Knowledge Base
Site Assessment Guide
From soil testing to greenfield baseline: everything before you start designing
Eight Steps to Complete Site Assessment
Understand Your Site Constraints
Before any calculation, you need to gather baseline site data. This information determines which discharge strategies are feasible and feeds into every downstream design decision.
Collect the following:
- Site area: Boundary of the development (hectares). Used to calculate total contributing runoff volume.
- Soil type (WRAP classification): Determined from borehole logs and soil analysis. Soils are classified 1–5, where Class 1 is free-draining and Class 5 is clay with very low infiltration.
- Local rainfall data (FEH or FSR): Annual Average Rainfall (AAR), 1-in-100-year storm depth, and storm duration. Usually extracted from the Flood Estimation Handbook (FEH) or regional guidance.
- Groundwater levels: Depth to water table from site investigation boreholes. Critical for assessing infiltration feasibility: typically need at least 1 m clearance between base of infiltration feature and groundwater.
- Receiving watercourse/sewer details: Location, distance, existing discharge consents, invert levels, and capacity. Required to assess surface water discharge or sewer adoption viability.
Why it matters: Missing or incomplete baseline data leads to design uncertainty, cost overruns, and regulatory rejection. Invest time at the start.
Classify Your Soil
Soil classification determines the infiltration rate and feedstock for greenfield runoff calculation. WRAP classification (1–5) and HOST classification (derived from borehole data) are the standard schemes used across UK drainage design.
Use the Soil Classification Calculator to:
- Input borehole data: soil texture, grain size, permeability testing results.
- Obtain WRAP soil class (1–5) and HOST classification.
- Export the classification to use as input in greenfield runoff and infiltration calculations.
This single classification serves downstream calculations: get it right once and reuse it throughout design.
Open Soil Classification Calculator →Run Infiltration Tests (BRE 365)
If your discharge strategy includes infiltration (S1 hierarchy), you must determine the site-specific soil infiltration rate. Field testing is always more reliable than desk-based estimates.
BRE 365 trial pit procedure:
- Excavate a trial pit to the base elevation of your proposed infiltration feature (typically 1–2 m depth).
- Fill from a bowser with water; record the water level in the pit at fixed time intervals (e.g., every 10 minutes).
- Measure the rate of water level drop: this is the soil infiltration rate q (in m/s).
- The test must meet the 24-hour half-drain requirement: water must drain to half its initial level within 24 hours for infiltration to be viable.
Poor drainage (half-drain > 24 hours) indicates infiltration is not feasible; you must pursue alternative discharge routes.
Open BRE 365 Test Logger →Screen for Groundwater Risk
Shallow groundwater or contamination can render infiltration infeasible or hazardous. Statutory consultation and environmental risk assessment are required; this tool synthesises those findings.
Check:
- Groundwater elevation relative to proposed infiltration depths.
- Groundwater and soil contamination risk from historical land use.
- Proximity to sensitive receptors (e.g., drinking water abstraction licences, designated groundwater protection zones).
- Groundwater body status under the Water Framework Directive.
The tool provides a pass/fail screening outcome; a fail indicates infiltration is ruled out and you must proceed to surface water discharge or sewer connection.
Open Groundwater Impact Screening →Delineate Your Catchment
SuDS design is area-dependent. You must define which surfaces drain to which features (roofed areas, hardstanding, landscaping) so that every flow path is accounted for and correctly assigned to the right calculation.
Map:
- Roof areas (m²) and their drainage route: green roof, tank harvest, or downslope conveyance.
- Permeable vs impermeable hardstanding and which areas drain to which swale or basin.
- Landscaped or open space that may intercept or infiltrate rainfall directly.
- Confluence points where flows combine before discharge.
The tool generates a catchment delineation plan and summarises contributing areas for each design node. This output feeds into every downstream calculation.
Open Catchment Delineation Tool →Calculate Greenfield Runoff Baseline
The greenfield runoff rate (QBAR) is your compliance benchmark for S3. It represents the peak discharge rate as if the site were entirely undeveloped, a natural baseline against which all designed discharge must be measured.
Inputs:
- WRAP soil class (from Step 2).
- Site area (hectares).
- Design rainfall: typically 1-in-100-year storm depth from FEH (before climate change uplift).
- Site catchment characteristics: urban influence, antecedent wetness.
The tool returns the QBAR rate in l/s. This is your S3 limit: you must not exceed this discharge rate to existing watercourses or sewers.
Open Greenfield Runoff Calculator →Apply Climate Change Allowances
Rainfall intensity is increasing due to climate change. Building Regulations and planning policy require design rainfall to be uplifted, typically 40% for housing, varying by region and water authority.
Steps:
- Identify your EA management catchment or water company area from postcode.
- Look up the regional climate change uplift (40%, 30%, or other %) from EA guidance or your LLFA.
- Apply the uplift to your 1-in-100-year design rainfall.
- Recalculate greenfield QBAR using the uplifted rainfall if required for storage design (this is normally done separately).
Climate change uplifts increase design rainfall by 30–40%, resulting in larger storage and higher discharge rates than historical baseline. This is a major cost driver in attenuation design.
Open Climate Change Comparison Tool →Assess Feasibility
Combine all site data to determine which SuDS components are viable for your specific constraints. Some sites can infiltrate; others must rely on surface water discharge or sewer connection. The feasibility screener synthesises soil, groundwater, catchment, and regulatory data into a clear pass/fail for each component type.
Component screening:
- Infiltration features (soakaways, trenches, swales): require good drainage, sufficient groundwater clearance.
- Permeable paving and green roofs: require adequate depth and maintenance access.
- Surface water discharge: requires a receiving watercourse and acceptable flow rate.
- Sewer discharge: requires sewer adoption agreement and capacity assessment.
Output: A feasibility matrix showing which component types are available, ranked by S1 hierarchy preference. This guides your design strategy.
Open SuDS Feasibility Screener →Checklist: Site Assessment Complete
Before you proceed to design, ensure you have gathered and validated the following outputs:
Last reviewed: 2026-05-10 · Sources cross-checked against the canonical registry at the time of publication. Standards may be updated; verify currency against the publisher before relying on any citation.