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Find out why it is important to have a drainage plan, things it should contain and how to use it to plan operations, emergency response and drainage servicing as well as why you should keep them up to date.


Drainage plans show the pollution pathway and will identify your drain and associated infrastructure arrangements. When verified, drainage plans are an essential document when managing your site operations and pollution risk management. This blog will discuss why you should have a clear, comprehensive and verified drainage plan.

Inform your trade effluent operations

If you discharge any water from your trade operations, you will need a permit or license to do so. If you discharge to surface water, you will require an Environmental Permit from the Environment Agency and if you discharge to foul water, you will need to seek consent from the Local Water Authority. As part of either application, you will have to provide information about your drainage system and at what point you discharge to the drainage network. A clear, comprehensive and accurate drainage plan will make the process of application much more straight forward.

Make sure everything is maintained

To ensure your operations don’t cause pollution and to reduce the chances of flooding, you should be maintaining your drainage networks. An accurate site drainage plan will ensure you maintain all drains, channels and gullies and service, and maintain your interceptors and other infrastructure. With a clear drainage plan, you will avoid the risk of missing key pollution control features.

Identify the pollution pathway

In order to prevent pollution from leaving your site, you should use your drainage plan to inform your pollution prevention plans. You should locate anything of high-risk, e.g. waste skips and oil and fuel tanks, away from drains and watercourses. In the event of a spill or leak, this will give you more time to intercept the pollution before it can leave your site. You can use an accurate drainage plan to take swift action if pollution does enter your drainage system as you will know exactly the route it will take. Hopefully, you can do something further along your drainage network to prevent the pollution leaving your site but, if you can’t, you will know exactly who to contact to report the incident. If pollution has entered surface water drains, you must inform the Environment Agency and you must contact the Local Water Authority if you contaminate the foul water system.

Mark your drains

Best practice is to paint your drains red and blue to indicate foul and surface water drains respectfully. You may also use other colours to show interceptors and any drains that lead to any on-site effluent treatment. A precise site drainage plan will make sure your marking is correct and will allow for quick response in the event of an incident. To effectively manage your site operations and your pollution risk, a drainage plan is an essential document to understand your underground networks and ensure your activities do not pose an avoidable risk. However, merely having a drainage plan isn’t enough. Your plan must be clear and easy to understand and the drains represented on them must be correct and verified.



An accurate and verified drainage plan will identify your site drains and associated infrastructure, giving you a clear understanding of your pollution pathway. Above we have discussed the reasons why you should have a site drainage plan. Continue to read to find out the top three things that should be included on your drainage plan.

Surface and foul drainage

Your drainage plan should identify all the underground drainage networks, clearly showing foul and surface systems. Foul drains (usually identified in red) lead to sewage treatment works, whereas surface water drains (usually identified in blue) lead directly into the environment. Your site drainage plan should show all manholes, rainwater pipes, gullies and drainage channels. In addition, the plan should show the drain flow; as in what direction the water flows underground and clearly showing the point at which any water leaves your site.

Interceptors and infrastructure

Any key features on your site should clearly be shown on your drainage plan. This may include oil interceptors, soakaways, storm water attenuation systems and water recycling equipment. All areas creating effluent, such as on-site vehicle washes, tray washes and your toilet facilities should also be shown on your plan. How effluent enters the drainage system and whether interceptors are on the foul or surface drainage network should be very clear.

Above ground gradients

Above ground gradients will show you at a glance how surface water will flow over your site and enter the underground drainage system. These should be shown clearly on your drainage plan and be used to respond quickly in the event of an incident or spillage. You can place spill and absorbent material in strategic locations to intercept pollution before it enters the drainage system. Having a site drainage plan is very important when considering how to prevent pollution. However, your drainage plan must be comprehensive and show all relevant details so you can use it to its full potential. As a minimum, it should show you your foul and surface water drainage, interceptors and site infrastructure and above ground gradients. As long as this information is verified and accurate, your drainage plan will be an essential part of your site environmental management and pollution prevention plan.



Your drainage plan is essential when planning your operations or considering changing the locations of some activities. It is very important to consult the drainage arrangements and check the above ground gradients to see where areas will run-off and drain to. This will help you consider the best locations for your activities and what pollution control measures and devises are most appropriate.

Oil and Chemical Storage

Oil and chemical stores have the potential to spill or leak, so it is essential that they are situated in areas that will cause minimal pollution in the event of an incident. Ideally, drains in the area should be connected to the foul sewer, not to unmade ground or surface water and the drainage network serviced by an interceptor (you can read about interceptors are in our fact sheet here). Stores should be on hard standings to prevent groundwater contamination and should be protected from vehicles. This could mean situating them away from vehicle routes or erecting barrier protection.

Fuel Islands

These areas are probably close to your fuel tanks, but may be serviced by different drainage. Due to the nature of fuel islands, they should drain to a foul sewer and covered by an interceptor. There is always a risk of spills, leaks and drips from refueling activities so it is very important that pollution is intercepted before leaving site.

Vehicle Wash

Any effluent generated by a site must have permission from the water company (read our blog here) and cannot be released to the environment through the surface water drainage system. So, vehicle washes must be connected to a foul sewer or drain to a fully contained system and the appropriate permissions must be in place before they are used. You may also want to consider an interceptor for the area that will intercept any fuel, oil and silt that is washed off the vehicles.

Floor Wash

Similarly, floor wash must only be discharged with permission. It could be discharged into the vehicle wash or could be discharged to another foul drain. You will need to check your drainage plan to confirm whether the discharge point leads to the same or a different point to confirm whether you will need one consent covering both discharges, or two consents for each discharge. You may also want to consider some method of preventing any solids from the floor wash being discharged into the drains and potentially causing a blockage.


It is very important to ensure your waste is stored in a way to prevent environmental pollution and you should consider your drainage too. Ideally, waste will be stored on hard standing, without run-off to unmade or surface water drains. Any liquid wastes should also be bunded to prevent escape into the environment.



An accurate and easy to understand drainage plan is essential when planning for incidents, especially spillages that can enter underground drainage and cause environmental pollution off-site. The drainage plan is an integral document in response plans and should be consulted to ensure that spill response is quick and effective.

Pollution Incident Response Plan (PIRP)

Measures should be taken to prevent pollution and avoid an accident or incident; however, you also need to plan what should be done if pollution does occur. A Pollution Incident Response Plan, or PIRP is a written document that includes all essential information for pollution incident response. There is no defined template for a PIRP, but as a minimum, it should include internal and external contacts, site location and defining features, a record of site storage, spill procedure, pollution control arrangements and a copy of your drainage plan. This document should be clear and easy to understand, trained out to your staff and easily accessible in the event of an incident.

What Can Your Drainage Plan Tell You?

Your drainage plan contains essential information for quick and effective incident response. The above ground gradients will tell you where pollution is likely to have entered the underground drainage system and therefore the route it will take and where may have left your site. The drainage plan will also highlight pollution control devises such as interceptors and you should also be able to see areas where you can intercept or divert pollution before it leaves your site.

Quick & Effective Response

The information provided in the drainage plan will allow your on-site spill team to respond quickly and effectively to avoid pollution. If a spillage has entered the drainage system, swift action should be taken to prevent it from leaving the site. This could include intercepting the water outfalls from site and holding the spilled material in the drainage system for later removal. In some cases, it will be unclear whether pollution has entered the drains and, if it has, how far it has gone and whether it has left site. In this case, drains in the vicinity should be checked for contamination. If pollution is evident, the drainage plan can be used to investigate how far the contamination has spread by checking manholes, interceptors and outfalls it may have travelled to.

Once the initial incident has been dealt with, the drainage system should be emptied and cleaned of all contamination. It will then be useful to check the previously contaminated drains and the system further along the line (as indicated in your drainage plan) to ensure no contamination has escaped site.



We have discussed its use when planning operations and for emergency planning. To finish off we will explore how your drainage plan can inform your drain and interceptor Planned Preventative Maintenance (PPM) schedule.

Location of Drains and Interceptors

Interceptors, sometimes called separators, are large underground tanks fitted on the drainage system designed to prevent oil from leaving your site. For more information on interceptors, read our fact sheet here. An accurate drainage plan will highlight all interceptors on your site on the foul or surface water drainage systems and indicate the areas that the interceptor services, for example, fuel storage or a vehicle wash. As a general guideline, interceptors should cover areas with a risk of oil contamination such as car parks, vehicle maintenance areas and refueling areas. When you have consulted your drainage plan to identify your interceptors, you should check that they are identifiable and easily accessible at all times. You may want to consider painting them with an I, or placing signs on them so their location is clear to everyone on site.

Drain and Interceptor Maintenance

Interceptors must be regularly checked and serviced to ensure they remain in operable condition and continue to provide pollution control. At this service, they should receive a physical inspection for integrity, an assessment of oil and silt accumulation and a check/ service of the associated electrical equipment. If necessary, the interceptor should be cleaned of accumulated oil and silt. The frequency of inspection, servicing and emptying will be dependent on site arrangements and the activities in the area. This could be every three, six or twelve months. For example, an interceptor that services a heavily used fuel island should be serviced more frequently than an interceptor servicing a small car park. Your drains will also need to be checked and, if necessary, cleaned. This can often be carried out at the same time as interceptor servicing, but should be considered even if you don’t have interceptors on your site. They should be clear and free-flowing at all times and free of debris.

Site Arrangements

Your drainage plan will indicate any areas that are not covered by an interceptor. If there are any areas not covered, depending on the risk, you may want to consider installing an interceptor or moving at-risk activities from that area. Checking the location of interceptors on your drainage plan is also essential when considering resituating activities or storage areas.

As we have demonstrated through the above entries, an accurate and easy to understand drainage plan is an essential document for operational efficiency and management. It can help you to plan your Planned Preventative Maintenance (PPM) schedule and ensure that drains and interceptors remain in operable condition and control pollution.



We have covered why your drainage plan should be used in live situations rather than being filed away, but what happens when there are changes on your site?

What changes affect your drainage plan?

Any site changes have the potential to affect your drainage arrangements, and therefore your drainage plan; it isn’t just physical changes to your drainage network. For example: • Extensions or changes to your building could mean that some manholes are now inside rather than outside and some drains may no longer be accessible • Moving operations to a different area e.g. vehicle washing, fuel tanks or waste sorting areas • Adding, changing or stopping trade effluent activities • Changing your drainage arrangements e.g. adding an interceptor, adding a manhole for effluent disposal etc.

What should you do with your drainage plan?

Following any changes, you should assess the impact on your drains and update your drainage plan. This could involve altering the building layout to add in any extensions or new buildings that have been built and showing how this affects the drainage network. You may need to move some areas around like vehicle washing or fuel tanks and make sure your labelling reflects the changes. You may have sample points for effluent discharge or discharge points labelled that need amending.

If you have physical changes to your drainage arrangements, you will need to alter the drainage plan to reflect this. It is essential that these changes are accurate and you may want to do some investigation to confirm the drains match up how you expect before making changes to your drainage plan.

Keeping everyone in the know

Once you have updated your drainage plan, it is essential that you update the necessary people to ensure everything is covered. You should inform your drainage contractor if it will result in changes to your regular planned preventative maintenance (PPM) schedule and you should also ensure your internal spill response team are made aware of the changes.

Changes to your drainage arrangements may impact your spill risk areas, so it is important to review these and your spill response kit to make sure you are covered in the event of an incident. You may need to move some spill kits around or get new ones for certain areas. It is also essential that you update all related paperwork along with your drainage plan. Key documents may be your Pollution Incident Response Plan (PIRP), Site Environmental Emergency Plan (SEEP), spill kit plan and spill response training.

If you make any changes to your site, it is essential to consider the impact of those changes on your drainage arrangements and thus your drainage plan. You should review the changes and ensure your drainage plan is updated accurately and the relevant people and other documents are updated accordingly.