November 22, 2021

Introduction

Floods are common phenomena in the world, resulting in widespread damage to property and life. Since time immemorial man has been trying to control floods, its need in the modern times has become acute because of the frequent floods. Villages, towns and cities situated on the banks of rivers suffer because of floods. In addition to damage to life and property flooding can lead to secondary consequences as long-term displacement of residents and creating increased spread of waterborne diseases.

Floods are one of the natural calamities that India faces every year in varying degree of magnitude. In India, there are many large rivers like the Ganges and the Brahmaputra which flood every year and destroy lives and property. The tributaries of these rivers originate in the Himalayas which bring a lot of water. There is a feeling of helplessness and hopelessness that nothing could be made to control the floods. Similarly, cities like Mumbai and Chennai flood every year. The present paper tries to understand the causes of floods and tries to give remedies. Flood control mechanisms have to be different to rural areas and to urban areas. A note to the readers that this is the initial thinking of the author and lot of research has to be done on the ways suggested by the author to control the floods.

Causes of Floods

Flood refers to an outflow of enormous water in a place and the situation is caused when the water becomes uncontrollable. Generally there are five types of floods. They are: river flood, coastal flood, storm surge, inland flooding and flash flood. A river flood occurs when water levels rise over the top of river banks. A coastal flood is the inundation of normally dry land areas along the coast with seawater. Storm surge is an abnormal rise in water level in coastal areas over and above the regular astronomical tide. An inland flood is flooding that occurs inland and a flash flood is flooding due to heavy rainfall which occurs between 3 to 6 hours. Massive rainfall, overflowing of water, collapsed dams; snowmelt, deforestation, climate change, storm surge, tsunamis and clogged drains are the important causes of floods.

Flood Control Mechanisms

Since ancient times, some methods of flood control have been practiced. These include planting vegetation to retain extra water, terracing hillsides to slow flow downhill, and the construction of flood ways. Since flood control activities are massive and varied, we are examining the flood control mechanisms for both rural areas and urban areas. Since the causes of floods are different and since physical structure of cities and villages are different, different flood control mechanisms are suggested.

Rural Flood Control Mechanisms

India is home for numerous rivers and among them Ganges and the Brahmaputra are the biggest. These rivers bring with them a lot of water specially during monsoon and every year people suffer due to floods. Floods in the Ganges are due to the silt it carries. The Ganges is one of the highest sediment load carrying rivers. The silt deposition is said to have raised the river's bed-level causing it to break embankments and flood the adjoining human settlements and farmlands.

One of the methods of controlling floods is to build embankments on the sides of the rivers. Though embankment has been successful in many instances, there is increasing evidence that they have made little difference to floods which have increased in fury and last longer. Various governments have found it hard to maintain the embankments and any breach in them would frequently offset benefits accrued over past many years in one go. Further, many governments or institutions have learnt the bitter lesson that when a heavily silt laden river is embanked, the sediment gets trapped within the embankments leading to successive rise of the riverbed necessitating raising of the embankments. In many instances, as a consequence of embankments the height of the river has become more than the adjoining agricultural lands. In such instances, these lands get flooded due to seepage of water and overflow of the embankments.

1. Research on the Flow of Water

With the rains a lot of runoff water ends up in creeks, streams, and rivers, flowing downhill towards the oceans. Stream flow is always changing. Rainfall causes rivers to rise, and a river can even rise if it only rains very far up in the watershed. Most of the rivers in India overflow during monsoons causing widespread damage to crops, property and lives. During these floods, the rivers carry a large amount of silt and sand. The sediment deposited on the riverbed impedes its flow, so it frequently erodes its banks and carves a new channel. For example, the Kosi River is known as the ‘Sorrow of Bihar’ as the annual floods affect about 21,000 km2 of fertile agricultural lands thereby disturbing the rural economy. It has been changing its course whimsically. So, proper research is required regarding the flow of water in rivers, the amount of silt carried, effects of flooding, change rivers course etc and proper solutions to be found to the problems of flooding.

2. Generous Sand Policy

For thousands of years, sand and gravel have been used in the construction of roads and buildings. Today, demand for sand and gravel continues to increase due to urbanisation and the need to have better homes. So, in order to build our modern cities requires massive amounts of sand for cement and other uses. Sand mining activities in rivers can have positive impacts. The positive impact comes from the availability of employment for local people, encouraging the local economy and encouraging development on a broader scale. At the same time, sand excavation is important along the river course, otherwise with the continued sand deposit there can be river bank erosion or floodplain problems. Sand mining lowers the riverbed and increases its water storage capacity, thereby, effecting physical and biological environments of the river system. Secondly, if we do not mine the sand deposits in the river bed then finally will be deposited on the sea bed. So, sand mining to a safe depth is advantageous.

Since the advantages of sand mining outweigh the disadvantages, every state should have generous sand policy. With this every state government can increase their revenue through the sale of sand and the construction of houses, roads, bridges, nay towns and cities can be built easily. In this process, the mining operators, in conjunction with cognizant resource agencies, must work to ensure that sand mining is conducted in a responsible manner.

3. Deepening the Rivers

Though stopping rivers from flooding is not completely possible but we can do a lot to mitigate flooding through de-siltation of the rivers. Siltation is a natural phenomenon which is very bad for the rivers and many of the inhabitants living on either side. The build up of silt will lead to a reduction in water depth of the watercourse. Silt removal can be done through various ways as through earthmoving machinery, silt dredging and silt pumping works. When the area is small as in small river earthmoving machinery can be used and if there is water, floating excavator can be used. When the river is broad and when lot of silt has to be removed can be done through dredges. The dredged silt can be transported through barges or pumps. However, deepening the rivers should not be done near the bridges because during the heavy flow during the monsoon can destabilize the pillars of the bridge.

The most significant problem to overcome with silt removal is what to do with the material. Removed silt can be used for the creation of platform to construct the elevated villages. It can also be used to construct elevated roads which will connect the elevated villages. It can also be used to raise the fields. Silty soil is considered to be one of the most fertile of soils. It is composed of minerals and fine organic particles. In India, silty soil is majorly called alluvial soil that is found near the river valleys. The silts collected by the water of the rivers enrich the soil bed and that’s why the name is silty soil. This soil is rich in potash and the plants or crops grow well on this soil.

Below is a case study of deepening of the Kajali River near Sakharpa village in Maharashtra. Due to desilting the river, the flood prone village became flood-free.

Lessons from Group of 5000 Who Made Their Village Flood-Free

With the help of Naam Foundation, the village of Sakharpa has carried out de-siltation work to restore the course of the Kajali River in Maharashtra to become flood-free for the first time in 70 years. In 2019, for the first time in 70 years, Sakharpa and Kondgao did not flood, despite the incessant rain and a river flowing between the two.

River Kajali flows through the middle of the Sakharpa village, and in 1948, similar heavy rainfall and floods were experienced in adjacent villages. The result was so devastating that the course of the river changed. Moreover, the river is a confluence of Kev and Gad rivers, which meet at the entrance of the two villages. The massive confluence resulted in silt deposition in the river, narrowing its channel and water carrying capacity. The change in the course of the river and silt deposition contributed to flooding of the village market area and caused widespread damage.

A survey was conducted for the de-siltation of the river for a distance of 1-km stretch of the riverbed. However, the expenses came to around Rs 24 lakh, and the gram panchayat had no money for the task. So, the villagers contacted Naam Foundation. With the help of Naam Foundation and other NGOs the work of desilting started with the help of earthmovers and other machinery. As a result the river width was increased to 60 metres and depth to 4 metres. The work done on the river showed results, as the village and marketplace did not flood despite incessant rains in July 2021. It was a big achievement for the villagers.

4. Elevated Villages

Various solutions have been experimented to solve the floods. In the Netherlands, there are floating villages where people live in house boats. In addition, the developers decided to ‘test the waters’ with a floating neighbourhood made up of two-, three- and four-storey floating homes connected together and linked to the land by a network of floating walkways. People of Golaghat district, one of the worst affected regions in Assam, have been building elevated bamboo houses to escape the floods. The author has witnessed man-made highlands in the Kaziranga National Park which have turned into islands of relative safety for the park’s animals as large parts of Assam remain inundated by flood waters of Brahmaputra.

As we have seen earlier deepening the rivers is the best solution to increase the capacity of water the rivers disperse out into the ocean and thus to solve the problem of floods. The tons of silt removed through dredging can be used to construct elevated villages. For the construction of elevated villages a plot equivalent to the existing village in area has to selected/acquired and this plot has to be raised by filling it with the silt dredged from the river at least by10 feet from the existing ground level. Since the process of raising the level of the ground takes place in summer, during the monsoon this
platform can be used for providing temporary accommodation and for providing relief measures in the initial years. Care has to be taken that this silt does not get eroded and this can be prevented by planting trees on the sides or covering them by gravel or stones. During subsequent years, the people can be encouraged to build new houses on the elevated platform by providing financial incentives. The physical structure of the village has to be planned. The village panchayat can oversee the work of allotting plots for the villagers and providing public utilities such as drinking water, electricity, drainage, toilets etc.

5. Elevated Roads

Elevated roads are effective against flooding because they are normally situated higher than the floodwaters reach. Elevated roads on top of a bank of a river are cheaper to construct by using the silt dredged from the river. The author has also witnessed people taking shelter by the sides of National highways during floods in Bihar because the height of the National highways is higher than the surrounding lands as a result they do not get flooded. As suggested earlier deepening the rivers is the best solution to flooding. After constructing the elevated villages/platforms remaining silt can be used to construct roads connecting elevated villages/platforms with the National Highways. In this manner all weather roads can be constructed.

While building these elevated roads care has to be taken that road crossings should have sufficient bridge openings and adequate culvert capacity so that roads do not disrupt drainage patterns. Roadways drainage systems must be updated accordingly when they are elevated. It is also important to routinely maintain roadways and waterways because debris can build up along bridges which adversely affect water flow and can lead to damage of surrounding structures and roadways. While constructing elevated roads, there may be rivulets which join the rivers. In such instances, the floodwaters of the rivers may enter the rivulets and flood the area. In such cases, flood gates can be constructed on the mouth of the rivulets so that flood waters do not enter the rivulets. After the flood water subsides in the rivers, these gates can be opened.

6. Building the Retaining Walls

River bank erosion is common during floods. In this process, fertile soil gets eroded and travels downstream. There it settles in a river potentially leading to floods. While river erosion is a naturally occurring process, human impact can increase its rate. Common contributing factors to river and stream bank erosion include clearing vegetation away from the river bank, flooding, stream and land use management and river redirection or debris in the channel.

While almost all rivers will have some amount of healthy river bank erosion, unstable rivers that have large amounts of erosion are cause for concern. Not only does river bank erosion impact the area where the erosion is happening but it can also affect life down the river. Erosion sends extra sediment downstream, which can change the river’s course. At the site of the erosion, issues can include loss of businesses and farmland, which can bring economic hardship to the area. Additionally, erosion can lead to homelessness and migration if people are forced to move. The most significant problem with river bank erosion is mass failure, which occurs when an entire section of the river bank collapses into the river.

There are two main approaches for river bank stabilization. Softer approach is bioengineering and vegetation. The simplest and most natural way of prevention of soil erosion is through planting vegetation. Erosion of soil from barren land is more; hence the soil must be kept covered by growing grass and planting trees on it. Additionally, plants can act as shock absorbers during heavy rainfall, which also slows the rate of erosion. Hard approach includes riverbank riprap and retaining walls. The protection of river banks in mountain rivers by riprap requires large blocks with weights typically exceeding 1 ton. These blocks have to be placed individually. Retaining walls are an amazing piece of engineering that helps keep the soil on a slope from falling into the river. They help prevent soil erosion and destruction of plant life. Retaining walls can be built with locally available stones either granite or laterite or by RCC. Further, the walls could be erect or sloppy. If they are sloppy there is less danger of wall collapsing. Sloppy walls can also be built in the form of steps which are long lasting.

Urban Flood Control Mechanisms

Flooding in the cities and the towns is a recent phenomenon caused by increasing incidence of heavy rainfall in a short period of time. Floods are caused because urban areas are covered with roads, concrete pavements and buildings which prevent seepage of water. Further, floods are caused by indiscriminate encroachment of waterways and wetlands, inadequate capacity of drains and lack of maintenance of the drainage infrastructure. As the urban population increases, so does the pressure on natural hydrological systems. Lakes and rivers are encroached upon and turned into apartment complexes, transit centers and sport stadiums.

India needs to prepare for urban flooding because rural to urban migration takes place at a rapid pace. This leads to crowded neighbourhoods, buildings being built haphazardly without much regard to planning where open, fertile, green, and wooded areas are replaced by impervious land cover like roads, concrete pavements and buildings which increases the risk of floods in urban areas. The following table gives the rate of urbanisation in India from 2010 to 2020.

Above table shows that more than one third of the total population in India lives in cities and the trend shows that the percentage increase of urban population goes on increasing. Taking the estimated population of India in 2020 to be 138 crores, every year urban population is increasing by 63 lakhs. Providing housing, sanitation and other necessities is a stupendous task. It also adds to the problems cities and also to flooding.

The towns and cities situated on the banks of rivers suffer because of floods. They cannot be shifted like the villages. So, they require separate methods of flood control. Following are some of the important urban flood control mechanisms.

1. Rain Water Harvesting

Rainwater harvesting can be considered as a major flood mitigation. Rainwater harvesting is the collection and storage of rain, rather than allowing it to run off. Rainwater is collected from the roof and other impermeable surfaces and redirected to a tank, cistern, deep pit, aquifer, or a reservoir with percolation, so that it seeps down and restores the ground water. Rainwater harvesting systems are used to address water shortages and urban flooding. Rainwater harvesting is regarded as one of the best available ways for sustainable water supply and also effective to recover natural hydrological cycles in urban area. Rainwater harvesting systems provide a source of ongoing water supply and reduce reliance on other water sources. Below, is a case study of rain water harvesting undertaken in Tamil Nadu.

Rain Water Harvesting in Tamil Nadu

Tamil Nadu is the first state in the country to try successful rain water harvesting. The rain water harvesting movement launched in 2001 was the brainchild of the then Honourable Chief Minister, Jayalalitaa. It has had a tremendous impact in recharging the groundwater table all over Tamil Nadu. Amendments made to Section 215 (a) of the Tamil Nadu District Municipalities Act, 1920 and Building Rules 1973, have made it mandatory to provide rain water harvesting structures in all new buildings. To consolidate the gains, various measures have been taken up for rejuvenation of rain water harvesting structures created already in both public and private buildings, besides creating new ones. Chennai is the first city in the country to have successful rain water harvesting.

2. Percolation Wells

In urban areas, large areas of ground are covered in concrete and tarmac. Rain is channelled straight into drainage systems, which can become overwhelmed. In order to prevent floods, similar idea to the rainwater harvesting is digging of percolation or recharge wells. In a large building compound a small well of around 3 ft diameter can be dug to a depth of 20 ft. If the walls of the wells are not strong, small concrete rings with pores can be inserted. Such a well should be closed with a thick perforated lid. It is better to dig the well in the corner of the compound all rain water should be made to flow into this well. Such a well will absorb and retain a lot of water. In summer water could be pumped out of it for washing the cars and for gardening. If the compound is large multiple percolation wells could be dug.

In the UK, the Flood Act of 2010 obliges builders to landscape developments so that water from roofs and driveways seeps into open ground rather than rushing into the water system. Guidelines suggest the ground should be able to absorb the water - a process known as infiltration. Creating a sponge city is another similar plan to prevent floods in urban areas. A sponge city is one that can hold, clean, and drain water in a natural way - using an ecological approach. Rather than funnelling rainwater away, a sponge city retains it for its own use, within its own boundaries. The uses include: irrigating gardens and urban farms, recharging depleted aquifers, replacing or replenishing the water used to flush toilets, and processing it so that it can be clean enough to use as drinking water.

3. Methods of Improving Drainage

Designing and constructing drainage systems require expert advice from engineers to make sure that water flows away quickly and smoothly. The detailed design of storm water drains should be carried out by engineers and take into account climatic and hydrological data. Stormwater drains should be designed to collect water from all parts of the area and lead it to a main drain, which then discharges into a rivulet or local river. Storm water drains are best constructed using a concrete lining. Earth drains are more likely to become clogged and overgrown, and cause problems with storm water flow during minor floods. Bottom curved drains or ‘U’ shaped drains are better compared to the drains which are levelled at the bottom. U shaped drains will facilitate smoother flow of water and cleaning. The drains must also be properly maintained and cleaned.

It is also essential to dispose of the sullage which consists of waste water from household sinks, showers, and baths. The sullage should not be connected to storm water drains because it can lead to the formation of stagnant pools and result in breeding sites for disease vectors, such as mosquitoes, increasing the risk of malaria, and other diseases.

Other Methods

Other methods include that can prevent flood in an urban area are: rooftop gardens, installing water infiltration, keeping the sewer system clean and permeable pavements. In fact, UK has developed a new concrete which can absorb water very fast i.e. it can absorb 1,000 litres of water per square meter in a minute.

Conclusion

Floods are one of the most disastrous acts of nature and impact human life in multiple ways. Both rural and urban flooding is a perennial problem in India and requires different solutions. Damages by floods in rural areas are more severe compared to urban counterparts due to poverty, limited infrastructures and access to resources. Carrying out rural flood control mechanisms such as deepening of the rivers, building elevated villages and roads require a lot of resources. Every year, India spends crores of rupees for mega projects as Express ways, Train freight corridors, Central vista project, Industrial corridors, Smart city project etc. Urban flood control mechanisms can be taken up with smart city projects and rural flood control method can be taken up as a major mega project because lot of money has to be allotted for the rural flood controls.