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An integrated inter-sectoral approach consists of two primary phases: 1. Hazard Vulnerability and Risk Assessments 2. Mitigation Strategy Planning.
Approach # 1. Hazard Vulnerability and Risk Assessments:
It establishes the baseline context for integrated decision-making.
It consists of four parts:
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1. Define the boundaries of the project area (entire country, one community, etc.)
2. Hazard identification
3. Vulnerability assessment
4. Risk assessment
1. Define the Boundaries of the Project Area:
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The coastal hazard assessment will be used as the foundation for long-term coastal management planning. Because of the multipurpose, multi-sectoral uses to which the assessment will be applied, a map of the study area is important.
Define Mapping Protocols:
Define scale and reconcile data sets for baseline variables and features for orientation which could include:
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i. Coastline and offshore limits of interest;
ii. Waterbodies (rivers, lakes, other inland waterbodies); topography;
iii. Major ecosystem features (forests, dunes, others);
iv. Major transportation linkages that may constrain the area available for planting; and
v. Land uses that impact (or possibly encroach into) ecosystems.
2. Hazard Identification:
Identify key hazards:
On a national basis, the probability of specific hazards occurring in individual communities will differ depending on such variables as climate, geology, bathymetry/ topography, coastal geometry and land-use patterns. For some hazards, the entire community will have similar susceptibility, such as from a cyclone.
For others, such as flooding, some portions of the community may be impacted more than others; for example, low-lying areas are more susceptible to inundation. For this reason it is important to obtain maps for as many types of hazards as possible and to clearly delineate the specific characteristics and small- scale location-based variables that will become important considerations when developing a mitigation strategy. Table differentiates hazard exposure between two countries.
Define incidence of previous disasters and document impacts:
There are extensive data on damage from the 2004 tsunami. In addition to the tsunami, it is important to analyse damage from lesser, but more frequent, events to begin assessing cumulative past losses. An electronic version of data sets will facilitate correlation of multiple variables using GIS. In some cases GIS maps may not be available, in which case it will be necessary to rely on qualitative information such as oral histories.
For each hazard, variables could include, but not be limited to:
i. Inundation boundaries;
ii. Other location indicators; and
iii. General characteristics including, but not limited to, secondary effects such as location of scour, sediment transport and others.
3. Vulnerability Assessment:
(a) Identify and Characterize Impacts from Prior Events:
Within the area identified during the Hazard Analysis, identify and characterize damage and impacts of prior disasters as well as those impacts that can be expected from future events such as coastal flooding, riverine flooding, landslides, cyclones and tsunamis.
(b) Correlate Effects with Coastal Geometry:
Tsunami behaviour varies in relation to topography, location (especially with respect to islands) and coastal geometry (Fig. 6.1). In areas where the coastline is relatively even, vegetation can buffer the effects of the wave; in areas where there is considerable articulation, wave forces can be considerably higher.
Some harbours may be in protected locations from the standpoint of wind energy because they are in narrow bays or have headlands that dissipate wind energy. On the other hand, such features can also focus or amplify the wave, in which case tsunami energy is focused against the infrastructure, causing extensive damage to harbour facilities and boats.
This appears to have been the experience of Hamatsumae on Okushiri Island, Japan, in 1993. Tsunamis treat rivers the same as harbours; once they enter the mouth, the wave travels significant distances upriver. Sri Lanka’s southeast coast, which is characterized by riverine estuaries, narrow-mouthed bays and lagoons bordered by sand dunes, flat sandy beaches and headlands, experienced significant damage from the 2004 Indian Ocean tsunami.
In four case study locations that suffered particularly severe damage, near shore transformation processes interacted with the shoreline geometry. In each location, tsunami waves were funneled into a narrow bay or lagoon where the younger sand dunes in low-lying land between older dunes were breached. The combined interaction contributed to extensive damage to buildings and the devastation of vegetation, including mangroves, palm trees and shrubs.
Tsunami effects are also increased in the lee of circular-shaped islands, as demonstrated by the devastation to Aone on Okushiri Island in 1993, and Babi Island in Indonesia.
Ecosystem features (offshore and onshore): Ecological features in the offshore and near shore environments have dual importance because:
I. They are prone to damage such as coral reefs, dunes and vegetation including trees, shrubs and grasses;
II. They can reduce damage farther inland. They are the first line of protection against most coastal hazards because they create friction, thereby mitigating the forces of strong winds and waves.
Offshore:
I. Coral reefs:
a. Reefs constitute natural breakwaters that can reduce coastal erosion.
b. The health of coral is indicative of many phenomena which need to be examined. For example, erosion elsewhere can result in sediment, which is being transported and deposited on the reef; unhealthy reefs can also be indicative of poor water quality, or of extreme turbulence during cyclone flooding.
II. Sand dunes, berms, wetlands and marshes
a. The presence and condition of dunes and/or berms/wetlands reduce impact and velocity.
b. The health of dunes and berms is indicative of various phenomena, including sediment transport, and deposition rate influenced by erosion, or (conversely) the stabilizing influence of planting programmes.
III. Indications of bank erosion from scouring:
Scouring and bank erosion impact vegetation by undercutting the buffer from direct wave impacts. It also results in sediment transport which can negatively impact the micro-ecosystem.
IV. Assessment of forests and other vegetation:
Presence and characterization, including recent changes of vegetation: dune grasses and creepers; shrub forests; paddy fields etc.
Artificial features (land use and infrastructure):
Land-use patterns are a reflection of changing demographics and settlement trends. In some instances, lack of institutional oversight contributes to, or even creates, unsafe conditions by allowing such practices as encroachment into floodplains, inadequate drainage provisions, filling of wetlands and destruction of coastal vegetation, including dune grasses and mangrove forests.
All of the above practices may further exacerbate the impacts of natural hazards including slope instability, erosion and siltation which, in turn, lead to increased frequency and losses from small- and medium-size disasters. Current conditions and practices must be documented as benchmarks that can be compared with past land-use patterns.
To monitor trends, the documentation will identify changes that have occurred during a specified time period, for example, over the last 25 years. Land management .practices that could influence the future will also be identified, for example, encroaching urbanization which threatens forested lands.
Inventory elements include:
I. Land use:
a. Built up areas including houses, hotels and related uses;
b. Changes in land use (e.g., abandoned fields, or paddies) or proposed changes (e.g., new hotels or harbour facilities);
c. Schools, hospitals, other buildings with potential human mortality or community importance;
d. Paddies and agriculture;
e. Cultural and archaeological sites.
II. Infrastructure:
a. Major roadways and railroads;
b. Ports and fishing harbours;
c. Coastal and shore protection (seawalls, dykes, etc.);
d. Others.
III. Interactions:
Interactions between the components of the coastal management area are important to identify. Such interactions include utilization patterns of the various ecosystem regions.
4. Risk Assessment: Potential Loss Assessment:
Risk provides the basis for decision-making and institutional acceptance of protective measures. Risk is calculated by correlating information derived from the Hazard Assessment and the Vulnerability Assessment, i.e., Hazard + Vulnerability = Risk. The characteristics of risk are then analyzed in terms of estimated probability of occurrence, magnitude and incidence of losses, which can be calculated both in quantitative or qualitative terms. Spatially correlate hazards and designate “hot spots” where multiple occurrences or types of events occur, for example, coastal erosion or coastal flooding.
Calculate Probability of Occurrence:
Frequency of events is an important indicator of both past and future loss patterns. Because cumulative implications are important, the analysis must consider not only a large event such as a cyclone or tsunami, but also multiple and less severe events such as winter storms.
Annualized losses over a ten- or 20- year time frame from lesser events may equal or even exceed the losses from a large event. The probability of occurrence is based on frequency, as documented by historical records and scientific evidence. The time period for re-occurrence is based on criteria selected for a specific plan, for example, over 30 years, the frequency that an event may occur will be of high, medium or low probability.
Communities in close proximity to each other often have different probabilities of hazard occurrence.
A comparison of two communities in the southern and eastern portions of Sri Lanka illustrates similarities and differences in probable occurrences:
Community # 1:
(Table 6.2) Hikkaduwa, is flat; prone to coastal and riverine flooding, bank erosion and storm surge.
Riverine flooding is often accompanied by channel migration with extensive sediment transport and/or deposition. The probability of coastal storms, riverine flooding and coastal erosion is high because the return period is annual. The historical experience of cyclones impacting Hikkaduwa is moderate; the geological evidence indicates that the probability of another tsunami impacting the area is also low, because the frequency is very rarely greater than every 15 years.
Community # 2:
(Table 6.3) Arugam Bay, on the other hand, is characterized by variable flat areas, which, unlike Hikkaduwa, are not prone to landslides. On the other hand, Arugam Bay is prone to both riverine and lagoon flooding. Some high probability events may have low consequence individually, but may occur many times each year.
Over a 20- or 30-year period, losses such as from coastal erosion could be significant. Conversely, the consequences (losses) from a single cyclone or a tsunami would be high. The consequences from the more severe event may —or may not —exceed the more frequent lesser hazards. Weighting of the consequences is therefore an important aspect of the risk assessment and the ensuing development of the mitigation strategy plan.
Tables and compare the probability/frequency and consequences of hazard occurrence for the two communities in Sri Lanka. Note that the vulnerabilities differ for the two communities, and thus, eventual priorities for mitigation strategies, such as forest planting, will also differ.
Comparison of characteristics and the approximate magnitude of potential loss under alternative event scenarios are important factors to help evaluate the consequences of various scenarios. The consequences should be evaluated in terms of the four variables identified during the Vulnerability Assessment: ecosystems, influences of geomorphology, and societal and economic variables (land use and infrastructure, existing protection) breakwaters, dykes, revetments, etc. demographic profiles, economic variables.
Approach # 2. Mitigation Strategy Planning:
It establishes the means to reduce the risk of losses. Such loss reduction is achieved through the application of mitigation tools and implementation strategies that address risk characteristics that are defined during the risk assessment.
The mitigation phase consists of two parts:
Part I: Identify mitigation tools;
Part II: Evaluate and select mitigation tools.
Part I: Identify Mitigation Tools:
A variety of actions to reduce the likelihood of losses are identified. Specific objectives and implementation priorities are tailored to community needs and the characteristics of hazard exposure.
Engineered Approaches:
Engineered barriers must be able to withstand overtopping wave forces at crest level. Such barriers are expected to remain stable during the progression of the storm event, including during tsunami run up and rundown. If such a system is breached at a weak point, there is a high possibility of progressive collapse leading to greater inundation.
Breakwaters and Seawalls:
A breakwater is an offshore structure providing protection from wave energy or by deflecting currents. A seawall is a hard coastal defense constructed onshore to prevent the passage of waves and to dissipate energy. Modern seawalls tend to be curved to deflect wave energy back, thereby reducing forces.
In the event of overtopping, designs typically incorporate drainage systems. Seawalls can be effective defenses in the short term. In the long term, however, the backwash tends to be reflected to the beach material beneath and in front of the seawall, which is erosive. Specific design solutions and ongoing maintenance are important considerations to reduce such negative effects.
Dykes and Levees:
A dyke (also known as a levee) is an artificial earthen wall built along the edge of a body of water such as a river or the sea to prevent flooding. Dykes are often found where low banks or dunes are not strong enough to protect against flooding. Dykes and levees require regular maintenance, which, if neglected, can have disastrous consequences.
Revetments:
Revetments on banks or bluffs are placed in such a way as to absorb the energy of incoming waves. They may be either watertight, covering the slope completely, or porous, to allow water to filter through after the wave energy has been dissipated. Waves break on revetments as they would on an unprotected bank or bluff, and water runs up the slope. The extent of run up can be reduced by using stone or other irregular or rough-surfaced construction materials.
Ecosystem Management:
Ecosystem management, including the use of vegetation, has been recognized as an important means to reduce exposure to multiple hazards. Non-structural tools through ecosystem management create friction to slow velocity; they constitute porous barriers against wind and waves.
The underlying purpose is to prevent or reduce the erosion of coastlines, estuaries and riverbanks through three main processes:
I. Functioning as a porous buffer by creating friction, thereby reducing wave action and current energy.
II. Binding and stabilization of the substrate by plant roots and deposited vegetative matter to reduce erosion.
III. Trapping of sediments.
Enhance Coral Reefs:
Coral reefs are the first line of defense to attenuate wave energy. Preserve and enhance dune formation and sand bars: Dune formation and restoration is achieved by stabilizing the soil. The first colonizers on bare sand are a species of plants known as creepers. Wind-borne sand collects in and around them as they grow, forming small hummocks, which are then colonized by fresh seedlings.
Gradually sandy hillocks are formed and additional species colonize and stabilize the sand, preventing wind-induced erosion. Gradually, the soil quality improves to establish suitable conditions for the growth of more substantial shrubs, which in turn create favourable conditions for the growth of trees.
Planned Forests (Porous Barriers):
Dense plantings of trees (planned forests) have multiple functions. The natural porous structure of littoral woodlands with deep roots generates a stable barrier against wind and wave forces. They can be an efficient natural energy absorber of steady flows and long waves.
They are also an effective means to stabilize banks from erosion and scouring. Such stabilization will also reduce downstream siltation Many communities impacted by the Indian Ocean tsunami have cited the presence of mangroves as positive contributions to the mitigation of wave velocity and amplitude. It is essential to recognize that some species of mangroves are more appropriate than others, because each species has differing characteristics. It is also vital to consider that the geometry of the site will influence the behaviour of the vegetation.
When the 1998 Papua New Guinea tsunami occurred, many people were killed or maimed as they became impaled on splintered mangrove trees. Others took refuge in palm trees that became flying missiles when uprooted. Reports indicate that people who took refuge in Casuarina trees survived.
A number of uncertainties remain to be investigated, including whether the palm trees were shallow-rooted or whether the instability resulted from geological conditions such as a shallow clay layer, or the width of the forest was too narrow.
Wetlands:
Wetlands of various types provide coastal protection functions which are similar to the protective functions of vegetation. Both features create friction, which slows the speed of the waves. They also create opportunities for water detention and retention.
Hybrid Strategies:
The relative effectiveness of mitigation tools is evaluated in relation to specific community benchmarks or goals and priorities, which are defined by local stakeholders based on the risk assessment. Priorities are established to minimize risk, based on the probability of occurrence(s) and/or anticipated consequences. Table 5.4 illustrates the correlation of goals with alternative mitigation strategies.
Table Correlating goals with alternative mitigation tools (using a tsunami as a sample hazard)
Part II: Selecting and Evaluating Integrative Mitigation Strategies:
Mitigation strategies are typically hybrid approaches that combine a number of measures to maximize benefits while addressing the unique characteristics and requirements of a site and a community. It is incumbent on each community to identify alternative actions potentially appropriate to its requirements, and to evaluate these strategies in relation to its unique priorities.
Integrative mitigation strategies:
No mitigation tool is responsive to all hazards or appropriate for all locations. Hybrid approaches integrate diverse tools, for example, forest planting with land use and infrastructure planning and vegetation management programmes. Mitigation entails difficult choices between competing claims on fragile areas.
Choices will involve tradeoffs and the need to reconcile opportunities for ecosystem enhancement or restoration such as forests, preserving wetlands, re-establishing dunes or mangroves; securing infrastructure; and re-establishing tourist, agricultural or fishing industries.
Evaluation criteria must address such variables as frequency of hazard occurrence, as well as consequences which are quantifiable (for example, the number of hectares of destroyed ecosystems, potential lives lost, cost to construct and maintain) and others that are qualitative (for example, social dislocation and opportunity costs in terms of lost opportunities).
A word of caution at this point is important. Land-use decisions pertaining to the coastal zone are invariably complex and often highly politicized. The thumbnail summaries below are only intended to exemplify complex considerations addressed by the decision-making process. They therefore do not capture the subtleties of political processes that erupt over allocation of scarce land uses.
Case Study:
Hilo, Hawaii Tsunami Reconstruction: Central Urban Core.
Background:
In 1946, Hilo, Hawaii, was struck by a tsunami generated by an earthquake in the Aleutian Islands; it was struck again in 1960 by a tsunami generated by the great Chilean earthquake. Both events inflicted significant damage on Hilo’s downtown urban core, located at the head of Hilo Bay. Because of its crescent shape, wave forces were focused at the narrow end of the bay. In both events, the tsunami overtopped the Hilo breakwater.
The two Hilo case studies illustrate differing approaches to hazard mitigation that have been adopted for Hilo’s coast. The differences, in part, reflect different timing for plan preparation; Project A was prepared in the immediate recovery period after the 1960 tsunami, while Project B (located outside of the urban renewal area but within the tsunami experience area) was prepared approximately 18 years after the tsunami.
Mitigation Concept:
After the second tsunami in 1960, a multicomponent plan was proposed to rebuild Hilo’s downtown core consisting of the following activities (Fig. 6.9):
I. Increase the height and length of the existing breakwater and create a large dyke along the waterfront to protect development.
II. Construct a redevelopment project outside the inundation area.
III. Dedicate damaged areas as open space for park use.
IV. Plant a dense tsunami “forest.”
Project status and evaluation considerations:
Breakwater:
Prior to the tsunami, Hilo was protected by a breakwater that had been designed and constructed against winter storms. It had been constructed between 1908 and 1929 upon a submerged reef in Hilo Bay. Immediately after the tsunami, the US Army Corps of Engineers (COE) approved funding to extend the breakwater by 4,000 feet (1,219 meters) and raised its height to 20 feet (six meters) above mean sea level.
During the 1960s and 1970s, the community debated the advisability of increasing the breakwater height, because modification of the breakwater would become a strong visual statement. Public opinion viewed the breakwater as a “towering” wall that would block views to sea. Business interests also questioned the aesthetics of the breakwater, which they feared could negatively impact tourism.