Building components · Drainage Elements
Outfall
Explains the outfall as the constraint that determines whether a drainage design is possible at all.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a outfall is
The outfall is where a drainage system ends: the connection into a public sewer, the discharge into a watercourse, or the point at which water is released into the ground.
It is the constraint the rest of the system is designed around. What the outfall will accept — whether a connection is permitted at all, and at what rate — determines whether storage is needed upstream, how large it must be, and in some cases whether the development can proceed in the form proposed.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not an outlet. An outlet is where water leaves one element into another within the system; the outfall is where the system as a whole discharges.
- It is not a soakaway. A soakaway is one type of outfall, discharging to ground.
- It is not an overflow. An overflow is a secondary route within the system for exceptional conditions.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- outfalls
- discharge point
- system outlet
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Whether discharge is permitted, which is a consent matter rather than an engineering one.
- The rate the receiving system or ground will accept, which commonly forces attenuation upstream.
- The receiving water level, since an outfall below it may be unable to discharge when it most needs to.
- Its physical condition, since an outfall is at the end of a system and is commonly the least visited part of it.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
A restricted discharge rate at the outfall is what makes attenuation necessary upstream, so the two are one design problem.
Consents and connection agreements for an outfall are the documents that establish whether a drainage strategy is viable.
Outfalls are commonly at boundaries, in watercourses or in awkward positions, and they are the part of a system least likely to be inspected.
Where the receiving water can rise above the outfall, the system can be prevented from discharging exactly when the flow is greatest.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Where it is met
Application contexts
Building contexts where this element is commonly encountered. Association is not recommendation: appearing here does not mean the component suits every instance of that context, or any particular one.
- Site Drainage
Clarifications
Commonly misunderstood
- That drainage is designed from the building outward. It is commonly constrained from the outfall backward, because that is where the limit sits.
- That an existing outfall can take additional flow. Additional discharge to a sewer or watercourse is generally a consent matter.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Where does this system discharge, and is that discharge permitted?
- What rate will the receiving system accept, and does that require storage upstream?
- Can the receiving water level rise above the outfall, and what happens then?
- When was the outfall last inspected, and is it accessible?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- No discharge rate, consent requirement, level or capacity is stated here.
- Discharge to sewers, watercourses and the ground is subject to consent and regulation; establishing what is permitted is professional work.
Drainage & Rainwater Elements
The gutters, outlets, downpipes, channels, chambers and traps that collect water, carry it and let it be inspected.
Browse all drainage elements entries →