Local Controls manage stormwater runoff in the local area, typically within the road reserves. Typical SuDS options include: bio-retention areas, filter strips, infiltration trenches, sand filters and swales.
Filter Strips Filter strips are grassed areas of land that are used to manage shallow overland stormwater runoff through several filtration processes in a similar manner to vegetated buffers. They may be strips of lawn. They act as pre-treatment options, especially to remove sediments prior to the runoff entering water bodies or bioretention areas. In order to prevent rutting it is vital that the flow is evenly spread across the entire filter strip
- The installation and maintenance costs for filter strips are relatively low;
- The layout of filter strips is quite flexible;
- Infiltration of the stormwater runoff helps to attenuate flood peaks;
- Filter strips generally trap the pollutants close to source; and
- Filter strips normally integrate well within the natural landscape to provide open spaces for uses such as recreation.
- The primary limitation of filter strips is clogging of the subsurface drainage media, which is generally a result of poor solid waste management and irregular maintenance practices;
- There is relatively limited potential for filter strip to remove fine sediments and dissolved pollutants;
- The stormwater runoff needs to be spread out in order for filter strips to operate optimally;
- Filter strips have minimal stormwater runoff storage capacity and are not very good at treating high velocity flows;
- Because filter strips are not able to manage high velocity stormwater runoff flows, they are not effective on steeply sloping landscapes.
Swales are shallow grass-lined channels with flat and sloped sides. They serve as an alternative option to roadside kerbs and gutters in low density residential areas but because they generally have a larger stormwater storage capacity, they help to reduce runoff volumes and peak stormwater flows. They are normally avoided in areas with a high population density as they require relatively large surface areas to function effectively.
- Vegetated swales are normally less expensive and more aesthetically pleasing then kerbs and their associated concrete- and stone-lined channels;
- The ponding that results from the runoff from adjacent impermeable areas is often completely infiltrated in-situ using swales;
- Swales retain particulate pollutants as close to the source as possible; and
- Swales generally reduce stormwater runoff volumes and delay runoff peak flows.
- Swales normally require a larger land area than conventional kerb and channel drainage systems;
- Swales have very limited removal capabilities for soluble pollutants and fine sediment;
- Swales are impractical on properties that have a relatively steep topography;
- Standing water in swales have the potential to result in the breeding of mosquitoes and the generation of foul odours; and
- If they are not properly maintained, failure is likely to occur more quickly with swales than with most other SuDS options.
Infiltration trenches are excavated trenches filled with rock void forming material. Infiltration trenches are designed to receive stormwater runoff from adjacent properties and transportation links such as asphalt roads and footpaths. The runoff is then detained and allowed to infiltrate over a period of time, usually not more than 24 hours.
- Infiltration trenches increase stormwater infiltration and corresponding groundwater recharge;
- Infiltration trenches decrease the frequency and extent of flooding;
- Infiltration trenches are particularly effective in removing suspended particulates from stormwater;
- Due to their relatively narrow cross section, infiltration trenches can be utilised in most urban areas, including brown-field or retrofit sites; and
- Infiltration trenches have negligible visual impact as they are generally below ground.
- Infiltration trenches are not appropriate on unstable or uneven land, or on steep slopes;
- If infiltration trenches are situated in coarse soil strata, groundwater contamination is a possibility;
- Infiltration trenches are prone to failure if sediment, debris and/or other pollutants are able to clog the gravel surface and/or backfilled aggregate material (Taylor, 2003); and
- They are restricted to areas with permeable soils.
Bio retention Areas
Bio-retention areas, also referred to as ‘rain gardens’ or ‘bio-retention filters’, are landscaped depressions used to manage the quality of stormwater runoff through several natural processes. Bio-retention areas normally incorporate a series of small stormwater management interventions such as grassed strips for infiltration, temporary ponding areas, sand beds, mulch layers and a diverse variety of plant species. They are particularly effective in managing stormwater runoff from minor and more frequent rainfall events. Excess stormwater runoff generated during major rainfall events is routed to other stormwater controls. Bio-retention areas are applicable for managing stormwater runoff on many sites, such as: between residential plots, parking lots, adjoining roadways, and within large landscaped impervious areas.
- Bio-retention areas are effective at the removal of most stormwater runoff pollutants Due to their flexible application characteristics, bio-retention areas are easily incorporated into a wide variety of landscapes;
- Stormwater runoff rates, volumes and flood peaks are effectively attenuated with the correct use of bio-retention areas;
- Bio-retention areas are generally satisfactory as a retrofit options; and
- Bio-retention areas can be made aesthetically pleasing.
- Bioretention areas are normally impractical in areas with steep or persistently undulating slopes;
- Bioretention areas are not suited to areas where the water tables are shallower than 1.8 m (Endicott & Walker, 2007);
- Bio-retention areas require frequent maintenance to remain aesthetically appealing;
- If there is poor housekeeping in the adjacent areas then there is an increased chance of clogging; and
- The construction costs incurred for bio-retention areas are generally higher than most other SuDS options (Wilson, et al. 2004).