Modelling a partially blocked flow control

Blocked flow control modelling means running the drainage model twice more: once with the control device partly obstructed and once with it fully obstructed, then showing on a plan where the water goes in each case. Lead local flood authorities ask for it because a vortex control with a 50 millimetre orifice is the single most likely point of failure in an otherwise sound surface water system, and because the consequence of that failure is measured in flooded ground floors rather than in litres per second.

It is a residual risk test, so it sits alongside the design event sizing rather than replacing it. The storage volume is still set by the 1 in 100 year event with a climate change allowance. The blockage run answers a different question, which is what the scheme does when the storage no longer empties.

Why the scenario is expected at all

Lead local flood authorities have been statutory consultees on surface water drainage for major development since April 2015, and their standing advice almost always includes a residual risk requirement. National policy reinforces it: planning practice guidance on flood risk asks whether development will be safe for its lifetime, which covers failure states as well as design states.

Defra's non-statutory technical standards for sustainable drainage deal with discharge rates, volumes and the treatment of exceedance flows. They do not prescribe a blockage percentage. That gap is filled by each authority's own guidance, so the first task on any scheme is to read the local requirement rather than assume a convention.

The other reason is practical. A Drainage Strategy that shows attenuation volumes and nothing else gives the drainage engineer at the council no way to judge what happens on the worst night of the adopted maintenance regime.

Degrees of blockage commonly modelled

Most authorities ask for some combination of the following, and several ask for all of them:

• partial obstruction of the control, often expressed as 50 per cent of the orifice area
• total obstruction of the control, so that the only outflow is the emergency overflow
• a blocked or surcharged outfall, representing a full receiving sewer or a watercourse in flood
• a blinded trash screen or inlet grating on an open feature

Each is run against the critical duration storm for the site, and the useful output is not a peak flow figure. It is a water level. Compare that level with cover levels, thresholds and the lowest point on the site boundary, and the answer either works or it does not.

Where the receiving system is tidal or a main river, the blockage run should be combined with a high outfall level rather than a free discharge, because those two failures tend to arrive together. The joint probability argument for treating them separately is available, but it needs stating explicitly rather than assuming.

Where the water goes when the control fails

Storage fills, the system surcharges, and water reaches the surface at the lowest manhole cover or the lowest gully in the network. That point is rarely where the designer expects it, which is the whole value of the exercise.

You need real levels to do this. A topographic survey with spot levels across the site, the access road and the receiving boundary, not a scaled architectural site plan. Where the scheme sits within a wider catchment, the flood model behind the Flood Risk Assessment should be consistent with the drainage model, and the same climate change allowance should appear in both.

Exceedance routes and threshold levels

An exceedance route is a deliberately designed surface path that carries water away from buildings to a place where ponding is acceptable. Roads with kerbs, swales, parking courts and areas of open space all work. The route has to fall continuously in the intended direction, which sounds obvious and is the most common thing missed on a sloping estate road with a crest partway along it.

Threshold levels are the other half of the answer. A finished floor level set 300 millimetres above the modelled flood level is a widely used freeboard convention, and many authorities apply 150 millimetres above adjacent ground level as a separate minimum. Both figures vary, so take them from the authority's own standard and say in the report which standard you took them from.

Where exceedance water has to leave the site, check that the receiving land can accept it and that you are not creating a liability under common law towards a downstream owner. A route that discharges onto an adopted highway needs the highway authority to agree, and that agreement is easier to get before the application than after it.

Consequences for basement and undercroft schemes

Basements change the arithmetic completely. Surcharged water finds the lowest opening, and on most urban schemes that is a car park ramp or a lightwell. Once water enters an undercroft there is no gravity route out, so the residual risk becomes a pumped system with a power supply, a duty and standby arrangement and an alarm, all of which need a maintenance owner.

The planning practice guidance on flood risk treats basement accommodation with particular caution, and sleeping accommodation below ground in higher flood zones is difficult to justify. For the drainage side specifically, the questions worth answering in the strategy are whether the ramp threshold sits above the modelled blockage level, whether the lightwells have an upstand, and what happens in a power cut. Property level measures such as demountable barriers and non-return valves belong in the residual flood risk discussion rather than in the drainage design, and the interaction between the two is set out in more detail for basements and sleeping accommodation.

Maintenance regimes that reduce the risk

A blockage scenario is a statement about maintenance as much as about hydraulics. The strategy should name who owns each component, who inspects it and how often, and how a jetting vehicle reaches the control chamber.

Three adoption routes exist in practice: a sewerage undertaker under a section 104 agreement where the assets meet its design and construction standards, the highway authority for highway drainage, or a private management company funded by an estate charge. Schedule 3 to the Flood and Water Management Act 2010, which would have created SuDS approving bodies in England, has not been brought into force, so check the current position before describing any statutory approval route.

Specify the inspection interval for the control chamber and the screen, not a general commitment to maintain. A control with a small orifice in a catchment with mature trees needs looking at more often than one on a hard paved industrial yard, and saying so in the report is what makes the blockage run credible.

Presenting the scenario so an officer can use it

Give the drainage engineer at the council four things and the consultation response gets shorter.

1. A table of the scenarios run, each with the storm event, the blockage assumption, the outfall condition and the resulting peak water level.
2. A plan with arrows showing the exceedance route for the total blockage case, with ponding areas hatched and depths annotated.
3. A level comparison listing every threshold, cover and boundary level against the modelled level, with the margin stated.
4. The maintenance schedule as an appendix, with the inspection frequency for the control device highlighted.

Then write one paragraph of residual risk conclusion in plain words. If the total blockage case floods the garden of plot 14 to 80 millimetres and nothing else, say that. An honest small number reads better than a claim that no flooding occurs anywhere, which no reviewer believes.

This is general guidance, not legal advice. Check the requirement with your lead local flood authority before relying on it. 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.