WHY DOES MUMBAI FLOOD EVERY YEAR (2)

HUMANITIESSEM5 Editedby, HarshiBhandari21109 GauriPanhale 21149 AashishUlhas 21153 OwaisShaikh 21176

WHY DOES MUMBAI FLOOD EVERY YEAR? Harshi Bhandari 21109, Gauri Panhale 21149, Aashish Ulhas 21153 & Owais Shaikh 21176 ABSTRACT The change in rainfall pattern and intensity is becoming a great concern for Mumbai. Many parts of the city are experiencing extreme rainfall events such as experienced on 26th July 2005 in Mumbai, India. For the appropriate design and planning of urban drainage systems in an area, Intensity Duration Frequency (IDF) curves for given rainfall conditions are required. The aim of the present study is to formulate a conclusive response to the rainfall flood we will be dividing our analysis into 3 main topics (a) Background (factual information about rainfall intensity, flood events, losses, etc in recent decades) (b) The causes of flood events (c) The possible ways to overcome the problem Maharashtra, India. Observed rainfall data from 1901 pertaining to Colaba and from 1951 of the SantaCruz rain gauge stations in Mumbai are used in the present study to derive the Crowd source data, with the help of this data we can arrive on an alternative and which are compatible even with the extreme rainfall of 26th July 2005 in Mumbai. Keywords ; Rainfall, Intensity, Duration, Return Period, Frequency, Flood Warning, Urban Flood BACKGROUND Flooding impacts millions of people globally each year and causes significant social and economic harm. Due to the intricate interaction of hydrology, human activity, and climate, floods are becoming more severe and frequent in most regions of the world at an alarming rate. Floods are divided into different types based on the mechanism of creation, as well as the spatial and temporal scales. Urban flooding is a particular kind of pluvial flood that happens when an urban area's drainage capacity becomes saturated. These floods can be brought on by a number of factors, including excessive rain, river overflows, coastal storm surges, sea-level rise, flash floods, groundwater seepage, sewer overflows, a lack of permeability, and poor city management. Extreme precipitation has increased due to global warming, and it will only become worse, following is a graph of extreme rainfall events in the years 2017-2021 with rainfall.

Figure 1: Extreme events in the years 2017-2021 with rainfall>150mm Due to the heavy rainfall events there have been many losses throughout the decade, which lead to infrastructure loss as well, the total direct losses have been recorded by the BMC since july 2005 to present day also the future (RP = Return Period)

Upper estimation of total losses (direct+indirect, including loss in housing services) due to various types of events in present-day and future conditions Also looking at the Modeled 'affected' exposures for different return period flood events for the Mithi Basin, in comparison to the simulated July 2005 event Area Population affected Affected exposure Mithi Basin CAUSES OF THE FLOOD EVENTS 1. Rising Sea Levels: Heavy rains to a magnitude of more than 240 mm are almost of a regular occurrence in Mumbai at the onset of monsoons. Urban areas get flooding by different types of floods like river floods, flash floods, coastal floods, release of excess water from reservoir or failure of dam on the upstream side, increase in intensity of rainfall, blockage of drainage system due to silting, dumping of waste material at the inlets of drainage, encroachment over natural drainage, water bodies etc. 2. Extensive Reclamation and Faulty Zoning Regulations: It is evident from the city’s planning history, that in the process of housing construction and setting up industries, the waterways that allowed the accumulated rain water to drain out, have been drastically reduced. The reclamation that was carried out originally only to link the seven islands of Mumbai was eventually performed to a greater extent to accommodate the ever burgeoning population of the city. Large slum colonies as well as planned constructions are being developed on the land reclaimed from the sea.

3. Mangroves that dotted Mumbai’s coastline have largely disappeared. They once served as an important component in absorbing water thereby, mitigating the impact of torrential rain. A network of storm-water drains, concrete embankments is being created along nallahs and pumping stations near the mouth of estuaries. Which has resulted in the narrowing of the width of streams and estuaries, shifting of high-tide lines and consequent loss of the mangrove ecosystems. While the project’s pumping facilities do eject large quantities of water into the sea, in some cases, they have been seen to shift flooding to other areas – the case of Juhu Moragaon fishing village is a notable example. 4. Incapable Storm Water Drains : The storm water drainage system of Mumbai was built largely in the days of the British Rule in 1860, when the population of Mumbai was merely one-tenth of what it is at present. This antiquated storm water drainage system is capable of handling rain intensity of 25 mm per hour at low tide. If the rain intensity exceeds 25 mm per hour and a high tide occurs, there is always a possibility of inundation. The city’s existing drainage system is designed to tackle a rainfall of normal intensity, with the assumption that there are no significant solids deposits in the drains. Most of the drains throughout the city have been found to be occupied by a substantial amount of garbage and other solid deposits. The resulting decrease in the capacity of the city’s storm water drainage system has been proved by the disastrous effects of the inundation that hit the city of Mumbai on 26th July 2005. 5. Reduction in the catchment area of Mithi river: The Mithi rivers catchment area being reduced due to extensive reclamation of land contributes a lot towards the flooding along its path. Path of the Mithi River being changed due to CSIA’s runway: The runway for CSIA (Chhatrapati Shivaji International Airport) has been elongated over the path of Mithi River which changes its path. During high rainfall the water runs down with great pressure. Instead of changing its path it climbs above the land surface and penetrates the city causing floods. There are a few measures put out by the MCGM already, and we think those measures should also be considered and acted upon. METHODS IMPLEMENTED BY THE MCGM: 1. The BRIMSTOWAD project which was started after the 26th July, 2005 disaster which included the following: 1. Increasing the capacity of the 'Storm Water Drains'. The BRIMSTOWAD project consisted of widening the storm water drains to a capacity of 50mm per hour with a run off coefficient of. 2. Setting up of Storm Water Pumping Stations at the SWD outfalls: As the problem of high tides arose the BRIMSTOWAD committee designed a plan for setting up pumping stations which discharged water by gravity flow during low tides & pumped the flood water with a capacity of 6 meter cube per second, during high tides.

THE POSSIBLE WAYS TO OVERCOME THE PROBLEM Here are a few ways that we think can help to overcome this city flooding issue- Stormwater wetlands There are small, easily implementable urban design and planning solutions that can reduce flooding. Simply ensuring that residential and commercial buildings are a few inches higher than the road Wetlands have been around for a long time but urbanization destroyed them. Researchers at the Massachusetts Institute of Technology have found that wetlands, which provide water treatment, flood protection, carbon storage, habitat, recreation and aesthetic value, can be constructed anew keeping desired hydraulic flow and ecology in mind. PLANTING AND PRESERVING OF MANGROVES: The previous storm water drains were designed taking into account the number of mangroves that were present in Mumbai they absorbed most of the water from the rains in the past but now-a-days Land reclamation over the mangroves has reduced the water absorption into the ground. Due to this concretization, water does not penetrate in and the soil isn’t able to absorb the water and as a result it remains stagnant over the land creating situations of flood. So there is a need to protect these mangroves and to replant them and strict rules should be made for their preservation. CLEANING OF MITHI RIVER: The first and foremost step is to clean the Mithi River. Mithi River being one of the major part of the city’s storm water drainage system is full with contaminants such as plastics, garbage from houses along its banks, effluents from commercial buildings such as the BKC along its banks, and many more. Due to this contamination, it is not possible to drain out the water during excess heavy rains and as a result flooding occurs. Upgrade flood warning and forecasting measures to “nowcasting”(Nowcasting is weather forecasting on a very short term period of up to 2 hours). Developing and implementing efficient early warning systems can help authorities and residents prepare for potential flooding events and take necessary precautions in advance. Create public awareness about warnings, teach people how to react and ensure self help grooming, collaborate with NGOs. Educating the public about responsible waste disposal, not blocking drains, and other flood prevention measures can lead to a more proactive approach to dealing with the issue. Natural Water Rejuvenation: Restoration and preservation of natural water bodies like lakes, ponds, and marshlands are essential to ensure water can be absorbed and retained during heavy rainfall, reducing the burden on the city's drainage system. resettle the encroachment, enforcement of rain water harvesting etc. to moderate the discharge. The storm water drain department should look into Installations of box drains , mini pumping stations, and micro tunneling to ensure rain water accumulation does not take place.

Regular Maintenance and Cleaning: Regular cleaning and maintenance of stormwater drains, culverts, and other infrastructure are essential to ensure unobstructed flow of water during monsoons. Sensible land use planning can prevent construction in flood-prone areas and encourage developments that are more resilient to flooding. Upgrading critical infrastructure like roads, bridges, and embankments to withstand heavy rainfall and flooding can significantly improve the city's resilience. BORES ALONG WITH THE STORM WATER DRAINS: As the work of increasing the size of storm water drains is going on, along with it, pile holes should be bored so as the water from the floods will enter the bores and will meet the groundwater table. This would prove to be helpful when the tide level will increase above the expected and the pumping stations would fail to pump the water out. PERCOLATION TANKS: It is said that Mumbai doesn’t has its own water source and the water to Mumbai comes from Nashik, if a percolation tank is designed and the flood water is diverted to it by means of gravity it could serve two purposes of providing a water source during scarcity of water and it will also reduce the flood impacts.

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