Bre Digest 365 Soakaway Infiltration Rate Calculation

Inputs

Results

Formula Basis: This calculator uses the methodology outlined in BRE Digest 365. It estimates the infiltration rate based on soil type and then calculates the required soakaway volume to manage a 24-hour rainfall event. The calculations involve determining the soil infiltration rate (f), hydraulic radius (Rh), and then the required storage volume based on the contributing impermeable area and a design rainfall depth.

Assumptions: The soil infiltration rate is derived from typical values for the selected soil type and BRE guidance. The depth to groundwater or impermeable layer is a critical safety factor. The 24-hour rainfall event intensity is based on UK meteorological data, typically around 75mm for design purposes.

Input Parameter	Value	Unit
Soil Type	—	Categorical
Depth to Groundwater/Sealing Layer	—	m
Soakaway Diameter	—	m
Soakaway Depth	—	m
Contributing Impermeable Area	—	m²

The BRE Digest 365 Soakaway Infiltration Rate Calculation is a method used in the UK to design and assess the suitability of soakaways for managing surface water runoff from impermeable areas. Soakaways are underground structures designed to collect rainwater and allow it to slowly infiltrate into the surrounding soil. This process is crucial for Sustainable Drainage Systems (SuDS), helping to reduce the risk of flooding, improve water quality, and recharge groundwater.

This calculation is based on the principles outlined in the Building Research Establishment (BRE) Digest 365. It helps determine how quickly water can drain away from a proposed soakaway, which in turn dictates its required size and its effectiveness in handling rainfall. Proper calculation ensures that the soakaway can manage a design storm event without overflowing or saturating the ground prematurely, which could lead to surface water pooling or groundwater contamination.

Who Should Use This Calculation?

• Civil engineers and drainage consultants designing SuDS for new developments.
• Building control officers and local authority planners assessing drainage proposals.
• Architects and developers incorporating sustainable drainage into site designs.
• Environmental consultants evaluating water management strategies.
• Homeowners or property managers looking to manage surface water runoff from hard surfaces like driveways or patios.

Common Misunderstandings:

• Unit Confusion: Infiltration rates can be expressed in mm/hour, m/day, or l/(m².s). It's vital to use consistent units throughout the calculation, typically mm/hr for the rate and m³ for volume.
• Oversimplification: Assuming any soil can accept any amount of water. Soil permeability varies significantly, and a site-specific assessment (or conservative estimate) is crucial.
• Ignoring Groundwater: Not accounting for the depth to the groundwater table or any underlying impermeable strata, which can severely limit drainage capacity.
• Underestimating Rainfall: Using local rainfall data that is not sufficiently conservative for a design storm event (e.g., a 1-in-30 or 1-in-100 year event plus climate change allowances).

BRE Digest 365 Soakaway Infiltration Rate Formula and Explanation

The core of the BRE Digest 365 methodology involves determining the characteristic infiltration rate of the soil and then using this to calculate the necessary storage volume for a given impermeable area and rainfall event. While the full digest covers various scenarios, a simplified approach for sizing often focuses on the 24-hour rainfall event.

Simplified Calculation Steps:

1. Determine the Characteristic Infiltration Rate (f): This is the key soil property. It's typically derived from Table 1 of BRE Digest 365, which assigns a rate based on soil type and a visual assessment of its permeability. For this calculator, we use representative values.
2. Calculate the Hydraulic Radius (Rh): For a cylindrical soakaway, Rh = Radius / 2.
3. Calculate the Soil Infiltration Rate (v) in m/s: Convert the characteristic infiltration rate (f, usually in mm/hr) to m/s. `v = f / 3600000`.
4. Calculate the Drainage Area (A): This is the area of the soakaway's side wall in contact with the soil. For a cylinder, `A = π * Diameter * Depth`.
5. Calculate the Effective Soakaway Diameter (Ds): This is used to ensure the calculation accounts for the soakaway's geometry in relation to the contributing area. It's calculated such that `Ds = sqrt(Impermeable Area / π) + Rh`. However, a more direct approach often used for sizing is to calculate the required storage volume first.
6. Calculate the Required Storage Volume (V_req): This is the volume needed to contain the runoff from a design rainfall event. A common design storm is 75mm over 24 hours. `V_req = (Impermeable Area * Design Rainfall Depth) – (Area_soakaway_base * Design Rainfall Depth)`. A simplified formula often used is `V_req = Impermeable Area * Design Rainfall Depth`. The calculator will use a common design rainfall of 75mm.
7. Check Soakaway Viability: The infiltration rate calculated helps determine if the soakaway can empty within a reasonable timeframe (e.g., 48 hours). The effective storage volume that can be infiltrated is `V_inf = A * v * Time`. The required storage must be less than or equal to the volume that can be infiltrated.

Key Formula for Storage Volume (Simplified for Calculator):

Required Storage Volume (m³) = (Contributing Impermeable Area (m²) * Design Rainfall Depth (m))

The calculator then compares this required volume to the volume that the specified soakaway can store and infiltrate over time.

Variables Table:

Variable	Meaning	Unit	Typical Range/Notes
Soil Type	Classification of the surrounding soil material.	Categorical	Clay, Sandy Clay, Loam, Sand, Gravel, etc.
Characteristic Infiltration Rate (f)	The rate at which water infiltrates into the soil.	mm/hr	Ranges from < 1 for clay to > 40 for gravel. Derived from BRE Digest 365 Table 1.
Depth to Groundwater/Sealing Layer (d_gw)	Vertical distance from the base of the soakaway to the highest level of groundwater or an impermeable layer.	m	Minimum 1.0m, often more. Critical for safety and performance.
Soakaway Diameter (D_sw)	Diameter of the cylindrical soakaway.	m	Typically 1.5m to 5m.
Soakaway Depth (H_sw)	Total depth of the soakaway structure.	m	Must be less than d_gw. Commonly 1.5m to 4m.
Contributing Impermeable Area (A_imp)	The total area of roofs, driveways, etc., draining to the soakaway.	m²	Highly variable, depends on property size.
Design Rainfall Depth (R_des)	The depth of rainfall assumed for the design storm event.	mm	UK typical design value: 75mm for 24 hours.
Soakaway Storage Volume (V_sw)	The total volume the soakaway can hold.	m³	Calculated as `π * (D_sw/2)² * H_sw`.
Required Storage Volume (V_req)	The minimum volume needed to contain the runoff.	m³	Calculated based on A_imp and R_des.

Practical Examples

Let's illustrate with two scenarios using the calculator.

Example 1: Standard House with Clay Soil

• Inputs:
• Soil Type: Clay
• Depth to Groundwater/Sealing Layer: 1.5 m
• Soakaway Diameter: 3.0 m
• Soakaway Depth: 2.5 m
• Contributing Impermeable Area: 250 m²

Results (from calculator):

• Characteristic Infiltration Rate: Approx. 1.0 – 1.5 mm/hr
• Effective Soakaway Diameter: ~ 11.3 m (Indicates the soil can accept water from a wide area)
• Volume for 24h Rainfall (75mm): 18.75 m³
• Required Soakaway Volume (min): 18.75 m³
• Calculated Storage Volume: 17.67 m³ (Volume of the specified soakaway: π*(1.5)²*2.5)

Interpretation: The specified soakaway (17.67 m³) is slightly smaller than the minimum required volume (18.75 m³). This suggests the soakaway might need to be slightly larger or deeper, or the impermeable area reduced, especially considering the low infiltration rate of clay soil. The effective diameter calculation shows that the soil's low permeability is the limiting factor, not the size of the contributing area relative to the soakaway.

Example 2: Large House/Development with Sandy Loam

• Inputs:
• Soil Type: Sandy Loam
• Depth to Groundwater/Sealing Layer: 2.0 m
• Soakaway Diameter: 4.0 m
• Soakaway Depth: 3.0 m
• Contributing Impermeable Area: 500 m²

Results (from calculator):

• Characteristic Infiltration Rate: Approx. 10 – 15 mm/hr
• Effective Soakaway Diameter: ~ 19.9 m
• Volume for 24h Rainfall (75mm): 37.5 m³
• Required Soakaway Volume (min): 37.5 m³
• Calculated Storage Volume: 37.70 m³ (Volume of the specified soakaway: π*(2.0)²*3.0)

Interpretation: In this case, the calculated storage volume of the soakaway (37.70 m³) is sufficient to meet the minimum requirement (37.5 m³). The sandy loam soil has a much higher infiltration rate, meaning the soakaway is more likely to empty within the acceptable timeframe (e.g., 48 hours). The effective diameter is significantly larger than the actual soakaway diameter, indicating good drainage capacity.

How to Use This BRE Digest 365 Soakaway Calculator

1. Identify Soil Type: Determine the dominant soil type in the area where the soakaway will be located. This is crucial as it dictates the soil's permeability. You can do this through visual inspection (texture, colour), simple soil tests, or ideally, through a percolation test as recommended by BRE Digest 365.
2. Measure Key Dimensions:
   • Depth to Groundwater/Sealing Layer: This is a critical safety parameter. Ensure this depth is accurately known and is significantly greater than the proposed soakaway depth.
   • Soakaway Diameter & Depth: Measure or specify the planned dimensions of your soakaway structure.
3. Determine Contributing Impermeable Area: Calculate the total area of hard surfaces (roofs, patios, driveways) that will drain into the soakaway. This can often be found from site plans or by direct measurement.
4. Input Values: Enter these values into the corresponding fields in the calculator. Ensure units are correct (metres for length, m² for area).
5. Select Soil Type: Choose the appropriate soil type from the dropdown menu.
6. Calculate: Click the "Calculate Infiltration Rate" button.
7. Interpret Results:
   • Characteristic Infiltration Rate (f): This gives you an idea of the soil's drainage capacity (mm/hr). Lower values indicate slower drainage (e.g., clay), higher values indicate faster drainage (e.g., sand/gravel).
   • Volume for 24h Rainfall: Shows the total volume of water runoff expected from the impermeable area during a typical design storm (75mm).
   • Required Soakaway Volume (min): The minimum storage capacity the soakaway must have to contain this runoff.
   • Calculated Storage Volume: The actual volume of the soakaway dimensions you inputted.
   Compare the "Calculated Storage Volume" to the "Required Soakaway Volume". If the calculated volume is less than the required volume, the soakaway is undersized. The "Effective Soakaway Diameter" gives context on how well the soil can accept the water relative to the contributing area.
8. Adjust and Recalculate: If the soakaway is undersized, you may need to increase its diameter, depth, or reduce the impermeable area draining to it. Use the "Reset" button to clear fields and try new values.
9. Copy Results: Use the "Copy Results" button to save or share the calculated figures and assumptions.

Selecting Correct Units: The calculator uses metres (m) for lengths and square metres (m²) for areas. The infiltration rate is expressed in mm/hr, and volumes in cubic metres (m³). Ensure your input measurements are in the correct units before entering them.

Key Factors Affecting Soakaway Infiltration Rate

1. Soil Type and Texture: This is the most significant factor. Clays have small particles and low porosity, leading to very low infiltration rates. Sands and gravels have larger particles and high porosity, resulting in high infiltration rates. Loams and silts fall in between.
2. Soil Structure: The arrangement of soil particles into aggregates affects pore space and connectivity. Well-structured soils with large pores allow faster infiltration than poorly structured ones. Compaction can destroy structure and reduce infiltration.
3. Depth to Groundwater Table: If the groundwater table is high, it reduces the depth of unsaturated soil available for infiltration, significantly limiting the soakaway's capacity and increasing the risk of it becoming saturated.
4. Presence of Impermeable Layers: Clay layers, bedrock, or even compacted subsoil can act as barriers, preventing water from infiltrating further downwards. The calculation must account for the depth to the first such layer.
5. Soakaway Geometry (Diameter & Depth): While the soil type dictates the fundamental infiltration *rate*, the dimensions determine the total *volume* the soakaway can store and the total surface area available for infiltration. A larger diameter or depth increases the potential storage and infiltration surface.
6. Condition of the Soakaway Walls: The "rumbles" or voids within a traditional rubble-filled soakaway provide the primary surface area for infiltration. The void ratio and the condition of these surfaces directly impact how quickly water can seep into the surrounding soil.
7. Vegetation and Surface Cover: While the calculator focuses on the soil itself, surface vegetation can influence water infiltration. Tree roots can sometimes enhance permeability in the immediate vicinity, but dense grass or vegetation cover can also slow down surface runoff reaching the soakaway.
8. Existing Soil Saturation: During prolonged wet periods, the soil may already be saturated, significantly reducing its ability to accept further infiltration, even if its "characteristic" rate is high. The design storm accounts for this implicitly, but extreme conditions can still pose a challenge.

Frequently Asked Questions (FAQ)

Q1: What is the difference between infiltration rate and percolation rate?

A: While often used interchangeably in casual conversation, "infiltration rate" typically refers to the speed at which water enters the soil surface, while "percolation rate" refers to the speed at which water moves downwards through the soil profile. For soakaway design, BRE Digest 365 focuses on the characteristic infiltration rate derived from percolation tests.

Q2: How accurate are the infiltration rates listed for soil types in BRE Digest 365?

A: The rates are indicative and based on typical classifications. Actual soil conditions can vary significantly due to factors like compaction, organic content, and structure. For critical applications, a specific percolation test (as detailed in BRE Digest 365 Appendix D) is strongly recommended to determine a site-specific infiltration rate.

Q3: What is a typical design rainfall depth for a soakaway?

A: For the UK, a common design rainfall depth for a 24-hour storm event is 75mm. However, this can vary depending on the location's susceptibility to extreme rainfall, the consequences of soakaway failure (e.g., properties at risk), and local planning authority requirements. Climate change allowances may also necessitate using higher rainfall figures.

Q4: My soil is very clayey. Can I still install a soakaway?

A: Clay soils have very low infiltration rates, making soakaway design challenging. It might require a very large soakaway, or it may be unsuitable. In such cases, alternative SuDS measures like permeable paving, rainwater harvesting, or connection to a surface water sewer might be more appropriate. A percolation test is essential.

Q5: What does the "Effective Soakaway Diameter" result mean?

A: The "Effective Soakaway Diameter" is a conceptual value derived from the calculation that represents the diameter of a hypothetical permeable circle in the ground that would have the same infiltration capacity as the actual soil surrounding your soakaway. If this value is much larger than your actual soakaway diameter, it indicates that the soil's permeability is good, and the soakaway size is the primary determinant of capacity. If it's similar to or smaller than the actual diameter, the soil's low permeability is the limiting factor.

Q6: How long should a soakaway take to empty?

A: BRE Digest 365 recommends that a soakaway should be able to empty within 48 hours following the design storm event. This ensures that it is ready to receive subsequent rainfall and prevents prolonged saturation of the surrounding ground.

Q7: Can I use this calculator for infiltration trenches or permeable pavements?

A: This calculator is specifically designed for traditional cylindrical soakaways. While the underlying principles of soil infiltration apply to other SuDS components, the geometric calculations (e.g., surface area for infiltration, storage volume) and design guidelines differ for infiltration trenches, permeable pavements, or bio-retention systems. Separate design methodologies exist for those.

Q8: What are the implications if my calculated storage volume is less than the required volume?

A: If the volume your proposed soakaway can hold is less than the volume needed to contain the runoff from the design storm (calculated based on the impermeable area and rainfall depth), then the soakaway is undersized. This means it's likely to overflow during heavy rainfall, potentially causing localized flooding. You would need to increase the soakaway's dimensions (diameter or depth) or consider reducing the amount of area draining to it.

Related Tools and Internal Resources

• Permeable Paving Calculator: Learn how permeable surfaces manage runoff and infiltration.
• Rainwater Harvesting System Sizing: Explore collecting rainwater for reuse instead of infiltration.
• Surface Water Runoff Calculator: Understand the factors contributing to surface water volume.
• BRE Digest 365 Official Guidance: Link to the authoritative document for detailed information.
• Flood Risk Assessment Guide: Understand how soakaway design fits into broader flood risk management.
• Geotechnical Investigation Services: Find information on professional soil testing for drainage projects.