Water logging is a common scenario in our cities. Every year, the heavy rains take their toll, leading to severe water logging. Rainwater infrastructure, if it exists at all, is not sufficient to guarantee drainage of the water and streets in low-lying areas get inundated. The problem is aggravated by the fact that drains do not have sufficient slope to draw water from surface water and channel it.
The monsoon in Mumbai
This monsoon, Maharashtra’s capital city Mumbai witnessed heavy downpours already. Residents are stuck in their homes as drains and potholes on the roads are choked; movement of people and vehicles is hindered by water-logging leading to traffic jams; pedestrians have to wade through flooded streets; and the urban poor, living in slums, or squatters have their homes damaged. As the water mixes with garbage, it becomes a health hazard. Uncovered manholes in an inundated street become invisible and cause a serious threat to pedestrians and two-wheelers. Commuters are hamstrung with train delays and rescheduled flights. Further, similar encroachments of Mithi River in Mumbai have also led to a serious water logging problem.
According to the weather bureau, Mumbai recorded a whopping 283 mm of rain on 19 June, pushing the total rainfall received so far in June to 537 mm. This is more than the monthly average of 523 mm; according to civic authorities, 283 mm of rain is normally received in 10 days. To worsen the situation, storm-water drains, designed to tackle intense rainfall, remain clogged with garbage and sewage, or simply do not exist because of encroachment. So the city becomes deluged even it receives less-than-normal rainfall.
Mumbai’s drainage system, more than 100 years old in many places, consists of 2,000 km of open drains, 440 km of closed drains, 186 outfalls and over 30,000 water entrances. The capacity of most of the drains is around 25 mm of rain per hour during low tide, which is exceeded routinely during the monsoon season in Mumbai, which witnesses over 1,400 mm during June and July. The drain system works with the aid of gravity, with no pumping stations to speed up drainage. Most storm-water drains are also choked owing to the dumping of garbage by citizens. On 17 June 2015, two pumping stations worth Rs 200 crore were inaugurated, but even these couldn’t take 300 mm of rain because of the garbage bunged in the pipes, which didn’t let the water run through.
The new concept of smart cities, which aims to address all urban issues, will certainly need to address the problem of water logging.
One way to tackle the problem and deal with the decreasing levels of groundwater could be rainwater harvesting, a technology used in ancient India for thousands of years. However, instead of reviving the drains, the Government has asked cities to use high-end technologies, pipes for instance, to divert rainwater to nearby rivers or seas using funds from the Jawaharlal Nehru National Urban Renewable Mission (JNNURM). But this is not the right approach. In the present scenario where cities are extracting huge quantities of groundwater and lakes are disappearing, we should use the runoff flowing through storm-water drains to recharge the groundwater.
The loss of storm-water drains in urban areas leads to the twin problems of reduced groundwater recharge and water logging. This can be solved by integrating green infrastructure into the storm-water management process. Designing an efficient drainage system with underground surface run-off collection pits, also a part of a rainwater harvesting system, is an option to get rid of water logging. The storm-water drainage system should always be separate from the sewerage system to prevent contamination.
To design a storm-water drain for any city, the following points should be considered:
- Rainfall data of the past 100 years (as per the expected return period /design service life of the structures)
- Surface runoff (based on land slopes, solid drainage capacity)
- Expected structures in the surrounding locality (dams, tall buildings, etc)
- Design slope of land, say 1:100 / 1:150, orientation of drains (main drain and sub drains)
- Hydraulic parameters (lined/unlined drain).
Deploying smart water management would best tackle the problem of water logging as automated systems would interpret data from water-logged chunks in the streets and provide an integrated operating picture with robust real-time analytics, modelling and decision support capabilities for clearing the stranded water.
Some best management practices to reduce the run-offs include green roof, filter strips, rainwater tanks and cisterns, permeable pavement, grass channel, bio-retention ponds and swales. Many countries have taken up these initiatives and have incorporated them in their policies. For instance, Santa Monica, California, in the US, requires all newly developed or retrofitted developments to manage 80 per cent of storm-water runoff from impermeable surfaces. Germany collects rain taxes for the amount of impervious surface cover on a property, which generates runoff and is directed to the local storm-water channel. So, the more rainwater is conserved, less is the runoff added to the storm drains. Less runoff allows for smaller storm-water channels, which, in turn, saves construction and maintenance costs at site. Thus, people get rain tax reductions by converting their impervious pavement into a porous one. In Germany, adoption of transparent surface water drainage charges and subsidies has encouraged a high amount of retrofitting of sustainable urban drainage (SUDS), particularly green roofs and water reuse systems.
Cisco has come up with an innovative tool, Flood Beacon, a device aimed at collecting and sharing real-time data about flood conditions, including water level, GPS location and fast accelerations indicating a sudden rise in water. The device contains sensors able to detect water level, location and can be anchored in high-risk flooding areas. It transmits live data through a cell phone network to a smartphone application or monitoring station, sending notifications when there's a risk of flooding, alerting local governments. It uses solar energy to stay charged. Another device, OptiRTC platform which monitors and controls water-levels in real-time i.e. the platform determines how much runoff is likely to occur and then takes action.
The India perspective
India receives most of its rainfall in just 100 hours and it is important to catch this water for water security throughout the year. There is also a need to manage the runoff flowing through the storm-water drains in cities to avoid water logging. Building rules and codes must reflect the maximisation of on-property storm-water use wherever possible.
Andhra Pradesh, Bihar, Goa, Gujarat, Haryana, Himachal Pradesh, Karnataka, Kerala, Madhya Pradesh, Maharashtra, Nagaland, Punjab, Rajasthan, Tamil Nadu, Tripura, Uttar Pradesh., Uttarakhand, West Bengal, Chandigarh, Daman and Diu, NCT Delhi and Puducherry have made necessary provisions in their building bylaws to make installation of rooftop rainwater harvesting systems mandatory. Chennai, Gurgaon, Mumbai, Surat, Kanpur, Hyderabad, Nagpur, Indore, Rajkot, Gwalior and Jabalpur have made it mandatory for new buildings that cover a certain area to have facilities to capture rainwater runoff and use it to recharge groundwater on the spot.
In 2012, the Jaipur Development Authority (JDA) planned a conventional dense graded asphalt parking lot at the Gandhi Nagar railway station and decided to include a porous asphalt area (about 340 sq m) as part of the parking lot. JDA planned to harvest rain through this porous asphalt area. A porous asphalt pavement, also called a permeable pavement, contains pores or separation joints that allow water to flow through and seep into the soil, thereby reducing runoff. The two types of parking lot pavements were divided by constructing a cement concrete partition wall. During the monsoon of 2013 a clear distinction was observed between the two. While the porous pavement absorbed the rainwater, the conventional pavement allowed the runoff to flow over it.
Another such example can be seen at the National Geophysical Research Institute (NGRI) in Hyderabad's Uppal Road. Huge amounts of runoff used to flow out of the 3 ha complex each year and load storm-water drains, which get choked during peak monsoons. In 2007, NGRI scientists planned to install groundwater recharge structures in their campus. They constructed nine rainwater harvesting ponds (rain gardens) to catch runoff. Each garden is 40 sq m in area and is circular in shape. Nine ponds have the capacity of holding 180 cum of rain.
These are just some of the ways forward if the problem is to be tackled decisively.