Attenuation Tunnels: Compare Storage and Installed Cost

Attenuation tunnels are chambers that store stormwater for planned releases or infiltration routes. Evaluate their usable storage, complete installation and maintenance scope and compare with a crate system before evaluating prices. A module price doesn’t establish the cheaper installed solution.

A quotation says “100 m³”, but does that figure include water in the tunnel, water between surrounding stones, or both? The answer to that can impact the quantities your contractor will have to purchase. This briefing is for drainage specifiers and procurement staff, and is to be used once the concept of the suitable drainage has been established by the project engineer.

Updated in 2026. Guidance examples identify their jurisdiction; worked volumes show assumptions, and aren’t based on a project design.

Before choosing a bid:

• Confirm whether the system infiltrates rainwater or releases it through a controlled outlet.
• Separate chamber volume from storage in stone pores; count each only once.
• Compare complete sections and maintenance arrangements, not just unit height or price.

What are attenuation tunnels designed to do?

Attenuation tunnels are buried chamber units in a stormwater management system. The shape of the chambers won’t determine where the water goes. A complete surface water design will have to address an infiltration route, a controlled discharge route, or a supported combination of the two, along with the inlet, outlet, surrounding materials, and access.

Start with the water route shown on the drawing. The infiltration guidance from Philadelphia describes storage within aggregate voids and infiltration to the surrounding soil. A controlled stormwater attenuation arrangement has a different outlet. Our soakaway versus attenuation tank guide covers this.

What is attenuation in simple words?

Attenuation means temporarily holding some incoming water so it leaves more slowly. In drainage systems, a storage space and a flow control work together to reduce the peak discharge reaching the next part of the network. Storage alone doesn’t prove whether the release rate can be achieved.

For a building or car park, this means reading the inlet and outlet arrangement along with the capacity figure. The wider sustainable urban drainage scheme may also include surface features. A buried unit is one component of water management, not a complete strategy for flood risk, downstream flooding or pollution.

Ordering an infiltration tunnel because its name contains “attenuation”, then telling the installer to invent a sealed detail is a common mistake. Understand the exact assembly and water route before ordering. Confirm the discharge consent or other applicable approval route with the project designer.

Separate tunnel volume, stone storage and usable capacity

Tunnel capacity and installed storage aren’t the same. Compare the space of the water-filled chamber and the credited pores in the surrounding stone at a shared design water level. Exclude unavailable space and check if the stone contribution is already included in the supplier’s system volume. Neither figure automatically represents the excavated space.

The Philadelphia subsurface detention manual separates chamber storage from stone storage and excludes permanent pools from detention credit. Its local calculation rules use a stone porosity of 0.40. That’s an attributed local rule, not a measured porosity for every aggregate delivered to a site.

3-Volume Storage Ledger

Ask each bidder to complete this ledger from the same level schedule. The three quantities that need to be reconciled are chamber water, stone-pore water and unavailable water. The other rows explain where these quantities come from.

Example Tender A: Let’s say we have 60 m³ of chamber water and 100 m³ of surrounding stone with an assumed 40% porosity. Stone storage would then be 100 m³ x 0.40 = 40 m³. Together they total 100 m³ before any deductions are made. If the design separately identifies 5 m³ of unavailable water within that total, the example leaves 95 m³. In this example, the storage would be 95 m³. These inputs and numbers are for teaching purposes only, and aren’t based on site findings or size recommendations.

If 40% porosity is applied to the entire excavation envelope, including the 60 m³ cavity, that would be double counting. It would be better to request the stone take-off as opposed to trying to reconstruct it from a marketing diagram.

Storm Manage’s geocellular attenuation tank specifications publish 190 L for an H400 unit and 237.5 L for an H500 unit. Four H500 units therefore total 950 L (0.95 m³) of published module capacity, before project-specific deductions. The published 95% unit void figure must not be applied to the whole excavation.

The 3-Volume Storage Ledger makes a headline total of 100 m³ auditable. To achieve this, chamber volume needs to be established, the actual stone-pore volume calculated, and unavailable volume identified. In an example, if 5 m³ unavailable water is assumed, the total usable water is 95 m³. This calculation doesn’t state the volume of storage required at site.

Can the complete installation fit your site?

A tunnel or crate installation fits only when the entire section meets the site levels, ground conditions and loading requirements. Unit height by itself fails to capture bedding, cover, pavement, pipe levels and ground water. Compare the exact models you wish to construct in the same drawing envelope before crediting either shape with a shallow installation advantage.

The following classification aids a civil engineer and contractor to split checks. It pertains to a residential, industrial or town centre project; it doesn’t state a universal cover depth or load-bearing capacity. See the attenuation design and installation overview for a broader discussion on build-up.

For England, clause 1.15 of the national sustainable drainage standards calls for at least 1 m between an infiltration feature’s base and the maximum likely groundwater level. That scoped provision is not a universal clearance rule for a sealed attenuation tank. Determine the anticipated route of the water prior to applying it.

Construction Check: Agree how vehicles will cross the installation while it’s being constructed. The Philadelphia construction guidance refers to temporary covers, clean aggregate and records before backfilling. A “high load-bearing” label doesn’t explain which vehicles will be allowed to cross the installation.

England’s cited 1 m separation belongs to infiltration design. A sealed tank will still require a groundwater, loading and stability assessment. Put the route of the water and the groundwater level in the same section drawing as the module, and do not treat a smaller external dimension as proof of a workable installation.

What should an attenuation tunnels cost quote include?

A complete attenuation tunnels quotation would define the complete installed scope against a shared hydraulic design brief. Compare storage, excavation, aggregate, containment, controls, connections, access and handover. Identify who pays for exclusions. A low supply price can be useful, but it isn’t a whole-project cost until those interfaces are priced.

Confirm the rainfall and inflow, and the climate change allowance, and the conditions of discharge and receiving waters, as appropriate. The TxDOT storage-routing procedure treats the inflow hydrograph, stage-storage relation and stage-outflow relation as separate inputs. Hence, equal usable m³ doesn’t necessarily indicate equal behaviour of discharge.

8-Line Installed-Scope Ledger

Use the following lines in each tender return. Require bidders to state which items are included, excluded, or priced separately, with a responsible party identified for each. Items in the schedule make omissions obvious and avoid assuming a generic online price is a project price.

In Hypothetical Tender A above, the 100 m³ headline became 95 m³ after the stated deduction. If another bid offers 100 m³ of usable storage, the two prices aren’t directly comparable. Return the volume schedules for reconciliation, and then ask the designer to confirm both hydraulic arrangements.

How much does an attenuation tank cost?

An attenuation tank can’t have a dependable project price without determining the required storage, the site section and the installation scope. A supply-only module rate can exclude some or all of the surrounding civil works. For comparable bids for the same usable storage and hydraulic duty, sum all priced exclusions and compare the totals, before selecting the lower total.

Finance can compare cost per credited m³ only after those boundaries match. Initially, construction costs need to be comparable; future maintenance budgets must be separate. As before, reliability depends on the owner being able to carry out agreed maintenance. The attenuation tank cost guide expands those cost drivers without substituting for a current quotation.

When considering the apparent saving, determine where exactly it’s occurring. Is it using less modules? Is it using less aggregate? Is it shortening the installation programme? Are there any efficiency or cost-effectiveness advantages? In these cases, the saving should survive the completed ledger and engineer’s design checks.

Keep sediment and cleaning access in the design

There has to be a way to remove sediment from storage and a place to collect it. Inspection alone doesn’t satisfy this requirement. Storage that can be inspected through an opening in the roof of a chamber is not necessarily reachable for cleaning. Access to cleaning equipment and the means to transport waste should be incorporated in the design, and a Maintenance Owner should be identified, before the system is covered up.

According to the Philadelphia Detention Manual, sediment can be difficult to remove from stone storage and systems with component grids. This is a reason to consider where sediment collects, rather than to assume that every tunnel is easy to clean, or that every crate is inappropriate. For background on US chamber and crate terminology, see the earlier comparison; its generic excavation ratios are not a basis for the quotation or excavation checks in this briefing.

What are the common problems with attenuation tanks?

Points to check should include sediment entering storage, use of dirty aggregate, damage during construction, inaccessible sediment removal points, and an outlet that doesn’t match the design. All of these should be considered failure conditions. The construction failure paths and the route the sediment takes through the operating system should be considered separately, rather than viewing the entire system as being “inspectable.”

Philadelphia’s construction guidance calls for protecting the installation from sediment and traffic while work is ongoing. England’s standards give separate considerations for access for maintenance and pollutant risk. A filter or silt trap that protects storage, by itself, doesn’t demonstrate that the water quality standards of the project are satisfied.

Hidden maintenance bottleneck map

The UK Health and Safety Executive recommends avoiding confined-space work where possible, so plan external inspection and cleaning where feasible. An access opening shouldn’t be considered an authorization to enter. Any necessary entry requires an assessment and safe plan in its own right. Small modular cells shouldn’t be considered walk-in spaces.

Purchase hold point: do not select a system merely because it stores the required volume if nobody can explain how the proposed installation will be inspected and cleaned.

Which supplier documents belong in the approval pack?

The approval pack should connect the supplied model to the specified installation and project’s design basis. It should include requested dimensions, storage information, load evidence, installation description and maintenance info. Test results for other models or manufacturers can’t prove the package quoted is adequate.

Use this sequence to evaluate an alternative offer. Inquire who will manufacture the specified unit and who accepts responsibility for each interface. A description of material, a concrete or high load-bearing capacity comparison isn’t a structural basis.

1. Identify the assembly — match model, materials, dimensions, connectors and drawing revision.
2. Reconcile capacity — check module litres, credited system m³ and the storage curve.
3. Check the installed section — match ground, traffic, temporary works and water levels to the proposed evidence.
4. Close the interfaces — confirm containment, outlet, treatment, access and installation responsibilities.
5. Agree the handover — retain inspection records, maintenance instructions and the named owner.

For hypothetical Tender A, include the total of 95 m³ usable capacity in the approval schedule. If a revised drawing again lists 100 m³, ask whether that means gross storage or whether the 5 m³ deduction changed. Keep the approved schedule and purchased unit connected.

For Storm Manage’s published figure of H500, 237.5 L is a starting point for module arithmetic. It doesn’t define long-term durability, groundwater stability, or the load case for your site. Request the evidence through the supplier’s evidence and design basis page, before accepting a substitution.

What is changing in stormwater drainage procurement?

Current drainage advice gives buyers a practical reason to make maintenance and handover explicit in procurement. The England National Standards of 2025 are non-statutory. They don’t certify a tunnel product and don’t define a universal cut off point for replacement. Use the applicable guidance in conjunction with the project’s requirements to clarify responsibilities.

The GOV.UK publication page dates the standards to 19 June 2025. The July update added a link to the archived standards. This change log is not evidence of a new tunnel test. For a UK project, check the right authority and scope, and do not extend the details of England’s document to other jurisdictions.

“we need to build SuDS in from the start”

Early design advice is applicable when you integrate underground storage with a wider landscape. Sustainability and ecological value and environmental protection can’t be inferred from a durable plastic unit alone. A sustainable development brief requires the designer to consider the entire runoff route, treatment and maintenance plan, and protection of receiving freshwater if required.

For a Storm Manage crate proposal, send the required usable storage, water route, site levels, groundwater information, traffic basis and delivery destination. The project input checklist assists in obtaining these for consultation. The quote should state what the supplier will provide and what will remain with the designer or contractor.

Frequently asked questions

Are attenuation tunnels and infiltration tunnels interchangeable?

No. Check the exact model’s supported duty and installed details.

An infiltration arrangement relies on suitable ground and a permitted route into the soil. A sealed attenuation arrangement needs supported containment and a controlled outlet. Similar-looking shells can be offered within different systems, so confirm the assembly, end details, connections and design responsibilities. A product name can’t establish that a generic liner conversion is acceptable for your project.

Do tunnels always need less excavation than crates?

No. Compare complete sections using matched usable storage and design conditions.

The relevant quantities include chamber dimensions, surrounding aggregate, bedding, cover, pipe levels and working space. A tunnel’s claimed capacity may include stone-pore water, while a crate quotation may state module capacity. Reconcile those definitions before measuring the excavation. Neither a lower unit height nor a higher published void percentage proves a smaller complete cut on a particular site.

Can an underground storage system sit beneath a car park?

Only where the exact system and installed section suit the project’s loading and ground conditions.

Ask the designer to check the proposed model, pavement, cover, bedding and groundwater basis together. Also identify the construction vehicles that may cross the area before the final surface exists. A general traffic label or a quoted unit strength doesn’t resolve those conditions, and evidence for a different model shouldn’t be transferred to the supplied assembly.

Does a 95% module void ratio mean 95% of the excavation stores water?

No. Module void ratio and excavation efficiency have different boundaries.

The excavation includes bedding, cover and working space. Use the credited water volume and complete civil-work quantities rather than applying a module percentage to the whole cut.

Which details make a tunnel-versus-crate quotation comparable?

Both bids need the same hydraulic brief, usable-storage basis and declared installation scope.

Begin with the engineer’s required storage and outflow conditions, not a supplier’s nearest standard package. Ask each bidder to identify its exact model, layout, volume schedule and installation drawing. Then reconcile aggregate, excavation, containment, connections, treatment, access and delivery against the eight quote lines above.

Keep exclusions visible and assign them to a responsible party. The owner should also confirm the inspection, cleaning and disposal arrangements. This review can expose an incomplete offer, but it can’t approve a design or guarantee a saving. Where two layouts change the storage curve or outlet arrangement, return them to the drainage designer before treating their nominal m³ as equivalent.