Swales are shallow channels designed to collect and slowly drain water, either storing or transporting surface runoff during high rainfall. They gather water from a catchment area, guiding it through grass or plants, allowing it to soak into the soil and slowing water flow. Swales can also enhance gardens as attractive features. 

Unlike traditional ditches, swales are wide and shallow, often appearing as linear grass depressions but can be planted to create garden features. Depending on the size of the catchment, they can be typically dry in good weather, and fill with rainwater during wet conditions. This influences the choice of plants most suited to this situation. 

Swales can be part of a wider drainage system, transporting runoff on the surface to another location, such as a water body or discharge to an existing drainage system. Being above ground, they are less prone to blockages, which are easier to detect and resolve. 

During rainfall, runoff can be lost from the swale by infiltration, evaporation, and transpiration. Swales work best with small gradients on their side slopes and along their length. Check dams can be placed across swales to reduce flow rate, minimizing erosion and providing temporary water storage, especially on a gentle slope. This encourages sediment to settle, reducing the risk of clogged drains and promoting infiltration into the surrounding ground. 

Swales can also help trap small amounts of oily residues and organic matter from runoff, breaking them down in the soil and vegetation, significantly reducing pollution. Designing swales to be dry between heavy rainfalls maximizes pollutant removal. Groundwater can be protected by placing an impermeable lining under the swale, with an overflow to an existing drainage system. Periodic maintenance may be needed to remove any build-up of toxins from polluted water. If the pollution risk is high, a seperate treatment at the source or inlet may be needed.

Working swale in the Woodland area at RHS Bridgewater

• ​Dry swales

Dry swales are tailored for managing water runoff in regions with minimal rainfall or a small catchment area. Instead of retaining water, these swales guide runoff away by promoting infiltration or controlled discharge.  

• Wet swales

​Wet swales are designed to hold water for longer durations. They typically include a permanent water feature or maintain a high-water table, making them ideal for locations with abundant rainfall or frequent storms, or a large catchment area.  

• Bioretention swales

• Shallow design: Typically, swales have a wide, gentle slope and a shallow depth, which slows the rate of water flow

• Planted surface: Swales are often planted with grass, perennials, or other vegetation to enhance water infiltration and look attractive

• Flexible: They can be designed in various shapes and sizes depending on the specific needs of the site and to suit aesthetics

Planting has enhanced the swales at RHS Bridgewater in the woodland area and the car park

Site Assessment

Before constructing a swale, a thorough site assessment is essential to determine: 

• Catchment area: How large an area will the water be coming from. This can be assessed from local knowledge and observation in a rainstorm or looking at and measuring the ground levels.

• Local rainfall patterns: A swale created in Essex or Suffolk is likely to receive around half the average rainfall than if it was created in Manchester. Rainfall can be found on the Met Office website.​

• Soil Type: Soil permeability and drainage characteristics will influence the design of the swale.

• Slope and terrain: The slope of the land will affect the design and effectiveness of the swale.

• Existing vegetation: Existing plant life and landscape features should be considered, to integrate the swale seamlessly into the environment (such as tree roots). 

• Pollution: Is the water likely to contain road runoff, or sediment from surface runoff? Rainwater collected directly from roofs usually has the lowest pollution load.

Design Considerations 

When designing a swale, several factors must be considered:

• Dimensions and slope: The width, depth, and slope of the swale should be tailored to the volume of runoff expected and the site’s specific conditions. Rounding the ‘shoulders’ avoids scalping to bare soil if the area is mown.

• Flow rate: Design should accommodate up to the expected flow rate and provide an overflow for when design flows are exceeded, and ensure effective management. Erosion can occur if the swale is not designed to accommodate high flow rates or the gradient of the swale is too steep. This will depend on the catchment area and rainfall.

• Vegetation selection: Choose appropriate plant species that can thrive in the local climate and soil conditions.

• Location: Swales should be located close to the source of runoff and can form a network within a garden (dependant on size) linking to storage ponds and wetlands.

• Inlet zones: Spreading the inlet zone either with multiple inlets or horizontally along the length of the swale can reduce the erosion risk and slow down the flow.

Construction Methods 

• Excavation: The swale is excavated in dry conditions to the required dimensions and slope. If the soil has clay content, excavation can risk creating a smeared soil pan that will reduce the infiltration of rain.

• Soil preparation: The bottom and sides of the swale are prepared with existing soil and compost to support plant growth and water infiltration.

• Planting: Vegetation is planted according to the design specifications, and additional features such as mulch or erosion control fabric may be added where needed. Mulch can be useful to retain moisture around the roots of new plantings in drier weather but be aware that thick layers of organic mulch may float away and cause blockages in very wet weather. If the soil needs improving, it would be best to use organic mulches in dry weather for swale plantings.

Routine Maintenance 

Regular maintenance is crucial to ensure the swale continues to function effectively: 

• Debris removal: Periodically remove large amounts of leaves, branches, and other debris that could obstruct water flow, but generally, small amounts of debris can be left, especially in drier weather to improve soil health and retain moisture. Litter can blow in on the wind

• Vegetation management: Monitor and maintain vegetation to prevent overgrowth and ensure proper coverage

• Inspection: Regularly inspect the swale for signs of erosion, sediment buildup, or other issues

Regular checks to remove any accumulations of debris and sediment which can block the flow of water, reducing the effectiveness of the swale, especially where they may enter a pipe such as under a footpath. 

Occasional weeding may be required during the first two years of the life of the swale. Remove by hand any unwanted plants, ensuring that you remove the whole plant, including the roots. As the plants in the beds mature, they will fill in any gaps and suppress growth.  

During winter, you may want to remove any dead or un­tidy plants, although it is good to leave some dead stems and seed heads for wildlife. However, leaving the canopy in place till the spring can provide insulation from frost, reduce the amount of rainfall reaching the swale and habitat for invertebrates.

Regular mowing is not required, but the bed may benefit from cutting occasionally. If required, cutting can be undertaken in late summer or autumn with a scythe or strimmer with particularly tough material cut by hand with secateurs.