Sustainable Drainage System (SuDS)- Different Design Options

Depletion of ozone layer and greenhouse effect as a result of high level of carbon emission in the air has presented a serious global threat.

Warming of climate is evident from increases in global average air and ocean temperatures, widespread melting of snow and ice, unprecedented flooding around the world and rising global mean sea level. We have already started seeing and might see more in future, falling crop yields in many areas, particularly in developing regions, depleted water reservoirs, sea level rise threatening many cities, very high intensity of storms, forest fires, droughts, flooding and heat waves.

The increased frequency of flooding is raising the public awareness of the importance of effective surface water drainage and the need for greater protection of property and life.

To address these impacts planners, engineers and technicians need to develop drainage systems that accommodate the increased demands of the climate. Potential solutions include provision of Sustainable Drainage Systems (SuDS)

SuDS are a sequence of water management practices and facilities designed to drain surface water in a manner that will provide a more sustainable approach than what has been the conventional practice of routing run-off through a pipe to a watercourse.

SuDS are the most effective way to manage rainfall where it falls. They comprise interconnected features that surface water flows through or into. Each feature reduces pollution, flow rate, volume and improves water quality. SuDS endeavours to mimic the natural catchment processes that can be found in the natural water cycle. This includes:

• Allowing water to soak into the ground (infiltration)
• Storing water and releasing it slowly (attenuation)
• Slowly transporting (conveying) water on the surface
• Filtering out pollutants
• Allowing sediments to settle out by controlling the flow of the water

This process can often result in the formation of attractive environments that provide pleasant open spaces and valuable wildlife habitats.

Following are a few design options for Sustainable Drainage System.

Rain gardens

A rain garden, also called a storm water garden, is a designed depression storage or a planted hole that allows rainwater runoff from impervious urban areas, like roofs, driveways, walkways and parking areas, the opportunity to be absorbed. A rain garden is designed to improve water quality in nearby bodies of water, to ensure that rainwater becomes available for plants as groundwater and to reduce demand on the existing piped drainage systems.

Filter Strips and Swales

Filter strips, also referred to as buffer strips, are small, edge-of-field tracts of vegetated land that are used to reduce the contamination of surface water. They are primarily used in at the edge of highways and car parks to control non-point source pollution.

Swales are often designed to concentrate or remove debris and pollution out of surface runoff water, but also to transfer flow throughout the site. They consist of a drainage course with gently sloped sides (less than 6%) and are often filled with vegetation. The shallow ditch design maximises the time water spends in the swale, which aids the collection and removal of pollutants, silt and debris.

Swales are also beneficial in groundwater recharge and are great storm water mitigation tools. Depending upon the topography of the land, a swale may have a meandering or almost straight channel alignment.

A common application is around parking areas or adjacent to highways, where substantial vehicle pollution is settled on the paving and then flushed by the first instance of rain.

Swales could be unlined or lined e.g. with concrete etc.

Filter Drains

A filter drain is also known as a French drain, blind drain, rubble drain, rock drain, drain tile, perimeter drain or land drain. It is a trench filled with gravel or rock or containing a perforated pipe that redirects surface water and groundwater away from an area. A French drain can have perforated hollow pipes along the bottom to quickly vent water that seeps down through the upper gravel or rock.

Permeable Pavement

Permeable paving is a method of paving vehicle and pedestrian pathways that allows for infiltration of surface water. In addition to reducing surface runoff, permeable paving can trap suspended solids therefore filtering pollutants from storm water.

Surfacing systems are available using permeable blocks (or permeable tarmac).

Underground Storm Attenuation Tanks

Underground storm water attenuation tanks allow for a high-volume storage of runoff in a small footprint area.

The tank is typically buried under a car park or other open land adjacent to the highway. In the latter case this underground tank may be preferable to a surface detention pond if other uses are intended for the land (e.g., a pedestrian area or park).

In other situations, a tank is used because installing a pond might pose other problems, such as attracting unwanted waterfowl or other animals. The tank will empty by infiltration (soakaway) into the underlying soil or as a controlled outlet to a watercourse or sewer.

Soakaway

A soakaway allows water to slowly infiltrate into the ground, dissipating into the groundwater.

The size and effectiveness of a soakaway is dependent on the infiltration rate of the soil surrounding the pit and the relative location of the groundwater level compared to the base of the structure.

Detention or Attenuation Basin

A detention basin or retarding basin is an excavated area installed on, or adjacent to, tributaries of rivers, streams, lakes or bays to protect against flooding and, in some cases, downstream erosion by storing water for a limited period.

These basins are also called "dry ponds", "holding ponds" or "dry detention basins" if no permanent pool of water exists.

Some detention ponds are also "wet ponds" in that they are designed to permanently retain some volume of water. In its basic form, a detention basin is used to manage water quantity while having a limited effectiveness in protecting water quality, unless it includes a permanent pool feature.

Detention basins provide general flood protection and can also control extreme floods such as a 1 in 100-year storm event. The outlet is generally a restricted flow drain from the basin.

Balancing pond or wetland

A wetland is a distinct ecosystem that is inundated by water, either permanently or seasonally, where oxygen-free processes prevail. The primary factor that distinguishes wetlands from other landforms or water bodies is the characteristic vegetation of aquatic plants, adapted to the unique hydric soil. Wetlands play several roles, sometimes referred to as functions. Among these are water purification, water storage, processing of carbon and other nutrients, stabilisation of shorelines, and support of plants and animals.

Constructed wetlands can be used to treat municipal and industrial wastewater as well as storm water runoff.