THE JOURNAL FOR THE SOCIETY OF GARDEN DESIGNERS

Dealing with drainage

Drainage elements can be aesthetically pleasing, such as these award-winning channel grilles. Photo: Lateral Design Studio


Contractor David Dodd explains everything you need to know about garden drainage


With climate change and the increasing occurrence of flash floods, how we manage rainwater within our landscaping schemes has never been more critical.

Traditional drainage practice was designed to move rainwater as rapidly as possible from the point at which it has fallen to a discharge point such as a watercourse. This has been proven to have a number of adverse impacts. Firstly, run-off from hard paving and roofing can increase the risk of flooding downstream, as well as causing rises in water levels and flow rates in watercourses. Secondly, surface water run-off can be contaminated with surface impurities like oil or organic matter. These may result in poor water quality in rivers. Finally, by diverting rainfall to piped systems, the amount of water infiltrating the ground is reduced, depleting water levels and reducing flows in watercourses in dry weather.

The Government requires that all new developments should ideally collect, treat and re-use storm water where it falls, and being compliant at managing surface water drainage within the property of your design might result in a nice rebate on your client’s water bill.

Consideration should always be given to sustainable drainage systems (SuDS), and regulations are changing the way we need to design paving, both in finished surface and substructure, when designing landscaping projects. A sustainable drainage system is designed to reduce the potential impact of new and existing developments with respect to surface water drainage discharges. It is worth noting that the term ‘sustainable urban drainage system’ is no longer the accepted term, as the ‘urban’ reference has been removed to also accommodate rural sustainable water management practices.

There are many varieties of SuDS, including reed bed treatment systems for polluted water, settlement ponds for sediment, permeable paving, infiltration and attenuation systems, ponds and wetlands. SuDS can be designed to be cost effective and to work with retained natural features such as ditches or ponds, forming an integral part of hard and soft landscaped areas. In this way, they can contribute towards an attractive scheme that enhances the nature conservation and amenity value of the development, whilst also recycling the valuable water source.

The benefits of SuDS are better flood control and better management of storm water at source; greater pollution control (with the use of filtration systems); and more availability of water during drought.


Surface water drainage

The four main types of surface water drainage are direct, French (piped and non-piped), linear (channel and slot) and gully pots. When designing paving, it’s always worth considering porous surfaces for drainage such as gravel (both loose and resin bound) or combining solid paving with more porous materials.

On small paved areas of 25 sq m or less, and if the levels permit, it may be possible to direct the fall of the paving towards a lawn, gravel area or planting bed. This can actually assist with irrigation. However, this can only work if the soil is free draining. If the soil is heavy clay and poor draining, and/or the levels don’t permit, the water needs to be directed towards a drainage point where it can be dealt with.

A French drain is simply a linear excavation backfilled to the surface with a loose granular material such as shingle. However, they have become more sophisticated, and are now usually lined with a permeable geotextile membrane. The most common place you’ll see these are alongside the hard shoulder of motorways.

A basic French drain can be put in along the edge of paved areas against retaining walls, steps etc, or where a change of level may trap surface water over a wide area. This will only be effective if the soil is reasonably free draining. For domestic situations, the width of a French drain should be between 100-200mm wide and a minimum of 300mm deep. A piped French drain is the same as the above, but with a perforated pipe installed. This is connected to a solid pipe leading to a soakaway or outlet, and is usually necessary on larger areas where a normal French drain may not be sufficient.

Linear drains are used along the edges of paving, where a French drain may not work or be desirable. They can be used around buildings, in front of garages and are also now required at the entrance to driveways to prevent surface water running out onto roads. These will take more surface water away quicker than a French drain or gully pot, and are generally more suitable for larger areas where the surface water needs to be taken away quickly. They are set to a fall (some manufacturers have a built-in fall) and are connected to a soakaway or outfall. Most linear drains are made of a lightweight polymer/ cement concrete and have a metal grille suitable to take vehicles. Plastic systems are available for pedestrian and domestic schemes.

A slot drain works on the same principle and may be more desirable than a grille; however, it’s worth noting that some manufacturers are now producing beautifully designed channel grilles which can be used as a decorative edging to paving.

Gully pots are usually used for downwater drain pipes, but can be useful where rainwater can be trapped between surrounding buildings/walls e.g. a courtyard. They are usually set in the centre of the area and all paving falls away from the buildings and walls towards the gully pot. The gully pot must be able to cope with the volume of water that may fall over the area.


Sorting out soakaways

The best way for surface water to permeate into the ground is via a soakaway, which will be connected to any of the above drainage systems. A soakaway is a hole in the ground that will disperse surface water back into the soil rather than having it run into our sewer systems. A traditional soakaway is a hole dug in an area away from the house, with the pipe from the collection source inserted into the hole and then backfilled with old hardcore. A layer of shingle is spread over the hardcore and topsoil spread for the finished level. Depending on the soil type, these can be reasonably efficient, although after a while, they can clog up with soil and cease to work.

Nowadays, if this system is being used, it is better to encase the hardcore with a geotextile membrane and a sandwich layer of shingle where the pipe enters the soakaway. This enables the water to filter through the shingle into the soakaway without soil washing through.

There are numerous other varieties from brick ‘lobster pots’ to industrial concrete rings, but the most common now are cellular systems such as AquaCell – manufactured modular cells, usually made from recycled polypropylene, very lightweight and incredibly strong. The cells lock together and are encased in a geotextile membrane to create a soakaway of any size required. They are also very useful for rainwater harvesting.

It is important to remember that soakaways rarely work on heavy clay soil and where the water table is high. Water will drain away eventually but very slowly, often over several months. Therefore, the two main factors to consider are the size of the area to be drained and the percolation rate of the soil. If a site is waterlogged, the water table is often high and a soakaway will not function at all. If this is the case, it may be necessary to dig down to the porous strata.


Five rules for soakaways

There are five main rules to follow when incorporating a soakaway.

  • 1)It must be in a position lower than the area being drained
  • 2)It must be at least 5m away from any building.
  • 3)It must be sited so that it will not saturate the foundations of any structure.
  • 4)The base of it must be permanently above the water table
  • 5)It must be far enough away from other soakaways/infiltration devices so as not to impair the other device’s function.

A trial pit may need to be dug to check the suitability of a soakaway, especially on clay soil. For an average garden project, 1.8m should be sufficient. Inspect the hole 24 hours later, and if there is no water in the pit, then the position should be suitable for a soakaway. If there is any water present, the depth will have settled at the water table level and it needs to be considered whether a soakaway is going to be possible or functional.

On the assumption that the trial pit is suitable, the next stage is to calculate the size of the soakaway required. An easy calculation that usually works is:

Area to be drained x (rainfall rate per hour/3000) = volume of soakaway

Therefore if a patio is 100m² and a stormwater rate is 50mm per hour, the size of the soakaway will need to be 1.66m³ (or 2m³ to be on the safe side):

100 x (50/3000) = 1.66

Remember that a soakaway must be a minimum of 1m³ and the volume required starts from below the inlet pipe. The volume calculated is for the actual water capacity, so if the soakaway is filled with hardcore or shingle the pit needs to be approximately twice the size.