Water and external systems · Drainage & Rainwater
Stormwater Attenuation System
A reference on how attenuation storage, the flow control that creates the need for it and the exceedance route depend on one another, and why the arrangement limits the rate at which water leaves a site rather than the amount.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What attenuation storage is
A stormwater attenuation system sits in the middle of a site's drainage. Everything collected arrives, is held briefly, and is let out through a device that restricts how fast it can go. The storage may be a buried cellular void, a tank, oversized pipework, a lined basin or a pond, and the restriction may be an orifice, a vortex device or a weir, but it is the pairing that makes the assembly a system.
The purpose is a rate, not an amount. Everything arriving eventually leaves; what the arrangement changes is the shape of the discharge over time, flattening a short intense flow into a longer gentle one so that the receiving sewer, watercourse or downstream network is not asked to take more than it can. That purpose is invisible in any of the parts on their own.
Its nearest sibling is the infiltration drainage system, and the test between them is whether there is an outflow to control. Attenuation always has one and exists to throttle it; infiltration has none and gives water to the ground. Take the flow control out of an attenuation arrangement and what remains is a tank with a pipe leaving it, passing water through at whatever rate it arrives and achieving nothing the site needed.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- detention storage
- balancing storage
- stormwater detention system
- flow-controlled storage
- surface water throttle and storage
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Limit the rate at which surface water leaves a site, so a downstream network is not asked to take a sudden flow.
- Hold the difference between what arrives and what is allowed to leave, for as long as that difference lasts.
- Empty again in time for the next event, which is why storage that stays full is a signal rather than a success.
- Give water beyond the design assumption a chosen route rather than an accidental one.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. It is distributed rather than stacked: what matters is that the path connecting these roles stays continuous and unobstructed, not the order they are listed in.
- ProtectionPre-treatment and silt interception
Traps, catchpits or a treatment stage upstream of the storage. What passes here becomes sediment inside the store and, more seriously, is what eventually reaches and obstructs the flow control, which is the narrowest opening in the whole arrangement.
- Cavity or voidTemporary storage volume
Buried cellular units, tanks, oversized pipes, basins or ponds, holding the difference between arrival and permitted discharge. It is a working volume rather than a reservoir, and its value lies in being empty when the next event begins.
- Control layerFlow control device
The restriction that decides the discharge rate and therefore creates the need for storage in the first place. It is the smallest opening in the system by intent, which makes it both the reason the arrangement works and the component most easily defeated by debris.
- Edge and terminationOutfall to the receiving system
The connection to a sewer, watercourse or other network. Its water level is not a fixed thing: when the receiving system is itself full, the outfall stops being an exit and the storage begins to fill from the wrong end.
- Edge and terminationExceedance and bypass route
The designed path for events beyond the assumption, whether a weir, a bypass or a shaped surface route across the site. It is chosen so that surplus goes somewhere tolerable rather than to the lowest doorway.
- Service zoneAccess, inspection and silt removal
Chambers, openings and clear routes for looking at and cleaning the store and the control. The flow control in particular has to be reachable, because checking it is the only way to know the system is still doing what it exists for.
- Primary supportCover construction, surround and formation
Where storage is buried beneath a car park, drive or landscaped area, the loads above pass around and over the void and the prepared formation beneath carries what is left. Neither has anything to do with drainage, yet uneven settlement in a store built from small units shows as water trapped in the void and a control no longer at the level assumed.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The flow control and the storage are the same decision seen from opposite ends. Tightening the control increases the volume that has to be held; enlarging the store makes sense only in relation to a control holding water back. Neither can be selected alone, and altering one after construction, by fitting a different plate or filling part of a basin, changes the behaviour of the other without it being touched.
That relationship also makes the smallest component the most exposed. A restriction fine enough to shape the discharge is fine enough to be obstructed by a plastic bag or a build-up of grit, and when it is, the store behaves as though the control were tighter still: it fills, then passes water over the exceedance route while the outfall runs almost dry. The upstream interception stage is therefore not a refinement but the thing keeping the control able to work.
The outfall imposes a condition the store cannot influence. A control discharges according to the difference between the water level behind it and the level in front of it, so a receiving sewer running full during the same storm reduces or reverses that difference. The store then fills from both directions, and an arrangement that behaved correctly in isolation is overwhelmed by something happening beyond the site boundary.
Emptying is the interaction that gets forgotten, because it happens when nobody is watching. The store is ready for the next event only once the control has passed everything held from the last, so events separated by less than the recovery time are met by a progressively smaller system. Silt accumulating in the base has the same effect permanently, which is why access provision and storage volume are related components rather than a convenience.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
Buried tanks, chambers and the surround to cellular storage are commonly encountered in these families, particularly where the store sits beneath a trafficked surface. Which form applies to a given store is settled by the structural design and the manufacturer's documentation.
Control layer
Where a store has to retain water rather than release it into the ground, sheet membrane families of this kind are commonly encountered as the wrapping that closes it. Whether a lining belongs at all follows from the drainage strategy rather than from a material preference.
Jointing and sealing
Joints in cast structures and the openings around inlet and outlet penetrations are commonly detailed with these families. A store retains only as well as its joints and penetrations do, and how those are formed belongs to the designer and the product literature.
Edge and termination
Covers, frames, screens and grilles at inlets, overflows and access points are commonly encountered in these families. Their arrangement follows the loading above and who is expected to open them, both of which are settled in the project documentation.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Arrival from the site collection network
Everything the store is asked to hold arrives through this system, and everything the flow control has to survive comes with it. Where surface collection has been extended or resurfaced after the store was built, the assumption the store was arranged around has quietly changed.
Read about Surface Water Collection →Outfall into the buried network
The outfall usually joins a buried network, and the junction is where the assumptions of both systems have to agree: the network's own capacity, its surcharge behaviour and its level together determine whether the control can discharge at all.
Read about Buried Drainage Network →Storage held at roof level instead
Holding water on a roof does the same job higher in the catchment and reduces what reaches the ground store. The pair are often considered together, since roof storage changes the arrival pattern rather than the quantity that eventually arrives.
Read about Roof Attenuation →Storage within the pavement construction
Pavement construction can itself provide storage, so the real question is whether the pavement discharges into the store or forms part of it. Treating both as independent stores generally overstates what a site actually has available.
Read about Permeable Paving →Disposal instead of controlled release
Where the ground will accept water, part of the store's contents can be disposed of rather than passed on, which changes what the flow control is asked to do. Both arrangements are commonly combined and are frequently mistaken for each other.
Read about Infiltration Drainage →Storage inside a pavement build-up
A store beneath a drive or car park sits within another system's construction, and the pavement's own support requirements often decide the depth and footprint left for storage before drainage is considered at all.
Read about Pavement Foundation →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- A buried store standing empty in wet ground is a large closed space with water around it, and ground water behaves differently from the water inside. How the store is held down and kept from taking on water it was never meant to receive is a structural matter.
- Durability
- Sediment settling in the base is a permanent reduction in working volume and is not visible from a cover. How quickly it accumulates is set by what the upstream interception lets through, so durability here is decided somewhere other than in the store.
- Maintenance and access
- The flow control has to be reachable and is the component most often built in behind something else. If it cannot be inspected, nobody can say whether a site is still discharging at the rate its permission was based on.
- Buildability
- Buried storage competes for the same ground as foundations, services and pavement construction, and it is usually the deepest and widest of them. That space is either coordinated early or discovered during excavation.
- Interfaces
- The behaviour of the receiving system during the same storm decides what the control can achieve. An arrangement worked out against a free outfall behaves quite differently once the network in front of it is surcharged.
- Documentation
- The discharge rate the arrangement was based on, the position of the control and the exceedance route are what a later owner needs. Without them, a change upstream can defeat the system without anyone realising a system was there.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- What discharge rate is this arrangement built around, and who set it?
- Where is the flow control, and can it be seen and cleaned without excavation?
- How long does the store take to empty, and what happens if a further event arrives before it has?
- What intercepts silt and debris before they reach the control, and how often is that stage emptied?
- What has been assumed about the receiving system during the same storm, and is that assumption recorded?
- Where does water go in an event beyond the design assumption, and what does it pass on the way?
- Does the store sit beneath a trafficked surface, and what happens to it if the loading above changes?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Attenuation does not reduce the quantity of water leaving a site; it changes how quickly it leaves, and everything arriving still departs.
- A storage void without a flow control is not an attenuation system, because nothing is being restricted and nothing needs to be held.
- Storage standing full is not evidence that the system is working, and on a store that should have recovered it is evidence that something has stopped.
- Adding storage does not compensate for an obstructed control, since it is the control that creates the storage requirement in the first place.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What discharge rate was agreed with the drainage authority, and what shows that the control delivers it?
- How is the flow control reached, and what does routine inspection of it actually involve?
- What was assumed about water levels in the receiving system during the design event?
- Where does exceedance flow go, and has that route been checked against the finished site levels?
- How would a loss of working volume to silt be noticed before it affects the site?
- Is the store buoyant when empty, and what holds it down in wet ground?
What this page does not do
- This entry states no storage volume, no discharge rate and no control size, all of which are outcomes of design work carried out for a specific site.
- Discharge rates and consents are determined by the relevant drainage authority, and what is acceptable differs between jurisdictions and catchments.
- Altering paved areas, roof areas or connections upstream changes what the store is asked to do, even where the store itself is left untouched.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Site Drainage and Rainwater Management Systems
Stages in the single route water takes from the surface it lands on to the point it leaves the site, each one defined by where it hands the water on.
Browse all drainage & rainwater entries →