The Basic Concepts
In this chapter, I’ll show you the basic elements of surface water drainage
I will also show you where to get more information on surface water drainage.
Let’s get started
What is Surface Water?
Water that gets accumulated at your rooftops, driveways, yards or any open surface at your property that does not get properly drained and is retained on the surface is considered to be Surface Water.
What is surface water flooding?
When rainwater or any kind of surface water accumulates on an impermeable surface (concrete for example) where water cannot penetrate into the soil, flooding occurs.
It can cause heavy damage to land and property. Not to forget the environmental hazards associated with flooding – it is a major cause of several waterborne diseases and water contamination.
Where to get information on surface water?
You can get information by looking at the surface water map provided by the Environment Agency.
You can also look at our flood risk map.
Using our flood risk tool, you can see three flood risk levels on surface water flooding.
- Very Low
- Low
- Medium
- High
Your house or development is likely to be at risk of surface water flooding if it is within the medium and high level.
You can check the depth of water for each level of flood water risk.
You can also check the hazard as per shown below.
Should you house or development be within a medium to high flood risk, you would need to manage the surface water.
Why should you prevent surface water flooding?
That’s what I’m going to cover in the next chapter of this guide. Keep reading…
Why should you prevent surface water flooding?
Sounds absurd?
But we think it is imperative for one to learn about the consequences of surface water flooding.
Let’s get started
1. Water Seepage
During flooding, the possibility of water seeping into the house cannot be ruled out.
This can cause severe repercussions resulting in damage to structure of the house, the furniture and other essential appliances at home.
Note that any surface in contact with flooded water should be removed from your home as it is likely to be contaminated with sewage.
2. Algal scum and mosses
When water logs in an area for a long time or when plaster gets in contact with water, there is a wide possibility of growth of algal scum and mosses, including bacteria from sewage.
These bacteria will have long term health implications if left to live within a habitable area.
3. Unhealthy for plants
Excess water stagnation can prove to be unhealthy for plants as there is not enough oxygen for the roots to breathe.
This condition can deter the development of the plants and if continued for a long-time, it may result in the death of the plants.
4. Insurance
Internal surface water flooding will increase the premium of your house insurance.
You may not be able to insure your house.
However help is at hand, you can get subsided insurance through Flood Re.
Also, you can get lost of insurance advice from the National Flood Forum. Or find a broker.
Legislation
By now you should have the following info:
- What is surface water
- The consequences fo surface water flooding.
Did you know?
The Department for the Environment, Food and Rural Affairs (Defra) estimates that:
- 3.2 million households in England are located in areas at risk of surface water flooding.
- On average, damage due to surface water flooding exceeds £300 million every year.
- By 2050s the cost associated with damage from surface water is likely to increase by about 40% this is because of a growing population and climate change which increases the intensity of heavy bursts of rainfall.
Because of this, there is legislation in place to protect us form the risk of flooding.
Surface Water Drainage Regulations
The key regulation are:
- Building regulation document H Drainage and waste disposal: Approved Document H
- The Environment Agency Discharges to surface water and groundwaterenvironmental permits
- CIRIA SuDS Manual (C753)
- The guidelines from the Lead Local flood authorities
- You need a flood risk assessment (FRA) to determine the risk of flooding
Surface Water Runoff Law –UK
The Environmental Law website states that:
“The law (common law) requires that you use your property or land in a way that does not increase the risk of flooding to a neighbouring property.
If you do carry out acts on your property that results in flooding to other people’s property, you may face a civil action.”
As part of the surface water drainage responsibility the law requires that you:
“Keep your drains clear in your property and to ensure that you do not drain water into your neighbour’s property or foul drain..
There is a natural right of drainage that allows water that flows naturally across your land to flow downhill naturally to your neighbour’s land. But you are not allowed to artificially channel water a way that will cause damage your neighbour’s land. If you do, you may face a civil action.
(Example: Yes – Rainwater that falls on your lawn is allowed to flow downhill through your neighbour’s land. No – You are not allowed to channel roof water through a down pipe on to your neighbour’s property.)
Maintain your flood defences (if you have any). If failure to maintain these defences leads to flooding, you could face a claim in negligence or nuisance.”
Surface Water Drainage Solutions
Now that we have seen the consequences and laws governing surface water,
let’s look at some of the solutions available for this problem.
These are the different types of surface water drainage.
1. Underground drains
Underground drains are surface water drainage pipes and fittings built below the ground level to which excess water from the surface and be transported.
From there, it is transmitted to a sewage treatment facility or a watercourse.
Pros
- Appearance of the property is not affected
- Prevents spread of diseases and pollution as opposed to an open drainage systems
Cons
- Least sustainable option
- Increase velocity of water and may produce flooding further downstream
2. Sump pumps or Pumping Stations
A sump pump is also a device that is used to remove the excess water out of your property through a system of pressurised pipes.
Most pumps have alarms embedded in them that alert the house owners when the water reaches a particular level. There are two types of sump pumps.
Submersible pumps – Submersible pumps are placed inside the sump pit which prevents overheating of the pumps. They are expensive and hard to maintain. This are the most reliable option.
Pedestal pumps – Pedestal pumps are designed with motors at the top of a long device and they are less prone to water damage. They are less expensive and easy to repair. However their maintenance is high and are noisy.
Tips for maintenance
The filter must be cleaned three to four times a year to prevent damage to the pump from debris.
Ensure the sump pit is cleaned once every year so that it is ready in the event of any unforeseen circumstances like flood.
3. Basins and Ponds
These surface water drainage solutions are most used in areas where there is land availability.
Detention Basins
Detention Basins are used to store the runoff water from surface areas and they are also helpful in settling pollutants.
They are used to control the flow of surface water run-off.
They are normally dry and they can also be vegetated depressions. They can be on-line or off-line components.
On-line where regular surface water flows through the basin and off-line where surface water can be diverted after flows attain a particular threshold.
Pros
- Simple to design
- Simple to construct
- Easy to maintain
- Can also be used for recreation
Retention Ponds
To store water for a longer period of time, retention ponds are used.
They use biological mechanisms to treat the pollutants in water and gradually transport the water downstream.
This is especially useful in urban settings where it supports vegetative growth across the shoreline.
Pros
- Highly suitable for urban areas
- Recommended in areas where groundwater is vulnerable
Note: Do not use existing ponds to treat run-off water. Always create new ones to avoid damage to the existing vegetation and wildlife.
4. Green Roofs and Rainwater Harvesting
Did you know that Green Roofs can considerably reduce the surface water run-offs?
Yes, especially when there is intense rainfall, Green Roofs can help prevent flash floods by absorbing water and releasing it through evaporation.
A study done by University of Toronto in the school’s Green Roof Innovation Testing Lab (GRIT Lab) demonstrated that green roofs have the capacity to capture an average of 70 percent of rainfall.
Green Roofs also help in reducing the noise levels and increasing the longevity of the roof.
There are two categories of Green Roofs – Extensive Roofs and Intensive Roofs.
We have created a blog about Rainwater Harvesting
Extensive Roofs have shallow substrate layers and therefore require low maintenance while Intensive Roofs have in depth substrate and therefore require high maintenance.
5. Soakaways
Soakaways are concrete pits or circular shaped lined with brick or plastic crates.
To drain relatively large areas, these can be plastic crates.
The excess surface water is moved to the surface water drainage soakaway with the help of a pipe or an open channel.
Soakaways should be located 5m away from buildings.
Pros
- Recharge of groundwater
- Easy construction and operation
6. Filter Strips & Permeable Surfaces
Filter strips of gently sloping vegetative land that is used to treat excess surface water by means of infiltration, filtration and sedimentation.
The vegetation absorbs nutrients while removing pollutants through filtration and sedimentation.
Filter strips are used along roads and serve as a pre-treatment of the surface water.
Permeable surfaces allow water to infiltrate directly into the ground.
7. Swales
Swales are shallow, vegetated channels, broad, and flat bottomed.
They are designed to convey excess water and remove pollutants.
The standard swale channel which is broad and shallow is usually covered by vegetation to slow the water.
In order to promote settling and infiltration, check dams and berms can be placed across the flow path.
Swales are located next to roads, replacing gullies and drainage pipes, in landscape areas and in other open spaces.
3 types of swales for different surface water management
- Standard conveyance swale: Used to convey runoff from drainage
- Enhanced dry swale: An underdrain bed of soil beneath, leaves the main channel dry and prevents channels from waterlogging.
- Wet swale: Can maintain marshy conditions in the swale’s conveyance channel
Pros
- Easy to integrate into landscaping
- The runoff rates and volumes are reduced
- The capital cost is low
- As the Pollution and blockages are visible, it’s easy to be dealt with
- Swales are efficient in removing pollution
8. Underground Attenuation Storage
Attenuation Storage Tanks provide temporary surface water storage below the ground before discharging to a watercourse or a sewer.
Pros
- Considerably high storage volume capacity
- They can be installed beneath areas such as public open spaces, car parks etc apart from drainage
Sustainability, maintenance, cost and pre-treatment performance are few things to be considered.
The attenuation storage tank has a chamber for flow-control that allows a certain quantity of water to be released from the tank in such a way that is at the green field rate or well below the brownfield rate.
Final thoughts
Increasing urbanisation is the primary cause of surface water run-off problems.
Construction of buildings and pavements have decreased the surface area of vegetative land which in turn has reduced the percolation of water through the soil.
Hence there is an increased demand for a proper Surface Water Drainage Solution.
The surface water flooding can be reduced by using Sustainable Drainage Systems (SuDS) techniques.
SuDS is similar to the natural drainage processes but they also help in treating the pollutants before the run-off water enters the watercourse.
The advantages of SuDS over traditional practices are
Run-off water speed management
– Water quality enhancement
– Groundwater Recharge
– Wildlife Habitat
– Environmental impact reduction
SuDS technique encompasses methods like filter strips and swales, permeable surfaces and filter drains, infiltration devices, basins and ponds – some of which we have seen above.
Summary
You can protect and manage the surface water of you development by implementing surface water drainage solutions such as sustainable drainage systems.
It is preferable to invest in a surface water drainage solution that allows you to enjoy your investment. For instance using water gardens instead of underground soakaways.
- Underground drains are built below the ground level to which excess surface water from the surface is transported
- A sump-pump is a device constructed at the basement of the house to remove excess water through a system of pipes
- Detention basins and retention ponds are used to store run-off water and are helpful in treating pollutants
- Green roofs absorb water during intense rainfall and prevent flash floods
- Soakaways are pits either square or circular used to drain excess run-off water
- Filter strips are strips of vegetative land that use the mechanism of infiltration, filtration and sedimentation to treat surface water
- wales are shallow, vegetative channels designed to transport water and remove pollutants
- Attenuation storage tanks provide temporary surface water storage below the ground before infiltration
Invest in a sustainable surface water drainage solution today to get rid of run-off problems tomorrow.
