- Storing runoff and releasing it slowly (attenuation)
- Harvesting and using the rain close to where it falls
- Allowing water to soak into the ground (infiltration)
- Slowly transporting (conveying) water on the surface
- Filtering out pollutants
- Allowing sediments to settle out by controlling the flow of the water
In short, we are trying to prevent large quantities of rainwater from rapidly entering the local stormwater drainage systems and overloading the local surface water infrastructure, such as streams and rivers. Clearly, overloading of this nature can, and has, caused devastating flooding and associated damage to property and loss of life in recent years. In addition to slowing down the transportation of this water, we are also trying to prevent the transportation of suspended solids and pollutants into the local surface water infrastructure. If there are enough Sustainable Urban Drainage Systems installed in a river catchment area the rate at which the water reaches the river can be significantly reduced and consequently so can the risk of flooding.
Flooding has cost the UK an average of £1.6 Billion per year in damages and hence the focus has shifted strongly towards prevention rather than just cure. Sustainable urban drainage solutions are an effective strategy in trying to combat the risks of potential flooding. 32% of flood damage occurs in the business sector whilst the remaining 68% affects residential properties. Flooding causes financial, physical & mental health problems for those affected related to loss of livelihood, property, and potential long-term relocation.
HOLDING BACK THE FLOOD WATER
TCS Geotechnics supply products for four different SUDS solutions which have different ways of providing water management namely; Infiltration soakaways, Attenuation tanks, Permeable paving and Balancing ponds. As indicated above all the solutions TCS offer are designed to slow the passage of rainwater to areas of potential flooding.
Attenuation tanks are created underground using highly porous polymer crates that create a void which are sufficiently structurally stable to support the weight from overlying soils and vehicles above. Consequently these systems can be installed under car parks and access roads as well as landscaped areas. Attenuation tanks are intended to collect and store storm water run-off during periods of high precipitation and then release that water gradually over an extended period, thereby slowing its progress to any watercourse of concern. During construction the proposed location is excavated and then lined with the non-woven fleece. An impermeable geomembrane liner is then installed and the attenuation crates are positioned inside that. Once the crates are installed, the geomembrane is then folded and welded along any joints around the tanks to create a waterproof seal. The wrapping of the non-woven geotextile is then completed create a cushioned barrier to inhibit puncturing of the geomembrane liner. The tanks are completed with an inlet and outlet to control the flow of water through the system and an air vent to prevent the build-up of air pockets.
Soakaway are designed and constructed in a very similar way to attenuation tanks and use the same polymer crate system to create the void. The key difference is the lack of impermeable geomembrane liner. Soakaways are generally designed for small usage, like isolated residential properties and permeable ground conditions, like sand or gravel. Typically they are employed to deal with roof run-off water from residential properties that goes directly into the soakaway tank from the guttering. A soakaway system’s function is to collect the rainwater and slowly dissipate it into the surrounding ground via a permeable non-woven geotextile that is wrapped around the tank. The water will eventually find its way into the water table in the ground and slowly enter a water source. In residential properties the system can be installed under the garden or driveway.
PERMEABLE PAVING SYSTEM
Large hardstanding areas like car parks generate significant quantities of run-off water during precipitation events. Residential developments, shopping centers, retail parks, hospitals, and business parks are good examples of locations that frequently use blockwork to create paved areas for parking, footpaths, and roads. Regular impermeable paving systems with blocks or asphalt have two main issues. Firstly, if properly constructed, water runs off these areas rapidly and directly into the stormwater drainage system which – as indicated above – can lead to flooding elsewhere. If poorly constructed or aging these impermeable surface systems may be uneven which will mean that water may not reach a drainage grid and therefore puddling and potential associated surface damage results. A permeable paving system should instantly drain surface water without the need for it to be directed towards a drainage inlet. Formpave Aquaflow permeable paving system provides a tried and tested solution for the management of surface water and drainage. TCS is the sole distributor of all the geosynthetic Formpave components that are used within the Aquaflow system. Formpave has two options available, infiltration (soakaway) and attenuation (tank), which operate similarly to the descriptions above
The Infiltration system is used when the ground is permeable (not clay) and not contaminated. Water percolates through the Formpave Aquaflow blocks and is filtered through the Inbitex geotextile which provides some cleaning action. The water is then allowed to flow through the underlying drainage stone to dissipate into the soil.
The attenuation system works much like a regular attenuation system. The blocks allow the water to percolate through to an underground section of the ground which is tanked using an impermeable SC Membrane which restricts water from entering the groundwater table. The water filtrates through the Inbitex and granular infill and builds up underground on top of the SC Membrane. As the contained water level rises the water enters the SC Findrain which is positioned in key places within the tank and directs the water to the nearest underground pipe and stormwater drainage system.
Balancing ponds are a cost-effective solution for managing stormwater run-off providing development space isn’t an issue. As the name suggests, a balancing pond is an open water area which fills during rainfall events and then the water is gradually allowed flow out to the main drainage system thereby reducing the impact of any storm and avoiding flooding. The most obvious geosynthetic application for balancing ponds is the use of geomembranes and geosynthetic clay liners (GCLs) for sealing the base of the pond together with the associated protection geotextile fleeces. TCS offer a range of HDPE, LLDPE, PP and GCL membranes and needle punched non-woven geotextile protection fleeces. Unlike attenuation and soakaways, balancing pond are visible at ground level and generally converted to areas of green open space, particularly around residential developments. With this in mind the appearance of these features is important and good vegetation growth will make them appear more natural. Erosion control products like coir matting is utilised on the sides and slopes of balancing ponds to offer an anchor for plants to grow and flourish.