Groundwater levels should be fully evaluated before determining the extent that infiltration can be used on a site. The Arun district is located on the coastal plain. Our area is typically low lying, and the superficial deposits generally consist of silty clays which are poorly drained. A high water table is common in the district, whether this is perched groundwater, the aquifer or springs.
Groundwater monitoring is critical to surface water drainage as it can render infiltration to ground, the highest priority disposal method (after water reuse), unviable. This is because national guidance stipulates that there must be 1m of unsaturated ground between the base of an infiltration component and the expected peak groundwater level.
Establishing peak groundwater levels, where infiltration is not viable, is also important, due to the risk that the groundwater can cause floatation (uplift) of drainage features and potentially lead to structural failure of those components.
It is recognised that national guidance is not prescriptive in terms of the duration or frequency of groundwater monitoring that is required for different scales of development. We believe this is due to the variation in geology and hydrogeology across the country, which makes the requirements for such monitoring unique to each area . This causes uncertainty and confusion to applicants, agents and their designers when making planning applications.
It is the designer's responsibility to demonstrate what the seasonal maximum groundwater levels are where sustainable drainage system [SuDS] features are proposed. This should include an assessment of relevant existing groundwater or borehole records, maps and on-site monitoring in wells.
As peak groundwater levels are variable across the district, they can only be reliably confirmed on a site through investigation during high groundwater level periods. This should include sufficient data collected to demonstrate seasonal groundwater levels to inform the assessment and design.
A designer is free to demonstrate the peak groundwater level in whatever way they choose. However, this must be demonstrated with sufficient accuracy so that flood risk is not increased by a proposed SuDS design. Our planning conditions applied to a planning permission often stipulate that winter groundwater monitoring is required. This is reflective of the fact that we do not believe that desk-based studies are sufficient evidence of groundwater levels at a local scale or representative of the most current ground conditions.
To provide clarity we offer suggestions of a minimum groundwater monitoring requirement dependent on the scale of the proposed development. If these requirements are met, then the duration and frequency of monitoring will not be challenged as part of the consultation and approval process. If a designer deviates from these suggestions, then their ground investigations may be challenged. This may result in further evidence being required.
It is important to recognise that if peak groundwater levels are not accurately demonstrated, then there can be significant delays to a proposed development if (further) site specific monitoring is required.
This is particularly critical if a site relies on the use of infiltration to ground as the only option to drain surface water. In those instances, the viability of infiltration is expected to be demonstrated prior to planning permission being granted. Groundwater monitoring is a critical element to that demonstration, alongside BRE DG 365 compliant infiltration testing. Please also see our guidance notes on infiltration testing.
Where site specific monitoring is suggested, monitoring points should be located on the site in areas where future drainage features like soakaways, permeable paving, swales, ponds or tanks may be positioned. In all cases we suggest that observations should be a minimum of once a week and additionally as required. This ensures that the impact of any significant rainfall event is captured.
The national standards for sustainable drainage design [NSfS] defines groundwater as water which is below the surface of the ground in the saturation zone, and in direct contact with the ground or sub-soil. No distinction is made between the aquifer and perched groundwater. Perched groundwater is a shallow accumulation of water that sits above the aquifer (which holds and transmits a large body of groundwater in a continuous and deep geological formation). Where groundwater is perched it is trapped by a low permeability layer like the clays in our district and remains within an elevated saturated zone.
It is important to recognise if water that is encountered in monitoring wells is in the saturation zone (standing water), or if it is transient and flowing through the strata to the water table beneath. Where water levels are higher than expected, particularly after rainfall, it is suggested that they are observed again later that day to see if transient water has been captured and the reading is anomalous. It is important to robustly protect monitoring points from surface water inundation to reduce the risk of errors in recording.
We do not object to the careful use of ground raising to enable this method of drainage if the formation is into natural and tested ground (not made ground). If ground raising is absolutely necessary, then it is important to consider the potential impacts of surface water runoff affecting adjoining land and property. Mitigation measures may be required as a result.
In many areas (particularly along the coastal plain), groundwater levels will be too high to allow for infiltration to ground. Groundwater monitoring for infiltration purposes may be abandoned at any time of year if levels will preclude the use of infiltration. However, groundwater levels must still be accounted for due to the risk of floatation. If a designer or applicant wishes to abandon groundwater monitoring early, we would expect groundwater levels to be assumed to be at ground level for the purposes of buoyancy calculations. This assumption can lead to over-engineering water storage features. It is more sustainable and preferred that peak groundwater levels are accurately demonstrated for all SuDS designs.
24 October 2025