Flood risk management: modelling blockage and breach scenarios

This guidance explains how to model blockage and breach scenarios as part of a Flood Consequences Assessment (FCA). It sets out the minimum requirements for assessing risks from defence failure, overtopping, and blocked structures.

Who this guidance is for

Consultants preparing flood consequences assessments
Developers creating proposals in areas at risk of flooding
Local planning authorities

Before you start

You must read this guidance alongside:

You must agree the scope of the flood consequence assessment or model with Natural Resources Wales before you begin any detailed work.

Uncertainties in modelling of breach or blockage should be carefully considered and the chosen approach fully documented.

When blockages must be assessed

You must model blockage when:

a culvert, bridge or structure or other structure could reasonably become blocked
blockage could change flood levels on or near a development site
Natural Resources Wales identifies blockage risk during scoping

How to model blockage

During a flood event, flood water can carry a significant amount of debris. A flood consequence assessment may therefore need to consider:

the likelihood of a blockage
consequences of that blockage

Hydraulic modelling of flood risk should include sensitivity testing to understand the full range of consequences of a blockage.

Whilst it is relatively straightforward to assess the impact of obstruction on upstream water levels, it is more difficult to decide on a credible degree of blockage. The likelihood of material accumulating depends on various risk factors including the:

type
size of structure
nature of the debris

In order to carry out a hydraulic analysis of blockage, it is necessary to make assumptions about the degree of blockage. The appropriate proportion of blockage is usually a matter for pragmatic judgement and often relies on local knowledge.

There is no definitive guidance, although some guidance for culverts is available:

Standard blockage proportions

Use these proportions to model blockages at bridges or culverts where no better information is available:

Culverts

Low: 30%
Medium: 67%
High: 100% (use 95% in the model to maintain minimum stability)

Bridges

Low: 5%
Medium: 25%
High: 80%

Lower value are applied at bridges as they are normally less prone to blockage. Local knowledge and engineering judgement or scientific evidence may be used to apply varying proportions if considered appropriate at a particular location.

Full justification for using an alternative value must be included in the flood consequence assessment.

Events you must model for bloackage

To understand the potential impacts of blockage, a range of flows and blockage proportions must be assessed. This must also consider how climate change will affect flood risk.

To meet TAN15 requirements you must assess blockage using at least:

1% Annual event probability (AEP) 1 in 100 year, plus central estimate of climate change
0.1% Annual event probability (AEP) plus central estimate of climate change

These must be assessed against the flood-free thresholds and the tolerable conditions set out in Figures 5 and 6 of TAN15, along with the requirement for “no increase in flooding elsewhere”.

Where a risk of blockage is included within the flood consequence assessment, it may be beneficial to assess the 1% AEP (1 in 100 year) risk against the upper end estimate of climate change. This will help provide a better understanding of how higher scenarios of climate change may impact a proposed development and can be used to inform resistance and resilience measures.

Factors such as vulnerability of development may need to be considered when establishing upper end requirements.

If the events set out in paragraph 3.8 (where is this?) are not included in the flood consequence assessment, full justification/ reasoning will need to be provided.

For example, an assessment against the 1000 year plus climate change (including blockage) event may not be necessary if the flood consequence assessment has already demonstrated the site cannot meet the TAN15 flood free thresholds in Figure 5 of TAN15. Or the tolerable conditions in Figure 6 during lower magnitude events.

It is also acknowledged that in some instances, the modelling may be so complex that it becomes unstable, and the corresponding outputs may not be reliable.

We recommend modelling a wider range of events (for example 50%, 10%, 3.3%, 1.33%) to:

fully understand of flood risk due to blockage
ensure there is no change in risk against smaller return events both to and from development
ensure there is no increase in flood risk elsewhere from lower magnitude flood events as a result of the proposed development

For simplicity in hydraulic modelling terms, if a blockage is included in a flood consequence assessment it must be in place for the full duration of the flood events indicated in paragraph 3.8 (where is this?). Its effect on flood risk over the whole event should be sufficiently detailed within the flood consequence assessment.

Overtopping and breach modelling

When you must assess overtopping and breach

You must always assess the likelihood and risk of overtopping or breach for any defence that protects a proposed development site.

The presumption is that once a defence is overtopped, the greater the risk of failure, especially for earth embankments (soft engineered defences).

The level of risk depends on several factors, including the:

timing
duration
peak flood level
depth/velocity (flood hazard)
volume of floodwater entering the area
local overland flow paths
site’s distance from the defence
how quickly floodwater will drain or be removed following an event.

In general, the sites most at risk of rapid inundation are those located immediately behind flood defences. If the failure of a defence is considered so unlikely it does not need to be assessed, then this must be supported by appropriate justification and evidence set out in an flood consequence assessment.

Agreeing breach location

Where an assessment of failure is required, the location and nature of any breach should be agreed with Natural Resources Wales and should generally be located as follows in terms of priority:

at any known areas of weakness (e.g. low-spots, the interface between soft and hard defences, outfall structures etc)
the location where the defence is closest to the development site

Ultimately any decision on the breach location will need to be guided by local knowledge including known asset condition and performance during previous flood events.

When a breach assessment is required

You must assess breach where:

the site benefits from flood defences
the site is within a TAN15 defended zone
a breach could create rapid or deep flooding
Natural Recourses Wales identifies breach risk at scoping

An allowance for climate change over the lifetime of development must be included.

TAN15 defended zones

The Flood Map for Planning identifies areas that benefit from Risk Management Authorities operated, managed and maintained defences.

These defences are considered to meet the minimum level of protection set out in TAN15. However, the Standard of Protection (SoP) afforded by them is our best estimate based on the most recent assessment data. This may be several years old and it should be recognised the SoP is likely to have reduced from when the defence was constructed.

For example, as a result of changes in hydrology, or due to the impacts of climate change. Hydraulic modelling is therefore likely to be needed to determine the current SoP.

TAN15 Defended Zones should not be considered automatically suitable for any type of development. The flood risks and consequences, including an overtopping and/or breach assessment must be assessed within a FCA to determine whether a development can comply with TAN15 acceptability criteria, be managed safely over its lifetime and ensure no increase in flood risk elsewhere.

If a site is within a TAN15 Defended Zone, you must assess defence breach unless the flood consequence assessment provides clear justification for why a breach assessment is not needed.

Breach widths

Breach widths vary depending on the nature of the defence.

This table summarises suggested breach width depending on location and defence type. This can be used as a starting point unless more appropriate site-specific information is available to justify the use of an alternative value.

**Breach widths by defence type**
Location	Defence type	Width (m)
Open coast	Earth bank	200
Open coast	Dunes	100
Open coast	Hard	50
Open coast	Sluice	Sluice width
Estuary or tidal river	Earth bank	50
Estuary or tidal river	Hard	20
Fluvial river	Earth bank	40
Fluvial river	Hard	20

Event timing and duration

You must model:

three tide cycles
or an appropriate fluvial duration (based on an estimation of the time lapse between the initial breach and subsequent repair. Even if this is a temporary solution)

The breach could be present for the whole event (i.e. is deemed to have occurred prior to the event peak).

Alternatively, breach initiation can be timed to coincide with peak water levels or at the point of overtopping (whichever occurs first). This approach takes into account rapid inundation of areas behind defences.

A sudden breach is often an issue for model stability and so defence height may need to be gradually reduced to the base level. The failure mode of a defence will be a function of the defence type.

Required breach scenarios

Paragraph 11.7 of TAN15 notes that a breach assessment should be undertaken ‘…against return periods up to and including the 0.1% AEP (1 in 1000 year) flood event, including an allowance for climate change’.

To meet TAN15 requirements, the flood consequence assessment should consider a range of scenarios where a breach could occur.

To assess against the TAN15 flood free thresholds and tolerable conditions (TAN15 Figures 5 and 6), and the requirement for ‘no increase in flooding elsewhere'.

You must model breach for the following, depending on development type:

Emergency services

0.1% AEP plus central estimate of climate change (rivers)
0.1% AEP plus upper central estimate of climate change (sea)

All other development

1% AEP plus central estimate of climate change (rivers)
0.1% AEP plus central estimate of climate change (rivers)
0.5% AEP plus upper central estimate of climate change (sea)
0.1% AEP plus upper central estimate of climate change (sea)

If a full range of events are not included in the flood consequence assessment, full justification and reasoning will need to be provided.

We recognise that it may not be necessary to undertake an assessment against the more extreme event, for example if it has been shown the defence has already failed in the 1% CC event.

We also acknowledge that modelling may not be achievable as hydraulic models can become unstable and may not provide reliable outputs for such an extreme event. However, a reasonable effort must be made to stabilise such models before discounting it and evidence of such must be set out in an flood consequence assessment.

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