Sustainable drainage system

A sustainable drainage system is designed to reduce the potential impact of new and existing developments with respect to surface water drainage discharges. The term sustainable urban drainage system is not the accepted name, the ‘Urban’ reference having been removed so as to accommodate rural sustainable water management practices.

The alternative drainage systems include the techniques used to minimize the impacts of urbanization on the natural water cycle. In the natural environment, some of the water seeps into the soil or passes through the evapotranspiration of the plants and the surplus drains into the rivers and lakes. However, urban growth causes impervious areas to emerge, so that the flow becomes more bulky and faster. Conventional drainage systems seek the rapid removal of water out of the urban environment. However, they are often unable to withstand the drained volume, which causes flooding.

Alternative drainage systems seek to reproduce as much as possible the natural conditions of the hydrological cycle through the retention and infiltration of soil water. These techniques can be applied locally, through storage structures, reuse of rainwater and creation of permeable areas in lots. In the urban environment, larger structures that temporarily store the volume or promote infiltration into the soil are also important in reducing total outflow, flow velocity, and sediment transport. These structures are generally easily integrated into the urban landscape.

In the natural environment, water cycle promotes the circulation of water between the atmosphere, surface and underground. The incidence of solar energy causes the evaporation of the water that forms the clouds that bring rain. Water also returns the atmosphere through the perspiration of plants. On the surface, rainwater can flow through the surface and, according to the topography, go to the valleys where the rivers are. Some of the water may also seep into the deeper layers of the soil, which subsequently return to the surface at springsor reaches the aquifers. The water stored in the aquifers usually act as reservoirs for the maintenance of river levels during periods of drought. This cycle occurs continuously.

A number of factors can alter the natural hydrological cycle, including human activities. Among these, urbanization causes the greatest impacts. Over the past few centuries, the human population has become increasingly populated with urban centers. This process becomes even more intense due to population growth. However, especially in developing countries, urban growth in metropolises and large cities occurs in a disorderly manner, leading to irregular occupations.

The construction of buildings and paved roads, among other characteristic structures in cities cause soil waterproofing. In this way, rainwater that previously infiltrates the soil becomes the surface runoff, whose volume becomes larger in relation to the natural environment. The greater volume of drained water quickly reaches the lower areas, causing flooding. In addition to economic losses, flooding poses risks to health and impairs the quality of life of the population.

The quality of the water flowing through a precipitation event carries with it a large amount of sediment and litter deposited on public roads and buildings. This charge is higher at the beginning of the precipitation. Although it is agreed that storm drainage and sewage networks should be separated, clandestine connections may introduce waste into the runoff, favoring the proliferation of waterborne diseases.

Increasing urbanization has caused problems with increased flash flooding after sudden rain. As areas of vegetation are replaced by concrete, asphalt, or roofed structures, leading to impervious surfaces, the area loses its ability to absorb rainwater. This rain is instead directed into surface water drainage systems, often overloading them and causing floods.

The idea behind SuDS is to try to replicate the drainage patterns of natural systems by using cost-effective solutions with low environmental impact to drain away dirty and surface water run-off through collection, storage, and cleaning before allowing it to be released slowly back into the environment, such as into water courses. This is to counter the effects of conventional drainage systems that often allow for flooding, pollution of the environment – with the resultant harm to wildlife – and contamination of groundwater sources used to provide drinking water. The paradigm of SuDS solutions should be that of a system that is easy to manage, requiring little or no energy input (except from environmental sources such as sunlight, etc.), resilient to use, and being environmentally as well as aesthetically attractive. Examples of this type of system are basins (shallow landscape depressions that are dry most of the time when it’s not raining), rain gardens (shallow landscape depressions with shrub or herbaceous planting), swales (shallow normally-dry, wide-based ditches), filter drains (gravel filled trench drain), bioretention basins (shallow depressions with gravel and/or sand filtration layers beneath the growing medium), reed beds and other wetland habitats that collect, store, and filter dirty water along with providing a habitat for wildlife.

Originally the term SUDS described the UK approach to sustainable urban drainage systems. These developments may not necessarily be in “urban” areas, and thus the “urban” part of SuDS is now usually dropped to reduce confusion. Other countries have similar approaches in place using a different terminology such as best management practice (BMP) and low-impact development in the United States, and water-sensitive urban design in Australia.

SuDS use the following techniques:

source control
permeable paving such as pervious concrete
storm water detention
storm water infiltration
evapo-transpiration (e.g. from a green roof)

A common misconception of SuDS is that they reduce flooding on the development site. In fact the SuDS is designed to reduce the impact that the surface water drainage system of one site has on other sites. For instance, sewer flooding is a problem in many places. Paving or building over land can result in flash flooding. This happens when flows entering a sewer exceed its capacity and it overflows. The SuDS system aims to minimise or eliminate discharges from the site, thus reducing the impact, the idea being that if all development sites incorporated SuDS then urban sewer flooding would be less of a problem. Unlike traditional urban stormwater drainage systems, SuDS can also help to protect and enhance ground water quality.

Conventional drainage systems
Conventional drainage systems are formed by a network of conduits that facilitate the maximum flow of water out of the city. However, the flow velocity is significantly high. With the waterproofing of urban areas, large volumes of water are destined from the buildings and pathways directly to the drainage network, which must be able to withstand large outflows occasionally. When they are not, water overflows and causes flooding.

For a long time, rainwater drainage systems were used as secondary and not strictly necessary for urban development. However, urban structures created by the Romans for more than two thousand years have drainage networks. In the nineteenth century, pipe networks began to be established in Italy in order to remove rainwater and also the sewage domestic, eliminating the use of septic tanks and areas of water accumulation, the occurrence of which was directly related to the proliferation of diseases and mortality of people and animals. Similar works spread throughout Europe as a preventive measure. Thus, it was assumed that the removal of stormwater and sewage from the city would be the best way to solve drainage problems.

However, the impacts caused by the rapid removal of water from the urban zone were not considered until the 1960s. In developed countries, the damages that this form of drainage brought to the environment were perceived, due to the large volume that was introduced in the water downstream, such as erosion and pollution of water resources.

The natural hydrological cycle consists of the balance between infiltration, evapotranspiration and flow, directly influenced by topography, vegetation cover and temperature, among others. The formation of cities strongly interferes with these variables. Alternative drainage systems seek to minimize the impacts caused by urbanization, promoting the infiltration of water into the soil and its retention. Alternative drainage systems have principles that are opposed to conventional drainage systems by seeking water retention rather than driving it out of the urban environment as quickly as possible. In this way, the total volume of runoff decreases, as well as water velocity and peak flow, making downstream areas less prone to flooding and erosion. The implantation of these systems also avoids the overload of the conventional drainage network. Other benefits include improvingquality of the water, which becomes less polluted when it passes through the natural filtering to infiltrate the soil. Its implantation also brings benefits to the urban landscape, since the reproduction of the natural conditions has a positive landscape effect.

In recent decades, studies on the application of alternative drainage systems have become important, especially in developed countries such as the United States and Canada (Low Impact Development, LID), United Kingdom (Sustainable Urban Drainage Systems, SUDS), Australia (Water Sensitive Urban Design, WSUD) and New Zealand (Low Impact Urban Design and Development, LIUDD), all with the same basic principles. In developing countries, however, quantitative control of urban surface runoff is poorly distributed and surface water quality control is practically nonexistent.

There are a number of procedures that favor the maintenance of natural runoff conditions, which include retention or infiltration of water into the soil. Strategies for the implementation of alternative drainage systems should be implemented jointly in order to promote greater efficiency. Its application is made according to local conditions, such as soil permeability, the characteristics of these soils and risks of contamination by chemical products. There is still some reluctance on the part of the public authorities regarding the implementation of these devices, mainly because their long-term behavior is not known. Its application must be in line with local conditions, including allied to conventional drainage networks.

They are based on a triple principle:

temporarily store rainwater upstream to buffer and slow downstream flows.
Infiltrate unpolluted water into the soil, whenever possible, to reduce downstream volumes
Separate and treat polluted water, stormwater and runoff water separately and ensure that those that have run on polluted substrates (roads, factory courtyards, polluted soils, etc.) are treated according to their their characteristics (load in microbes, detergents, heavy metals, pesticides, road pollutants, etc.)

Which involves:

manage and, if possible, purify the water as close as possible to its point of fall, with passive solutions (not dependent on pumps, valves, valves and pipes that may become clogged, etc.), from the roof, for example with vegetated terraces, or near the house, with systems of valleys and restoration of functional wetlands (such as natural lagunages that also purify water, which is not the case for sewers collecting rainwater)
avoid or limit runoff, which is a powerful factor in water pollution and rapid transfer of pollutants downstream and into the sea (water turbidity has become one of the most critical parameters for water courses. water from northern France and many urban or agricultural areas plowed).

Local control
Local control of runoff refers to alternative drainage techniques applied to or near urban lots. These techniques include storing water on the roof. On buildings, relatively flat areas can be used to temporarily store rainwater. However, for its application, proper care with insulation and structural capacity to support the weight should be considered.

Green roofs, a soil cover that allows the growth of plants on the roof, have a relevant storage capacity, in addition to favor evapotranspiration and improve the quality of the water that is filtered by the soil layer. In this way, they reduce the maximum flow and the total volume drained.

Rainwater reuse can also have a positive impact on reducing runoff as it decreases the amount of water leaving the lot. Stored water, while not potable, can be used in a variety of household tasks, while also saving treated water.

Infiltration devices
These systems favor the entry of water into the soil through infiltration. The infiltration trenches are linear trenches filled with stones, which allow water penetration and also favor their storage. They are generally integrated into the environment where they are deployed. In addition to promoting improved soil quality through filtration, infiltration trenches also promote aquifer recharge. French drainsare similar to the trenches, but inside it passes a drilled pipe, which allows the water to escape from it and infiltrate the soil. Flooded ground ditches can be used to collect rainwater and temporarily store it until full infiltration occurs. They are mainly used along highways, but can be used in gardens and even incorporated into leisure areas. Infiltration wells are deeper and less extensive than infiltration trenches, but are also filled with stones. A large amount of sediment in these devices can compromise its efficiency, since the particles block the passage of water.

Permeable pavements allow the passage of water through itself, which can be stored under the pavement or infiltrate into the underground layers. Among the types of pavement are permeable asphalt and permeable concrete, both used in light traffic areas, such as parking lots. Semipermeable pavements are formed by hollow concrete blocks, whose cavity is filled with granular material, which also facilitates the passage of water to the soil. bio-retention beds, or “rain garden”, are designed to collect rainwater from the surrounding area and allow it to infiltrate or reduce the flow velocity, which reduces the amount of sediment carried by the water, in addition to having a landscape function. In general, green areas with the presence of grass and trees in urban areas are able to significantly reduce flow velocity and to reduce its volume through infiltration. They can also retain a large amount of sediment, so periodic maintenance is required.

Retention devices
The retention devices have the main purpose of storing the water coming from the flow and later releasing it with less flow and speed. Retention basins have the advantage of being easily integrated into the urban environment, being possible to use them as a leisure area, since their filling occurs only with the precipitation event and, after cessation, empties. However, they are not efficient for the removal of pollutants. Infiltration basinsare similar but, besides storing the water, they allow their infiltration into the soil, reducing the drained volume. Usually grassy areas are kept dry. These devices, however, have the disadvantage of accumulating a lot of sediment and garbage that is loaded and deposited in these places, requiring periodic cleaning.

Artificial wetlands are areas especially filled with earth, stone and gravel, kept saturated with water, where aquatic plants are introduced. They are cheap to build and bring many benefits. In addition to considerably reducing flow velocity, water quality is improved by removal of soluble or particulate contaminants (including metals) by biological action and sedimentation. They also provide shelter for wild species and can serve as educational and recreational areas. However, its handling must be done with care, because the living beings that normally inhabit there are sensitive.

Fight against floods and droughts
Replenishment of groundwater
Reduced cost compared to conventional solutions (pipes, pumps)
Less polluting discharges into the natural environment
Reliability (by developing the most self-sustaining passive systems and eco-technical solutions possible, through natural ecosystem processes.

Examples of technical “Alternative”
They combine various solutions such as

roadways – reservoir
whose highly porous material is designed to temporarily store rainwater, with slow release to clog floods. The water is purified – to a certain extent – by percolating through the bacteria in the substrate.
Equivalent buried structures can receive the water from the pavements, injected by appropriately disposed drains if the coating is watertight.
After storage, if there is a risk of pollution, the water can be evacuated to an outlet intended for its purification (purification plant or natural lagoon depending on the type of risk…)

Absorption well
they are injection wells in the aquifer. They therefore require the water to be very clean, which is why the infiltration wells are preferred to them, the water being purified by percolating into the soil and / or a purifying substrate prepared before reaching the aquifer.

Ditches and / or valleys
they allow storage in the open air before infiltration and / or evapotranspiration by plants that purify the water of nitrates, phosphates and some of its pollutants;

Draining trenches
linear structures, superficial with a buffer volume allowing temporary storage of water that can then be treated, lagooned or infiltrated into the ground.

Infiltration basins vegetated
(It can even be a flood garden, designed so that there is no island where children might be surprised by the rise of water): they are a size more important than previous solutions, and positioned to collect massive influxes of runoff water, before purifying them and slowly seep into the soil after temporary storage.

Slabs Honeycomb engazonnables
In HDPE for example, resistant to UV and for some models 100% recycled and recyclable. Properly laid, they allow 90% sodding or revegetation by wild flora (provided you do not grow woody). Some models facilitate the flow of earthworms from one cell to another. Rainwater is partly purified and infiltrated into the soil. supporting up to 200 tonnes per m², they prevent rutting and possibly slope erosion. If the number of vehicles is not too important, they allow to build green car parks, or real permanent vegetated roads (in some eco-districtsfor example). Horses may be embarrassed by the unusual sensation they experience on this soil.

Conditions for Success
It is a good transversality of the services of road, sanitation, green spaces, which must work upstream and can usefully associate the services of a landscaper and an ecologist to develop an ecological and thus differentiated management favorable to a better management and purification of water. ADOPTA has shown that these solutions provide very significant savings if the following conditions are met:

the solutions are based on a relevant diagnosis;
they take into account the growing climate risk, as well as the constraints and opportunities of the sites concerned;
they are integrated in a coherent way and from the upstream of the projects;
they are integrated at the watershed scale.

Source from Wikipedia