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.
Stop your garden flooding by raising the surface level above the floodplain, the Pros and Cons.
The plus side of raising the soil level.
You will have risen above the flooding problem.
You and your kids won't be walking on a bog.
Plants will grow again.
The underlying cause will still be there.
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
A six foot fence erected on the new level will be eight foot high on the neighbours side, probably needing planning
Other important considerations.
Adding topsoil to permanently saturated soil without first laying a drainage course of clean rubble will create one big
If you want to grow potatoes, carrots, parsnips etc. you will need a deep bed of soil.
Don't bury manholes
Constructing a drainage course before adding the top soil
How you raise the surface level of a garden outlined here.
Raising the level of a garden or lawn by 18
inches will require,
Retaining walls need to be built around the complete area being raised. These
can be built using breeze blocks set on a sound footing.
Slope the surface very slightly to allow surface water to drain away.
Weep holes for drainage need to be formed every 2 meters and every 1
meter at the lowest end of the slope.
Rubble. Clean, chunky, dust free rubble to form the drainage course can be
added when the retaining wall cement is set solid.
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 will work like a wick or sponge, if allowed to come into contact
with mud, drawing water up to the surface.
Top tipIf 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.
Adding topsoil when raising 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
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