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SuDS Components CIRIA C753 §19 · BS 8545

Tree Pit / Stockholm System Calculator

CIRIA C753 §19: Stockholm-style structural soil tree pit sizing for SuDS.

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1
Site & Tree
Parameters
2
Pit Design
Dimensions
3
Storage & Hydrology
Calculations
4
Results
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Site & Tree Parameters ?

Project Information

Between 1 and 50 trees

Site Configuration

Shared Site Parameters

These parameters are shared across the Catchment. Update them here or in your site profile.

Source: EA May 2022 Climate Change Allowances Table 3 (peak rainfall intensity, surface water). Choose by time horizon + band: 2050s central 20 % / higher 25 % / upper 35 %, 2080s central 25 % / higher 30 % / upper 40 %, for 3.3 % AEP (30-year) sizing. For 1 % AEP (100-year) use the higher column (e.g. 2080s upper = 50 %). Check EA guidance for your river-basin district.
📍 UK overview map for site context

Pit Design ?

Pit Dimensions

Tree Root Ball

Pit Void Properties

Drainage Properties

Typical infiltration rates per BRE 365: Sand 10⁻⁴–10⁻³ m/s (≈0.4–4 m/hr), Loam 10⁻⁶–10⁻⁵ m/s, Clay 10⁻⁸–10⁻⁷ m/s

Groundwater & Environmental Constraints

Design Guidance (BS 8545:2014)

  • Minimum pit depth: 0.6m (1.0m preferred)
  • Small tree pit: 8m³ minimum
  • Medium tree pit: 15m³ minimum
  • Large tree pit: 25m³ minimum
  • Structural soil system allows shared soil volume across trees
  • Modular systems provide maximum void ratio (90%+)

Storage & Hydrology ?

Design Rainfall

Flow Calculations

Calculations based on CIRIA C753 and Defra SuDS standards. Climate change uplift applied to rainfall intensity.

Inflow Volume

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m³

Storage Capacity

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m³

Storage Balance

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m³

Infiltration & Drainage

Infiltration Outflow

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m³/event

Half-Drain Time

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hours (max 24h per BRE 365)

Interception & Evapotranspiration

S2 Interception Volume

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m³ (5mm standard rainfall)

S2 Compliance

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Status

Evapotranspiration (indicative water balance)

Supporting estimate only, based on FAO-56 reference evapotranspiration for UK lowlands. Not applied to the S2 compliance verdict above.

ET Rate

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mm/day (≈ annual ET ÷ 365)

Annual ET Volume

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m³/yr (potential, from pit area)

Interception Recovery

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days to restore 5 mm store by ET

Hydrology Assumptions

  • Inflow Volume = (Impervious Area × Rainfall Depth × (1 + Climate Change %) ) / 1000
  • Rainfall Depth = Intensity (mm/hr) × Duration (hours)
  • Storage Capacity = Pit Length × Width × Depth × Void Ratio × Number of Trees
  • Infiltration Outflow = Infiltration Rate × Pit Base Area × Duration (if enabled)
  • Half-Drain Time = Storage Volume / (Infiltration Rate × Base Area)
  • S2 Interception = Impervious Area × 0.005m (5mm standard)
  • Evapotranspiration (indicative) = FAO-56 reference ET0 ≈ 550 mm/yr (UK lowland) × pit surface area; potential upper estimate, not applied to the S2 verdict
  • Climate change factors align with Defra 2025 guidance

Results & Summary ?

Design Summary

Summary will appear here after calculation

Compliance Status

Design Recommendations

Standards Referenced

  • CIRIA C753 §19 – Bioretention systems (tree pits as bioretention subtype)
  • Trees in the Townscape Guide (Trees & Design Action Group)
  • BS 8545:2014 – Trees: from nursery to independence
  • Stockholm Method / Silva Cell design specifications
  • Defra 2025 National SuDS Standards (S2, S5, S6)
  • BRE 365 – Hydraulic properties of soils (drainage testing)