Calculator Constants

Adjust the key constants used in the BRE 365 and CIRIA C697 calculations:

CIRIA C697 - The SUDS Manual (2015)

CIRIA C697 provides comprehensive guidance on sustainable drainage systems (SUDS), complementing BRE 365 with additional considerations:

Key SUDS Design Principles:

• Water quantity management - Managing runoff volumes and peak flows
• Water quality treatment - Removing pollutants through filtration and sedimentation
• Amenity value - Providing visual and recreational benefits
• Biodiversity enhancement - Creating wildlife habitats and green corridors

Soakaway Design Considerations from CIRIA C697:

• Integration with overall site drainage strategy
• Treatment requirements for different pollution risks
• Long-term maintenance and performance monitoring
• Climate resilience and adaptive capacity
• Multi-functional design opportunities

Performance Standards:

• Hydraulic performance - As per BRE 365 but with additional freeboard considerations
• Water quality - TSS removal ≥80% for most applications
• Structural integrity - Design life of 100+ years with appropriate maintenance
• Safety - Safe access and emergency overflow provisions

1. Fundamental Design Equation (BRE 365)

The core principle of soakaway design is based on the water balance equation:

• I = Inflow from impermeable area (m³)
• O = Outflow infiltrating into soil (m³)
• S = Required storage volume (m³)

2. Rainfall Inflow Calculation

• A = Impermeable area drained to soakaway (m²)
• R = Design rainfall depth for storm duration (m)

Design rainfall values are determined from meteorological data using return period factors Z1 and Z2 from Tables 1 and 2 in BRE 365.

3. Infiltration Outflow Calculation

• a_s50 = Internal surface area at 50% effective storage depth (m²)
• f = Soil infiltration rate determined from soakage trial pit (m/s)
• D = Storm duration (seconds)

4. Soil Infiltration Rate Testing

Infiltration rate must be determined through soakage trial pits:

• V_c75-25 = Volume of water between 75% and 25% effective storage depth (m³)
• t_c75-25 = Time for water level to fall from 75% to 25% effective storage depth (s)
• a_s50 = Internal surface area at 50% effective storage depth (m²)

Test Requirements:

• Trial pit 1m to 3m long, 0.3m to 1m wide
• Same depth as proposed soakaway
• Fill and drain three times to near empty
• Record time for each drainage cycle
• Use lowest infiltration rate from three tests

5. Climate Change Considerations

Both BRE 365 and CIRIA C697 emphasize the importance of climate resilience:

BRE 365 Recommendations:

• 40% increase in rainfall intensity (updated from 30% based on latest UKCP data)
• Based on UKCP projections for 21st Century
• Applied to design rainfall values before calculation
• Accounts for increased intensity of extreme rainfall events

CIRIA C697 Additional Considerations:

• Adaptive management - Design systems that can be upgraded if climate projections change
• Resilience planning - Consider failure modes and emergency overflows
• Temperature effects - Account for increased evapotranspiration and soil moisture changes
• Seasonal variation - Design for winter storage and summer infiltration capacity

6. Performance Criteria

Half-drain time requirement:

The soakaway must drain from full to half-full within 24 hours to provide capacity for subsequent storms.

Minimum infiltration rate:

• f ≥ 1 × 10⁻⁶ m/s for acceptable soakaway performance
• Below this rate, soakaways become impractical

7. Construction Requirements

• Minimum 5m clearance from building foundations
• Above groundwater table - minimum 1m clearance
• Geotextile lining to prevent fine particle migration
• Granular fill material (20-40mm graded gravel)
• Access provisions for inspection and maintenance
• Surface water only - no foul drainage connection

8. Design Process Summary

1. Determine rainfall ratio from site location (Figure 3, BRE 365)
2. Calculate design rainfall using Z1 and Z2 factors
3. Conduct soakage trial pit testing
4. Calculate inflow volume (I = A × R)
5. Calculate outflow volume (O = a_s50 × f × D)
6. Determine required storage (S = I - O)
7. Size soakaway dimensions
8. Verify half-drain time ≤ 24 hours
9. Apply climate change factor (typically +30%)

9. References

• BRE Digest 365 (2016) - Soakaway design
• CIRIA C697 (2015) - The SUDS Manual