including all calculations, deemed compliance analysis, and recommendations.

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