Flood Zone Mapping
Floods are a recurrent phenomenon, which cause huge loss of lives and damage to livelihood systems, property, infrastructure and public utilities. In India, out of the total geographical area of 329 million hectares (mha), more than 40 mha is flood prone. The Central Water Commission (CWC) has given detailed estimates of economic loss and loss of human and cattle lives due to floods from 1953-2016. This data reveales that the decade of the 1970s was the worst in terms of loss of human and cattle lives due to floods in India. It is also apparent that flood related damages show an increasing trend – the average annual flood damage in the last 10 years period from 1996 to 2005 was Rs. 4745 crore as compared to an average of Rs. 1805 per annum for the previous 53 years. This increase can be attributed to many reasons – a steep rise in population, rapid urbanization, growing developmental and economic activities in flood plains coupled with global warming.
Recent reports suggest that between 2011 and 2014, Arunachal Pradesh suffered an average annual loss of 10% of the GDP. The state has been reporting significant flood damages since early 2000s. Sikkim witnessed annual average losses of 1.7% of the GDP, and Meghalaya at 1.5% of the GDP during the same period. For Himachal Pradesh, annual average losses between 2011 and 2013 stood at 1.8% of the GDP. Hilly regions suffer more due to flash floods which are difficult to predict and also cause landslides.
There are mainly four types of flood – riverine, coastal, urban and flash floods. A brief explanation of these four kinds follows.
Riverine Floods: This type of flooding affects land areas along the banks of rivers in their lower reaches. Flooding along the Kosi River as it passes through Bihar is caused by overflow of water along the channel. This is a typical example of a riverine flood. The 2008 Bihar flood which was one of the most disastrous floods in the history of Bihar, affected more than 2.3 million people was caused due to a breach in an embankment in Nepal. The flooding also resulted in the river changing its course. The major riverine flood prone regions in India are the basins of the Ganga, Brahmaputra, Narmada-Tapti, Krishna, Godavari, Mahanadi and Cauvery rivers.
Coastal floods: This type of flooding is caused by the ocean water driven inland by storms, cyclones, tidal waves and tsunamis.
Urban flood: Urban flooding can occur on account of several reasons including heavy rainfall, sudden release of water from a bund or dam, tidal waves, etc. However, the main underlying cause is usually the slow absorption of water by the land. The 2016 flooding of Chennai and the 2017 flood in Mumbai were examples of urban flooding.
Flash flood: Flash floods usually occur without warning in a very small interval of time. Flash floods are usually caused by heavy rainfall or release of water from a dam. The disastrous floods in the Uttarakhand in the last few years were examples of flash floods.
What is a Flood Zone Map: A floodplain or flood plain is a low-lying area of land adjacent to a stream or river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge. A flood zone map is a map that shows areas identified to be susceptible to floods. These geographical areas called flood zones are given a specific ranking depending on the estimated flood risk. A ranking of the flood zone will determine how much damage is likely to be caused during a flood event, and what possible measures will be required for effective management. The primary benefit of such maps is to communicate and manage flood-related data.
Making and maintaining an accurate ﬂood map is a complex and expensive process. Since the flood zones are required to be mapped with a high degree of accuracy, land development and natural changes to the landscape or hydrologic systems create the need for continuous map maintenance and updates. To manage flood risk and minimize future disaster relief costs, the nation, through the National Disaster Management Authority (NDMA), has invested significant resources in mapping ﬂood hazard areas. Transforming ﬂood maps into digital format can greatly enhance their utility since digital maps are more versatile for ﬂoodplain management and other uses because they are easier to update.
Data required for Floodplain Mapping:
Land Surface Reference Information: Land surface reference information describes streams, roads, buildings, and administrative boundaries that show the background context for mapping the ﬂood hazard zone. The older paper maps contain only vector data (points, lines, and polygons) to describe all land surface reference features. Modern maps typically use a digital orthophoto or a high resolution satellite image as the base map, supplemented by planimetric vector data for key map features (e.g., roads, building locations and other infrastructure) and administrative boundaries (e.g., city or taluk boundaries) that cannot be observed in photographs and satellite images.
NOTE: An orthophoto is an aerial photograph from which all relief displacement and camera tilt effects have been removed, such that the scale of the photograph is uniform and it can be considered equivalent to a map.
Topographic Data: Topographic data are the most important factor in determining water surface elevations, base ﬂood elevation, and the extent of ﬂooding. Topography defines the shape of the land surface, which is important in defining the direction, velocity, and depth of ﬂood ﬂows. Topographic data have traditionally been derived from topographic maps or by land surveying. The very large areal extent of modern ﬂoodplain maps, which cover tens of thousands of kilometers of streams and shorelines, demands that land surface elevation data must be derived from mapped sources, not from land surveying.
Land Subsidence: Land subsidence generally occurs when groundwater is mined in an unplanned manner. This is more evident in areas that are underlain by fine-grained sediments. Decline of groundwater table causes a vertical compression of sediments, leading to subsidence of the ground surface. There can be indirect effects such as a change in gradient of streams or drains. Subsidence not only leads to damage of infrastructure (roads, bridges), but also increases the vulnerability to flooding, particularly when the drainage systems of the city or town is ineffective. Since such ground subsidence is a continuous process, data on ground subsidence needs to be collected periodically in order to keep the flood zone maps current.
Other Data: For coastal flood zone mapping, additional data on hurricane, bathymetry, wave and storm surge (water that is pushed toward the shore by the force of winds) are also integrated into the model. The output is used in geographic information systems to delineate the predicted ﬂoodplain area. All the inputs have uncertainties that may affect the accuracy of the resulting ﬂood zone map.
Flood Zone Mapping by NDMA
In India, the frequency of major floods is more than once in five years. Eighty per cent of the precipitation takes place in the monsoon months from June to September. Rivers derive heavy loads of sediments from the upper reaches of their catchments and deposit these in the lower reaches, causing congestion of their own channels, which worsens the flood situations in these areas. Cyclonic rains and cloud bursts cause flash floods, particularly in hilly areas, inflicting huge losses. Another complex dimension is added to the problem on account of the fact that many rivers causing damage in India originate in neighboring countries.
Continuing and large-scale damage due to floods necessitates that we develop an effective system for responding to floods. The National Disaster Management Authority (NDMA) has prepared the Executive Summary Guidelines to enable the various implementing and stakeholder agencies to effectively address the critical issues for minimizing flood damage. As part of this effort, the NDMA has produced a flood zone map of the entire country. Floods have also occurred in areas, which were earlier not considered flood prone.
Flood Zone Map of India
The major flood prone regions in India are Punjab, Haryana, most of the Gangetic plains including Uttar Pradesh, North Bihar and West Bengal, the Brahmaputra valley, coastal Andhra Pradesh and Orissa, and South Gujarat. Among the severely affected areas are the Brahmaputra valleys, north Bihar (Kosi River and north Gangetic plain) and lower West Bengal.
Apart from these, floods affect large areas in the following belts:
(i) The lower courses of rivers in the north Indian plains get silted and change their courses. These areas lie in the states of Punjab, Haryana, Himachal Pradesh, Delhi, Rajasthan, Uttar Pradesh, Bihar and West Bengal.
(ii) The tributaries of the Indus – the Jhelum, Satluj, Beas, Ravi and Chenab – cause floods in Jammu and Kashmir, Punjab, Haryana, western Uttar Pradesh and Himachal Pradesh. Inadequate drainage in parts of Haryana and Punjab is the main cause of inundation.
(iii) Certain areas in central India and the peninsula get flooded by the Narmada, Tapti, Chambal, Godavari, Krishna, Cauveri and Pennar.
(iv) Certain areas along the east coast get flooded due to cyclonic storms.
According to the World Meteorological Organization (WMO), sea levels have been rising about 3 mm per year since 1993 – totaling a 200 mm increase (7.87 inches) in global averaged sea level since 1870. The Global Flood Map at http://globalfloodmap.org/India uses NASA satellite data to show the areas of the world under water and at risk for flooding if ocean levels rise. Use this interactive map by entering any number of inches – the map will show what areas would be flooded or would be at risk. The left side bar will display the number of people displaced. Click on individual points for current elevation, elevation after sea level rise, and the number and percentage of people losing homes.
This website is hosted by
Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)