Source Controls manage stormwater runoff as close to its source as possible, typically on site. Typical SuDS options include: green roofs, rainwater harvesting, permeable pavements and soakaways.
A roof that is deliberately covered in vegetation may be described as a ‘green roof’ A wide range of vegetation may be used on green roofs, but the types are often limited to small shrubs and grasses for structural reasons. Flat roofs often incorporate a thicker layer of vegetation or roof gardens that promote general rooftop accessibility and other forms of outdoor recreation. The use of vegetative roof covers and roof gardens is an important source control for stormwater runoff. Green roofs also offer a range of additional benefits, most notably in the form of insulation and amenity value.
- Green roofs may be established on both existing and new buildings
- The insulation characteristics of green roofs help to regulate building temperatures with consequent savings of energy
- Green roofs can significantly improve amenity and biodiversity where they are implemented
- The implementation phase for green roofs requires experienced professionals that are competent in waterproofing and plant requirements
- Green roofs are generally more costly than conventional roof-runoff practices due to their added structural, vegetative and professional requirements
- Green roofs may only be used on steep roofs (>20°) if additional support systems such as horizontal strapping are provided
Permeable pavements refer to pavements that are constructed in such a manner that they promote the infiltration of stormwater runoff through the surface into the sub-layers and/or underlying strata. There are many alternatives for the load-bearing surface material including: specially designed concrete block pavers (CBP), brick pavers, stone chip, gravel, porous concrete and porous asphalt. The latter two are also referred to as porous pavements. In places with suitable climates and low traffic loading, even grass can be used with or without reinforcement as the situation demands. Patented open celled concrete grass pavers or cellular plastic grids are often used for the reinforcement of the grass surface layer. The permeable paving surface is suitable for pedestrian and vehicular use, and can be modified to carry heavier loadings.
- Permeable pavements reduce stormwater discharges rates and volumes from impervious areas
- Permeable pavements increase the ‘usable’ area on specified developments by utilising, inter alia, roadways, driveways and parking lots as stormwater drainage areas
- Stormwater runoff stored in permeable pavements can be used to recharge the groundwater table and for several domestic purposes Lined permeable pavement systems can be utilised where foundation or soil conditions limit infiltration processes
- The implementation of permeable pavements is generally limited to sites with minimal slopes
- Permeable pavements should not be constructed over fill materials, as these soils could fail when saturated
- If managed incorrectly, there is a great potential for clogging by fine sediment, which significantly reduces the effectiveness of the specified system
- The pollutant removal ability of permeable pavements is significantly lower than most other SuDS options
Soakaways are excavated pits, normally square or circular in shape, packed with course aggregate or other porous media. They are similar to infiltration trenches in operation, but usually have are submerged and act as source rather than local controls to manage the runoff from residential houses. Soakaways are most commonly used for the temporary storage of stormwater runoff which is then infiltrated into the ground.
- Soakaways can handle relatively high volumes of groundwater recharge
- Soakaways significantly decrease both the runoff volume and rate
- Soakaways are particularly effective in removing particulate and suspended stormwater runoff pollutants.
- Soakaways are not suitable in areas where infiltrating water would negatively impose on adjacent structural foundations or adversely affect existing drainage characteristics
- Soakaways cannot be used where the ground water table is near the surface
- Sub-drain piping systems must be utilised when soakaways are implemented in very fine silt and clay stratum because of the low infiltration rates