Tag Archives: Flood Risk

Application for Discharge of Drainage Condition

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Application for Discharge of Drainage Condition

We have recently been engaged to discharge drainage conditions in Wiltshire. The planning approval contains 2no. drainage related conditions that we are applying to remove at the same time.

What does the condition look like?

“No development shall commence on site until a drainage strategy has been submitted
to and approved in writing by the local planning authority. The drainage strategy shall
include, as a minimum, the following information:
– Calculations and drawings for the drainage system design showing designated holding areas and conveyance routes based on no flooding on site for a 1 in 30 year rainfall event.
– Calculations and drawings for the drainage system design showing designated holding areas and conveyance routes based on no flooding on site for a 1 in 100 year plus climate change rainfall event in respect to a building (including basement) or utility plant susceptible to water within the development;
– Calculations which demonstrate that the required 20% betterment against greenfield rates has been achieved for all storm events between the 1 in 1 year and the 1 in 100year return period storm events.
– A clearly labelled drainage layout plan showing the pipe networks and any attenuation ponds. The plan should show any pipe node numbers referred to within the drainage calculations.
– A plan showing the cross sections and design of any attenuation pond and its components”

What is our response to that?

Our response to this will include a custom written report that sets out the requirements step by step, using plans and calculations.

How we can help.

If you need a planning condition related to drainage discharged then please do contact us. We provide a free of charge 15 minute consultation to help understand your requirements. Our reports are kept as simple as possible, and are typically well received.

Flood Defences for Bradford on Tone

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Where is Bradford on Tone?

Bradford on Tone is a small settlement on the River Tone. It is situated to the west of Taunton, and has a population of 622.

Where are the Flood Defences?

The below map shows the location of the flood defences (mapping under PMCL No. LAN1001865) :

flood defences bradford on tone

Are the Flood Defences Effective?

To a point. But they are not very good.

The defences are recorded as being in fai to good condition. The southern embankment has a crest level of 31.68mAOD, and this would be overtopped given a 1:20 year flood event. It provide protection against a 1:10 year event just.

These scenarios do not account for climate change that will increase flow volumes in this river by 4.7%.

A 1:100 year event is perhaps more appropriate level of defence, and cities such as London will for the most part have 1:1000 year protection.

1:10 Year Explanation

This last section made with gemini AI

A 1:10 flood event, also known as a 10-year flood, is a flood event that has a 1 in 10 chance (10% probability) of occurring in any given year. It’s important to understand that this does not mean the flood will occur exactly once every 10 years, nor does it guarantee that it won’t happen more frequently.  

Here’s what you need to know about 1:10 flood events:

  • Probability: A 1:10 flood has a 10% chance of occurring in any single year. This means that even if a 1:10 flood happened this year, there’s still a 10% chance it could happen again next year.  
  • Recurrence Interval: The “10-year” part refers to the average recurrence interval, which is the average time between events of a similar magnitude. However, this is just an average, and floods can occur more or less frequently due to natural variations in weather patterns.  
  • Flood Risk: 1:10 floods are considered relatively frequent events. They can cause significant damage to property and infrastructure, and pose a risk to people’s safety. 

Incorrect Flood Zone 3b on Flood Mapping

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Incorrect Flood Zone 3b on Flood Mapping

This is about as niche as it gets . . . . but here we go!

We have recently seen a couple of examples where Cornwall Council’s flood mapping (which is different to flood map for planning) gives some very strange results for Flood Zone 3b.

How Flood Zones Normally Appear

Perhaps the most familiar flood mapping for those “in the trade” is the flood map for planning. Typically we see the darker blue (more often flooding)  Flood Zone 3, fringed with lighter blue (less often flooding) Flood Zone 2.

screen shot from flood map for planning

Screen shot from flood map for planning.

In flood mapping it is always the cases that a “more often” flood outline (shaded area) never go outside of the “less often” flood outline. Look:

Flood Zones from Some Older Mapping

In the above slide the dark blue areas do not extend beyond the light blue areas. They can extend up to the edge, but not beyond.

Flood Zone 3b on Cornwall Council Flood Mapping

So when we first saw the mapping from Cornwall Councils’ own mapping service below we knew something was wrong with it. Apart from the strange colour scheme, you can see Flood Zone 3b Extends well beyond Flood Zones 2 and Flood Zone 3a.

So there is something wrong with the modelling of Flood Zone 3b at this site.

Incorrect Flood Zone 3b – Cornwall Mapping

The image below shows the beginning of our investigations in to the error. We made a detailed study of elevations in the vicinity for starters and then wrote a report to explain why the site in this instance should not be included in Flood Zone 3b.

Detailed Contour Zones on Google Satellite Images

And we are happy to say that the argument we politely put forward worked well, and the permissions was granted.

If you have issues with a site that appears to be included in FLood Zone 3b, when maybe you think it shouldn’t be then please do contact us and we will be glad to help. We can deal with this issue as part of a flood risk assessment or as a bolt on.

We have seen strange behaviour like this in Cornwall Council’s flood mapping, and also in Bristol City Council’s Flood Mapping.

 

Southwest Environmental Limited: Your One-Stop Shop for Flood Risk Assessments

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Flood Risk Professionals

Flooding is a natural disaster that can cause significant damage to your property. It can also lead to business interruption and loss of income. That’s why it’s important to have a flood risk assessment in place to protect yourself from the financial and emotional costs of flooding.

a man is cycling along a flooded street in england, the water is brown and about 10cm deep, he is not looking hwere he is going and behind him there is a row of shop one is painted yellow with the words cartridge world written on the sign, the buildings in the background look quite old maybe 100 years old of more

Image: johnda CC BY-SA 2.0

Southwest Environmental Limited is a leading provider of flood risk assessments in Somerset. We have a team of experienced professionals who can help you assess your flood risk and develop a plan to mitigate it. We offer a range of services, including:

  • Flood hazard mapping: We can use our advanced mapping technology to identify areas that are at risk of flooding.
  • Flood risk assessment: We will assess your property’s vulnerability to flooding and identify potential risks.
  • Flood mitigation planning: We will develop a plan to reduce your flood risk, such as installing flood barriers or making changes to your property’s drainage system.

Why Choose Southwest Environmental Limited?

  • We are experienced: We have been providing flood risk assessments for over 15 years.
  • We are qualified: Our team of professionals is qualified to assess flood risk.
  • We are affordable: We offer competitive rates for our services.

Contact us today to schedule a flood risk assessment.

Don’t wait until it’s too late. Protect your property from flood damage with a flood risk assessment from Southwest Environmental Limited.

Flood Risk Assessment – Newmarket

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Writing a flood risk assessment if carefully done is a largely scientific process. By “scientific” we are alluding to the fact that the content is grounded in fact, or at least as far as the available data is accurate.

If a flood of a certain depth is “forecasted” when viewing available flood data for the site then we can raise the building up. Facts and reactions to those facts. This is what we like.

One part of the Flood Risk Assessment is the Sequential Test. This “non-scientific” part of the Flood Risk Assessment we could do without. It is a policy based, almost artistic endeavor that relies on the matching of unqualified of opinions, sometimes resulting in mismatches.

Image: johndal CC BY-SA 2.0

The sequential test relies on the following ingredients in order to work:

  • search area
  • search criteria

Both of these ingredients are open to debate in terms of their size and type respectively, and as such we have to be careful to follow established guides that have been issued by some of the more proactive administrative authorities’ so that we can demonstrate plausibility.

If you would like to discuss a Flood Risk Assessment for a Project, or a Sequential Test, then please do get in touch.