Water and external systems · Drainage & Rainwater
Site Surface Water Collection System
A reference for the part of site drainage that happens above ground and ends at the inlet, covering how surface shaping, catchment boundaries and interception points determine where run-off actually arrives.
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 surface water collection is
External surfaces do not drain; they deliver. Water lands on paving, hardstanding or graded soft ground and travels across it until something intercepts it. This system is the arrangement of those surfaces together with the points placed to receive them: shaped falls, kerbs and edge restraints that split a site into catchments, point inlets, linear channels, and interception at level changes.
The boundary against a below-ground drainage network is physical and easy to test on site: this entry ends at the inlet. Everything above the grating, meaning where water goes, how quickly, and whether it arrives at the inlet at all, belongs here; everything from the grating onwards inside a pipe belongs to the buried system. Linear channels and slot drains are a component format within this entry rather than a system of their own.
Its parts are unusually vulnerable to later works. A resurfaced yard, a new planting bed, a raised threshold or a fence line laid across a catchment boundary can send water somewhere it was never intended to go without anybody touching the drainage. That is why a clear, unaltered inlet can begin flooding after work was done nowhere near it.
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.
- yard drainage
- hardstanding drainage
- external surface water drainage
- site run-off collection
- paved area drainage
- linear channel drainage
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.
- Decide where water crossing external ground is collected, rather than leaving that to whatever shape the ground happens to take.
- Divide a site into catchments so no surface is asked to carry water from an area that was never meant to reach it.
- Intercept flow before it reaches a threshold, a level change or a boundary where arriving water becomes somebody else's problem.
- Trap silt and leaf litter where they can be lifted out, ahead of buried pipework where they cannot.
- Hand water on to conveyance, infiltration or storage at a point the designer chose.
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.
- Finish surfaceFalls-formed surface
The paving, hardstanding or graded soft area that both takes traffic and moves water. It is the largest component and the one most often altered later, and the shape it is given determines whether the inlets placed on it receive anything at all.
- Edge and terminationKerbs, upstands and edge restraints
The elements deciding which catchment a drop of water belongs to. A kerb is not only a restraint holding a surface together; it is the boundary keeping flow on the side of the site where a receiving point was provided for it.
- Drainage planeGullies, pot inlets and gratings
Point receivers set at low points. Each depends entirely on the surface delivering water to it, which is why a gully at a spot that is no longer the lowest point is an opening in the ground doing nothing while water passes by.
- Drainage planeLinear channel and slot drain
A continuous receiver used where flow arrives along a line rather than at a point: across a driveway, along a building edge, at the head of a ramp. It removes the need for the surface to deliver to one spot, at the cost of a longer element to keep clear.
- ProtectionSilt trap, sump and leaf basket
The interception keeping grit, leaf litter and detritus out of the buried network. It works by being emptied, which ties this component more closely than any other to whether anybody is actually looking after the site.
- Edge and terminationThreshold and level-change interception
Channels and gratings placed where a surface meets a doorway, a ramp head or a retained edge. They exist because the consequence of arriving water is worst at these places, not because more water arrives at them.
- Service zoneConnection to the receiving system
The outlet from each inlet into conveyance, infiltration or storage. What that outlet leads to, and how the receiving system behaves when it is full, decides whether the inlet keeps accepting water during the events that matter.
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 surface and the inlet are one device. An inlet does not collect; a surface delivers, and the inlet receives only what arrives. This is why most useful thinking about site drainage is really about shaping: a low point in the wrong place, a settled panel of paving, or a threshold raised during a later refit will each send flow past an inlet that is otherwise perfectly sound.
Kerbs and edges arbitrate between catchments, so they decide how much any one inlet is asked to take. Removing a kerb during landscaping work, or adding a raised planter that dams a route, merges two catchments so that one receiver is asked to take what was shared while the other sits dry: a change in behaviour caused by something nobody thinks of as drainage.
Silt interception and the buried network trade places with each other. Anything trapped at the surface can be lifted out; anything that passes settles in pipes and chambers where removal is disruptive. A design that omits the trap has quietly moved a recurring task into a place where it will be done rarely, late and expensively.
The receiving system sets the ceiling on all of it. When conveyance, storage or infiltration downstream is full, inlets stop accepting and the surface reverts to whatever shape it has, which is why the shaping question and the discharge question are the same question asked at opposite ends. Water then leaves along the route the levels dictate, and thresholds are where that route is tested.
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.
Finish surface
Collecting surfaces are commonly encountered in these families and in bound surfacings. How a surface behaves once it settles, opens at its joints or is patched is a matter for the paving design and the material documentation.
Edge and termination
Kerbs, edgings and upstand elements defining catchment boundaries are commonly encountered in these families. Whether an edge holds its line under traffic and ground movement is settled by its bedding and haunching design.
Drainage plane
Channel bodies, gully tops and gratings are commonly encountered in these families and in polymer concretes. How a grating behaves under the traffic it will see, and whether its openings are acceptable in a pedestrian route, is for the designer.
Jointing and sealing
Bedding and jointing around inlets and channel units are commonly encountered in these families. Whether a rigidly jointed or a flexible arrangement is used where a unit meets the surrounding surface belongs with the paving designer.
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.
Unbound paving around an inlet
Jointed paving on a laying course settles and shifts differently from a channel unit bedded rigidly in concrete. The ring of paving around every inlet is where that difference first appears, as a slight sink that moves the local low point away from the grating.
Read about Flexible Paving →Surfaces that take water downwards
Where a surface is intended to accept water through its joints it stops being only a conveyor, and the inlets on it serve a different purpose. Mixing permeable and sealed areas without deciding which drains to what is a common source of unintended loading.
Read about Permeable Paving →Thresholds and level access
A level threshold removes the step that used to keep surface water out, so interception in front of the door carries a duty the construction previously carried. The channel, the fall away from the door and the threshold detail form one arrangement.
Read about Door Threshold Interface →The inlet as a handover point
Everything downstream of the grating belongs to the buried network, including whether it can accept flow while surcharged. An inlet is only as useful as the system it discharges into at the moment that system is most heavily loaded.
Read about Buried Drainage Network →Surfaces above a retaining structure
Water allowed to run over the head of a wall arrives behind it. Interception above a retained edge is part of keeping the wall's own drainage doing the job it was designed for rather than the job the finished levels have given it.
Read about Wall Base Drainage →Discharge into the ground
Where collected water is taken to a soakaway or a filter trench, the silt washed off the surface arrives with it. What the surface delivers therefore determines how long the ground interface keeps accepting water at the rate it was designed around.
Read about Infiltration Drainage →
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
- Standing water against a building base, at a threshold or over a retained edge is the usual consequence of a collection arrangement that no longer matches the levels. What the fabric behind is made of decides how serious that becomes.
- Durability
- Surfaces settle, joints open and edges are clipped by vehicles, and each of those changes the shape water follows. Deterioration in this system shows itself as altered flow paths well before it shows as a broken component.
- Movement
- Rigidly bedded inlets and channels sit within surfaces that move seasonally and under load, so the junction between them is where cracking and lipping appear. How that junction is arranged is a design decision, not a finishing detail.
- Maintenance and access
- Gully pots, silt traps and channel gratings work only by being emptied. A design that places them where lifting a cover means closing a route or moving a vehicle makes it likely they will not be emptied at all.
- Interfaces
- The most consequential interfaces are with things nobody classes as drainage: kerbs, planters, fences, thresholds and later resurfacing. Each of them can redirect a catchment without a pipe being touched.
- Buildability
- Setting out levels so every point on a surface genuinely delivers to a receiver is harder than placing the receivers, and it is the part most often compromised when a surface is built to suit the levels of its edges.
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.
- Which catchment does each inlet serve, and what on the ground actually defines that catchment?
- Does every part of the surface deliver water to a receiver, or only most of it?
- What happens at thresholds and level changes when the receiving system cannot accept more?
- Is silt intercepted somewhere it can be lifted out, or is it going straight into buried pipework?
- How will later resurfacing or landscaping be prevented from merging catchments?
- Do gratings sit safely within pedestrian and wheeled routes as well as taking flow?
- Where does a linear element earn its place instead of a point inlet, and who keeps it clear?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Drainage problems are blamed on blocked inlets when the inlet is clear and the surface has simply stopped delivering water to it.
- A linear channel is read as a stronger gully, when it answers a different problem: flow arriving along a line rather than at a point.
- Site drainage is treated as work that ends when the pipes are laid, though most of it is levels, edges and the shape of surfaces.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- How were the catchment boundaries set, and what physically holds them on site?
- What does the surface do when the receiving system cannot take more?
- Where is silt intended to collect, and how often does that need attention?
- Are the inlets positioned at the low points the finished levels will actually produce?
- What interception protects thresholds, ramps and retained edges here?
- If this yard is resurfaced in future, what has to be preserved for the drainage to keep working?
What this page does not do
- No falls, sizes or capacities appear here; how much a surface can shed and where receivers belong is a design calculation for a qualified professional.
- Discharge across a boundary, into a watercourse or into a public system is a matter for the relevant authority and the parties concerned.
- Gratings and covers in trafficked and pedestrian routes carry safety considerations belonging with the designer and the product documentation.
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.
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 →