This year,Mumbai was caught unawares. Two big showers in the first 10 days of monsoon and the city received over 20 per cent of its annual average rainfall of 2,409 mm. By July-end,Mumbai received more than 80 per cent of its average rainfall for the full year. This put unprecedented pressure on the citys capacity to drain water,causing craters on roads and leading to a public and political outrage. While competitive pothole politics – with each party taking to the streets and filling the pits on the roads – lent itself to a good public spectacle,there has hardly been any informed discussion or debate on issues that really matter.
Are we in for worse times in the coming years with a front loading of monsoon heavy rains in the first few weeks,instead of an even rainfall through the season? If that be so,then the citys civic administration better be prepared for the worst. While rapid urbanisation and increase in population puts pressure on utilities and resources of the city to provide basic necessities,be it food,water and shelter,large-scale construction in the city as well as its extended suburbs has resulted in increased concretisation and diminishing green cover. When rainfall is in abundance and that too in a short period,two aspects of urban planning are critical one,the ability to flush water into the sea (in a city like Mumbai) and two,to create spaces for holding water so that a significant quantity of water percolates into the ground.
As per BMCs 1991 Development Plan
275.26sq km or 63%
of Mumbais total area of 437.81 sq km
can be developed rest 27% is covered under forests,wetlands and water bodies
As per 2001 Census
Built-up area was already
265 sq km,leaving only 10 sq km of land available for further development.
Mumbai has to keep expanding to accommodate influx of people into the city from not just the state hinterland but from across the country. This means more and more land is taken up for housing,commercial sector and for expanding infrastructure.
As per the BMCs 1991 Development Plan for Mumbai,out of the citys total area of 437.81 sq km,275.26 sq km or 63 per cent can be developed while the the rest 27 per cent is covered under forests,wetlands and water bodies. 2001 Census data shows that the built-up area of the city up to 2001 was already 265 square km,leaving only 10 square km of land available for further development within the city limits.
Data sourced from the Indian Institute of Human Settlements (IIHS) based on the latest Census 2011 figures,however,reveals that while the population of Mumbai has increased by around 15 per cent from 2001 to 2011,the built-up area has gone up by over 35 per cent. The population density,according to the 2011 census,shows there are 20,694 people per square km of the city,making it the most populated city in India and seventh of 10 most populated urban agglomerations in the world.
The Development Plan documents this shift in land cover primarily from coastal wetlands (from 29 to 19 per cent) and forest/agricultural lands (from 32 to 19 per cent).
As a result of this excessive concretisation and ultimate increase in the impervious surface,the infiltration of rain into the earth has been significantly arrested along with the quantum of evapotranspiration from grass cover and tree cover.
IIT Professor Kapil Gupta,who is also a member of the UNESCO international working group on sustainable water strategies,says these conditions lead to a rise in the urban island heat effect,and incidence of rainfall. In green areas,trees have a cooling effect that helps lower the temperature of surrounding areas by 3-4 degrees Celsius. Cutting off trees reduces this cooling effect and also the amount of infiltration of water into the soil. Concrete structures radiate a lot of heat and raise the temperature in the built-up areas by about 4 to 5 degrees Celsius. This warming is further enhanced by vehicular emissions, he says. The urban heat island effect has also been studied by the US National Aeronautics and Space Administration (NASA),which found that it led to an increase of precipitation by at least 50 per cent within and around urban areas,Gupta adds.
Data from the BMCs disaster management cell shows that since 2004,the average number of days of rainfall at around 65 mm has risen from around 10 till 2008 to around 15 days. The average number of high-tide days has similarly risen from around 20 days to now roughly 24 days over the last four years.
The situation of Mumbai is unique. No other mega city in the world receives as much rainfall in as little a span of time as Mumbai,so it is not possible to implement the strategies of other mega cities of the world, Gupta says.
The citys underground drainage system is still what
Britishers planned over a
To date,it is equipped to handle only
265mm of rainfall per hour.
While unbridled urbanisation in Mumbai has proceeded at a remarkable pace,the citys underground drainage system is still what the Britishers planned over a century ago. To date,the citys drainage system is equipped to handle only 25mm of rainfall per hour.
Proper drainage is crucial for for long life of roads. While there are many operational reasons for poor-quality roads,bad drainage causes the premature failure of roads. It is well known that the rate of road deterioration increases if the water content of the granular material increases. This failure is caused due to several reasons,like increase in moisture content,decrease in strength,mud pumping,formation of waves and corrugations,stripping of bitumen,cutting of edges of pavements,frost action etc, says Professor Jalindar Patil,head of the civil engineering department at the RMD Sinhagad School of Engineering,Warje.
Though the civic body has undertaken the widening and deepening of four rivers,the citys natural drains,in the suburban part of Mumbai,the complex underground storm water drain network of about 340 kms in the island part of the city,a naturally low-lying area,poses a problem. There is no scope in the island city to expand the drainage network. So,we have focused on rehabilitating old drains in dire need of repair,installing more dewatering pumps here and properly cleaning the system. The carrying capacity will still remain at 25mm of rainfall per hour but we are trying to ensure that at least this much will be drained off effectively, says L S Vhatkar,chief engineer of the BMCs storm water drains department.
So far,the BMC has overhauled about 18 kms of the island citys drainage network in need of urgent repairs. Work on 64 kms of crumbling drains is still pending.
In addition to repairing its drains,the BMCc pet project,the Brihanmumbai Storm Water Drain (BRIMSTOWAD),is expected to double the drainage capacity of the city. Four of the eight pumping stations constructed under this project are dedicated to the island city alone. They are located at Haji Ali,Love Grove,Cleaveland Bunder,and Britannia. Three other stations Irla,Gazdarband and Mogra are situated in the western suburbs while a pumping station at Mahul has been planned for the eastern suburbs. When these are completed,the drainage capacity of Mumbai will be raised to 50 mm per hour. Two pumping stations at Irla and Haji Ali are already operational and have reduced waterlogging in the area significantly, Vhatkar said.
According to the corporations conservative estimates,once BRIMSTOWAD is fully operational,it will reduce the number of flood days to two times during one monsoon season provided rainfall does not exceed 50 mm per hour and the height of waves during high tides are not over 4.5 metres.
However,a fact-finding committee report on the 2005 deluge,led by Dr Madhavrao Chitale,says that while this may be adequate for minor roads in the city,the civic bodys Rs 3900-crore panacea still falls short. Important roads must be kept flowing even under lesser rainfall intensity to avoid serious traffic congestion and to ensure quick evacuations from critical areas such as schools and hospitals. For more severe conditions,contingency plans will have to be kept ready and acted upon, Chitales report says.
A senior civil engineer who has previously worked with BMC and who is a member of the standing technical advisory committee (STAC),an advisory body to the civic bodys roads,traffic and bridges departments,says,Merely constructing bigger drains is not enough. The civic body has to ensure that the drainage system is cleaned systematically through the year,especially since annually over Rs 100 crore is spent on the exercise.
In addition to poor quality de-silting,the civic body in its flood preparedness guidelines has noted that man-made problems such as land reclamation and encroachments along the four rivers – Mithi River,Dahisar River,Poisar River,and Oshiwara River – aggravates the problem. …reclamation of the riverine wetlands… encroachments inside the riverbed as well as on the banks and holding ponds,have choked and constricted the water courses and aggravated flooding risks, read the guidelines.
Concretisation in Mumbai has shrunk open spaces that have
as holding ponds for water.
In view of the rapid concretisation in Mumbai,open spaces have correspondingly decreased. Apart from the cooling effect of green open spaces,these plots function as excellent holding ponds for water that help increase Mumbais groundwater stores and prevent water pressure on roads. Parks such as Oval Maidan,Shivaji Park,Rani Baug etc,created with this purpose in mind,serve as good water detention pools during the monsoon season.
Chitales fact-finding report,however,says the citys capacity for soil-infiltration has severely reduced. Concretisation has led to an increase in the cover of impervious surfaces in Mumbai thereby restricting the percolation of water in the soil.
Chitales report observed an increase in the run-coefficiency from 0.5 to 1 since as early as 1970s. This means that while earlier,50 per cent of the rainfall fell on impervious surface and was flushed out of the city through the drainage network,the rest 50 per cent percolated in the soil. The new co-efficient of 1 means no rainwater percolates at all now. It either runs off into the citys already weak drainage system without any soil infiltration or is retained on the roads,causing their early ruin.
In the national guidelines on urban flood disaster management,we have recommended that parks and stadiums be constructed in low-lying areas of a city. These holding ponds could be integrated in the landscaping in such a way that is even pleasurable for people to visit the parks and sit by the ponds. This will curb flooding and damage to property, Gupta says.
Drawing the example of two holding ponds created along the upstream reaches of the Mithi River,he says,There has not been any major incident involving the upstream Mithi river despite heavy rainfall. However,the downstream river may face problems from time to time because of tidal water flow from the Arabian Sea,especially during high tide days.
In a 2012 study on open spaces conducted by the Mumbai Metropolitan Region Environment Improvement Society (MMR-EIS),it was found that of the total of 3,246 existing and proposed open spaces in the city as marked in the DP,835 spaces or 26 per cent are occupied by structures and hence are not in use as open spaces. The study further found that 756 open spaces,roughly 23 per cent,are not developed and are lying vacant.
While forming the DP,the BMC had expected the residents of some of these areas,which include temporary slum settlements and dilapidated cessed buildings,to be rehabilitated within 20 years,thus freeing up land to be converted into open spaces, says Neera Adarkar of Adarkar Associates,which had conducted the survey for MMR-EIS,adding that the vacant open spaces already function as holding ponds since construction is not allowed based on the land reservation category.
If the BMC acquires its open spaces in low-lying areas on a priority basis,they would make for great holding ponds in the monsoon season. With proper land-levelling,not only can these spaces help the rain infiltrate the soil,but these will also help harvest water that can be later used, she says.
BMCs new mantra is
concretisation of roads,which it thinks will reduce water
retention on roads.
To date,it has completed concretisation of
574 kms of the roughly 1950 kms of road network under its care in Mumbai.
Increased water retention on roads caused by the run-off coefficiency of 1 and a drainage capacity for 25 mm of rainfall per hour has led the BMC to believe that concretisation is the best solution for better roads in the city. To date,the corporation has completed concretisation of 574 kms of the roughly 1950 kms of road network under its care in Mumbai.
Concretising roads is the best solution for Mumbai. The sub-base for construction of these roads is already of good enough quality. These roads can withstand damaging factors for a longer period. Apart from quality issues,trenching work carried out haphazardly by over 35 different utility agencies,leakages from underground water and sewerage mains,and bad water drainage are the causes of road damage. While BMC is taking up the projects of ducting the underground utilities and revamping its water and sewerage mains,these will take some time. But a good drainage system is absolutely essential to complement the construction of new roads, the STAC member said.
BMC made rainwater harvesting mandatory for all new buildings with an area of over 1000 square metres.
it extended the rule to buildings over
300 sqm. But the problem remains with
A lesser known answer to the citys drainage problems and by extension,its pothole problems,is rainwater harvesting – where the water cannot be drained,measures should be adopted to contain it. Watersheds not only aid eco-friendly conservation efforts but also,in view of the run-off coefficiency of 1,curb water holding on roads.
A UK study has found that 1000 litres of water stored in rooftop or ground level tanks can reduce overflows to the roadside drains by 40-50 per cent. In Portland,Oregon,USA,providing pervious cover on the rooftops of new constructions to compensate for the loss of green cover on the ground has been made compulsory and it has been found this is an effective way to sustain the groundwater recharge, Gupta says.
In 2002,BMC made rainwater harvesting a condition for all new buildings with an area of over 1,000 square metres to obtain completion certificates. In 2007,it extended the rule to buildings over 300 square metres. However,so far,the civic body has yet to show a serious inclination for implementing the rule. Latest BMC figures show the rule has been followed in 42 buildings in the island city,594 in eastern suburbs and 2,530 buildings in the western suburbs.
A study published by the Observer Research Foundation,released in May at the hands of Mayor Sunil Prabhu,has reported that most new buildings do not have rainwater harvesting facilities.
The violation is more rampant in the city and the eastern suburbs,while the western suburbs have an implementation rate of around 52 per cent, the study says. The report,prepared by Rishi Aggarwal with Janki Pandya,claims the members of the BMC rainwater harvesting cell were unresponsive to sharing implementation data. The cell is short of manpower and needs to improve qualitatively and quantitatively, it says.
A senior civic official on the condition of anonymity says,The BMC is more concerned with improving its own water supply network and working out the other water-related problems. While water harvesting is important,we are not as stringent because the civic administration does not view it as essential. Generally,as a concept it is seen as more viable in the upper-middle class sections of the city.