Four working group sessions were held in parallel to develop “diagnostic tools” to assist in identification of situations under which forests and trees would be suitable to protect coastal areas against: (i) tsunamis; (ii) cyclones; (iii) coastal erosion; and (iv) wind and salt spray. Groups were asked to assume the roles of experts called upon to develop a tool to assist coastal planners in deciding where trees could be planted to protect a coastline from one of the other natural hazards. The steps suggested to construct the tool were as follows:
Suggestions were made for possible initial criteria/conditions as follows:
Criteria/conditions appearing further into the diagnostic tool were suggested as follows:
The point was made that in most cases, the appropriateness of trees or forests for coastal protection may depend on several factors that must be weighed against one another. Further quantification, analyses or assessments could also be required and, in such cases, it was suggested that relevant comments be recorded. In cases where specific information on the capacity of trees/forests to protect against a specific hazard was unknown, it was suggested that required information be noted.
The tsunami working group began with the same objectives as the other groups, but in spite of the concerted efforts all members, the meeting ended in deadlock with little movement towards the objectives owing to fundamental disagreement over the validity of the question. Several opposing points of view were fielded by different sections and individuals. In summary:
Other points included:
Limitations on using trees and forests as coastal protection measures against tsunamis:
Interactions between trees/forests and tsunamis:
The wealth of information and viewpoints makes summary problematic. Overall, the consensus during the workshop was that trees and forests can provide protection against tsunamis. The main questions, therefore, revolve around the quantity and quality of forest necessary to provide protection against a tsunami under a set of local conditions, and whether the result can be implemented under the prevailing socio-economic conditions. To make the diagnostic tool operational, and notwithstanding questions of feasibility, it would be necessary to bring together detailed information quantifying the physical parameters that define the effectiveness of trees and forests in protecting against tsunamis. This basic information could then be applied at the local level prior to implementation of analyses to assess environmental, social and economic costs and benefits of trees and forests in comparison with other forms of coastal protection.
The diagnostic tool developed by the cyclone group is given below. The key decisions relate to the size of the expected storm surge. If the surge is expected to be above five metres, other measures should be taken as trees and forests will have little impact on influxes of water of this magnitude. There is a further important point: Where coasts are eroding, trees will not secure the coastline in the long term and engineered measures are required. This point also relates more generally to the other coastal hazards. Other key points relating to the potential protection provided by trees and forests against cyclones were:
A major misapprehension concerning the effects of cyclones is that wind is the main threat to life. As stated earlier, water kills many more people than wind and this has important repercussions on the protective functions of trees and forests. Because flood levels are likely to remain for a day or more, porous structures such as forests will not prevent the influx of water. They will, however, attenuate wind-driven waves. Given these observations, it is clear that a broad breadth of tree cover would be necessary to provide protection from cyclones. It is commonly held, for example, that the Sunderbarns in Bangladesh afford cyclone protection and recent data from Orissa, India indicate that a 0.5-kilometre band of mangroves can protect lives with a statistical confidence level of 95 percent, whereas a 1.5-kilometre strip can protect lives with a confidence level of 99 percent (Saudamini Das, unpublished data). More data are likely to emerge to further inform on this issue, but with the large areas of land required for tree planting, apropos protection of people and assets, it would be more likely that the above arguments would be used in calls for conservation rather than the establishment of forests exclusively as protective measures against cyclones.
Exhibit 1 Cyclone diagnostic tool
The diagnostic tool developed for wind and salt spray followed a similar approach to that adopted by the cyclone group. Information required for the tool relating specifically to wind and salt spray included the nature of the local winds, whereby coastal shelterbelts would only be appropriate where onshore winds occurred. It was also noted that deep-rooted trees should be used to enable water uptake and to provide adequate support in sandy soils; moreover, they should also be sturdy and salt-resistant. Further information could be included in the tool to specify the degree to which salt spray and wind effects could be alleviated by using shelterbelts and also the area over which benefits would be experienced. It should be noted in this context that salt may be suspended throughout the boundary layer and transported inland. Larger crystals of salt are likely to fall out of the air mass under less turbulent conditions, but smaller particles may be carried far inland. Therefore, the effects of coastal shelterbelts may be fairly localized and tree planting would probably be best for protecting specific valuable structures or assets. For additional information see Takle et al. (this volume).
Exhibit 2 Wind and salt spray diagnostic tool
The coastal erosion group developed a tool based on a coastal protection strategy including possible short- and long-term measures in combination with prioritization of areas according to the severity of erosion. The group agreed that economic, environmental, social and cultural values are likely to be subjects of protection from coastal erosion, and that national governments should determine priorities, depending on their specific situations.
In the short term, it was proposed that engineering solutions should be targeted at critical areas, according to prioritization of all areas. In the long term, guidelines, shoreline management plans within Integrated Coastal Zone Management (ICZM) initiatives and coastal laws should be developed to support short-term measures and overall aims. The following levels were categorized to facilitate prioritization:
According to this prioritization, critical and significant erosion areas are protected first to prevent loss of land, natural resources and heritage, to avoid displacement of population and to maintain/improve tourism value.
Once consensus is reached on the areas in which protection is needed, processes are initiated to consult local residents and stakeholders, collect and analyse data and develop an understanding of erosion and key coastal processes in the area. Following this process, available options, including hard engineering, soft engineering, reforestation and combined measures are considered for a period according to the urgency of the situation, with critical situations requiring more immediate action. Points considered include:
In summary, it was noted that reforestation is not appropriate for all areas and the following criteria are mandatory:
Following this assessment, an identification of minimal data requirements for reforestation is carried out and reviews are undertaken by experts at critical decision-making stages throughout the process. A note was made that the tool focuses on continental coastlines and mangroves, and that small islands require different approaches and considerations as the environmental conditions and guiding principles are different.
Little information was included in the diagnostic tool on the circumstances under which trees could be suitable for arresting or reversing coastal erosion. In relation to this, it should be noted that trees may not decelerate the pace of erosion at all on naturally eroding coastlines, and therefore, local circumstances should be reviewed before decisions are made.
Exhibit 3 Coastal erosion diagnostic tool
A number of elements contained in each diagnostic tool were generic and could be used for a variety of coastal hazards with minor adjustments made to accommodate technical information. The following questions were central:
To further develop and operationalize these diagnostic tools, a greater level of technical detail is required to quantitatively define the relationships underlying points 2 and 8 above. This information has proved to be unavailable in forms readily accessible to foresters and policy-makers, but discussions during the workshop indicated that there is further information available to quantify the degree to which trees can protect against a coastal hazard under given sets of coastal conditions. A number of the technical experts present at the workshop supported follow-up work to unearth, consolidate and make public such information, and by doing so, resolve some of the controversy over the protective capacity of trees and forests — especially in relation to tsunamis.