Coastal Zone Natural Disasters

Managing Natural Disasters in Coastal Areas- an overview

S K Srivastav

President, CIMO & Addl. Director General of Meteorology, India Meteorological Department

Introduction

Natural disasters are events caused by the forces of nature that adversely effect on human settlements, and environment. The coastal zone is the transitional area between land and sea. It is defined, as a strip of land and sea of varying width depending on the nature of the environment and management needs. It seldom corresponds to existing administrative or planning units. The natural coastal systems and the areas in which human activities involve the use of coastal resources may therefore extend well beyond the limit of territorial waters and many kilometres inland. The worldwide average width of the coastal zone on the terrestrial side is said to be 60 km. The zone occupies less than 15% of the Earth's land surface, yet it accommodates more than 60% of the world's population. Furthermore, only 40% of the one million-km of coastline is accessible and temperate enough to be habitable. As a result, coastal zones are marked by above-average concentrations of people and economic activity. This coupled with their proximity to the ocean makes them one of the most sensitive zones for natural disasters. In the future, it is estimated that about 600 million people will occupy coastal flood plain land below the 1000-year flood level by 2100 (Nicholls and Mimura (1998).

Every year natural disasters result in considerable damages to life and property not only in the coastal zones but also in other vulnerable areas. 2001 has been a year of average losses, as compared to 1999 with record number and 2000 with few losses. Figure 1 gives the numbers of natural disasters and their damage estimates. As coastal zones are more prone to natural disasters there must be better management policies to adjust and cope with such events. In most countries throughout the World, coastal zone management has traditionally been practiced in a sectoral fashion, based upon political or socio-economic boundaries. More recently, the concept of Integrated Coastal Zone Management (ICZM) has led to the idea that coastal zones would be more effectively and efficiently managed on the basis of information inputs from various sectors including improved weather warning and management systems. Remote sensing, Geographical Information Systems (GIS) and the Information Technology offer potential solutions for ICZM. Remote sensing has, for a long time, been an invaluable source of environmental data and information. It has many advantages, including the capability to collect data over large tracts of land at any one time, and at a range of scales, from low-resolution data covering large areas, to high-resolution data covering smaller areas. Moreover, remote sensing provides a unique ability to collect both terrestrial and marine data in one seamless dataset. With the vast amount of data available about the coast, and with the potential to add even more data into the system (including remotely sensed imagery, and online GIS), there also needs to be a new, more simple, but also more effective way of gaining quick and easy access to this data and information.

Figure 1. Natural disasters – numbers and their damage estimates during the years (Topics 2001, Annual Review: Natural Catastrophes in 200 by Munich Re Group)

Potential Natural Hazards to Coastal Zones

There are a wide variety of meteorological phenomena, which pose a threat to the coastal zones. They could be roughly listed the following:

Floods/flash floods, cloud burst, heavy precipitation
Tropical cyclones and their associated storm surges
Severe convective storms - thunderstorms, hailstorms, tornadoes, lightening, dust storms, sand storms
Heat wave and cold wave
Snow avalanches
Sea erosion

The spatial and temporal scales of these hazards vary widely from short-lived, violent phenomena of limited extent (e.g. severe thunderstorms), through large systems (e.g. tropical cyclones). These events can subject large regions to disastrous weather phenomena like strong winds, heavy flood-producing rains, storm surges and coastal flooding, heavy snowfall, blizzard conditions, freezing rain and extreme hot or cold temperature conditions for periods of several days. With this wide variety of the scales of weather phenomena, the requirements of meteorological and hydrological forecasting for effective early warning of these hazards also vary spanning over a very broad spectrum. These can range from very short range forecasts of less than one hour in the case of severe thunderstorms and flash floods; through short and medium range forecasts of from a few hours to days for tropical cyclones, heavy rains, extreme temperatures and high winds.

The various natural hazards pertaining to coastal zones can be summarized as below:

Tropical cyclones

Tropical cyclones are intense vortices in the atmosphere with strong winds circulating anti-clockwise around a low-pressure area in the Northern Hemisphere and clockwise in the Southern Hemisphere. They are capable of causing massive destruction in three ways: by high winds; heavy rainfall causing inland flooding; and storm surge inundating low lying areas. Tropical cyclones form in the warm tropical oceans where the sea surface temperature is at least 26.5 C. They may last with destructive power for two weeks or more where a large open sea is available. The storm surges are by far the greatest killers in a cyclone. The Bay of Bengal region has the distinction of having experienced the world’s highest recorded storm tide of 41 feet (1876 Bakerganj cyclone near Megna estuary, Bangladesh). As a result of storm surge, sea water inundates low lying areas of coastal regions causing heavy floods in the coastal areas, eroding beaches and embankments, destroying vegetation and reducing soil fertility. Very strong winds may damage overhead installations, dwellings, communication systems, trees etc., resulting in loss of life and property. Flood and tidal waves due to storm surges pollute drinking water sources resulting in shortage of drinking water and causing out-break of epidemics, mostly water borne diseases. Past records show that very heavy loss of life due to tropical cyclones have occurred in the coastal areas.

Floods

Floods are excessive accumulation or flows of water, which result from heavy rainfall. They include flash floods, which are caused by rapidly rising and falling rivers and overland flows resulting from the rapid run-off of intense rainfall from upland areas (usually hilly upstream); river floods in which river water spills over adjoining land; rainwater floods from the ponding of rainfall run-off and the raised ground water-table flood plain depression; tidal flooding, usually saline from the outflow of coastal rivers at high tides; and storm surge floods associated with the passage of tropical cyclones.

Excessive runoff from the catchment due to heavy rainfall, concentrated in short time periods resulting in the spilling over the banks of the river channel may be due to inadequate channel capacity. Heavy sediments brought from the catchments are deposited on the riverbed reducing the channel capacity thereby increasing the flood risk, Breaching of embankments/failure of a dam results in flood inundation. Human interference with the floodplain disturb the flow regime and increases the flood risk, The drainage congestion caused by natural processes and human activities further aggravates the flood problem.

Climate Change and sea-level rise

Many coastal areas are already experiencing coastal erosion, sea-water intrusion and sea-flooding. The increased sea-levels due to climate change may aggravate these problems. Productive coastal ecosystems, coastal settlements and islands will be vulnerable to these pressures. Tropical and sub-tropical coastlines, which are under significant population pressures, will be significantly impacted by these changes. Salinization of potable ground water, inundation of coastal zone and erosion will be exacerbated. Climate change will also directly impact coastal wetlands, especially mangroves, coral reefs, lagoons and estuaries. Large scale bleaching of corals has been already reported. The extent of impacts will depend on the rate of sea-level rise. Strict protection; preparation of action plan for each component, regeneration, and providing space for backward movement of species are some mitigating measures.

Effects of climate change on coastal has been mostly considered from the standpoint of sea-level rise. Increase in air-sea and sea-surface temperatures, changes in wave characteristics, storminess and tidal regions will also impact coastal zones. Sea-level rise will impact the low-lying areas such as deltas and coastal plains. More severe storm-surge flooding, saltwater intrusion and sedimentation will be some of the potential impacts of climate change on coastal areas.

Tsunami

Tsunami’s (Japanese for "harbor wave”), are large waves generated by earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, which can savagely attack coastlines, causing devastating property damage and loss of life. Coastal zones are always prone to such tsunami hazards caused by natural phenomena occurring at regions far removed from the actual coastal zone. Although, tsunami is sometimes referred to as "seismic sea wave" because of its frequent association with an earthquake it can also be caused by a non-seismic event, such as a landslide or meteorite impact.

Tsunamis are characterized as shallow-water waves, with long periods and wave lengths. They are often observed with wavelengths in excess of 100 km and period on the order of one hour. Due to their large wavelengths, tsunamis propagate at high speeds, great transoceanic distances with limited energy losses. As a tsunami approaches shore, it begins to slow and grow in height. They inundate and flood hundreds of meters inland past the typical high-water level, damaging homes and other coastal structures. Despite energy losses on approaching shore, tsunamis still reach the coast with tremendous amounts of energy with great erosional potential, undermining trees and other coastal vegetation.

Observational aspects

Surface weather

A dense network of observations is required to assess and mitigate weather-related hazards to the coastal zone. Also, ocean based data collections through buoys is necessary. Installation of Automatic Weather Stations over coastal areas will immensely help monitoring and warning coastal zones about hazardous weather. It is important to consider the topography and vulnerability of the coastal zone to severe weather before deciding on the weather station network requirements. The network should also be capable of disseminating the weather data collected on real time basis for use in forecasting offices. This will enable quick diagnosis and planning of mitigative measures. Severe weather systems affecting coastal areas mostly have their origins in the seas and oceans adjoining the coast. It is therefore necessary to enhance the data collection efforts over the oceans. Although space based and remotely sensed data is available is important to have actual observation at the surface. Ship observations and data from ocean buoys are crucial while decisions are taken on track forecasts of storms. Both in-situ and remote sensing techniques have been evolving during recent past. Automatic Weather Stations (AWS) and Automatic Weather Systems (AWSs) have taken an operational status in some of the important development in Ground Based Remote Sensing Techniques. The monitoring system for specific type of natural disaster need a combination of several techniques as some of these pointedly observe whereas some others provide supporting observation to evolve warning mechanisms. This needs high degree of expertise and investment.

Satellite observations

Satellite measurements systems have been continually evolving to enable better monitoring of synoptic systems over the oceanic regions adjacent to coastal areas. Satellite based systems are used to obtain both images and quantitative information about meteorological features of the lowest 20 km of the atmosphere. Space-based observations together with surface-based networks provide invaluable monitoring of weather and climate, providing prior information on natural hazards. Since the 1960s, when the first weather satellites were introduced, the space-based observational systems have evolved substantially. They have been particularly useful in monitoring coastal zones and biological activity as well as weather hazards associated to coastal zones.

Doppler Radar and other Radar networks

Radar observations are very useful in detection and monitoring severe weather, which could be potentially disastrous. Since there is a wide range of weather phenomena, a variety of radar characteristics are used according to requirements. A reliable radar network can provide adequate means for monitoring severe weather over a large stretch of coastal zone. The present generations of Doppler radars, which can identify and provide a measure of intense winds associated with tropical storms and thunderstorms downbursts, have enhanced our capabilities. Radars can also provide good description of heavy precipitation associated with mesoscale weather systems often encountered in coastal areas with topography. Optimal network of radars can therefore help in monitoring hazardous weather capable of damaging effects on coastal zone. Figure 1 shows a Doppler radar image of a weather system over the southeastern coast of India.

Figure 1. A typical PPI (Z) of a weather system over Southeast coast of India.

Monitoring System for Coastal Hazards

1.For effective monitoring and early warning about coastal zone natural hazards there should be an integrated monitoring system. The system will incorporate latest technologies of meteorological instruments coupled with suitable networking mechanisms and dissemination system. The systematical diagram below illustrates the components of such a system.

Coastal Zone Monitoring System

2.It is imperative from coastal zone management that the integrated observations, both from land and sea, should be evaluated. The monitoring of sea state and other parameters is a complex, logistically difficult and expensive proposition. Sea platforms like the data buoys, instruments on board ships, etc. need to be deployed supplemented with merchant navy ships. Recent development of polar orbit satellites has unanimously complemented with inaccessible data sets. However, standardization and calibration of weather parameters yet remains a problem. Development of new techniques like ARGO floats are providing very useful means in collection of valuable sub-surface sea data. Consolidated efforts are required to homogenize the entire land and sea data in order to make best use in Atmosphere – Land – Ocean coupled models to evolve a reliable forecasting system.

Integrated Management Strategies for Coastal Zone

Integrated management strategies to ensure safety against natural hazards such as high waves, storm surges, and tsunamis have to be evolved and operationalized. In urbanized coastal zone vulnerability of support systems like waste management facilities, water supply and roads need to be taken into account. Disaster prevention facilities along the coast may have to be strengthened keeping in view present and potential (including those due to climate change) damage threats. Increased water table due to sea-level rise may weaken coastal structures and predispose them to other natural hazards like earthquakes and storm related damages. If we carefully examine the process of risk reduction we arrive to the following five basic steps:

Evolving efficient monitoring and surveillance system
Evolving accurate early warning systems
Putting in places an efficient warning dissemination system
Evolving pre-disaster hazard, vulnerability and risk assessment inventories
Evolving efficient post-disaster management and mitigation strategy.
Public Awareness.

In addition to conventional techniques, a number of new techniques have emerged in the recent past in the area of whether and climate monitoring. Evaluations of high-resolution area specific models have been developed. Their operational uses have become possible due to high-speed bulk data crunching computers available at lower prices. Thus, in addition to surveillance system have provided effective tool in providing early warning for some of the natural disaster effective. Rapid strides in satellite communication have resulted into fail-safe warning dissemination system. The technological interventions are showing the results for evolving effective meteorology’s for risk assessment and management during pre and post disaster periods. However, this area is still in evolution phase and lot of work needs to be done.