Eco Tips

Rainwater retention to reduce flooding

There are a number of small ways in which we as householders, and more especially those of us with gardens, can increase the amount of rainwater our domain can retains. This can reduce the risk of flooding in two ways.

  1. By keeping water out of, or by delaying the water entering into, the storm water drains we can help prevent them from overflowing – a common cause of flooding in urban areas. 
  2. By increasing the water absorbency of our grounds, we can delay the time and speed with which rainwater enters  local streams and/ or the water table. This will reduce the pressure on the capacity of local streams and rivers, so reducing the risk of flooding. 

How can we do this?

  • Install water butts. These collect water before it enters either the ground of storm water drains. If, advance of heavy rainfall, the butts are empty, then corresponding volumes of water can be held back from entering the ground or drains. Whilst one single butt may not make difference, a 1000 water butts each with a capacity of 200litres can hold back 200,000 litres – 200 cubic meters – of water. This does require some focus by the householder on rain forecasts. A less precise solution would be to leave the water butt tap half open or it to a leaky hose. Either way it is important that you ensure the empty butt is weighted down to stop it blowing over – either leave some water in the bottom or put in some bricks or out a heavy lid on top.
  • The RHS suggests using rainwater from butts to wash cars and paths in the winter and plants in the summer.
  • Avoid areas of bare earth. Ground that is covered with plants will slow the passage of rainwater into the water table. Rainfall will be interrupted by leaves and rainwater will absorbed by roots and by any mulch covering the ground. 
  • Compacted soil is less able to absorb rainwater – where the ground is regularly trafficked create a path that uses porous materials – eg gravel, bark, stepping stones.
  • Ensure a range of planting so that there are always some plants covering the ground and some plants with leaves.
  • Cover areas of bare earth – eg before planting begins or in the autumn after harvesting  – with a mulch or with a quick growing green crop – otherwise known as green manure – such as alfalfa, clover, mustard.  https://www.rhs.org.uk/soil-composts-mulches/green-manures
  • Avoid hard surfaces if possible. Where a hard surface is needed (as a patio, for car parking etc) you can opt for a porous hard surface such as gravel or paving stones spaced out with gaps for drainage. You could also incorporate a soak-away to catch and collect water that runs off the area. 
  • If you have a pond, keep it filled up with rain water. Run a hosepipe from a water butt so that when it rains the pond benefits from rain gathered over a larger area. 

Green Tau: issue 30

Flooding in the Thames is a threat that comes from both upstream and downstream

12th January 2022 

World wide action to avert the worst effects of the climate crisis are aiming to keep global temperature rises to 1.5C. Whilst that is the target spoken about by governments and businesses, Climate Action Tracker, analysing actual actions being taken, predict that we are on track for a global temperature rise of 2.7C (https://www.bbc.co.uk/news/science-environment-59220687). This level of temperature rise will lead to a faster melting of ice caps and glaciers, causing sea levels to rise  and more extremes bouts of weather, increasing the severity and frequency of floods.

Here in south east England sea levels are projected to rise by approximately 1.4m  – a little under a meter if the  temperature rise is brought below 1.5C (https://www.metoffice.gov.uk/research/news/2019/uk-sea-level-projections-to-2300). The Met office also predicts that 1 in 10,000-year flood events are likely to occur more than once a year by 2300. 

The Thames is tidal all the way up to Teddington Lock. At present the Thames Barrier is raised when ever there is a high tide that threatens to flood London. High tides are caused not just by the movement of the moon, nor by rising global sea levels. They are are also affected by weather systems. For example the height of a tide coming in from the North Sea can be increased by an on shore wind or by  a low pressure system over the sea. Rising global temperatures that are causing rising sea levels and more frequent and intense weather systems means that the time will come when  the Barrier will no longer be able to hold back an incoming high tide. The Corporation of  London is considering plans to tackle this, including increasing the height of the Barrier and raising the height of the walls that bound the Thames – eg  along The Embankment – by a further meter. There is debate as to whether this should be with glass so as not to obstruct the view of or from the river. 

When the Barrier is raised to slow an incoming high tide, the water that is held back has to go somewhere – that is somewhere in the Thames estuary downstream of the Barrier. There is a flood storage area at Tilbury and on both sides of the Estuary there are area of marshland and farmland where flood waters can flow. These are in between  other at risk built-up areas which have their own flood defences. Such provision will need to be reviewed as tide levels rise.

The Thames Barrier is also closed when there is a risk of flooding upstream. The Thames has a large  drainage basin that extends all the way up into Oxfordshire and the Cotswolds. Heavy and/ or persistent rain anywhere in the area can raise river levels  such that places such as Windsor, Thames Ditton, Richmond, Hammersmith risk being flooded. In such circumstances the Barrier can be raised at low tide, to prevent sea waters entering the lower part of the Thames so creating a reservoir where high volumes of water from upstream and be stored until the risk of flooding is lowered. The protection offered by this scheme is not going to be sufficient to cope with predicted rises in water levels due to the climate crisis.

The following map shows areas of London that will lie below the anticipated annual flood level based on the IPCC’s climate change forecasts. (https://sealevel.climatecentral.org/maps/


Boroughs along the Thames, such as Richmond, are having to develop plans to both limit or avert factors that contribute towards the risk of flooding and to constrain the adverse effects such flooding will cause. In part they will be dependant upon actions take by other people and organisations further upstream. For example, if further a.one the Thames, houses are built on the flood plain, then water that might otherwise be held or absorbed by that flood plain, will instead flow on down stream exacerbating the problem in Richmond. Similarly if surfaces that are currently – eg covered with grass and other vegetation – are replaced by hard surfacing such as tarmac, then again more water will flow more rapidly off the land and into the river. In a similar way areas of woodland are cleared, then less water will held in the land and instead will add to the volume in the river. Conversely recreating water meadows to allow flood waters to accumulate safely up stream, replacing hard surfaces with soft one, and planting trees, can all help reduce the volume and speed with which water drains off the land and into the river. In the Old Deer Park there are plans for the ‘re-wilding ’ of the land allowing formation of seasonal ponds/ lagoons and the growth of water-loving plants that increase the water absorbing capacity of the land. You can read more about this here – http://thames-landscape-strategy.org.uk/what-we-do/rewilding-arcadia/