Groundwater and hydrology screening
A screening of how a basement or deep excavation could affect groundwater and surface water flows.
About this service
A basement is a concrete box pushed into the ground, and water has to go somewhere. Groundwater may flow around it, rise behind it, or be held back from neighbouring gardens, while paving over a garden for a basement can increase surface water run-off. Groundwater and hydrology screening looks at whether a proposed basement or deep excavation is likely to cause these effects, and whether more detailed study is needed.
Councils with basement policies usually include hydrology and groundwater questions in their Basement Impact Assessment screening. Sites near London's buried rivers, near springs on the slopes of north London's hills, on river terrace gravels, or in areas with a history of surface water flooding attract particular scrutiny. London Clay itself has low permeability, but perched water in sandy layers and made ground above it is common.
We review published geology, borehole and groundwater records, flood maps and site information, then answer each screening question with evidence rather than assertion. Where a potential effect is identified, we assess it and recommend mitigation, and we say clearly when further investigation is needed. Fees start from £750 + VAT, and our typical turnaround is 10 working days, confirmed in writing with the fee.
When you need it
- Your BIA needs a hydrology and groundwater screening
- The site is near rivers, springs or a high water table
- The council has raised concerns about flooding or water flow
What's included
- Review of geology and groundwater records
- Screening against council criteria
- Assessment of effects on groundwater and surface water
- Mitigation recommendations
How it works
- Collect background dataWe gather geological maps, nearby borehole logs, groundwater level records, historic river routes, surface water flood maps and any site investigation already carried out on or near the site.
- Screen against council criteriaWe answer the council's screening questions on groundwater flow, surface water, flooding and nearby watercourses, giving clear reasons and evidence for each answer rather than a simple yes or no.
- Assess potential effectsWhere screening flags a concern, we assess how the basement could change groundwater levels or surface water flows, including cumulative effects with neighbouring basements in the same street.
- Recommend mitigationWe set out practical measures such as drainage around the basement, permeable paving, attenuation or design changes, and identify any further ground investigation or monitoring needed.
What you receive
Lost rivers, perched water and cumulative basements in London
Several of London's rivers now run in culverts or sewers, including the Fleet, Tyburn, Westbourne and Effra, and their old valleys can still carry shallow groundwater. Gravel and sand layers can hold water above the clay, so a site that looks dry on a desk study may still meet water during excavation. A ground investigation with standpipes gives real water levels over time and is often recommended where screening raises doubt.
Councils also consider the combined effect of many basements in one street, which together can act like a dam to shallow groundwater flow. Surface water matters too, as basements often remove garden soil that once absorbed rain. Our SuDS strategy work can address this, and the drainage strategy guide explains what councils typically expect.
Frequently asked questions
What is a hydrology screening for a basement?
It is the part of a Basement Impact Assessment that checks whether a basement could affect groundwater flow, surface water, flooding or nearby watercourses. It answers screening questions set by the council, using evidence from geology, groundwater records and flood maps, and identifies whether a more detailed assessment is needed.
Can I build a basement with a high water table?
Yes, but it needs more careful design and construction. Excavation may require dewatering, retaining walls must resist water pressure and be watertight, and the basement may need to resist uplift. Effects on neighbouring groundwater also need assessing. Ground investigation to confirm water levels is essential before detailed design.
Will a basement cause flooding to neighbouring properties?
A single basement in London Clay rarely changes groundwater significantly, but in permeable ground, or where several basements cluster, shallow flows can be diverted or backed up. Surface water run-off can also increase. Screening identifies these risks early so the design can include drainage or other mitigation.
Does every basement need a groundwater screening?
Not every one, but most London councils with basement policies require it within a Basement Impact Assessment. The level of detail depends on the borough and the site. Check the council's basement guidance or validation list, or ask us to confirm what applies to your property.