There is a need to look at alternatives for pollution abatement other than hard- engineered solutions. These include exploring nature-based solutions to minimise the pollutant load entering rivers, drains and other water bodies. Designing an appropriate riparian buffer is a great example of mitigating pollution entering rivers and other water bodies, writes Vishakha Jha, Senior Environmental Specialist, NIUA.
The wastewater, when left untreated in all probability, ends up polluting rivers, water bodies, drains and groundwater. All these years, cities have looked at wastewater treatment through typical engineered solutions such as sewage treatment plants, both centralised and decentralised, designed to improve waste water quality before it gets discharged into surface waters. But let alone being cost intensive, these treatment systems often due to lack of proper effluent treatment or sustained treatment, can further add to the problem. For instance, the effluent discharge of nutrients leads to deterioration of receiving waters, excessive algal growth and potential eutrophication of water bodies, thus impacting the aquatic ecosystem.
There is a need to look at alternatives for pollution abatement other than hard-engineered solutions. These include exploring nature-based solutions to minimise the pollutant load entering rivers, drains and other water bodies. Designing an appropriate riparian buffer is a great example of mitigating pollution entering rivers and other water bodies.
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The word riparian in Latin refers to the riverbank. In other words, it is an interface between land and river, or any other water body, an area where land meets the natural river course. Typically, it is a longitudinal stretch of vegetation on either bank of the river or drain or around water bodies. Vegetation in this buffer can include native grasses, macrophytes, sedges, climbers, shrubs and trees amongst other vegetation. Even though these buffers can be developed for water bodies or along drains, their nature, design and features may vary from those developed for rivers.
These riparian buffers are not just any vegetative stretches. They act as shock absorbers for the river or natural drain and its ecosystem against various detrimental developmental activities. In the process, they also help restore and maintain the physical and biological integrity of the river. It is this wide range of co-benefits that make them so unique.
The lack of any filter between the land and river would imply mixing of pollution on the land with the river water. Moreover, river edge, when left unregulated in urban areas, often become susceptible to human activities such as uncontrolled farming, grazing, and industries adding to the pollution. A healthy riparian stretch can help protect the rivers from such human land-use practices and forms of encroachment. The vegetation additionally enhances river water quality, capturing the surface run-off, trapping pollutants and providing protection from solid waste elements.
Other than the environmental benefit of improving water quality, such riparian stretches also offer manifold co-benefits. First, it serves as a source of food and energy for the aquatic ecosystem and therefore, enhances biodiversity. Second, it also helps reduce soil erosion. Soil erosion of stream bed and banks amalgamated channel instability leads to preponderance of sedimentation in most rivers. A healthy vegetated riparian buffer reduces the levels of sediments. They slow down overland runoff, filter out sediments, nutrients, pathogens, and toxins. To put it simply, by creating roughness along the surface of the ground, the vegetation decreases the water velocity, allowing time for water to infiltrate the soil and for sediments to drop out. Third, it helps reduce the impact of flooding, especially through the thick vegetative strips that help absorb excess water. Fourth, it also influences the temperature of the river water and the microclimate. The trees of riparian buffers shade the water of the river, moderating water temperature. The protection, management and restoration of the functions and values of riparian buffers is a unique cost-effective approach to improving wastewater quality, sedimentation and nutrient reduction goals of the river ecosystem.
Another unique aspect of these buffers is they will not just vary city to city but may vary from one area to another, even within a city. This is because every watershed is unique, and thus, riparian buffer projects will vary having its own goals. In fact, this is one of the major reasons why there are no set guidelines for riparian stretches that will fit all cities. Cities such as Colorado, Victoria, New York, Nashville have dedicated riparian management plans and guidelines but those are contextualised only for their own cities.
Given that every city has different needs, developing riparian buffers may not feature on top priority for all cities. There are several socio- economic factors and other drivers that will influence how cities need to approach developing riparian stretches and for which purpose. The foremost driver would typically be identification of suitable riparian stretches in the city. In most cases, from a pollution abatement perspective, these stretches should be developed close to polluting industries, pollution hotspots, and areas with presence of solid waste dumping sites. Other drivers would include establishing the width of a riparian buffer, its profile, activities permissible or restricted within these buffers amongst others.
Ideally, the riparian buffer for a river should be a continuous stretch with a width of twelve to fifteen metres, in some cases even extending to 30 metres. This width will depend on the slope, depth of the river, soil conditions and so on. Therefore, it varies from city to city. For example, the state of Oregon in the USA has guidelines for riparian buffers indicating a width of 15-30 metres, while Victoria city in Australia has riparian buffer guidelines of 10-20 metres. Some Indian cities are also adopting riparian guidelines. For example, Kanpur, which is the first city to prepare an Urban River Management Plan, has adopted a riparian buffer of 30 metres. Delhi, on the other hand, in its Draft Master Plan 2041, has proposed riparian vegetation of 25-30 metres along the identified stretches of river Yamuna.
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The planting strategy, for a riparian buffer, is critical for it to achieve its intended purpose. For instance, the dominant purpose of the streamside zone, the zone closest to the river, is to slow down runoff, reduce erosion, and create a habitat for invertebrates and other wildlife. Therefore, the plants in this zone should typically include trees with dense root systems. Similarly, the purpose of the middle zone is to remove pollutants from the subsurface flow of water. And for the outer zone, it is to remove pollutants from the groundwater. Consequently, the plantation for each of these zones may vary. Other than accounting for soil conditions, water depth, nature of ground profile and its surrounding uses, the use of native plant species cannot be overemphasised. These types of plant species naturally occur in a region or habitat and have co-evolved over geologic time with other plants and animals, serving vital ecological roles and creating a balanced plant community that supports indigenous wildlife. Thus, it is very critical for the sustenance of the riparian ecosystem.
The riparian buffers are also being used globally in combination with soft engineering solutions such as constructed wetlands that use wetland vegetation, soils, and their associated microbial assemblages to treat wastewater. Another hybrid yet naturalised variation to riparian is developing rip-rap edges that constitute boulders or large crushed stone lining along with natural edge, enabling many ecological interactions to take place while also protecting the river from pollution and erosion. While nature-based solutions such as riparian buffers are unsung heroes of healthy rivers, it cannot be a plant and walk away activity. To maximise its benefits and ensure its health, cities also need to plan for its maintenance and monitoring consistently.
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