Blockage scenario in a surface water drainage design

A blockage scenario drainage design test asks a simple question: if the flow control silts up or the trash screen clogs, where does the water go. It is separate from the rainfall design event, and it is a sensitivity test rather than a design case. Lead local flood authorities ask for it because flow controls do block, usually with leaf litter in autumn or with a plastic bag at the worst possible moment, and a scheme that only works with a clear orifice is not a scheme.

The usual outcome is a 200 mm change to a weir level. The usual cost of not doing it is a surcharged basin and a letter from a neighbour.

Why blockage is assessed at all

Attenuation works by holding water back and letting it out slowly through a small opening. The smaller the permitted discharge rate, the smaller the opening, and the more easily it blocks. A site restricted to a low greenfield run-off rate may need an orifice of 75 mm or less, which is about the size of a drinks can.

When that opening partially blocks, the storage fills faster than it empties. Two things then happen in sequence. The water level in the basin or tank rises above the design level, and then it leaves by whatever route is available, which may be the designed overflow or may be the car park, the access road or the garden of the house at the low corner.

The blockage test exists to make sure the first route is the designed one. It also tests the structural assumption, because a tank designed for a water level 300 mm below its cover may not be designed for a surcharged head to ground level.

Lead local flood authorities became statutory consultees on surface water drainage for major development in 2015, and the blockage scenario is one of the standard items on their checklists. Treating it as an afterthought at the end of the drainage calculations is how a strategy ends up going back for a second round.

Common assumptions and where they come from

There is no single national figure, which is the first thing to understand. The assumptions in common use are drawn from the industry guidance on sustainable drainage and on screens and culverts, and from individual authorities' published design guides.

The design event paired with the blockage matters as much as the blockage percentage. Testing a half-blocked control against the 1 per cent annual probability event with the climate change allowance is a demanding combination and some authorities ask for it. Others pair a full blockage with a shorter return period on the basis that two rare events together is not a reasonable design case.

Ask which combination the authority wants before running the model. Running the wrong one is a week lost.

Components most likely to block

Experience is reasonably consistent about where the trouble is.

• Small orifices, particularly below about 100 mm, which is why many authorities set a minimum orifice size and prefer a vortex control at low flow rates.
• Trash screens on culvert and pipe inlets, especially where upstream land use generates leaves or litter.
• Inlets to below-ground tanks, where silt accumulates because the inlet is the first place velocity drops.
• Permeable paving that has not been swept, where the surface clogs and the system behaves as impermeable.
• Filter drains and bioretention surfaces receiving silt from an unstabilised construction phase, which is the single most common cause of a system that never performs as designed.
• Swale check dams and level spreaders, which collect debris by design.
• Flow control chambers with no maintenance access, because the one thing certain about an inaccessible chamber is that nobody will clear it.

That last point is where the drainage strategy and the maintenance plan meet. A blockage scenario that assumes clearance within hours is only credible if somebody can reach the chamber with a van, which is why screen position, headwall geometry and maintenance access belong in the same conversation as the hydraulics, as the note on trash screens, headwalls and maintenance access sets out.

Consequences for exceedance routing

When the control blocks, the system needs a designed way to fail. That means three things on the drawing.

A defined overflow at a stated level, sized to pass the inflow with the control out of action. Usually a weir on the basin edge or a high-level pipe from the tank, set above the design water level and below any point where water would enter a building.

A route from the overflow to somewhere water can stand without harm. Across a grassed area to a swale, down an access road to an area of hardstanding, or into a secondary basin. The route has to be drawn with levels, and the depths and velocities along it should be checked against the flood hazard rating so that someone can walk out of it.

A statement of what the receiving area tolerates. A car park flooding 150 mm deep for an hour is usually acceptable. The same depth across a pedestrian route to a block of flats generally is not.

Where the exceedance route crosses the site boundary, the assessment has to show no increase in flood risk to third parties, and that is a point the authority will press. A Drainage Strategy showing exceedance flow arrows without depths has not answered the question, and on a site already in a flood zone the interaction with fluvial levels belongs in the Flood Risk Assessment rather than being left to the reader to combine.

Agreeing the scenario with the authority

Four questions, best asked at pre-application stage in writing.

1. What blockage assumption does the authority apply to flow controls, and is it an area reduction or a discharge reduction?
2. Which design event is the blockage scenario paired with, and is the climate change allowance applied?
3. What is the acceptance criterion: no flooding to buildings, a maximum depth in trafficked areas, or no increase in off-site flows?
4. Does the authority have a minimum orifice size or a preference for vortex controls?

Some lead local flood authorities publish design guides answering all four, and others answer only on consultation. Where the authority has published standing advice, the note on standing advice from planning consultees explains when that advice stands in place of a bespoke response.

Record the answers in the strategy. An assessment that states the assumption it tested, and attributes it, is much harder to argue with than one that presents a result without saying what produced it.

An illustrative assessment on a small site

The following is illustrative only and not taken from a real scheme. Take a 0.4 ha residential site discharging to an ordinary watercourse at a restricted rate, with a shallow attenuation basin and a single flow control chamber containing an orifice plate.

In the unblocked design case the basin fills to its design water level during the critical event and empties within the required drain-down time, with 300 mm of freeboard to the basin crest.

Apply a half blockage to the orifice and the outflow roughly halves, because orifice discharge varies with the open area. The basin level rises and the available freeboard is partly consumed. If the remaining freeboard is still above the overflow weir invert, nothing leaves the basin and the design is tolerant of the blockage. If the level reaches the weir, the weir operates, and the question becomes where the overflow goes.

Apply a full blockage and the basin fills with no outflow at all. The weir operates for the duration of the event, and the overflow route carries most of the inflow. On a site where that route runs across a lawn to a ditch, the outcome is acceptable. On a site where it runs along the frontage past three front doors, the design needs changing, and the change is usually a 200 mm reduction in the weir invert plus a regraded swale rather than a larger basin.

That is the point of running the scenario. It identifies a low cost level change early instead of an expensive redesign at technical approval.

What to show on the drawing

A reviewer should be able to see the blockage case without opening the model.

Show the flow control chamber in detail, with the orifice or vortex unit, its invert level and the design discharge. Show the trash screen with its bar spacing, its area and its maintenance access. Show the overflow weir with its invert level, its length and the calculated head over it in the blocked case. Show the exceedance route with arrows, depths and the receiving area, with levels along the route. Show maintenance access to every control structure, with a hardstanding or mown route a vehicle can use.

And add a note on the drawing stating the blockage scenarios tested, the design events they were paired with, and the acceptance criterion applied. One note saves a round of consultation.

This is general guidance, not legal advice; check the position with your lead local flood authority or a qualified professional before relying on it. To see which flood and drainage documents a site is likely to need, enter the postcode on PlanScope.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.