With flash flooding events becoming more and more frequent each year, it is definitely worth considering having water management systems incorporated in to your garden design.
If you have ever attached a water butt to a downpipe from your house roof and monitored how quickly it can fill during an average rainstorm, you will know the shear quantities involved. I have a water butt in my garden, which can capture 250 Litres of water in approximately 15 minutes during a heavy downpour, from one downpipe. If you multiply this up by the number of downpipes you have on your house, greenhouse, garden shed and garage, the sheer quantity of water entering your drains in a very short period of time is shocking. No wonder flash flooding can occur, particularly with so many gardens, driveways etc. being covered in in impermeable hard landscaping.
However, much can be done to both slow down the rate of flow and utilise the water collected within the garden setting, using clever and attractive water management design solutions.
The use of rain chains instead of downpipes and storm planters at the base of downpipes or chains can slow down the rate of flow. These planters are usually constructed at the base of the downpipe and are filled with layers of stone and gravel with topsoil and a variety of plants suited to growing in wet and dry conditions including temporary flooding. Water falling on to the vegetation from the downpipe, runs off on to the soil below and filters down through the layers of gravel and stone. An outlet at the top of the planter allows collected water to flow out once the planter is full. This water can then be allowed to flow in to another water feature, be allowed to enter in to the standard drainage network or infiltrate in to the ground below, if it is a suitable free draining soil type.
The rate of water flowing through the planter is reduced dramatically as it infiltrates through the soil and gravel layers. Water is also lost to evapotranspiration from the plants and soil surface.
Rainwater run off can also be stored in tanks (usually stored underground) for later use. Being used in the house for all non-drinkable needs, such as toilet flushing, washing machines and garden watering.
Rainwater run off can also be used to create a ‘rain garden’, with a series of rills, swales leading to a retention and infiltration pond. The definition of a ‘rain garden’ is where environmental benefits and aesthetic consideration bring purpose and individuality to a garden or landscape. They follow the principle of the natural water cycle by using landscape to slow down the rate at which water is lost to the public drainage system.
Using a porous or permeable paving on driveways and other hard landscape areas also helps to reduce the amount of surface water pouring off these surfaces directly in to the drainage system or causing flooding.
Green or sedum roofs on sheds, garden buildings and where possible on house roofs, also helps to reduce the rate of rainwater run off. Vegetation and it’s substrate acting in a similar way to the storm planters, by absorbing water. Once saturation has been reached, excess water is released more slowly in to downpipes and drainage system (if not being used elsewhere). Evapotranspiration from the planted surface also helps to remove water.
Swales, are shallow depressions or channels lined with free draining materials and plants which can tolerate both flooding and dryness. These can be used to retain and dissipate surface water run-off and work well on their own or in combination with storm planters, ponds etc.
All these water management features can be easily incorporated in to the garden setting, whether existing or new.
For further reading I highly recommend ‘Rain Garden’ by Nigel Dunnett and Andy Clayden. Published by Timber Press