How to raise the surface level of a garden at risk of flooding.

What causes gardens to flood in the UK?

 

These are the five main reasons low lying gardens flood.

  1. Heavy sustained rainfall. The simplest explanation is heavy rain. Wherever you live, you are surrounded by drains and public sewers designed to drain the rainwater away into water treatment plant owned and operated by the water and sewerage companies. In most cases, it does its job. However, sustained heavy rains over a long period can overwhelm these systems, especially where roadside drains have been neglected and become blocked and the water doesn’t drain away as quickly as it needs to causing the water in the drainage systems to back up, and the water rise. Unfortunately if your garden is lower than the surrounding properties it's heading your way! Into your garden!
  2. Rivers overflowing. You may not have had heavy rain in your area. However, if you live near a river where areas upstream have had heavy rainfall, it could lead to flooding where you live, especially if the river has become blocked down stream by debris or fallen trees.
  3. Nowhere for the water to go. Roads in our cities and towns, car parks our own and neighbours driveways and patios are mostly made of concrete and other impermeable material. Meaning there is no ground for water to sink into anymore. So, where is the water going to go? It is going to flood low-lying gardens. 
  4. Melting Snow and Ice. Melting snow and ice has to go somewhere. A combination of the factors above mean once again, it's heading for you.
  5. High water table. I have explained this in more detail here. Water table information 

These are just a few examples of common causes of floods, but sometimes you need to look a little closer to home! Clogged or broken pipes, leaking gutters, down pipes discharging into the ground instead of being connected into the drainage system, dripping outside taps, impermeable material used to construct our driveways and patios. Mud and other rubbish brushed over kerbside drains.

 

Before setting about raising your garden, it might be worth considering a couple of other options. One option described here Building a Bund or Levee  really can protect your home from flooding. You can find other solutions in the website menu.

Step by step guide to raising the surface level of a garden

In this guide to raise the surface level of a garden I have explained;

  1. Benefits of a raised garden level.
  2. Problems associated with raising a low lying garden.
  3. Retaining wall frame and sub base.
  4. Preparing the topsoil.
  5. What is the floodplain or flood plain?
  6. Contact for our drainage services
Flooded garden before being raised
Flooded garden before being raised.

The benefits and problems associated with raising the surface of your garden above the floodplain to stop flooding

The benefits of raising a garden;
  • You garden will be above the floodplain and associated flooding problem.
  • You and your kids won't be walking on a bog.
  • Plants will grow again.
The problems associated with raising a garden will be;
  • The underlying cause will still be there and you could be creating one big bog unless you install a drainage system.
  • Depending on how high you intend to raise your lawn or garden, you might need to get permission or at the very least consider the effect on the environment and people around you.
  • Where will the flood water flow to ?
  • Raising a lawn or the complete garden surface above the flood-plane will almost certainly have an effect on yours and others privacy, you could end up  2 feet higher and your neighbours 2 foot lower.
  • Unless you raise the height of boundary fencing or hedging you will be looking down on them and they will be looking up at you. 
  • A six foot fence erected on the new level will be eight foot high on the neighbours side, probably needing planning permission.
 
Other important considerations. 
  • Adding topsoil to permanently saturated soil without first laying a drainage course of clean rubble (explained below) will create one big bog garden.
  • If you want to grow deep rooting plants like potatoes, carrots, parsnips etc. you will  need a deep bed of soil. 
  • Don't bury manholes.
What is a floodplain.
A floodplain or flood plain is an area of flat or nearly flat land that is adjacent to a stream or river, stretching from the banks of the river or stream to surrounding higher ground  and experiences flooding during periods of heavy rainfall and run off from higher ground.
The run off can be from higher ground further up-stream. 

 

Put simply, a floodplain is an area near a river or a stream which floods when the water level rises.

Building the walls to make a frame to hold and retain the raised garden

Raising the level of a garden or lawn by 18 inches will require;

  • 18 inch high retaining walls will need to be built around the complete area being raised. These can be built  using breeze blocks or any material that won't rot set on a sound footing. Bear in mind you won't see the retaining wall, just the top, as it will be filled with soil. Strength and durability are more important than looks. For large areas building a bund may be the answer, this page describes how to construct a bund. 
  • Slope the surface of the frame very slightly toward where you intend the water to drain away. This should not be your neighbours garden or the footings of your house.
  • Weep holes for drainage need to be formed every 2 meters and every 1 meter at the lowest end of the slope.

Top tip If you have got good access to the area you are raising leave a gap in the wall wide enough to get a digger and tipper lorry through. It's a lot easier if you tip the rubble and then the soil directly onto the area being raised. Then finish the wall.

Constructing a drainage course before adding the top soil

  • Drainage course. This is a layer of clean chunky rubble laid to keep the new soil separate from the saturated soil and to assist drainage from the newly raised surface.  
  • Builders rubble, clean broken brick, broken slabs can be used for the drainage course.  Spread the rubble evenly and compact it. You may need to add several layers.
  • A wacker plate is the quickest way to compact the rubble. Driving a digger backwards and forwards over the area works too.
  • Depth of drainage course, lay and compact enough layers of broken brick etc. until its deep enough to separate the new top soil from the saturated soil below. This will be a minimum of six inches.
  • Clean gravel. Use clean gravel to fill the gaps between the compacted rubble to stop the new soil falling through to the mud below.  Soil works like a wick or sponge if allowed to come into contact with mud, drawing water up to the surface.

Adding topsoil to raise the surface of a garden.

  • Buying Topsoil.  The cheapest ways to buy recycled topsoil is in bulk from a local supplier (this type of topsoil cannot be guaranteed to be weed or clay free) or from a building site (make sure you are not using a mix of top and sub soil). The safest way to purchase loamy topsoil is from a specialist supplier.
  • Spreading the soil. If you can, ask the supplier to tip the soil in even piles over the area.
  • Raking out the soil. Choose a dry day if you can and shovel the piled earth evenly over the area and then rake it level.
  • Firming the soil.  Choose a dry day and firm the surface following the advice on this page.  Its important not to compact the surface or you will end up with a poorly draining lawn or garden.
  • Settling. The soil will settle as it dries out, so slightly overfill or top up as necessary.
  • Calculating quantity of  topsoil. Measure the length, width and depth of the area you need to fill with soil in metres and multiply the three figures together to get the volume in cubic metres. 75 cubic metres = 1 tonne
Some alternatives to raising the level of a garden at risk of flooding. 
flowerpotman.com

There are Pros and Cons involved in owning a home built on a floodplain. Pros; such as boating on the Thames, fishing, bird watching, the land is rich and fertile, relaxing sound of running water. You will probably pay significantly less for the property than for a similar house on the hill, possibly out weighing a major Con, higher insurance premiums. Other cons are: Sewers filling with river or groundwater and backing up into the house, Having to move and refit all electrical points and switches, if you are a worrier lying awake all night every time it rains. There are a lot of pros and a lot of cons. Thoroughly check it out before you buy.