Flood Risk Assessment Guide 2021
This is a complete Flood Risk Assessments guide in 2021.
Yes, I’ll show you where to find the basic flood risk information
But you are also going to learn how to assess flood risk and apply the latest national guidance
So if you’re looking to improve your understanding of flooding, you’ll love this flood risk assessment guide.
The Basic Concepts
In this chapter, I’ll answer the question: “What is a Flood Risk Assessment?”
I’ll also show you when a Flood Risk Assessment is required when applying for planning.
Let’s get started
What is a Flood Risk Assessment?
A Flood Risk Assessment (FRA) is a report that evaluates the impact of different flood sources.
The flood sources can be rivers, sewers, groundwater, reservoirs, canals and surface water run-off. In this flood risk assessment guide we will show how to assess and protect from these sources of flooding.
In England, the Environment Agency recommends that your flood risk assessment should
- Show how the flood risks affect the development
- Prove that the development type is appropriate for the location
- Determine the flood resistant and resilient protection
- Confirm that the development does not increase flood risk elsewhere
It seems complicated; we will go through each of these items through the blog. Each of the chapters in this flood risk assessment guide gives clear guidelines on these items.
What is a Flood Risk within this flood risk assessment guide?
The risk of flooding or flood risk is a combination between the probability and consequence of the flood.
Each flood source has its own probability; we compiled them for you in the table below!. This is the only flood risk assessment guide that has the risk probability tabulated for each type of flooding.
The consequence is the impact to your development. These can be loss of use, damage to buildings or nature, loss of income, damage to infrastructure. The BBC has created the picture below showing the impact of flooding
For each flood source, we need to determine their risk. Luckily most of this information is available online
We will show how to find it!.
There are certain flood types that are top secret.
One of them is the internal flooding from sewers. The DG5 register is protected within the water industry.
Imaging knowing whether a house has flooded internally, its cost will drop immediately!.
Discover Water UK has cool stats about internal sewage flooding for each water company
However, the Strategic Flood Risk Assessment, from your local council, can give you sufficient information to make an assessment.
On the other hand, there are flood risks that a less relevant for small housing developments. Flooding from Reservoirs is one of them.
Unless you are building a warehouse that storage chemical products or inflammable liquids. Hopefully you will be using an expert by now!
When is a detailed Flood Risk Assessment required?
Before we start looking at how to evaluate your flood risk, we should check if you actually need a flood risk assessment. This flood risk assessment guide will help you determine the need of a flood risk assessment.
You need a flood risk assessment when:
- Your development is located within flood zone 2 or 3 river and sea flooding this applies to minor developments such as extensions, porches and even dormers! or had a change of use for example it was an office and now it is a dwelling
- Your site is greater than 1 hectare (ha) within flood zone 1
- Your development is smaller than 1 ha within flood zone 1 but it is affected by other sources of flooding such as surface water drains, reservoirs, canals, groundwater flooding.
- The development is within flood zone 1 but is located within a critical drainage area
You might be wondering:
Where do I find all the sources of flooding information, exactly? What is flood zone 1 2 or 3?.
That’s what I’m going to cover in the next chapter of this flood risk assessment guide. Keep reading…
Finding Flood Risk Information
In this chapter, I’ll answer the question: “What is a Flood Risk Assessment?”
I’ll also show you when a Flood Risk Assessment is required when applying for planning.
Let’s get started
How to check if your development in the river flood zone?
You can check if your development is in the flood zone by using the relevant agency flood maps.
For England you should use the flood risk rivers and sea planning. Get into the site and download the flood map for your site.
The flood maps will give you the basic details of the overall location of the development in the flood zone.
If you development is anywhere within the blue area then your site may be affected by flood water.
The map does not give you the flood depth at property level.
It is possible that the level of the property is higher that the flood level!.
The flood level is given directly by each of the Environment Agency. We will show how to order it below.
Before we move on,
I would like to explain the 4 flood zones.
- Flood Zone 1 has the lowest probability of flooding (less than 1 in 1000yr)
- Flood Zone 2 has a medium probability (btw 1 in 100 and 1 in 1000yr)
- Flood Zone 3a has a high probability (higher than 1 in100yr)
- Flood Zone 3b ( higher than 1 in 25yr)
The higher the probability the more likely your site will flood. For instance flood zone 3b is usually the river bed.
I created this short video; hopefully it will explain better.
How to obtain the river flood levels?
You need to send an e-mail to firstname.lastname@example.org with the site location plan including the obtain the flood levels for free.
You will get the results within 20 working days.
Flood Risk From Surface Water
The Surface Water flood risk map is produced by the Environment Agency.
You can access this map by entering the post code and then following the link to the surface water map.
Once you get to the map, you will notice that the drop down menu in the flood risk has different options.
These options relate to the risk of surface water. From this drop down menu we are only interested in extent and depth (high,medium or low)
Flood Risk From Reservoirs
The maps showing the risk from reservoirs is also part of the Environment Agency’s datasets.
This map is part of the drop down menu of the Surface Water Flood Map
We are only interested in the Extent item
Flood Risk From Sewers and Groundwater Flood
It is not possible to find online whether a property has been flooded internally.
It is possible to get an idea of the number of flooding incidents in you neighborhood
To do this, you should find your local council
Enter your post code and your are in. Follow the link to the city council / borough council or district council.
Search for Strategic Flood Risk Assessment
Sometimes is better to search for it in google once you know the local council
Once you get to the document. You need to identify the appendices where the information is available
In this case, the information we are after is in appendix A and B
The appendices in the Strategic flood risk assessment, usually don’t have sufficient detail to make an assessment of individual properties.
They give us an indication of the potential flooding.
In some councils all this information is digital. Such as in The West London boroughs of Brent, Barnet, Ealing, Harrow, Hillingdon and Hounslow.
Critical Drainage Areas
Critical drainage areas are the most difficult areas to identify when completing a flood risk assessment.
In theory, there should be a list from the Environment Agency with all the critical drainage areas.
It is possible that such list exists, so far it has not been found.
The EA has produced a map showing a partial recount of the critical drainage areas.
The best way to find the critical drainage areas is by looking at the strategic flood risk assessments.
The critical drainage areas cover most of the town.
Now, we have all the information to make an initial flood risk assessment.
Which leads us to our next topic…
Assessing Flood Risk Information
By now you should have the following info:
- Flood Risk River and Sea Map – Done
- Product 4 – Done
- Surface Water Map – Done
- Groundwater Maps – Done
- Critical Drainage – Done
This step-by-step flood risk evaluation will give you sufficient information to complete a basic flood risk.
In short, you will know the level of risk that of your development
Assessing Flood Risk For Rivers and Sea
This assessment varies substantially depending on the flood risk vulnerability classification of your development and whether you site is affected by a tidal river, protected from a flood defence.
Something to note. This assessment is only my technical opinion based on the experience when dealing with the local councils and the Environment Agency.
I have prepared it to give you guidance and to make you aware of the complexities of assessing flood risk.
The first key item to determine is the flood level Let’s get down to it.
The Flood Risk Vulnerability Classification
The Planning Practice guidance gives us the the vulnerability of the development.
You just need to know the use of your development and check out the tables below.
For example, if you project is building wind turbine, your development vulnerability is “essential infrastructure”.
You can click (>) on the table below to find more development such as housing (which is more vulnerable).
Now, that you have the flood risk vulnerability classification. You can check the location of the site within the flood zone
Site Location within the Flood Zone
There are two set of data to determine the flood risk
- The environment agency flood map for planning
- The product 4 information
The Environment Agency flood map shows that this property is in flood zone 3
The flood maps for planning gives us an indication of the flood risk however it does not take into account the properties floor level.
We now need to check the flood level in relation to the property.
In some cases, specially if the development is near the edge of the flood zone, the site can be outside of the flood zone.
Flood Levels are given by the product 4 information (already requested above).
If by any reason, the EA has given you the product 4 information. Then a river modelling should be undertaken.
The flood level is independent from flood defences. Even if your site is protected by flood defences the assessment should take into account the flood level.
The product 4 information gives flood levels at certain nodes, these nodes are noted on Maps
and also tables
From the product 4 information pick the flood level related to the 1% AEP level from the table flows and levels
Use the point closest and most perpendicular to your site, if it is possible.
This is one contentious element when dealing with the Environment Agency.
On tidal rivers, the development are mostly protected by a flood defence to a suitable climate change allowance.
This makes the flood level less important as tidal water should be contained by the flood barriers.
The level to take is the Breach Water level (Inundation level) and the hazard flood maps. Let’s continue reviewing this flood risk assessment guide
Climate Change Allowance
Climate Change allowances depend on the flood vulnerability classification, the lifespan of the development and the river catchment where the development is located.
The EA has a detailed guide on assessing climate change how to make the assessment.
Depending on the risk vulnerability of your development and the lifespan you will need to assess the H++, Upper End or Higher Central
- The 20% increase in river flows is no longer in use
- For dwelllings you will be looking at 35% to 70% climate change increase
- You should assess the 2050 and 2080 levels and its impact on the housing development
- You can obtain the climate change level by creating a discharge level curve. Extrapolating the levels (intermediate assessment)
In some cases you can use the basic assessment in which the EA gives you the depth that can be added to your flood level.
This information should be within the product 4 pack otherwise just use the levels for 35% and 70%
Pro Tip: Agree with the EA the type of assessment required before proceeding to the assessment.
Now we have our Flood Plus climate change allowance level.
We are ready to stablish in which flood zone our development is located. Now this flood risk assessment guide will show you how to do this.
Comparing the Flood Levels and Site Levels
The flood levels need to be compared against the ground levels and the levels of the property.
You can do this completing a Topographical Survey
We can also determine the ground levels by using LIDAR data from the Environment Agency or LIDAR data for other suppliers
The LIDAR data is not as reliable as a topographical survey but it allows to identify the flood risk in the development at an early stage and without investing substantial money.
Now that you have the flood levels and the levels at the property, You can confirm if you place is located within Flood Zone 1, 2, 3a or 3b.
Flood Risk Compatibility Assessment
You can now use the location of the development in the flood zone and the flood risk vulnerability of the development to check if your development is considered to be inappropriate or a exception test required.
If your development is considered to be inappropriate for the flood zone within which the site is located then planning permission would normally be refused.
Assessing Flood Risk For Surface Water Run-off
The surface water run-off maps were not made to determine the flood risk at a property level.
It just give us an indication of the potential flood within the area surrounding the property.
In this picture, it shows that area around the property is at risk of surface water flooding.
Although we can see that the property is within the medium to low risk of flooding. We can’t take this as a given.
This data does not take into account the location the location of sewers, drainage channels, pumping stations that can reduce the impact of the flooding.
Nevertheless, we can see the potential flow paths of water.
We can now look at the Medium flood risk depth.
It shows that the gardens have depth of flooding up to 900mm deep.
We now look at the depth for low risk
The low risk depth also shows that depth of flooding is up to 900mm and occupies most of the properties gardens.
We can conclude that there is a high likelihood that there is a water path on the gardens and that the level of the house is outside of the potential surface water path.
Assessing Flood Risk For Groundwater
You should look at the register of groundwater flood incidents and the flood water susceptibility map produced for the Strategic Flood Risk assessment.
If this information is not available you can download the groundwater susceptibility map from BGS map showing the likely hood of groundwater flooding.
Groundwater flood risk is divided into 3 levels. Potential GW flooding Below Potential GW flooding Above and Limited potential for GW flooding. In this flood risk assessment guide we will not get into the detail as its control will be part of the flood resilience blog.
These 3 levels should be used as a guide to undertake more investigations such as boreholes in order to provide further information on:
- The proposed development’s groundwater flood risk and also from/as a result of the
- You should look at SUDS methods also need to be considered as part of the assessment and limited permeability may not be suitable for infiltration SUDS
- Depending on the construction of the development, you should also look at the construction methods which may interfere with groundwater flood risk and affect flood risk to adjacent
And finaly, you should include the effects of climate change that may have on the water-table and how this could affect the groundwater within the development.
Managing Flood Risk
Managing flood risk is one of the important elements in a flood risk assessment.
And for good reason:
If you can’t manage the flood risk you can’t have a development
And as for the development, it has a direct impact on the layout and architectural design of the buildings.
Flood Risk For Rivers and Surface Water Management
For this flood risk assessment guide, the golden rules are:
- Aim to stablish the final floor level of the new houses as high as possible from the flood level+ CC if you can.
- Create space for water, allow for flow paths and integrate water within the fabric of the development
- You can break up your development user allow for less vulnerable areas (shops etc) closer to the areas at higher risk of flooding.
Determining the Final Floor Levels of Dwellings
In general, there are two options of final floor levels.
The options depend on the water compatibility of development.
Option 1: Avoidance. Lifting the FFL 300mm above the 1% AEP + CC (allowance)
This approach is used for more vulnerable developments and developments where access is not compromised.
Option 2: Water entry, resistance, resilience or repairable. The FFL is the same as the ground level.
We use this approach mostly for water-compatible developments and small extensions.
The water resilience design should follow the detail given in the Improving the Flood Performance of New Buildings Flood Resilient Construction document.
Creating Space For Water
In this area, you will need to look for options to integrate green areas, evacuation routes and the flood zones
There are great examples in the Netherlands and in Germany about integrating flooding within the build environment.
Flood Risk From Groundwater Management
Groundwater flooding although is not as prevalent, it is a complex issue.
I suggest if you a building a basement within an area of potential (above or below) for ground water flooding. Then an site investigation is undertaken as early as possible.
If you have a site that has potential for above groundwater flooding, then work should be completed to clearly allow for potential for this flooding.
For instance by locating the ground floor well above the ground level or building a concrete slab and allow for flotation.
Flood Risk Mitigation within Critical Drainage Areas
The mitigation within critical drainage areas depends on the prevalent flood risk. i.e. surface water run-off, groundwater or sewerage flooding.
In most cases, it relates to a combination of these three flood risks with a greater emphasis on sewerage flooding.
You can mitigate the impact of surface water sewer flooding by providing a sustainable drainage systems that has adequate capacity to manage and control this flood.
You can also alleviate the foul sewer flooding by providing sufficient capacity and non-return valves or pump NRVs for your site.
Site Access and Egress
Safe Site access and egress must be provided for all developments affected by flooding.
The evacuation paths should be above the 1%+CC or the breach flood levels +CC.
In order to evaluate the safe paths, you should use the also use the flood water depth and velocity together with the flood risk hazard map provided by the Environment Agency in the product 4 package.
The flood hazard map should be compared against the hazard to people in relation to flood depth and velocity matrix
The different classifications will have an impact on the users of the site
This categorisation help us to determine the planning of safe access and exit for your development.
If you have a site that it is not possible to safely exit or access the site.
Then you could look at designing a ‘safe haven’. This is the most likely scenario in areas where there is a flood defence breach.
Assessing Off-Site Impacts
Now that you have managed the flood risk on you site.
We are going to determine the off-site impacts.
By the way this is set out in the NPPF
There are 3 items that you need to look at.
Surface Water Management
Include the use of Sustainable Drainage Systems in your development. In this flood risk assessment guide, we can see that the basic drainage parameter can be established by knowing the soil condition, proximity to a watercourse and flood risk, let’s dive in.
You don’t need to calculate the volumes and areas
You should give the principles of draining the site using Sustainable Drainage systems.
Did you know your local water company will give you a wastewater management discount if the SuDS are adopted.
Ideally the SuDS should reduce the run-off to the greenfield flow rate. Each LLFA has their own standards!.
SuDS also help on flood conveyance and also in the management of flood and movement of water (conveyance) within you site.
Flood Flow Conveyance
It refers canalisation of flow in your site which in turn affect areas up and down stream of the site,
The University of Tulane has created an article explaining the type of channels.
The key is to avoid is straight, deep and smooth channels. The channel can sometimes be created by roads or retaining elements.
For complex site, it is recommended that a 2D model should be created to determine the flood conveyance risk
Now we move to calcualte the storage volume taken from the flood zone
Flood Compensation Storage
This is one of the most contentious items with the Environment Agency.
As Flood Compensation should be level for level and should take into account how water enters and leaves the flood compensation areas.
Level per level compensation, is complete by creating cross sections and slices every 200mm showing the volume taken and the volume “created” at the same level. The easiest way is to create a 3D model of the site.
For small developments, a cross section showing the levels is usually accepted by the EA.
How water enters the compensation volume is important as it also has an impact on the river flow during time of flood.
Flood Compensation is provide to the level of 1% (1 in 100year) + Climate Change. For flood compensation under buildings and structures
The Environment Agency prefers to have voids 1m wide every five meters on all sides of the building.
Some local councils, may allow compensation at small depths under the buildings.
This is usually when surface water flooding is very deep and there is sufficient space (more than 1.2m ) under the building’s floor.
This flood risk assessment guide is based on the most up-to-date feedback received when submitted our flood risk assessments.
Determining Residual Flood Risk
The residual flood risk refer to
- Site access and public safety and Flood Warning and evacuation Let’s dive into it.
- We almost have assessed flood risk for a site
Site Access and Public Safety
Safe access should be provided above the 1% +CC level. This is applicable to all new site with new dwelling houses.
The increase in surface water could be seen as a increase risk on public safety , specially in areas of high surface water levels.
Include SuDS and you development will be ok.
Flood Warning and Evacuation
Check that the development is in an area with the service to receive early warnings of imminent flood hazard.
Create a Flood Warning and Emergency Response Plan for the site.
Now I want to turn it over to you: What do you think is the part that is more helpful?
Do you have sufficient info to learn more about flooding?
Let me know by leaving a quick comment below right now.