Natural drainage systems

Natural Drainage Systems as Green Infrastructure in Settlements


In agricultural lands drainage usually occurs naturally. Excess water flows from the fields to natural water bodies such as rivers and lakes or gets absorbed into the soil by natural processes.

In urban areas however, we build complex artificial water drainage systems. These drainage systems eventually carry pollutants such as heavy metals, paint and oil into natural water bodies as part of urban waste.

It is not possible to treat all of urban waste water without a concerted effort towards the endeavour. Storm water in urban areas can run off and compile pollutants that flow into natural water bodies and also corrode channels of water, endangering habitats.

The adverse impacts of the run off of storm water adding to the urban waste water in drainage systems can be offset to a certain extent by instituting natural drainage systems. Natural drainage systems clean storm water run off and the compilation of pollutants with the help of natural sources such as trees and soil to act as sinks within lived spaces.

Components such as ponds, storm water cascades and vegetated areas act to absorb water, filtering and preventing the accumulation of many pollutants. This offers a better alternative than artificial drainage systems that seeks to dispatch or store contaminants, polluting water bodies.

Natural drainage systems form part of what is called green infrastructure in urban spaces and can also be a landscaping technique. In this there can be certain principles that can be followed that can enhance the prospects for waste water management through the use of natural drainage systems.

In addition to urban spaces, natural drainage systems can also much more readily be a feature of rural landscapes, and can act to negate some of the adverse environmental consequences of the run off of pesticides and other chemicals into the hydrological cycle. In both urban and rural approaches, there are certain differences that depend also upon the foundational topography of the region.

Some Principles Concerning Natural Drainage Systems

The most important principle that can be considered in natural drainage systems is in providing a corridor for natural streams to flow through areas without interruption or construction. Providing this corridor for the flow of streams provides certain advantages in terms of the drainage patterns for the area. These include the abatement of flooding, less erosion of the banks of streams, less sedimentation, better groundwater recharge, greater moisture content in soil, filtration of pollutants and excess nutrients and better soil fertility with better habitats for animals.

A principle to remember with rural environs is that there is a better chance that man-made structures can be made to fit in with the surrounding environment rather than become the dominant element of the countryside.

For example, if building plots are situated in a random distributed pattern in a predominantly densely wooded environment, there shall be a significant reduction in vegetation and absorption of water. However, a compact arrangement of the same quantity of building requirements can allow more area for vegetation to flourish in the same area, thus increasing water absorption and enhancing the hydrological cycle.

The emphasis in rural areas as regards vegetation needs to be the conservation of continuous agricultural and wild green spaces, given the lack of congestion and population found in urban areas. This ensures a greater amount of vegetation that can act to absorb water, maintain the richness of the soil and contribute to the overall hydrological cycle. In urban areas, it is not possible to have wide running open green spaces like in rural areas. However, the planning aspect of urban areas must include areas marked as centres and areas marked as green spaces within the local area, such that there is a mixed and sporadic combination of constructed and green spaces.

Soil fertility is important even in urban spaces, as this provides a base from which green spaces can thrive in urban environments. This allows lush and healthy vegetation to be present even in urban areas, performing beneficial hydrological functions.

This requires urban centres to be compact and preserve space along with provision of easy transit such that spaces can be earmarked to be developed as green spaces (Greenway Connections, 2013). In both urban and rural areas, zoning regulations must be in place for designated areas such that spaces can be smartly planned. However, drainage in constructed spaces has better chances at being sustainable if plans for natural drainage systems are included at the earliest possible stages of development of an area.

Green Infrastructure and Sustainable Drainage

Concrete installations in constructed spaces can introduce impervious surfaces that prevent percolation of water into soil and provide artificial channels for the flow of water out to natural repositories of water. Conventionally, run off water is channelled through pipes and storm drains towards a location for depositing them or they are discharged to natural water bodies. Other than being expensive feats of engineering, these installations act to modify the hydrological cycle of significant portions of land. This can have adverse environmental impacts.

These effects can be abated somewhat with the installation of green infrastructure. Green infrastructure can include installations and practices such as tree plantation, stream daylighting (replacing underground pipes with an open underground stream), bioswales (vegetated ditches), rain gardens (landscapes with native vegetation), roof downspouts (to drain rooftops), green roofs (that absorb rainwater on rooftops with the help of soil and vegetation) and finally permeable concrete that allows water to percolate into the underlying soil (Greenway Connections, 2013).

These act to facilitate the natural absorption of rainwater in constructed spaces, thus lending some balance to the hydrological cycle. This induced absorption must attempt to emulate an area’s pre-development hydrological cycle, and if not all, restore at least a certain part of it.

Natural drainage systems are included in the broader category of the various methods of sustainable drainage, that aim at correcting the damaging effects of artificial drainage systems with sustainable solutions. The category of sustainable solutions in drainage systems is a broad category and can be applied even to water treatment before discharging it in repositories. This water treatment however can also involve the natural processes of biological degradation, adsorption, filtration and sedimentation.

Sustainable drainage offers many benefits over conventional drainage systems, and these can include (i) lessening of pollution impacts on water bodies, (ii) assistance in the prevention of flooding, mainly due to impermeable constructions, (iii) partial restoration of the natural hydrological cycle of an area, (iv) allowing better groundwater recharge, (v) having settlements more compatible with the environmental topography of an area, (vi) lessening the load on existing drainage systems, and (vii) providing a construction aesthetic (Environment Agency, undated). With climate change on the horizon, such sustainable solutions for managing rain water run off are imperative, and act to provide sustainable habitats both to human settlements and for wild habitats.

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