Advanced Soakaway Sizing Engine (ASSE)
Complete User Guide & Technical Reference Manual
Methodology Summary (BRE Digest 365 & CIRIA C753)
This tool performs soakaway design calculations in accordance with BRE Digest 365 (2016) and CIRIA C753. The design objective is to ensure the soakaway has sufficient volume to store stormwater runoff from a specific storm event (typically 1-in-10 or 1-in-25 year return periods) minus the volume lost to infiltration during the storm.
Key Principles
1. Mass Balance ( Inflow (I) - Outflow (O) = Required Storage (S) )
2. Inflow Calculated using the Rational Method ( I = A × Rainfall )
3. Outflow Calculated via soil infiltration ( O = as50 × f × Duration × 60 )
4. Half-Empty Check The structure must discharge half its volume within 24 hours to be ready for subsequent storms.
5. Cover & Clearance Minimum 1.0m cover above structure and 1.0m clearance to groundwater (BRE Digest 365).
1. Introduction to ASSE
The Advanced Soakaway Sizing Engine (ASSE) is a sophisticated web-based engineering tool for designing stormwater soakaway systems in accordance with international standards including BRE Digest 365 (2016) and CIRIA C753.
Design Objective: To calculate optimal soakaway dimensions that safely manage stormwater runoff from specific design storm events while ensuring regulatory compliance and cost-effectiveness.
What is a Soakaway?
A soakaway is an underground structure designed to temporarily store surface water runoff and allow it to infiltrate into the surrounding soil. Properly designed soakaways:
- Prevent local flooding during storm events
- Recharge groundwater aquifers
- Reduce strain on conventional drainage systems
- Provide sustainable stormwater management
Key Features of ASSE
- Multi-Standard Compliance: Designs compliant with BRE Digest 365 and CIRIA C753
- Climate Resilience: Incorporates climate change projections for future-proof designs
- Geotechnical Analysis: Comprehensive soil classification and infiltration assessment
- Visualization Tools: 2D schematics and 3D models for design validation
- Professional Reporting: Export capabilities to PDF, Excel, and Word formats
- Cost Estimation: Integrated cost analysis for budget planning
Important Notice: The ASSE tool provides engineering guidance based on standard methodologies but does not replace site-specific geotechnical investigation and professional engineering judgment. Always verify soil conditions with field testing and consult local regulations.
2. Methodology & Design Standards
Design Standards Implementation
The ASSE tool implements methodologies from the following internationally recognized standards:
BRE Digest 365 (2016) - Soakaway Design
Provides comprehensive guidance on soakaway design for small developments, including calculation methods, soil infiltration testing procedures, and design criteria for various applications.
CIRIA C753 - The SuDS Manual
Offers detailed guidance on sustainable drainage systems (SuDS), including infiltration devices and their integration into wider drainage strategies for both new developments and retrofits.
Fundamental Design Principles
- Mass Balance Principle: Inflow - Outflow = Storage Requirement
- Critical Storm Approach: Design for the storm duration that produces maximum storage requirement
- Hydraulic Efficiency: Optimize geometry for effective infiltration
- Environmental Protection: Maintain groundwater quality and prevent contamination
- Structural Integrity: Ensure adequate cover and clearance requirements
Design Storm Selection
ASSE supports design for two standard return periods:
- 1-in-10 Year Storm: Standard design for most residential and commercial applications
- 1-in-25 Year Storm: Enhanced design for critical infrastructure or high-consequence applications
| Application Type | Recommended Return Period | Climate Change Factor | Notes |
|---|---|---|---|
| Residential Gardens | 1-in-10 year | RCP 4.5 (2050) | Standard residential applications |
| Commercial Parking | 1-in-10 year | RCP 4.5 (2050) | Car parks, driveways |
| Critical Infrastructure | 1-in-25 year | RCP 8.5 (2050) | Hospitals, emergency services |
| High-Value Developments | 1-in-25 year | RCP 8.5 (2080) | Luxury residential, commercial |
3. Step-by-Step Workflow
Project Setup
Rainfall Data
Catchment Definition
Soil Analysis
Climate Considerations
Dimension Input
Calculation
Results Review
Export
Project Information Setup
Begin by entering basic project details that will appear in all reports:
- Project Name: Descriptive identifier for the project
- Client: Name of client or organization
- Engineer: Responsible design engineer
- Date: Design date (auto-populated with current date)
- Design Standard: BRE Digest 365 & CIRIA C753 (default)
Soakaway Configuration
Select the appropriate soakaway structure type based on site conditions and project requirements:
| Structure Type | Typical Applications | Porosity Range | Advantages | Limitations |
|---|---|---|---|---|
| Rectangular (Granular Fill) | Standard residential, small commercial | 0.25 - 0.35 | Cost-effective, simple construction | Lower void ratio, larger footprint |
| Circular (Concrete/Granular) | Space-constrained sites, retrofit applications | 0.25 - 0.35 | Structural integrity, space efficiency | Higher cost, specialized installation |
| Plastic Geocellular Cells | High-volume applications, limited depth | 0.90 - 0.95 | High storage capacity, lightweight | Higher material cost, potential for collapse |
Rainfall Data Input
Define the design rainfall characteristics:
- Rainfall Region: Select from predefined regions (UAE, KSA, Qatar, Egypt) or use custom data
- Return Period: 1-in-10 year or 1-in-25 year storm event
- Intensity-Duration-Frequency (IDF) Data: Tool provides regional data or accept custom values
Rainfall Data Example - Dubai (UAE)
Pro Tip: Use the "Add to Project List" feature to compare multiple design alternatives before finalizing your selection. This allows for optimization based on cost, space constraints, and performance.
4. Input Parameters & Data Requirements
Catchment Parameters
Define all impermeable surfaces contributing runoff to the soakaway:
Surface Area
Runoff Coefficient (C)
Surface Type
| Surface Type | Runoff Coefficient (C) | Typical Range | Notes |
|---|---|---|---|
| Roofs | 0.8 - 1.0 | 0.8 - 1.0 | Depends on roof material and slope |
| Pavements | 0.7 - 0.9 | 0.7 - 0.9 | Concrete or asphalt surfaces |
| Roads | 0.8 - 0.95 | 0.8 - 0.95 | Higher values for impermeable surfaces |
| Lawns | 0.05 - 0.35 | 0.05 - 0.35 | Depends on soil type and slope |
| Gravel | 0.15 - 0.30 | 0.15 - 0.30 | Permeable but some runoff |
Geotechnical Parameters
Characterize soil conditions through classification, testing, or standard values:
Soil Type
Infiltration Rate (f)
Groundwater Level
Seasonal Variation
Structural Parameters
Define the physical characteristics of the proposed soakaway:
Length (L)
Width (W) / Diameter (D)
Porosity (n)
Minimum Cover
Actual Cover
GW Clearance
Data Quality Note: The accuracy of ASSE calculations depends heavily on the quality of input data. Always use site-specific measurements where possible, particularly for soil infiltration rates and groundwater levels.
5. Calculation Methodology
Fundamental Mass Balance Equation
Where:
- \( S \) = Required storage volume (m³)
- \( I \) = Inflow volume during storm (m³)
- \( O \) = Outflow volume through infiltration (m³)
Inflow Calculation (Rational Method)
Where:
- \( A \) = Total catchment area (m²)
- \( C \) = Runoff coefficient (weighted average)
- \( R \) = Rainfall depth (m) = \( \frac{I_{mm/h} \times t_{min}}{60 \times 1000} \)
Outflow Calculation (Infiltration)
Where:
- \( a_{s50} \) = Infiltration surface area at half depth (m²)
- \( f \) = Soil infiltration rate (m/s)
- \( t \) = Storm duration (minutes)
Infiltration Surface Area
For rectangular soakaways:
For circular soakaways:
Storage Volume Calculation
For rectangular soakaways:
For circular soakaways:
Where:
- \( L \) = Length (m)
- \( W \) = Width (m)
- \( D \) = Diameter (m)
- \( H \) = Depth (m)
- \( n \) = Porosity (void ratio)
Critical Storm Determination
ASSE performs iterative calculations for each storm duration to identify the critical duration that produces the maximum storage requirement:
For rectangular soakaways - similar equation for circular
Half-Empty Time Verification
This ensures the soakaway will be ready for subsequent storm events and prevents long-term waterlogging.
Calculation Example
Given: Rectangular soakaway (L=3m, W=2m), Soil infiltration f=1×10⁻⁵ m/s, Porosity n=0.3, Required storage S=5 m³
Infiltration area at half depth: as50 = (3 + 2) × H = 5H
Half-empty time: ts50 = (5/2) / (5H × 1×10⁻⁵) = 50,000 / H seconds
Convert to hours: ts50 = (50,000 / H) / 3600 = 13.89 / H hours
For compliance (ts50 ≤ 24h): 13.89 / H ≤ 24 → H ≥ 0.58 m
6. Soil Classification & Properties
USDA Soil Texture Classification
The ASSE tool uses the USDA soil texture triangle system for soil classification based on particle size distribution:
| Soil Type | Typical Infiltration Rate (m/s) | Particle Size | Drainage Characteristics | Suitability for Soakaways |
|---|---|---|---|---|
| Gravel | 1×10⁻³ to 1×10⁻⁴ | > 2 mm | Excellent | Excellent |
| Sand | 1×10⁻⁴ to 1×10⁻⁵ | 0.05-2 mm | Very Good | Very Good |
| Loamy Sand | 1×10⁻⁵ to 5×10⁻⁶ | Mixed | Good | Good |
| Sandy Loam | 5×10⁻⁶ to 1×10⁻⁶ | Mixed | Moderate | Moderate |
| Loam | 1×10⁻⁶ to 5×10⁻⁷ | Mixed | Fair | Fair |
| Silt Loam | 5×10⁻⁷ to 1×10⁻⁷ | 0.002-0.05 mm | Poor | Poor |
| Clay Loam | 1×10⁻⁷ to 1×10⁻⁸ | < 0.002 mm | Very Poor | Very Poor |
| Clay | < 1×10⁻⁸ | < 0.002 mm | Impermeable | Unsuitable |
Particle Size Distribution Analysis
Determine soil type by percentage composition of sand, silt, and clay:
| Particle Type | Size Range | Drainage Influence | Identification Method |
|---|---|---|---|
| Gravel | > 2.0 mm | Enhances permeability | Visual inspection, sieving |
| Sand | 0.05 - 2.0 mm | Good drainage | Gritty feel, doesn't stain |
| Silt | 0.002 - 0.05 mm | Reduces permeability | Floury feel, slight staining |
| Clay | < 0.002 mm | Very low permeability | Sticky when wet, hard when dry |
Field Percolation Testing
For accurate design, conduct field percolation tests following this procedure:
- Test Hole Preparation:
- Excavate test hole to proposed soakaway depth
- Typical dimensions: 300mm diameter, 1.0m depth
- Ensure sides are vertical and base is level
- Saturation Phase:
- Fill with water and allow to saturate for 4+ hours
- This ensures natural soil conditions are replicated
- Measurement Phase:
- Refill hole with known volume of water
- Measure drop in water level over time (typically 1-6 hours)
- Record multiple measurements for accuracy
- Calculation:\( f = \frac{V}{A \times t} \)
Where V = volume infiltrated (m³), A = internal surface area (m²), t = time (s)
Field Testing Best Practice: Conduct percolation tests at multiple locations across the site to account for soil variability. Test during typical groundwater conditions (not extreme wet or dry seasons).
Design Limitation: Soakaways generally require infiltration rates > 1×10⁻⁶ m/s. For rates < 1×10⁻⁷ m/s, consider alternative drainage methods such as attenuation tanks or swales.
7. Climate Change Considerations
The ASSE tool incorporates climate change projections to ensure long-term resilience of soakaway designs against future rainfall patterns.
Representative Concentration Pathways (RCPs)
ASSE uses IPCC climate models based on different emission scenarios:
| Scenario | Description | Global Temperature Rise | Application |
|---|---|---|---|
| RCP 4.5 | Intermediate scenario with emissions peaking around 2040 then declining | ~2.4°C by 2100 | Standard residential and commercial projects |
| RCP 8.5 | High emissions scenario with continued growth throughout 21st century | ~4.3°C by 2100 | Critical infrastructure, high-value assets |
Climate Change Factors
Rainfall intensity adjustment factors based on IPCC AR6 projections:
| Scenario | 2030 | 2050 | 2080 | 2100 | Confidence Level |
|---|---|---|---|---|---|
| RCP 4.5 (Low) | +5% | +10% | +15% | +18% | Medium |
| RCP 4.5 (Medium) | +8% | +15% | +22% | +25% | Medium-High |
| RCP 8.5 (Medium) | +10% | +20% | +30% | +35% | High |
| RCP 8.5 (High) | +12% | +25% | +40% | +50% | Medium |
Design Recommendations by Project Type
| Project Type | Recommended Scenario | Time Horizon | Additional Considerations |
|---|---|---|---|
| Standard Residential | RCP 4.5 (Medium) | 2050 | Consider 2080 for buildings >50-year design life |
| Commercial/Industrial | RCP 4.5 (Medium) | 2050 | Use RCP 8.5 for critical operational areas |
| Critical Infrastructure | RCP 8.5 (Medium) | 2080 | Include additional 10-15% safety factor |
| High-Value Development | RCP 8.5 (Medium) | 2050 | Consider custom factors based on local studies |
| Temporary Structures | Current Climate | N/A | No climate adjustment for <10 year design life |
Regional Considerations: Climate change impacts vary significantly by region. Middle Eastern regions may experience higher increases in extreme rainfall intensity compared to global averages. Always consult region-specific climate models for critical projects.
Limitation Note: Climate factors in ASSE are based on globally averaged models. For precise regional projections, consult local meteorological agencies or climate research institutions.
8. Compliance Checks & Requirements
Half-Empty Time Requirement
BRE Digest 365 requires that soakaways discharge half their volume within 24 hours:
This ensures the soakaway will be ready for subsequent storm events and prevents long-term waterlogging.
Cover Requirements
Minimum soil cover above the soakaway structure:
This provides:
- Protection against surface loading
- Prevention of surface water short-circuiting
- Adequate space for landscaping
Groundwater Clearance
Minimum clearance between soakaway base and groundwater table:
This prevents:
- Contamination of groundwater
- Reduced infiltration capacity during high groundwater conditions
- Hydraulic connection between surface runoff and groundwater
Seasonal Groundwater Variation
Account for seasonal fluctuations in groundwater levels:
Always design for the highest anticipated groundwater level (typically in wet season).
| Compliance Check | Requirement | Standard | Consequence of Failure |
|---|---|---|---|
| Half-Empty Time | ≤ 24 hours | BRE Digest 365 | Prolonged waterlogging, reduced capacity for subsequent storms |
| Cover Depth | ≥ 1.0 m | BRE Digest 365 | Structural damage, surface water short-circuiting |
| Groundwater Clearance | ≥ 1.0 m | BRE Digest 365 | Groundwater contamination, reduced infiltration |
| Infiltration Rate | > 1×10⁻⁶ m/s | CIRIA C753 | Inadequate drainage, potential flooding |
11. Frequently Asked Questions
Soakaways generally require infiltration rates > 1×10⁻⁶ m/s. For rates between 1×10⁻⁷ and 1×10⁻⁶ m/s, consider larger soakaways or alternative designs. Rates below 1×10⁻⁷ m/s are generally unsuitable for conventional soakaways.
Select scenarios based on project criticality and design life:
- Standard projects: RCP 4.5 with 2050 time horizon
- Critical infrastructure: RCP 8.5 with 2080 time horizon
- Short-term structures: Current climate conditions may suffice
If ts50 > 24 hours:
- Increase infiltration surface area (wider or shallower design)
- Verify soil infiltration rate with field testing
- Consider soils with higher permeability if possible
- Add additional outflow pathways or multiple soakaways
- Consider alternative drainage methods if adjustments fail
The regional data in ASSE is based on published meteorological records and represents typical values for each region. For critical projects or sites with unique microclimates, always use site-specific rainfall data when available.
Yes, ASSE supports multiple design scenarios. Use the "Add to Project List" feature to compare different soakaway configurations and generate a comprehensive project report with multiple designs.
ASSE is designed for small to medium catchment areas typically up to 10,000 m². For larger catchments, consider multiple soakaways or consult a drainage engineer for specialized design approaches.
ASSE includes a "Seasonal GW Variation" parameter that allows you to specify the expected fluctuation in groundwater levels. The tool designs for the worst-case scenario (highest groundwater level) to ensure year-round performance.
Yes, ASSE provides professional PDF, Excel, and Word report exports that include all input parameters, calculations, and compliance checks suitable for submission to regulatory authorities.
Need Additional Support?
If you require further assistance with the ASSE tool or have specific technical questions, please contact:
- Technical Support: support@ahmedesmail.com
- User Manual: Online User Guide
- Training Resources: Video tutorials and example projects available on our website
Feedback Welcome: We continuously improve ASSE based on user feedback. Please share your suggestions for enhancements or report any issues you encounter.
