CHAPTER 2 PROTECTION FROM CYCLONES

Thematic paper: Role of forests and trees in protecting
coastal areas against cyclones

Table 2.4 Most destructive cyclones to hit India (modified after De et al. 2005)

Table 2.5 Most destructive cyclones to hit Bangladesh (modified after Karim, 2006)

The damage and destruction generated by these cyclones have not decreased. Loss of life, however, tends to show a decrease because of better weather forecasts and warnings, their dissemination, and disaster management strategies put in place by national weather services in conjunction with the significant role played by the World Meteorological Organization (WMO) through its regional meteorological centres (RMCs) that deal especially with tropical cyclones.

2.5 Western Pacific Ocean tropical cyclones

Table 2.6 Frequency of typhoons in Southeast Asian countries (1985–1989)

3 CYCLONE IMPACT CHARACTERISTICS

3.1 Return period calculations

Figure 2.3 Location map of Orissa and the inundated areas of Orissa
after the 1999 supercyclone

Table 2.7 IMD cyclone classification by sustained windspeed

Table 2.8 Twentieth century cyclones in Orissa, India

Table 2.9 Return periods for flooding in Orissa:
(a) nineteenth century, (b) twentieth century

Figure 2.4 (a) Cyclone tracks impacting the coast of Orissa from 1971 to 2000
(b) six synthesized generic cyclone tracks (Chittibabu et al., 2004)

Figure 2.5 Total water level on a 50-year return basis (Chittibabu et al., 2004)

Figure 2.5 Total water level on a 50-year return basis (Chittibabu et al., 2004)

Figure 2.6 Bangladesh storm surges:
(a) Areas most affected by historic cyclones (Karim 2006)
(b) computed 100-year return period surge levels (Kabir et al., 2006)

3.2 Vulnerability to cyclones

Dube et al. (2004) created models to simulate the storm surgesfrom past cyclones in the head of the Bay of Bengal (Orissa, West Bengal and Bangladesh) and discussed how each of the storms that were modeled formed and how the storms affected the specific areas. Dube et al. (1997) also discussed storm surges in the Bay of Bengal and why the area is affected to such an extent by extreme sea levels. The reasons were summarized by Ali (1979) as follows:

• coastal waters (shallow bathymetry extending tens of kilometres offshore);
• convergence of the bay;
• high astronomical tides;
• thickly populated low-lying islands;
• favourable cyclone tracks impacting perpendicular to coastline; and
• innumerable inlets and river systems.

Figure 2.8 Per capita income of different states in India and risk level. Dark red areas equal low per capita income and yellow areas equal high per capita income

Figure 2.9 (a) Hatia Island locations of simulations (b) surge variations at fixed locations 600 m inland from the shoreline with different forest widths
between the locations and the coastline (Kabir et al., 2006)

Table 2.10 Summary of computed storm surge reductions by mangrove bands of various widths (Kabir et al., 2006)

According to the model, the most effective area of forest cover for the 1970 cyclone would have been along the southern tip of Hatia Island. This location would have seen a decrease in surge of 0.45 metre with a 600-metre strip of mangroves and a 0.18-metre decrease at the test location 600 metres inland for a 133-metre strip of mangroves. The site in the middle would have seen little decrease in surge height (0.15 metre with a 600-metre strip). The base flooding with no mangroves was 6.2 metres at this location 600 metres inland. Hence, a storm surge height reduction of seven percent was achieved by adding 600 metres of forest between the shoreline and the test location 600 metres inland. In the case of the northernmost site, the surge level would have increased slightly due to the forests’ trapping of flow coming from other parts of the island. This illustrates the importance of site characteristics in determining that the effect of the forest will vary according to the site characteristics. Similarly, coastal forests can also funnel flows along creeks, thereby increasing surge heights in some cases. The main conclusion is that narrow coastal forests have minimal effects on the storm surge height and inland flooding. In order to significantly reduce the impact of the storm surge —usually the most devastating cyclone hazard — kilometres of coastal forests are required. Few other storm surge reductions by mangrove models were readily available to the authors.

Figure 2.11 Kendarapara and Bhadrak districts of Orissa, India, with the location of Bhitarkanika Conservation Area and the extent of mangrove forests

The report aimed to measure the economic losses attributed to the 1999 supercyclone relative to the prevailing socio-economic conditions of the study villages. It evaluated the extent of damage caused in areas that were under the umbrella of mangrove forests and areas that were not, in the wake of this supercyclone. In 1971, an embankment was created along the entire Orissa coast to prevent seawater intrusion into reclaimed paddy fields. Therefore, the report also studied the effectiveness of such artificial structures in providing storm protection, as opposed to mangrove forests.

Hence, the following three situations were identified: (1) A village in the shadow of mangroves; (2) a village not in the shadow of mangroves and with no embankment; and (3) a village not in the shadow of mangroves, but with an embankment on the seaward side. Bankual village was in the shadow of a mangrove forest, Singidi village was neither in the shadow of mangroves nor protected by an embankment from storm surge and Bandhamal village was not in the shadow of mangroves, but had a seaward side embankment. The report indicated that the intensity of the impact of the 1999 cyclone on these villages should have been fairly uniform, as all the three selected villages were equidistant from the seashore and had similar aspects. The two villages outside mangrove cover were located close to each other, but both were far from the mangrove forest in order to eliminate any effect of mangrove forest presence.

Table 2.11 Basic description and mean values of the variables (per household)
in the three study villages in Bhitarkanika Conservation Area, India
(US$1.00 = INR 45, August 2004)

Although only indicative, the report shows that the damage attributed to the cyclone was more extensive in the village further away from the mangrove shadow. The embankments constructed in 1971, after a previous cyclone, to prevent the intrusion of salt water into agricultural fields and villages were ineffective during the high storm surge; in fact, they acted as a barrier to runoff when the water was receding. The embankments suffered a number of breaches that resulted in the flooding of villages such as Bandhamal, which was surrounded on all sides by the embankment. Singdi village, with no mangrove cover and no embankment, suffered the highest level of field inundation; however, the seawater receded quickly, resulting in less damage to agricultural crops. Bankual village, which was in the shadow of mangrove forest and had minimal embankment around it, suffered the least. Although this study is not conclusive, the lack of breaches in the embankment closer to the forest is indicative of the protection provided by mangroves to the embankment. In areas far from the forest, several breaches in the embankment were observed. Water levels were higher and the flooding was of longer duration in Bandhamal.

Extensive Casuarina plantations established as a storm protection measure along the Orissa coast were ineffective in preventing damage; rather, they caused destruction to Olive Ridley sea turtle (Lepidochelys olivacea) nesting beaches. The cyclone uprooted almost all the trees in the immediate vicinity of the coast and caused much damage to trees several kilometres inland. However, mangrove forests and trees in the shadow of mangrove forests remained intact. The report contends that the vulnerability of many coastal communities to cyclones is heightened by the removal of mangroves for development, agriculture and habitation purposes. Mangrove forests are natural buffers against storm surges and protect tropical shores from erosion by tides and currents. Ecological functions such as storm protection may be very important components in the total economic value of a wetland and may constitute almost 80 percent of the estimated value. These major benefits are often the principal reasons for restoring mangrove forests along much of the low-lying deltaic coasts. In the aforementioned study in Orissa, there was a 20 to 30 percent reduction in repair and maintenance costs of sea dyke systems due to the presence of mangroves in front of the dyke. The report realized that the artificial sea defenses were not only expensive to build and repair, but they were also, in many cases, ineffective.

3.4 Cyclone damage to mangrove forests

Plate 2.1 Mangrove damage after Hurricane Mitch on the Bay Island of Guanaja, Honduras: (a) defoliation (b) uprooting (c) erosion of shallow roots
(Hensel and Proffitt, 2000)

The negative example of the Bay Island of Guanaja shows that natural recovery is not always possible, even after several years in the aftermath of a cyclone. Unfortunately, cyclones tend to hit the most vulnerable coastlines with storms of various magnitudes within a few years. The possibility of multiple cyclone hits within a few years, or even within the same cyclone season prior to any significant recovery of mangrove forests, cannot be neglected when considering mangroves as protective shields.

Conclusions

The main conclusion is that narrow coastal forests have minimal effects on storm surge height and inland flooding. In order to significantly reduce the impact of the storm surge —usually the most devastating cyclone hazard — several kilometres of coastal forests are required. Mangroves are more efficient at attenuating surface waves and wind as well as providing protection against erosion. Typically, the wave energy is attenuated by a factor of two within 50 metres in front of the mangrove forest at normal sea level. The obstruction density caused by the mangrove wood structure decreases rapidly with height and, therefore, as the water level increases because of the storm surge there is proportionally less flow resistance and less reduction of wave energy. Unfortunately, this effect severely reduces wave attenuation with increasing cyclone intensity, storm surge height and wave period. Finally, the possibility of multiple cyclone hits prior to significant mangrove forest recovery needs to be considered.

ACKNOWLEDGEMENTS

This paper was sponsored by the Food and Agriculture Organization of the United Nations.

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Field study presentation: Cyclone disaster mitigation in Bangladesh

M. Alimullah Miyan, South Asian Disaster Management Center, Bangladesh

Bangladesh has witnessed many natural calamities. Between 1797 and 1998, 67 major cyclones occurred. The 1991 cyclone killed approximately 139 000 people and generated economic losses of US$2.07 billion. The coastal zone comprises 19 southern districts with 35 million inhabitants
(28 percent of the country).

The Bangladesh Meteorological Department provides cyclone warnings. The Standing Orders for Cyclone constitute the basic plan for addressing cyclone disasters with guidelines for action at various stages during calamitous events. Elaborate institutional arrangements are in place to deal with disasters. The most dedicated agency is the Cyclone Preparedness Programme, which is an organization of 33 000 volunteers in the field who mobilize people at the community level to cope with cyclones. Structural mitigational measures such as cyclone shelters, coastal embankments and improved housing, as well as non-structural mitigation measures including coastal afforestation, public awareness, community preparedness, local level contingency planning and social mobilization are in place to a limited extent. Coastal afforestation activities were initiated in 1966. The protection afforded by natural mangrove forest has prompted afforestation in the five coastal districts. A coastal zone policy has been formulated with emphasis on sea dykes and afforestation, planned tree plantation, social forestry, plant care and maintenance, afforestation of new chars (accreted land) and their conservation. A number of recommendations have been made to: improve storm surge forecasting; enhance public awareness; increase investment in warning presentation; promote information dissemination and comprehension; simplify warning contents; address knowledge and attitudes; carry out surveys on cyclones; and integrate disaster management in development planning.

The conclusions pointed to the need for mainstreaming disaster management activities, increasing the interface between disaster and development, and conducting sustained awareness and advocacy programmes. All of these issues call for a higher level of investment in preparedness, shelter construction, afforestation, institutional arrangement, policy formulation and community involvement for improved cyclone disaster mitigation.

Field study presentation: Evaluation of storm protection functions — a case study of mangrove forest in Orissa, India and the 1999 super cyclone

Saudamini Das, University of Delhi Enclave, India

The research aimed at making a quantitative assessment of the storm protection functions of mangrove forests. In October 1999, the state of Orissa was battered by a super cyclone that caused tremendous loss of life and property in the coastal region. It was widely reported in the national as well as international press that areas with mangrove forest on their coastlines experienced comparatively less damage. Using village level human casualty data, cyclone damage (taking into account wind velocity, storm surge velocity and the socio-economic status of the villages separately) was estimated; an attempt was made to evaluate how many human lives had been saved by the presence of mangroves in some areas.

Key points and observations emphasized in the discussions

Cyclones are composite phenomena, consisting of winds, multiple storm waves, storm surges (which generally last from several hours up to a day) and inland flooding generated by heavy rainfall. This inland flooding often causes more fatalities than the wind. Cyclones generally cause massive erosion or deposition – both long- and cross-shore. In recent years important discussions on multihazard approaches for coastal protection have been ongoing; however, it should be noted that often coasts vulnerable to cyclone storm surges are less (or the least) vulnerable to tsunamis, and therefore require different protective measures.

Coastal forests and mangroves can help to mitigate cyclone damage, but are not effective against all hazards. Their mitigation effect is limited (or absent) during cyclones of categories exceeding 4 on the Saffir–Simpson Scale and super cyclones. Narrow belts of coastal forest do not reduce storm surges; several square kilometres of forests or coastal wetlands are required to significantly absorb massive inland inundation caused by cyclones of high intensity. Preliminary analysis of the 1999 Orissa super cyclone showed that the mitigation effects of Casuarina plantations were lower in comparison to mixed indigenous forests.

The following factors were identified:

1. Coastal forests are efficient in reducing wind and storm wave impacts up to a certain level.
2. Coastal forests and mangroves in narrow belts do not reduce storm surge efficiently; they can, however, help to decelerate flooding velocity and trap floating debris.
3. A dense forest can reduce 0.5 metres of surge for each kilometre of forest.
4. Extensive wetland systems are also important as mitigating measures and the loss of wetlands makes the coast more vulnerable. Mangroves and coastal wetlands must be preserved.

When planning a green belt for protective purposes, several factors should be taken into account.

1. Space availability: A very wide belt of forest is needed to ensure an appropriate reduction of cyclone strength. Because the coasts of several Asian countries (for example, Bangladesh) often have very high population densities, this space may not be available and consequently a green belt cannot be prepared to the required width. In this case environmental and hard engineering structures should be used in combination with – or as an alternative to – the green belt.
2. Social involvement in planning and management of protective forests is essential for the long-term success of the project. The green belt should also be a source of additional yield to local communities (for example, controlled harvesting of wood and non-wood forest products, ecotourism, etc.), which will avoid illegal felling and damage to the green belt.

Additional factors which would benefit the ultimate goal of saving lives and resources in cyclone-prone countries are as follows:

1. The creation or enhancement of an appropriate cyclone warning system (together with reliable storm forecasting) at the national level, which should be easily understood by rural people and which should also extend to remote areas and communities.
2. Investment in public awareness on cyclone mitigation, including the protective role of forests (knowledge–attitude–practice).
3. The encouragement of local community participation in prevention and rehabilitation activities (for example, volunteer groups).
4. or initiation of institutional arrangements (for example, collaboration between disaster managers and other government agencies).