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Raising awareness of dewatering regulation

04 November 2021

Rules recently changed around dewatering - excavating below the water table. What permits need to be in place before projects are started?

Raising awareness of dewatering regulation
Avoid project delays by making sure you have the right dewatering permits in place. Image credit: Shutterstock

Groundwater lowering for construction (aka dewatering) is a common requirement for both small- and large-scale projects involving excavation below the water table.

In the past, most dewatering was done without needing specific regulatory permission. That all changed in 2018, when dewatering was brought into legislation for England and Wales through amendments to the Water Resources Act 2003.

This means that many projects involving excavation below the water table now need approval (a Water Abstraction Licence) from the Environment Agency or Natural Resources Wales.

Raising awareness of dewatering changes

My experience is that some clients and contractors are unaware of the change in legislation or perhaps do not fully understand the timescale needed to prepare a robust application and obtain regulatory approval.

This lack of awareness can have significant implications for project programme, and lead to unforeseen delays and costs.

My aim in this article is therefore to raise awareness of technical requirements and timescales of regulatory approvals for dewatering, including the requirements to obtain a Water Abstraction Licence and associated Environmental Permit to discharge the water.

Water Abstraction Licence

There are still exemptions for some small-scale dewatering activities. This includes sump pumping for less than six months duration and less than 100 cubic metres per day, provided the dewatering will not impact a protected site such as a SSSI.

However, where the duration or rates of dewatering exceed these limits, a Water Abstraction Licence will be needed.

The Water Abstraction Licence application includes a set of forms which are in themselves reasonably straightforward, requiring information such as company details of the applicant (usually principal contractor) and anticipated dewatering rates.

What is the Hydrogeological Impact Appraisal?

The application must be supported by a Hydrogeological Impact Appraisal (HIA). Guidance on the content of the HIA has been provided by the Environment Agency.

In summary, the HIA should include a conceptual understanding of the groundwater system and a survey to identify water-related features that could be affected by the dewatering e.g. water supply boreholes, rivers and protected sites.

The assessment is backed up by calculations of the dewatering rates and the predicted impact (groundwater level drawdown) on nearby water features.

A tiered approach is taken, with the level of detail, time and cost depending on the sensitivity of the site (presence or absence of rivers, protected sites etc.) and the scale of dewatering.

For moderate-scale dewatering that's not in a highly sensitive area, simple calculations and logical arguments can be used to support the application.

For sites where high dewatering rates are needed and/or in sensitive areas, the HIA may require extensive ground investigation, water features survey(s), pumping test(s), and stream/spring monitoring. This is likely to take several months.

Ground investigations need to be specified, tendered, and delivered. Access to third-party land may need to be negotiated for water features surveys. A pumping test may require a separate consent from the regulator and the test may need to be several days or weeks long. Stream and/or spring flow monitoring may need to be carried out over several months to capture seasonal variations. A numerical groundwater model may also be needed to assess the impacts with sufficient confidence to satisfy the regulator. Numerical groundwater models can be used to predict the effects of longer term and/or higher rates of dewatering, where it is not practical to be tested directly by observation.

Once the HIA is completed and submitted, the regulator has up to four months to assess the application.

In cases where a third-party asset would be affected by dewatering, such as a private water supply spring or borehole, a written derogation or compensation agreement is needed prior to the regulator issuing a licence.

When an Environmental Permit will be needed

Many dewatering activities will also need an Environmental Permit to discharge the water to a river or other watercourse.

This is a separate application and one that may require coordination between several teams; the principal contractor, specialist dewatering sub-contractor, hydrogeologists, hydrologists, and perhaps aquatic ecologists.

Hydrogeologists estimate the dewatering rates and analyse the groundwater chemistry including potential contaminants. Hydrologists assess whether the receiving watercourse can accommodate the dewatering volumes. Aquatic ecologists and/or geochemists may be needed to if the receiving watercourse is sensitive to changes in flow and/or water chemistry.

Again, the level of detail, time and cost needed to complete the assessment will be determined by the environmental sensitivity of the receiving watercourse, the dewatering rate and if there are contaminants in the groundwater.

Why the right permits need to be in place

Small projects in particular can suffer proportionally large programme delays and cost overrun if the relevant regulatory permits are not in place. This is not in anybody’s interest, since many schemes are ultimately paid for by public money (e.g. taxpayers, water company customers etc.).

Some projects such as road and railway cuttings require permanent dewatering to prevent flooding or damage to infrastructure. The Environment Agency’s position statement describes situations when an Abstraction Licence is, and is not, needed.

In short, a licence isn’t needed if the water is transferred to a river or other watercourse by gravity without being pumped, and there are no significant impacts on protected sites or other water users.

But in many cases, there still needs to be an assessment that demonstrates there will not be a significant impact. Again, this can lead to delay and unforeseen cost.

There can also be a perceived inconsistency where a project has planning approval, but further detailed assessment of dewatering may indicate unacceptable impacts. In this case, additional mitigation, or design change, may be required.

How to avoid project delays and problems

To avoid these problems, I believe there is scope for improvement on all sides.

Contractors and their advisors (consultants) need to allow time in their project programme to collect relevant data (ground investigation, water features surveys, pumping tests, spring/stream flow monitoring) and for the regulator to do its job in protecting the environment.

Early consultation with the regulator is strongly advised to discuss and agree the scope and duration of investigation and baseline monitoring. Regulators could request conditions at planning application stage that stipulate an Abstraction Licence is needed for dewatering.

This would alert the client and contractor and trigger the assessment process to begin early in the project programme. Consultants also need to understand the practicalities of dewatering and have the skills to assess impacts on a range of environmentally sensitive features.

  • Stephen Smith, Principal Hydrogeologist, Arcadis Consulting (UK) at Arcadis Consulting (UK)