Who this guide is for
- Self-builders and developers setting a site drainage strategy
- Owners of sites where water has nowhere obvious to go
- Renovators adding hard surface to a plot already drained
- Anyone briefing a drainage designer before levels are fixed
- Readers who want the site-scale decision rather than the puddle-scale one
The destination sets the problem
Three destinations exist in the published entries: the ground, a watercourse, or a piped system belonging to somebody else. Whether discharge is permitted at all is a consent matter rather than an engineering one, and the rate a receiving system or the ground will accept is what commonly forces storage upstream.
Because of that, the drainage strategy is not something to work out once the layout is settled. It is one of the things that settles the layout, and the outfall is the first item to establish rather than the last to connect.
Giving water to the ground
An infiltration structure has no designed outflow connection. Everything entering leaves through the surrounding soil, and the overflow exists for the day that stops being true. The disposal is done by the soil rather than by the structure, so the useful question is not how much it holds but how quickly it recovers between events.
That makes the ground itself the governing variable, and its position relative to foundations, retaining structures, slopes and boundaries a geotechnical question rather than a layout convenience, because the arrangement deliberately wets the ground around it.
Leaving at a restricted rate
Attenuation storage always has an outflow, because its purpose is to limit the rate at which water leaves rather than to dispose of it. Everything arriving eventually leaves; what changes is the shape of the discharge over time, flattening a short intense flow into a longer gentle one.
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 later changes the behaviour of the other without it being touched.
What the receiving system does during the same storm
A flow 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 beyond the site boundary.
The same applies to an outfall into a watercourse. An outfall below the receiving water level may be unable to discharge exactly when it most needs to, which is why what has been assumed about that level, and whether it is recorded, is a question worth asking directly.
Everything upstream is downstream of this decision
Once the destination and rate are known, the surfaces follow. A permeable pavement can be built to dispose of water into the ground, to throttle it through a flow control, or to hold it above a liner and pass it on entirely, and a pavement built for one of these and expected to deliver another disappoints predictably.
The same is true of a rain garden, which is a disposal arrangement where its base is open to the ground and a treatment and conveyance arrangement where it is lined. The visible feature is identical in both cases, and only the drawings tell them apart.
- Whether a surface can be permeable at all depends on where its water may go
- Whether a feature infiltrates or is lined depends on the same decision
- Whether a store needs a flow control depends on whether there is an outflow
- Whether a roof can attenuate depends on what the ground strategy still needs from it
Exceedance is part of the destination question
Every arrangement has a condition beyond its assumption, and the route water takes then is a design output rather than an accident. Omitting it does not make surplus disappear; it relocates it. Water backs up the inlet, fills the collection system behind it and reappears at whichever inlet sits lowest on the site.
On a sloping plot that is frequently the inlet closest to the building, which turns an arrangement at the far end of a garden into water at a threshold. The mechanism is invisible in any drawing showing the structure alone.
What to establish before levels are fixed
The order that avoids rework is destination, then rate, then levels, then surfaces. Establish where the system may discharge and whether that is permitted; establish what rate is acceptable and who set it; then set finished levels that can reach it; and only then choose what the surfaces do.
Doing it the other way round produces the familiar outcome of a site whose levels are attractive, whose surfaces are chosen, and whose water has nowhere it is allowed to go.
Site water destination checklist
- 1Establish the candidate discharge points: ground, watercourse or piped system
- 2Establish whether discharge to each is permitted and who decides that
- 3Establish what rate the receiving system or the ground will accept
- 4Find the outfall level and whether the receiving water can rise above it
- 5Ask what has been assumed about the receiving system during the design event
- 6Where the ground is the destination, ask what infiltration testing supports it
- 7Where a rate applies, ask where the flow control sits and how it is reached
- 8Set finished levels only once the discharge point and level are known
- 9Decide what each surface is doing before choosing what it is made of
- 10Ask where water goes in an event beyond the design assumption
- 11Check what that exceedance route passes on its way
- 12Record the discharge rate, the control position and the exceedance route for later owners
Common mistakes to avoid
- Designing the layout first and looking for a discharge point afterwards
- Assuming an existing outfall can take additional flow
- Treating a storage void with no flow control as an attenuation arrangement
- Specifying a permeable surface before knowing whether the ground may receive water
- Overlooking what the receiving sewer is doing during the same storm
- Leaving the exceedance route undrawn and therefore chosen by the levels
- Adding paved area later without revisiting what the destination was sized around
When to involve a professional
- Discharge to sewers, watercourses and the ground is a consent matter
- Drainage is commonly constrained from the outfall backward
- Ask what discharge rate was agreed and who set it
- Ask where exceedance flow goes and what it crosses
- Requirements vary by location and project; verify with your professionals
Frequently asked questions
Questions readers ask about this topic
Why does the outfall come first rather than last?
Because it is the constraint the rest of the system is designed around. What the discharge point will accept, and whether a connection is permitted at all, determines whether storage is needed upstream and how large it has to be. Everything else is downstream of that.
What is the difference between infiltration and attenuation?
Look for a designed outflow connection. An infiltration structure has none and gives water to the ground, so its overflow exists for the day the ground refuses it. Attenuation always has an outflow and exists to throttle it, so storage standing full means opposite things in the two.
Does attenuation reduce how much water leaves my site?
No. Everything arriving eventually leaves; what the arrangement changes is how quickly, flattening a short intense flow into a longer gentle one so the receiving system is not asked to take a sudden flow. The quantity is unchanged.
I am only adding a patio. Does this still apply?
Adding paved area changes what the existing arrangement is asked to handle, and the entity records note that changes upstream alter what arrives without anyone touching what is buried. Whether that matters on a particular site is a question for the drainage designer.
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