Recently every monsoon leaves us concerned about the flood situation in the country. With every year, a larger part of the country seems to be affected by the danger of floods.
The recent Mumbai floods prove that even the supposedly well planned metro cities are not flood-proof. Additionally, cities like Bangalore, which have traditionally enjoyed moderate climate, have also been witnessing considerable flooding and waterlogging in the last few years. To remedy this, the Bruhat Bengaluru Mahanagar Palika initiated a demolition drive in 2016 to clear the encroachments on the storm water drains in and around the city. However, the project could not be completed and the city yet again suffered miserably during the monsoons this year.
The smaller towns and villages are at even greater risk due to lack of as robust an infrastructure and correspondingly lower resources to deal with any emergency situations. Looking at the year 2017 alone, more than a million people have been rendered homeless due to floods.
It is natural for the common man to blame unplanned development or inefficient drainage systems. However this is not a problem of shortsightedness alone, though admittedly a better job of planning out the city infrastructures could certainly have been done. Our cities and towns are growing at an unprecedented rate (studies estimate that 70% of the non residential building infrastructure in cities is yet to be built), and when coupled with climate change, even the best laid plans haven't been able to cope.
Yet all is not lost. Even with the aforementioned problems, the damage is not irreversible yet. At its core, excessive waterlogging after a rainfall happens due to rainwater not being sufficiently absorbed into the ground. Large parts of the urban area have ended up being covered by housing, roads, pavements, concrete and other non permeable surfaces. This leaves very little open surface area that can actually absorb the water. Additionally, the loss of plant and forest cover has resulted in the depletion of natural moisture retention capacity of the soil. This has resulted in soil erosion, change in natural drainage patterns and further loss of vegetation.
Thus while widening and protecting the storm water drainage system is one solution, it needs to be complemented by rainwater harvesting and more environmentally aware building practices.
Rainwater harvesting helps conserve rainfall by collecting and filtering rainwater through percolation via different natural materials of varying permeability. This filtered water can then either be utilised by people (depending on the purity levels) in activities like horticulture, cleaning etc or it can be allowed to replenish the groundwater on site. This in turn mitigates soil erosion and protects the natural hydrological cycle while reducing the demand for groundwater.
Rainwater harvesting has been around for a thousand years and even today serve as the primary source of water in many semi arid areas. Baolis, networks of manmade ponds and waterways are examples of such techniques as used in India.
Thus, returning to our original topic of discussion, conventional stormwater drainage systems are mainly designed as flood-prevention measures. Green infrastructure (GI), on the other hand, is a more holistic waste-management approach which treats and recycles stormwater. It is estimated that almost 95 percent of the runoff water from the rains can be managed using the GI strategies.
Most importantly, GI attempts to mimic the natural water recycling patterns of an area and recreates it. It uses the knowledge of soil conditions, flora and other local resources to restore some of the natural processes of the area in the ecology of the building, resulting in low-impact development.
The goal here is to manage water as close to the source as possible using soil and vegetation based systems. These strategies help integrate the site with the surrounding watershed, are appropriate to the local ecosystem and climate, and deliver ancillary benefits like water reuse, habitat creation, and species diversity. By protecting existing natural resources that serve to reduce the generation of runoff, the volume of runoff can be greatly reduced. Natural resources on a building site like healthy uncompacted soils, riparian buffers, tree canopy, etc. must thus be protected from disturbance during the construction period in order to be effective later.
IGBC, LEED, GRIHA, or their equivalent premier rating systems in the world, recognise these issues and challenges. To begin with, it is necessary to establish the volume of runoff that is to be managed on site as a measurable goal. This can be done by choosing a percentile of precipitation events that must be managed such that there is no surface discharge from the site. Once this is established, Best Management Practices can be used to achieve the measurable goal. EPA’s National Menu of Stormwater Best Practices provides a host of options to look into.
Techniques include, but are not limited to, infiltration, storage and re-use, bioretention, open-grid pavement, and the reduction of impervious area. Infiltration may not be feasible in some cases based on the soil or geological conditions of the site. Karst geology and areas where water infiltrates at less than 1/2 inch (13 mm) per hour are two examples of situations that can be unfavorable for infiltration. The engineer, landscape architect, or rainwater professional will ultimately determine the best solution for the project’s unique conditions. Continued maintenance of all rainwater management strategies is thus very important in order for them to remain effective over time.