Maintaining a stormwater basin helps preserve its effective storage volume, hydraulic performance and the quality of discharged water. The gradual accumulation of total suspended solids (TSS), sediment and debris can reduce storage capacity, cause blockages and affect the flow control system. Where stormwater is potentially polluted, maintaining pre-treatment systems also helps limit the transfer of pollutants to the drainage network, ground or receiving environment.
An effective maintenance strategy should therefore combine regular inspections, cleaning, sediment removal and checks of hydraulic equipment.
Stormwater basin maintenance in 5 key points
Objective: preserve effective storage volume, flow control and the quality of discharged water.
Monitor: TSS, sediment, sludge, debris and any hydrocarbons carried by surface runoff.
Inspect: access chambers, grilles, drains and inspection channels, pre-treatment systems and flow control devices.
Maintain: remove debris, extract sludge and sediment, and carry out hydro-jetting when the accumulation of deposits makes it necessary.
Record: extracted materials must be characterised and sent to an appropriate waste treatment route. Where classified as hazardous waste, the applicable waste tracking requirements must be followed.
What are the risks of poor stormwater basin maintenance ?
A poorly maintained stormwater basin can gradually lose some of its hydraulic performance: sediment and sludge occupy storage volume, pipes can become blocked and flow control devices may no longer provide the intended discharge rate.
Surface runoff does not carry water alone. As it flows across the catchment, it collects material from roofs, roads, car parks, landscaped areas and other developed surfaces.
Some of this material is transported as total suspended solids (TSS) before potentially settling within stormwater management systems, depending on its characteristics and hydraulic conditions.
What are total suspended solids (TSS) in stormwater ?
Total suspended solids (TSS) are solid particles carried by surface runoff without being dissolved in the water. They can originate from dust and particles deposited on surfaces, soil erosion, organic matter, and wear from roads and vehicles.
During rainfall, runoff carries these particles from roofs, roads, car parks and developed areas into drainage networks and stormwater management systems. Some suspended solids can also carry pollutants attached to the particles.
When water velocity decreases, particularly in areas where settlement can occur, some of the suspended solids gradually settle. These settled particles form sediment, which can accumulate with other materials at the bottom of the system and contribute to the formation of sludge.
The three terms should therefore be distinguished:
- TSS: particles carried in suspension in the water;
- sediment: particles that have settled out of the water;
- sludge: deposits accumulated at the bottom of the system that may need to be removed during cleaning operations.
How does sediment and sludge accumulation reduce effective storage volume?
The gradual accumulation of deposits at the bottom of a stormwater system occupies part of the volume originally available for storing stormwater.
Over time, this can result in:
- reduced effective storage volume;
- slower or obstructed flows;
- blockage of certain pipes;
- disruption of flow control devices;
- resuspension of deposits during significant rainfall events;
- deterioration in the quality of discharged water.
Maintenance is therefore not simply about keeping the system clean: it helps maintain the hydraulic performance established during the design process.
Silt traps and oil separators: when is pre-treatment required?
Stormwater that may contain high levels of sediment or be polluted by hydrocarbons can require pre-treatment before storage, infiltration or discharge. The appropriate system depends on the pollution risk and the requirements applicable to the project.
A silt trap encourages heavier particles and sediment to settle before they reach downstream stormwater systems.
An oil separator is designed to retain non-emulsified hydrocarbons present in collected water. These systems may be used on certain industrial sites, traffic areas, car parks or other surfaces where an identified pollution risk exists.
However, an oil separator should not be considered automatically necessary for every stormwater management project. The need for treatment should be determined according to:
- the type of surfaces drained;
- the activities carried out on the site;
- the level of pollution risk;
- the method of discharge or infiltration;
- applicable regulations;
- requirements set by the relevant drainage authority or local authority.
Some sites subject to specific environmental regulations may, for example, be required to treat potentially contaminated stormwater.
Why should an oil separator be maintained?
An oil separator that is saturated with sludge or hydrocarbons can no longer perform its treatment function effectively.
Maintenance includes checking the level of sludge, the quantity of accumulated hydrocarbons and the operation of associated equipment, including the automatic closure device, coalescing device where fitted, and the inlet and outlet of the separator.
The frequency of emptying depends on the site and the applicable regulations. Specific regulated facilities may, for example, be subject to requirements concerning the maximum permitted accumulation of sludge and minimum inspection or cleaning frequencies.
What maintenance schedule should be used for a stormwater basin?
Maintenance frequency depends on the design of the system, its environment, its use and the quantity of material carried by stormwater.
Regular monitoring can include checking:
- the accumulation of debris, sludge and sediment;
- the condition of access chambers and accessible equipment;
- pre-treatment systems;
- flow control devices;
- whether sediment removal or hydro-jetting is required.
Maintenance frequencies should be adapted to site characteristics, the requirements of the installed equipment and any applicable regulatory obligations.
Additional inspections may also be necessary following an exceptional event, accidental spill or identified malfunction.
How are stormwater basins cleaned and hydro-jetted?
Cleaning involves removing sludge, sediment and debris accumulated within the system. Hydro-jetting uses pressurised water, together with suction, to clean accessible pipes and equipment within the stormwater drainage network.
The method of intervention depends on the design of the basin, its accessibility, the level of accumulated material and the equipment installed. Depending on the system configuration, maintenance may include the following steps.
1. CCTV inspection
The operator identifies areas of accumulation, checks access to chambers and determines which parts of the network require intervention.
A CCTV inspection provides a visual assessment of accessible parts of the network and can accurately locate defects or problems before or after hydro-jetting.
Introduced through an access chamber or other dedicated access point, the camera can help identify:
- deposits and sediment accumulation;
- obstructions or blockages;
- debris;
- possible deterioration of pipes;
- defects or anomalies at connections.
A CCTV inspection can also be carried out after hydro-jetting to assess the condition of the network and check the effectiveness of the cleaning operation.
For underground stormwater basins, the ability to use CCTV equipment depends directly on the design of the system and the accessibility of the areas to be inspected.
2. Removal of sludge and deposits
Accumulated sediment and sludge are removed using a vacuum tanker or other suitable equipment. Particular attention should be paid to settlement areas, access chambers, pre-treatment systems and low points.
3. Hydro-jetting of pipes
Pressurised water jets are used to dislodge deposits from pipe walls and move them towards a location where they can be removed by suction. Network accessibility is therefore an important consideration from the design stage onwards.
4. Post-cleaning inspection
After cleaning, pipes, access chambers and equipment are checked to identify any damage, remaining obstruction or other anomaly.
5. Waste removal and traceability
Extracted materials must be characterised and sent to an appropriate treatment or disposal route according to their nature.
Where extracted material is classified as hazardous waste, particularly where hydrocarbons are present, its handling and traceability must comply with the waste regulations applicable to the site.
Records relating to maintenance operations and waste removal should be retained in accordance with applicable requirements.
How should the basin's flow control devices be inspected?
Even a thoroughly cleaned basin can fail to operate correctly if its flow control system is blocked or damaged. Checking the discharge flow control system is therefore an integral part of maintenance.
Inspection can include:
- vortex flow control devices;
- calibrated orifices;
- valves;
- outlet pipes;
- flow control chambers;
- non-return devices where fitted.
Why should maintenance be considered at the stormwater basin design stage?
A stormwater basin is easier and less costly to maintain when access to its equipment has been considered from the design stage. Access chambers, inspection possibilities, accessible settlement areas and network layout all directly affect future maintenance operations.
This is particularly important for underground stormwater basins, where equipment cannot be directly viewed from the surface.
The objective is to provide:
- inspection access to the different levels of the system;
- access to areas where sediment may accumulate;
- access for hydro-jetting equipment;
- the ability to remove sludge;
- access to inlets and outlets for inspection;
- access to flow control devices.
See our complete guide to stormwater attenuation basins to consider these requirements from the design stage.
A maintenance plan combined with a design that allows inspection and cleaning therefore helps preserve effective storage volume, discharge flow control and water quality throughout the operating life of the system.
Sources et References :
- BSI – BS EN 858-2:2003 – Separator systems for light liquids (e.g. oil and petrol): British Standard covering the selection of nominal size, installation, operation and maintenance of separator systems for light liquids such as oil and petrol.
- GOV.UK / Environment Agency – Waste duty of care: code of practice: Official guidance on the legal responsibilities for managing, storing, transferring and documenting controlled waste in England and Wales.
- Environment Agency – Waste classification technical guidance (WM3): Technical guidance for the classification and assessment of waste, including the identification of hazardous waste and the properties that determine how it must be managed.
- Environment Agency / DEFRA – Classify different types of waste: Official guidance and resources for identifying and classifying different waste streams, including hazardous and oil-contaminated waste.
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