Coastal Vulnerability Index

By: T Srinivasa Kumar
Scientific study of the natural hazards and coastal processes has assumed greater significance after the December 2004 tsunami as the country learnt lessons from the high impact damage to life, property and environment. The nation’s want for reliable coastal vulnerability information has created a need for classifying coastal lands and evaluating its hazard vulnerability.
Planning n Mitigation

Coastal regions of our nation are facing tremendous population and developmental pressure for the last four decades. According to the 1992 estimates of United Nations more than half of the world’s population lives within 60 km of a shoreline. In the 1950s there were only two mega cities – New York and London, which notched upto 20 by 1990, and as recent projections predict, it is likely that we have 30 mega cities by 2010 with a population of 320 million. According to United Nations Environment Programme (UNEP) report the average population density in the coastal zone rose from 77 people per sq km in 1990 to 87 in 2000 and was projected as 99 for 2010.

Collectively, this is placing additional demands on coastal resources as well as exposing more people to coastal hazard.  About 200 million people were estimated to live in the coastal floodplain in 1990 (in the area inundated by a 1 in 1000 year flood) – it is likely that their number increases to 600 million by the year 2100. Furthermore, global climate change and threat of accelerated sea level rise exacerbate the already existing high risks of storm surges, severe waves and tsunamis. Over the last 100 years, global sea level rose by 1.0 to 2.5 mm/year. Present estimates of future sea level rise induced by climate change, range from 20 to 86 cm for the year 2100, with a best estimate of 49 cm. It has been estimated that a 1 m rise in sea level could displace nearly 7 million people from their homes in India (IPCC WG1, 2001).

Officials and resource managers responsible for dealing with natural hazards need accurate assessments in order to take informed decisions before, during, and after hazard events. Such study or analysis of risk is increasingly being presented with the intention of contributing data to physical and territorial planning specialist as an ingredient within the decision making process.

Disciplines such as geography, physical, urban or territorial planning, economics and environmental management helped to strengthen what is perhaps an applied science approach to disasters. Maps became more and more common due to greater participation of geologists, geotechnical engineers, hydrologists and other experts. They were able to provide required data for the adequate identification of the danger or hazard zones, according to the area of influence of natural phenomena. Also tools such as GIS have facilitated identification and analysis.

 

Methodology

Vulnerability may be defined as internal risk of a subject or system that is exposed to a hazard and corresponds to its intrinsic predisposition to be affected, or to be susceptible to damage. In general, the concept of ‘hazard’ is now used to refer to a latent danger or an external risk factor of a system or exposed subject. Hazard can be computed mathematically as the probability of occurrence of an event of certain intensity in a specific site, during a determined period of exposure. Vulnerability, however may be mathematically expressed as – feasibility that the exposed subject or system may be affected by the phenomenon that characterises the hazard. Risk, therefore is the potential loss to the exposed subject or system, resulting from a combination of hazard and vulnerability. Risk may be expressed in a mathematical form as the probability of surpassing a determined level of economic, social or environmental consequence at a certain site and during a certain period of time.

Although a viable, quantitative predictive approach is not available, the relative vulnerability of different coastal environments to sea level rise may be quantified at a regional to national scale using basic information on coastal geomorphology, rate of sea level change, past shoreline evolution, etc., to estimate the coastal vulnerability index (CVI). This approach combines the coastal system’s susceptibility to change with its natural ability to adapt to changing environmental conditions, and yields a relative measure of the system’s natural vulnerability to the effects of sea level rise. The method uses a rating system that classifies the coastal area based on degree of vulnerability – low, medium and high.

The method of computation of CVI in the present study is similar to that used in Thieler and Hammar-Klose (1999), Thieler (2000) and Pendleton et al., (2005). In addition to the 6 parameters used by earlier researchers, the present study uses an additional geologic process variable, i.e. coastal regional elevation. The seven relative risk variables used are shoreline change rate, sea level change rate, coastal slope, mean significant wave height, mean tidal range, coastal regional elevation and coastal geomorphology. Most of the above parameters are dynamic and require a large amount of data from different sources to be acquired, analysed and processed. Once each section of coastline is assigned a risk value for each variable, the CVI is calculated as the square root of the product of the ranked variables divided by the total number of variables (Pendleton et al., 2005).

Article 6 Map 1

Results

This is the first study to look at vulnerability on synoptic scales (1:1,00,000) that covers the entire Indian coastline. The resulting map is shown in Figure 1. The general trend shows that the northern parts of the coastal states: Tamil Nadu, Andhra Pradesh, Odisha, Kerala, Maharashtra, and Goa indicate high and very high vulnerability indices as compared to the southern and central parts of the states’ coastlines – Gujarat being an exception. The north south trend is also apparent in the Andaman and Nicobar Islands. Lakshadweep Islands indicate high to very high indices due to the sea level and terrain elevation of the region, with Minicoy recording very high vulnerability index. The Gulfs of Kambhat and Kachchh in Gujarat show very high vulnerability indices, with the inlets of Kachchh showing localised vulnerability. Sunderban in West Bengal shows high and very high vulnerability index in majority of its locations, while the north eastern patches show low vulnerability indices, due to mangroves in slightly elevated regions. It has been well documented that the mangroves break waves, dissipating the energy and hence acts as a natural barrier.

The study depicts vulnerable areas as per the seven parameters considered. These maps are therefore not maps of total vulnerability, but of essential aspects constituting overall vulnerability. They depict the problematic regions, and therefore further attention should be directed to these regions to analyse their vulnerability in the context of nested scales and on higher resolution. Use of additional parameters such as cyclone, storm surge and coastal flooding will add an additional dimension to the current study.

The coastal vulnerability maps produced using this technique serve as a broad indicator of threats to people living in coastal zones. This is a objective methodology to characterise the risk associated with coastal hazards and can be effectively used by coastal managers and administrators for better planning to mitigate the losses due to hazards as well as for prioritisation of areas for evacuation during disasters.

Coastal vulnerability index (CVI) = √ [(a*b*c*d*e*f*g)/7] Where

a                =                risk rating assigned to shoreline change rate

b               =                risk rating assigned to sea level change rate

c                =                risk rating assigned to coastal slope

d               =                risk rating assigned to significant wave height

e                =                risk rating assigned to tidal range

f                 =                risk rating assigned to coastal regional elevation

g                =                risk rating assigned to coastal geomorphology

 

The CVI values are categorised into very high, high, medium and low vulnerability coasts based on the equal interval of the CVI percentile.

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