Guwahati is the gateway to the North Eastern part of India. It is the biggest city of the state of Assam. It is located at 26.1445° N latitude and 91.7362° E longitude and covering an area of 216 sq. km with a population of 1.91 million (2016). The population density of the area is 4400 persons per square kilometre. Guwahati lies between the banks of River Brahmaputra and in the foothills of the Shillong plateau.
Water logging is a crucial problem in Guwahati and certain areas of the city get inundated within a very short duration of rainfall. Rapid urbanization rate with increased number of residential apartments, commercial buildings, streets and various other establishments have tremendously increased the percentage of sealed areas compared to the open spaces. This has resulted in lesser infiltration of water into the soils. Moreover, in order to accommodate the increasing population of the city, unplanned urban expansion takes place which ultimately lead to severe encroachments in the hills, low lying areas, wetlands and reduction in the forest cover. As a consequence due to the cleared hills and loss of wetlands, artificial or flash floods and water logging are frequent in the area. Several areas of Guwahati get waterlogged by flash floods due to heavy rains which disrupt the normal life. The drains in the area get silted up with silts carried with storm water along the hills and flooding the streets and other neighbourhoods.
The purpose of this study is to identify a possible solution considering the present scenario of Guwahati area and working on the development of a sustainable urban storm water sewer system that would be sufficient enough to accommodate the whole of wet weather flow during the heavy downpours and the rainy season at large. This study also focuses on ways to mitigate the undesirable consequences from the unplanned construction of sewage systems and on ways to prevent flooding in the area by application of various new techniques.
The objective of this study are:
1. To identify the potential reasons for flooding within the city
2. To find out ways to reduce the flooding and utilize the water for solving the problem of water scarcity in certain localities.
3. To propose feasible solutions to prevent the problem of water logging in the city of Guwahati
The area under study is Guwahati. It is located at 26.1670° N latitude, 91.7730° E longitude. The area is plain with elevation ranging from 45 m to 55.5 m high above mean sea level.
The plains area comprised of silt, sand, clay and red clay. The soil in Guwahati are a mixture of non-cohesive material grains like quartz, feldspar and mica, and lump of stone. ( Saikia and Saikia 2004)
The climate of Guwahati has a wide range of temperature which generally varies between the months December and January having the minimum temperature to June and July having the maximum temperature. The humidity in Assam is higher and falls next to Arunachal Pradesh and Meghalaya. The relative humidity ranges between 50% to 94%.
The average annual rainfall in the state of Assam is 2818mm whereas in Guwahati city, the average annual rainfall is just 1766mm which is lesser when compared with the whole of Assam.
The temperature in Guwahati varies between 11° in winter to 37° in summer.
GIS is an important tool when it comes to disaster mitigation process. In a GIS, data about various geographical features are organized into layers such as slope, drainage, recharge, rainfall, runoff, buildings, roads, railways, etc. These layers help in modelling and to study the landuse of the area in detail. A flood zoning map prepared with the help of Arc GIS based information system is presented here which would be greatly helpful while preparing for the upcoming floods and decide upon which techniques (structural or non-structural) measures would be implemented to mitigate the upcoming floods. Guwahati has an average of 147 rainy days and it is a highly flood prone area. The Brahmaputra flowing through Guwahati has a danger level of 49.68m above m.s.l.
The following zones were quantified during the period of research
1. Chronically inundated zone
2. Occasionally inundated zone
3. Rarely inundated zone
4. Inundation free zone
The area covered by these zones are calculated and it is found to be 52.34 percent of the Guwahati area is flood prone. 16.27 percent falls under the chronically inundated category, 13.34 percent under occasionally inundated category and 22.73 percent under the rarely inundated category.
The major roads and areas severely affected from flood inundation problem were identified based on field observation and can be listed as follows:
1) Geetanagar / Hatigaon Chariali upto Narengi
2) G.N.B Road from Guwahati Club to Noonmati ( Except New Guwahati Area )
3) R.G Baruah Road
4) Maligaon / Durgasarobar
5) Guwahati College approach road
6) Nabagraha Road and its nearby areas
7) Along the Kanwachal road , particularly the southern part
8) Nabin Nagar / Anil Nagar /Ambikagiri Nagar / Tarun Nagar /Lachit Nagar upto Bhangagarh ` Page 27 of 71
10) Christianbasti area of G.S Road
11) Rukminigaon , Mathura Nagar and some low-lying area in Beltola
12) Some areas on the A.T Road
13) Santipur – Bharalumukh area
14) Fatasil Ambari
15) B.Baruah Road
17) Lamb Road , Ambari
18) Gandhibasti area
19) B.K Kakoti Road
20) Kachari Garigaon
PROBLEMS AND DISCUSSION
The North Eastern part of India is extremely susceptible to natural hazards like floods, earthquakes, landslides etc. During the monsoon season every year, the region experiences havoc due to the flooding of river Brahmaputra and erosion along its banks. Many villages in the area are heavily flooded, affecting thousands of families, many people lose their lives every year and millions are rendered homeless. The flood wreaked havoc causing devastation of croplands and livestock. The river embankments are breached; bridges and culverts are washed away with the river flowing above the danger level. It pose a great threat of security to the inhabitants.
The flood not only affects the rural areas, but also the biggest city in the North Eastern part of India, Guwahati. It experiences severe flash floods every year during heavy downpours. Guwahati, being the major centre for the entire North east suffer a severe impact from these floods leading to disruption of normal life. Heavy downpours cause water logging in the low lying areas of the city where there are no planned drainage systems to discharge the water. Often it is seen that water enters into the households in areas such as Anil Nagar, Tarun Nagar, VIP road, Bhetapara, G.S Road area and so on.
The following are the potential reasons for flooding in the area:
· One of the reasons for flooding in those areas is the lack of proper dimensioned drainage systems. The drainage systems in the area are not sufficient enough to accommodate the amount of water which ultimately leads to over topping of the drains with dirty water flowing on the streets and even inside the houses. Also, the feeder drains connecting to the main channels run overflowing and due to siltation of drains, the carrying capacity of the drains are reduced greatly.
· The percentage of sealed surfaces in the area is high when compared to the greener areas. Therefore, there is more runoff than infiltration.
· Topography of the city is difficult to have required gradient in many part of the drainage system.
The drainage system in Guwahati is dependent mainly on the already existing natural drains. The drainage condition is not very convincing as they are mostly covered with solid wastes like plastics, food materials, wood etc. and sewage. The Water Resources Department had already prepared an extensive plan for regular cleaning and management of these drains, which are the main channels for draining the water out of the Guwahati city during flash floods.
The existing drainage system of the city are shown as below:
· Bharalu Basin (catchment area: 100 sq. km)
· Silsako Beel Basin (Catchment area: 92 sq. Km)
· Deepar Beel Basin(Catchment area: 144 sq.km)
· Kalmoni Basin (Catchment area: 66.5 sq.km)
The existing natural drain channels are:
· Bharalu-Bahini river system
· Mora Bharalu River
· River Basistha
· Lakhimijan Channel
· Bondajan Channel
· Khanajan River
· Kalmoni River
v The Government in response to these flooding problems took some productive remedial steps such as installation of high capacity pumps in low lying areas, construction of flood walls, sluice gates on the channels to restrain the flow of water.
v Few of other proposed interventions are still in process, in order to solve the problem of flooding like, construction of storm water sewage systems to carry off excessive discharge during wet weather flow into the main channels by considering the per capita consumption of the people and water consumption pattern, re-sectioning of existing channels, clearing the channels from solid waste and controlling the discharge of sewage into the channels.
v Encroachment or filling up of the major water bodies adversely affect the water retention capacity of the major Beels (wetlands). Eviction, excavation, fencing demarcation, should be done.
v Another possible solution in order to solve this problem could be construction of permeable pavements which would help in the infiltration of water into the ground and at the same time help in the ground water recharge. The permeable pavements can be incorporated in parking lots and in the side walks.
v In open spaces wherever available, grass, woody trees etc. should be planted in order to help in infiltration process.
v Construction of swales, bio retention ponds, detention basins wherever possible should be done as the excess flood water can be directed to these locations which would ultimately infiltrate into the ground and add on to the water table.
v Construction of Rain water harvesting systems in every households would reduce the heavy runoff generated during heavy downpours significantly. This water could also be later used for domestic purpose during scarcity period.
v The roads should be constructed with a gradual gradient so that during heavy rains the water flows in a particular direction and ultimately flows into the channels.
v The hilly areas should be prevented from encroachment and cutting as it leads to flow of soils into the drains, thereby reducing the carrying capacity of the drains.
v Strict laws should be implemented on the use of plastic bags as disposal of these bags into the soils and landfills pollute the soils and prevent the water from infiltrating into the soil
v The Municipal Corporation should introduce strict penalties for disposing garbage into the drains in each localities. The garbage disposal system in Guwahati should be revised and new strict laws should be implemented.
v Early flood warning systems should be installed which could be helpful in minimizing the losses incurred due to these frequent flash floods in the city.
v And lastly, people should be made aware of the consequences of these frequent flash floods. There should be mock drills performed in each locality for evacuation purpose and also disaster awareness workshop.
The problem of flash flood in Guwahati is very serious as it hampers the daily life of the people living there. Rainfall of strong intensity for a short duration leads to water logging in the city due to insufficient capacity of the drainage systems. Every year at least 3-10 lives are lost in Guwahati flash floods.Guwahati, being the hub of urban development, trade and commerce, education in the whole of North Eastern part of India suffers great economic loss during these floods. The Government of Assam needs to take some serious measures inorder to solve this burning issue of flash flood in the city. The problem requires scientific planning by the responsible authorities of the government of Assam for productive flood inundation and water-logging management techniques taking into account the present situation of Guwahati. The Government must come up with new schemes and ideas to protect, maintain and preserve the natural water bodies, channels and green cover in the hills and the surrounding areas.