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This chapter describes methods for integrating natural hazard assessments into natural resource evaluations, with emphasis on land-use evaluations. It also explains the role of ecosystems in naturally mitigating or intensifying hazardous events and how this role is affected by development.

During the initial stages of a regional development study, a region's problems and potentials are diagnosed. An assessment of the natural resource base is fundamental to any development planning and project formulation effort. This provides baseline information that will help in formulating a strategy and identifying projects. Land-use studies, including present land use and land capability, are part of these evaluations and require mapped information on resources and natural hazards. The planning process should identify all assumptions and reveal potential conflicts between current and proposed development activities and natural hazards. For example, deforestation on unstable soils may increase landslide activity upstream of a reservoir, resulting in high siltation, and shorten the life of the reservoir. Execution of an agricultural scheme in a flood plain may result in flooding of the project or in excessive expenditures to mitigate the effects of the flood. Although hazard assessments should take place throughout the planning process-especially during land-use evaluations-the evaluation of natural hazards generally receives minimal attention.

Natural hazards influence the security and viability of projects and communities. Furthermore, because they influence land use, they should also influence land-use decisions. The first objective of this chapter is to provide guidance for integrating natural hazard assessments into land-use evaluations. Among the many natural services provided by ecosystems is the mitigation of natural hazards. For example, a coral reef causes large waves to break some distance from the shoreline, reducing the impact of tropical storms; but if harbor development breaks down the coral, the natural protection is lost. This chapter examines the mitigating effects of ecosystems and the precautions necessary to ensure that unsound development does not undermine that effect.

A second objective of the chapter is to provide a synthetic view of the natural mitigation of natural hazards. By way of introduction to the detailed material on individual hazards and their assessment in Part III, it briefly examines the implications of development for the natural mitigation of all major hazards by setting them in a realistic, albeit hypothetical, landscape. This composite system examines the implications for volcanic activity, earthquakes, landslides, hurricanes, flooding, and desertification of upland (highlands, piedmont), coastal or lowland, near-shore (reefs and estuarine) and marine (open sea) ecosystem. Thus a second objective of the chapter is to provide a synthetic view of the natural mitigation of natural hazards.

Both objectives contribute to an overriding objective of promoting the consideration of natural hazards in the context of the system in which they occur.


The methods of land-use evaluation used in Latin America and the Caribbean demonstrate the difficulties in understanding nature and the limitations the planner's training, experience, and interests bring to decisions concerning land use. Land-use evaluation methods are always subjective, as can be observed by comparing the results of the application of a number of methods currently in use. Several of these methods were reviewed by Posner et al (1982) in their preparation of a land classification system for the steep lands of the northern Andes. With notable exceptions, the methods reviewed emphasized soil analysis. Since most soils specialists are also agronomists, the results are skewed towards agriculture.

The land-use evaluation methods generally used in Venezuela, Nicaragua, and Mexico are based on the methods of the Soil Conservation Service of the U.S. Department of Agriculture (USDA, 1938). These methods are widely accepted, but they have been criticized as inappropriate for developing countries. Also included in the review cited above was an evaluation method developed in Central America in the 1960s by the UN Food and Agriculture Organization (FAO). This method was also extensively criticized, as having "a flatland bias," and was replaced by a method developed in Africa that was based on the number of growing days for several crops. These two and the "Integrated Ecological Land Capability Classification" (IELCC) method developed in Latin America are major examples of methods that are not based solely on soil analysis and slope characteristics.

The IELCC method is based on the World Life Zone System of Ecological Classification by Holdridge (1967). It has been adopted as the official land classification system in Peru and has been used to map virtually all Central and South American countries. Of all the land-use evaluation systems in use in Latin America, this one is perhaps the most "complete" (Tosi, 1988) in that it includes bio-climate, land gradient, and micro-relief observations as well as alternative levels of technology that could be used in land management. Factors that influence social and economic risk as well as soil depth and texture, stoniness, soil permeability, fertility and pH, accelerated soil erosion, salinity, and flood hazards are then analyzed to suggest land use at a local level.

Modern technology is also used in the evaluation of land-use capability. The French Overseas Scientific and Technical Research Organization has mapped a large portion of the Andean highlands in Ecuador using satellite imagery. This information is used in regional development programs. A computerized mapping system (1:1,000,000 scale) was designed by the International Center for Tropical Agriculture in Colombia to support land-use decisions in the lowland tropics of Latin America.

The purpose of these methods is to assess the characteristics of a site in order to make decisions concerning its capability and/or suitability for use. Several conceptual problems add to the deficiencies of current land-use evaluations, starting with the terms "capability" and "suitability" themselves. Although they are often used interchangeably, they do not mean the same thing (AAAS, 1983). "Land-use capability" is the more general term and makes reference to limitations such as the degree of stoniness or slope that can negatively affect use. "Suitability" on the other hand, refers to qualities that permit specific land uses such as irrigation or the production of a certain crop. The term "capability" is heavily identified with the U.S. Soil Conservation Service method and its agricultural bias. In order to avoid confusion in some contexts, the term "suitability" may be preferable (FAO, 1976). A more significant problem arises from the capacity of current technology to render almost any land area "capable" of almost any use if the necessary investment is made (Hawes and Hamilton, 1980), although specific areas are more physically "suitable" for a given use than others. Land-use decisions are based on a number of factors in addition to the physical landscape.


1. Limited Emphasis on Cultural Components
2. Lack of Standard Procedures to Incorporate Information about Risk from Natural Hazards

Current land-use evaluation methods and their application are extremely limited for two main reasons: they show little interest in the cultural components of the landscape, and they lack standardized procedures that would make manifest the relationship between proposed land uses and natural hazards.

1. Limited Emphasis on Cultural Components

Although land-use classification systems are still generally based on physical data (Beek, 1978), most writers and practitioners acknowledge the importance of socioeconomic data in making land-use decisions. Less emphasis is placed on cultural factors, which are often more important than physical and even economic and social characteristics in determining land-use patterns.

For example, in Saint Lucia, areas that are potentially productive according to soil and slope parameters and to the prevailing social and economic factors do not sustain the activities that a land-use evaluation would assign to them. The reason these areas are not used is the fear people have of the fer-de-lance, which was introduced to the island and has taken refuge in these areas. This fear is so great that the national agricultural development plan had to include a project to eradicate this viper so that the area could be put into agriculture production.

A typical cultural bias which intensifies hazardous phenomena in many parts of Latin America, favors livestock ownership, because of the prestige and authority it brings. People will own as many head of livestock as they can afford, preferably cattle, even if the biotic, climatic, edifice, economic, and social characteristics of the area are unfavorable for grazing (Clausen and Crist, 1982). Poorly managed grazing often results in the intensification of such natural phenomena as erosion and mass movement of soil in much of Latin America.

Cultural factors that affect land use include information, technology, and any number of biases and taboos. Production of a given land unit depends on the knowledge of the resource manager, local taboos, the availability of appropriate technology, and the willingness of the local culture to accept the proposed technology and land use. Because cultures can be remarkably different from one another, land-use evaluations cannot be standardized for similar physical conditions. People living and working in a given space often disregard the proposals of studies on the physical parameters of the area. Evaluations can only suggest the potential for production and loss under a specified land use; they cannot dictate a decision, which depends on the characteristics of the populations affected.

2. Lack of Standard Procedures to Incorporate Information about Risk from Natural Hazards

A significant limitation of all land-use capability evaluation methods is that they do not adequately portray the risks that natural hazards pose to development activities. Yet reviews of resource evaluation methods (McRae and Burnham, 1981; AAAS, 1983) indicate that most do discuss natural hazards briefly and that it should be easy to incorporate information about them throughout the planning process. Numerous studies have been made on the assessment and display of specific hazards such as landslides (Varnes, 1985; Brabb and Harrod, 1989), earthquakes (Blair and Spangle, 1979; Jaffe et al., 1981; Brown and Kockelman, 1983; Kuroiwa, 1983), flooding (U.S. Water Resources Council, 1972; Waananer et al, 1977), tsunamis (Houston, 1980; URR, 1988), and volcanoes (Booth, 1979; Crandel et al., 1984). However, there is no standard method for assessing natural hazards in resource evaluations for development planning. Different methods are a response to specific concerns about individual hazardous phenomena.


1. A Systems View
2. Systems Attributes

Since approaches to the evolution of individual natural hazards are detailed elsewhere, this section looks at them from the point of view of the system in which they occur, within the context of land-use evaluations.

1. A Systems View

The combination of attributes of a landscape and the linkages between them can reinforce or restrict possible uses of the landscape. Hence, landscapes should be regarded and studied as systems (Chapman, 1969; Steiner and Brooks, 1981; Rowe and Sheard, 1981; Steiner, 1983). A systems view takes in a broader array of attributes and linkages than is normally considered in current land-use evaluation methods (see Figure 3-1), including as it does the relationships between natural phenomena, development activities, and natural elements (Hawes and Hamilton, 1980; FAO, 1976; Posner et al., 1982). Merely listing the important natural elements-slope, exposure, climate, evapotranspiration rates, surface water availability, and others (see Figure 3-2)-though helpful, is an incomplete approach that fails to integrate natural hazards information into land-use evaluations.

For purposes of land-use classification, all landscapes must be thought of as systems which provide goods and services for the satisfaction of human needs. Any aspect of a system structure and function that is of human interest can be classified as a system good or service (OAS, 1987). Photosynthesis, for example, produces biomass that becomes wood and then, through human activity, timber. If the system attribute is dangerous to human activity (e.g, high wind or heavy precipitation), it is considered a hazard. However, since the needs of humans vary, individuals will value system attributes and processes differently; goods and services valued by some may have no meaning for others. For some, the danger inherent in a specific system attribute makes it a service (e.g., rapids to run, mountains to climb). On the other hand, some phenomena are always hazardous (e.g., lava flows). It is these that must be considered in land-use evaluations.

The analysis of the goods, services, and hazards of a system, together with the needs of its population, permits the identification of alternatives not normally defined in land-use evaluations. This is consistent with the purpose of a systems analysis land-use evaluation, which is to formulate a strategy that includes the use, improvement, and conservation of the region's potential goods and services. Figure 3-3 is a regional model with examples of internal and external linkages.

Human needs involve nutrition, shelter, and personal or collective security. Landscapes contain structures and elements that are hazardous and that can negatively influence the secure appropriation of the goods and services indicated by a land-use evaluation. Figure 3-4 lists attributes of ecosystem structure and function that provide a wide array of goods and services that satisfy human needs; Figure 3-5 identifies other attributes that are hazardous.

Figure 3-1 - An Environmental Complex

Source: Based on Billings, W.D. "Physiological Ecology" in Annual Review Plant Physiology, vol. 8 (1957), pp. 375-392.



Related Land Qualities

Air temperature

Frost risk

Precipitation, including distribution and intensity

Erosion, flooding, moisture availability

Wind speed and direction

Evapotranspiration, storms, wind erosion

Net radiation


Hail and snow

Climatic hazards



Depth to water table

Drainage and aeration

Frequency of flooding

Drainage and aeration

Soil texture and stoniness

Ease of cultivation, moisture availability, drainage, aeration, water/wind erosion, permeability

Visible boulders/rock/outcrops

Ease of cultivation, moisture availability

Soil depth

Moisture availability, rootability, ease of cultivation

Soil structure, including impermeable layers, crusting, compaction

Water/wind erosion, rootability, moisture availability

Organic matter and root distribution

Moisture availability, water/wind erosion, ease of cultivation

pH (reaction)/CaCO2/gypsum

Soil fertility, soil alkalinity

Clay mineralogy

Water erosion, ease of cultivation

Soil chemistry

Fertility, nutrient availability, toxicities

Soil permeability

Drainage and aeration, moisture availability

Available water capacity

Moisture availability


Water erosion, flooding

Soil salinity

Drainage, toxicity, flood

Soil parent material

Fertility, nutrient availability, including deficiencies and toxicities

Source: Adapted from McRae, S.G., and Burnham, C.P. Land Evaluation (Oxford: Clarendon Press, 1981).

Figure 3-3 - Regional model showing examples of internal and external linkages

2. Systems Attributes

a. Linkages and System Function
b. Limiting Factors
c. Buffering
d. Thresholds

a. Linkages and System Function

It is not just the basic components, but the linkages between them, that make a system. They are the "tubes," "wires," and "connections" that relate one component of a system to another. The "First Law of Ecology" concerns linkages: "Everything is related to everything else" (Commoner, 1971). The sheer number of interconnections present in any given system makes methods that can identify these linkages valuable tools for planners (Steiner and Brooks, 1981).

A basic array of linkages can be identified for any ecosystem: terrestrial, marine, or urban. In all cases, these linkages have to do with the flow of material, energy, or information between components. It is important to identify and evaluate linkages between as well as within ecosystems (Karrnd Schlosser, 1978). The characteristics of a lake ecosystem, for example, depend on all the human activity around that lake, including activities that take place in the rivers that feed it and the chemical characteristics of the precipitation in its watershed. Exchanges of material and energy between ecosystems also influence the nature, timing, and severity of hazardous events. Earth tremors causing landslides many miles from the epicenter, and heavy rainfall-or the thawing of snow and ice-hundreds of miles upstream causing major flooding downstream, are two examples of linkages between seemingly unrelated ecosystems.

Most severe hazards involve the flow of energy (ecosystem function) rather than its storage (ecosystem structure). Despite this fact, structure and function are often studied separately. Consequently, land use is not studied in a system context, and the hazard analysis suffers accordingly.

Of equal importance are linkages between physical and biotic attributes of an ecosystem on one hand and social, cultural, and political factors on the other. The construction of a road or the urbanization of upstream areas will have a major influence on the flood hazard for that watershed. Ecosystem dynamics include human-induced and natural phenomena. Natural hazards, such as an excess or scarcity of water, can be intensified by human activity both inside and outside the system being studied. Unfortunately, off-site activities and events that can influence the project area are seldom considered in land-use evaluations.



1. Potable water (surface and ground)
2. Industrial water (surface and ground)
3. Irrigation water (surface and ground)
4. Timber
5. Firewood/charcoal
6. Construction material from wood (posts, beams, etc.)
7. Ornamental plants (indoor, landscaping, dry)
8. Vegetable fibers (rope, cloth)
9. Medicinal plants
10. Food for humans (fruits, nuts, sap, shoots, seeds, gum, honey, leaves)
11. Food for domestic animals
12. Food animals for human consumption
13. Aquatic plants for human consumption
14. Condiments (spices, salt)
15. Plant chemical substances (dyes, stains, waxes, latex, gums, tannins, syrups, drugs, etc.)
16. Fertilizers
17. Aquatic precious/semiprecious materials (pearl, coral, conchs, mother of pearl)
18. Materials for artisan work (rock, wood for carving, fibers for basket making)
19. Metallic minerals (bauxite, ores, nuggets)
20. Non-metallic minerals (asbestos, clays, limestone)
21. Construction materials (sands, clay, cinders, cement, gravel, rocks, marble)
22. Mineral nutrients
23. Mineral dyes and glazes
24. Hides, leather, skins
25. Other animal materials (bones, feathers, tusks, teeth, claws, butterflies)
26. Other vegetative material (seeds, pods)
27. Live fish (ornamental)
28. Live animals for pets and zoos
29. Live animals for human work
30. Live animals for research
31. Fossil fuels (crude oil, natural gas, coal)
32. Other fuels (peat, other organic matter, dung, biomass)
33. Livestock forage protection


1. Nutrient cycling

2. Nutrient storage

3. Nutrient distribution (floods, dust, sediment transport)

4. Photosynthesis

5. Respiration

6. Oxidation

7. Adaptation

8. Self-regulation

9. Competition testing and design (population control, evolution)

10. Mineral cycling

11. Habitat for local land, air and aquatic animals, insects, and other life forms (feeding, breeding, nursery, shelter)

12. Habitat for migrating land, air, and other life forms (feeding, breeding, nursery, shelter)


1. Windbreak

2. Shade

3. Recreational use of water (swimming, boating, skating, water-skiing, sailing, surfing, snorkeling)

4. Recreational use of land (hiking, climbing, sports)

5. Recreational use of air (flying, gliding, parachuting, hang-gliding, kiting)

6. Recreational use of animals (sport hunting, sport fishing, horseback riding, insect collecting, photography, observation)

7. Recreational use of ecosystem (sightseeing, tourism)

8. Scientific tourism

9. Exploration

10. Wealth accumulation and speculation

11. Spiritual development

12. Historical values

13. Cultural values

14. Early warning system (weather, climate change, hazardous events)

15. Moisture modification

16. Temperature modification

17. Light modification

18. Ultraviolet and other radiation filtration

19. Storage of life from adaptive (genetic) information

20. Other scientific values


1. Energy sources (wind, solar, hydro, tidal, biomass, geothermal)
2. Dilution of contaminants
3. Decomposition of contaminants (oxidation, evaporation, dissolution)
4. Transport of contaminants (wind, water, animal consumption, air and water dilution)
5. Storage of contaminants
6. Erosion control
7. Sediment control
8. Rood control
9. Other control of water regime
10. Ground water recharge
11. Space for urban, industrial, agricultural occupation, roadways, canals, airports
12. Physical sites for structures
13. Climate control and protection
14. Disease control and protection
15. Storm buffer

Source: Organization of American States (OAS). Minimum Conflict: Guidelines for Planning the Use of American Humid Environments (Washington, D.C.: OAS General Secretariat, 1987).


1.Diseases and plagues (viruses, bacteria, flukes, parasites, fungi)
2.High water
3.Avalanches (landslides, landslips, debris flows)
4.Wind (tornadoes, hurricanes, cyclones, dust storms)
5.Natural erosion and sedimentation
6.Temperature extremes
7.Extremes of humidity
12.Fog, mist
14.Solar radiation
17.Toxic chemicals, gas concentrations
18.Nuclear radiation
24.Expansive soils
25.Noxious vegetation (poisonous plants, invader species)
26.Poisonous animals (reptiles, insects)

Source: Organization of American States. Minimum Conflict: Guidelines for Planning the Use of American Humid Tropic Environments. (Washington, D.C.: OAS General Secretariat, 1987).

Furthermore, not enough attention is paid to a number of natural impediments to development beyond stoniness, slope and occasional flooding. Structural components (soil texture, depth, and distribution; slope; vegetation density and type; base rock; and precipitation and temperature) are emphasized at the expense of the functional processes of the system (hydrological cycle, track and timing of storms, photosynthesis and respiration, shear strength of soil, rhythm, succession, and energy dissipation).

b. Limiting Factors

Natural phenomena may have positive effects on development or they may have negative effects and be limiting factors (see Figure 3-5). Removing a limiting factor-say, by reducing soil moisture through drainage or adding to it through irrigation-allows further growth and development. The action that removes a limiting factor, called a "trigger factor," creates chain reactions that can be far-reaching. For example, a landscape that can sustain a specific number of livestock under a given level of management may deteriorate or improve as a result of fire or heavy rainfall. This event, in turn, can initiate a chain of events leading to overgrazing, erosion, sedimentation, and flooding, on one hand, or to increased production of edible vegetation, fewer insects, and control of both plant and animal diseases on the other. Some natural phenomena can be limiting factors because they occur infrequently or not at all; inadequate precipitation is a good example.

Phenomena like high water are often considered limiting factors and are classified as natural hazards, but they can have positive effects on the proposed land use. For example, the Necholandia rangelands of the Pantanal area of Brazil have very sandy soils and soil nutrients are rapidly depleted with infiltration of precipitation. However, annual flooding of these soils for lengthy periods replenishes nutrients in the soils and sustains vegetation. Figure 3-6 lists other examples of natural phenomena with positive and negative attributes.

c. Buffering

Ecosystems are continually adaptive to change. This adaptability is attributable to a number of ecosystem characteristics such as species diversity and physiological variability, storage capacity, and cycling rates of nutrients and other materials. The resistance of an ecosystem to outside perturbations is high. Swamps, reservoirs, floodplains, and soil absorb and slowly release water, reducing the extremes of high and low water. Forests buffer high winds and temperatures and reduce soil drying, erosion, and slope failure. Buffering mechanisms are important information for land use planners concerned with natural hazards. Again, the Pantanal region of Brazil provides an excellent example. This large area of swamps and lakes absorbs the Upper Paraguay River flood water and slows its arrival at the confluence with the Parana some six months later. Were it not for this buffering capacity, flood waters of the Parana and the Paraguay Rivers would reach the lower sections of the Parana River at the same time and cause catastrophic flooding.

d. Thresholds

The point at which an effect is manifested is called a threshold. Every system has limits, and despite buffering mechanisms, the components and processes of a system will eventually fail if pushed beyond the threshold. For example, soils move despite being covered by vegetation if rainfall is intense and the slope steep, or they may remain stable under increasing grazing pressure until vegetation cover is reduced below a threshold level.


1. Preliminary Mission
2. Phase I Activities
3. Phase II Activities
4. General Recommendations

As has been said, incorporating the consideration of natural hazards early in the planning process can minimize their negative effects on development projects. A systems approach identifies hazards by looking at limiting and trigger factors, thresholds, buffers, and internal and external linkages.

Information about a study area's natural hazards needs to be examined during the various planning stages (see Figure 3-7). The process of iteration focuses the planning studies on important factors.

1. Preliminary Mission

The definition of the major land units (river basins, sub-basins, watersheds, and life zones) is required at this stage. Satellite imagery is particularly useful for this activity. Time and money can be saved by using lower resolution imagery because of the possibilities it affords to identify potential off-site influences and linkages to other systems.

Conceptual modeling of the region to evaluate important internal and external linkages is also useful. Data obtained through local informants and through available literature are very important to the process. Both upstream and downstream linkages (influence on and influence from the study area) should be identified. A team working at this level defines the work plan, team makeup, and terms of reference for experts to work in the next stage.

2. Phase I Activities

During the Phase I analysis, major ecosystems should be defined in more detail. This will require, for example, evaluations of flood frequencies and water surface levels by a geomorphologist or fluviomorphologist to look into the system's buffering mechanisms and to locate, identify, and quantify factors that influence the water level. The nature and extent of streams and river valleys should also be evaluated in terms of flood hazard and flood control possibilities. Other specialists should identify threshold levels of system attributes that will ameliorate hazards and man-made features that influence the frequency, elevation, and duration of high water. Estimates of stream channel filling should be made, and slope stability and potential erosion under different scenarios should be examined. A scale of 1:250,000 or larger for maps will probably be required to outline floodplains and identify problem areas where floods or other hazards need to be studied in more detail (See Chapter 8.)

Similar evaluation of geological hazards may be necessary (see Chapter 11). The analysis of off-site and on-site seismic-prone systems will involve the identification of past earthquake intensities. The geologist will need to study the location and direction of active faults and identify probable fault ruptures. Micro-zonation technique will identify the most vulnerable areas. Similarly, a detailed study of volcanic hazards should incorporate information on the extent of previous ash falls, tephra falls, and lava flows. The proximity of a volcano to the project area and to large bodies of water must be considered because water intensifies the violence of the eruption and accelerates the velocity of lava or ash flows.


Natural Phenomena

Positive Effects

Negative Effects


Bring water, nutrients, sediments and propagules.

Remove structures.

Low temperature

By slowing down processes, allows for conservation and storage.

Freeze can be lethal.

High temperature

Accelerates processes, particularly respiration and recycling.

Can be lethal; reduces species diversity.

Heavy rains

Trigger phonological events in deserts; relieve salinity in coastal environments; redistribute nutrients.

Remove structures and can cause other stresses such as flooding, which affects gas exchange of wetlands sediments and turbidity in aquatic systems.


Makes nutrients and moisture more available; reduces competition.

Removes structures.


Allows higher gross productivity in mangroves up to seawater concentrations.

At values higher than 35 parts per 1000, increases respiration rates and decreases transpiration net production rates.

Volcanic eruptions

Allow for better nutrient, moisture, and competitive environments.

Suffocate and kill plants and animals.


Removes competition; triggers phonological events.

Increases energy maintenance costs; temporarily decreases the number of taxa and individuals.

Water flow

Transports nutrients and oxygen; removes toxics; redistributes larvae.

Removes structures; causes high energy maintenance costs to biota.

Tidal extremes

Redistribute nutrients, sediments, organic matter, and organisms.

Expose organisms to lethal conditions.

Source: Adapted from Lugo, A. Stress and Ecosystems (1978).

Figure 3-7 - Orders and types of soil surveys, their characteristics, data sources and uses

Planning is a dynamic process that responds to the dynamics of local systems. Land-use mapping should reflect this. Map overlay techniques are appropriate, and special hazard maps can be developed if they are not available (Giuesti, 1984; Singer, 1985; see Chapters 4-6).

3. Phase II Activities

The most appropriate scales for an action plan and project formulation during Phase II are between 1:20,000 and 1:60,000. In the case of floods, the geomorphologist or fluviomorphologist would further define thresholds for erosion and infiltration of precipitation, and examine changes in floodplains and peak discharge frequencies due to human intervention, both on-site and in linked ecosystems. In the case of seismic activity, development projects should be steered away from the most vulnerable areas. A typical recommendation in the development of new areas would be to restrict uses in zones that have exhibited significant ground movement to low-density functions such as agriculture or parks. Additional suggestions should be made for mitigation measures in already developed areas.

4. General Recommendations

1. Include specific hazard-related terms of reference for specialists working in the Preliminary Mission and in Phase I (e.g., hydrologist, soils specialist, environmental management adviser). These terms of reference should include the need to develop and analyze information at the points of interaction between sectoral activities. Include hazard-related terms of reference for technicians who will be responsible for project formulation in Phase II.

2. Add the short-term participation of a geomorphologist, hydrologist, or geologist to look into areas that have been shown to be problematic during an earlier study phase.

3. Evaluate proposed uses of floodplains, with special attention to downstream consequences that may result from a loss of flood-water storage capacity caused by development activities. Upstream activities should be evaluated for the same reasons even if they fall outside of the region being studied.

4. Look at the projects being considered under different scenarios of potential development in linked ecosystems.

5. Evaluate the influence of the projects being considered on other activities of the ecosystem, including buffering and threshold characteristics.

6. Account for changes in the hydrologic regime that will be induced by the creation of impervious surfaces (e.g., urbanization, road surfaces, soil compaction from trampling by livestock, change in vegetation cover).

7. Be explicit in all instructions concerning land-use capability or suitability, including statements on the technology requirements for development projects.


1. Ecosystem Boundaries, Watersheds, and River Basins
2. Ecosystems and Associated Hazards

1. Ecosystem Boundaries, Watersheds, and River Basins

The discussions in Chapters 8 to 12, focusing on man's relationship to each of the principal natural hazards, demonstrate that actions taken in the name of development often exacerbate hazard impact and prescribe actions that can be taken to mitigate damage. Here the focus is on the natural services of ecosystems that serve to reduce the impact of hazards. It follows logically that one strategy of hazard mitigation is to maintain the natural capacity of ecosystems to accomplish this. Secondly, in contrast to Chapters 8 to 12, this section discusses all the hazards simultaneously in the context of the natural ecosystem in which they occur. Again, it follows that the mitigation strategy is to maintain the natural functions of the ecosystems intact.

To put the hazards in the context of ecosystems, a hypothetical composite system has been imagined which includes several ecosystems: uplands (highlands, piedmont), lowlands, coastal lands, near-shore waters (estuary and reef), and marine waters (open sea) and the development activities representative of each. Such a place would approximate a small volcanic island, part at low elevation and arid, and part at sufficient elevation to catch moisture-laden winds from the sea. The island would experience, if at a high enough latitude, that given the variations in its elevation, both high and low temperatures extreme enough to influence development activities would occur. It would be located near an extensive fault zone and would contain a variety of development possibilities and a number of natural services that would help protect these development activities from natural hazard events. It should be added that there are real places very much like this.

The hypothetical system is made up of "watershed" or "catchment" subsystems and coastal subsystems. The term "watershed" is variously defined, and is sometimes used interchangeably with "river basin." As used here, these terms refer to two entities that differ significantly in complexity. A watershed is a system of streams that discharge all their water through a single outlet. Watersheds may range in size from a few hectares up to thousands of square kilometers, but each, whether large or small, is more or less homogeneous with respect to its geology, soils, physiography, vegetation type, and climate. A river basin, on the other hand, is made up of a number of component watersheds, among which there may be great variation (see Figure 3-8), and its hydrograph responses is therefore complex.

In such systems, water and gravity are the two major natural components that integrate system structure and function (the specific combination of components and processes that define a given system). Their influence on development activities in terms of the natural events they can present (seismic forces, hurricanes, mass movements, etc.) is generally forgotten by planners. Only when valuable downstream development is threatened or damaged by landslides, drought, floods, or sedimentation is attention shifted upstream or uphill.

The hypothetical composite system also includes the coastal zone where terrestrial, marine, and atmospheric processes create a greater range of hazards than in most other well-defined geographic areas. Combined with the likely presence of population centers, productive agricultural lands, communication routes, buildings, etc., the risk in such a zone for heavy losses in lives and infrastructure when hazardous events occur is ever present. 1/

1/ Though not included in this discussion, significant development activities and infrastructure also exist at sea (sea-bearing mining, off-shore oil rigs and pipelines, shipping lanes, transoceanic communication links, fishing and whaling activities, security patrols, research and monitoring, refuse dumping, incineration at sea, recreation and tourism, etc.), and these too should be seen in terms of their vulnerability to hazardous events.

Watersheds and coastal systems, of course, do not occur independently. By their very nature they are integrated and must be seen as a whole. Indeed, the concept of "expanded" watershed, which includes upstream, coastal, and near-shore characteristics, is relevant particularly where offshore hazards such as hurricanes, tsunamis, and storm surges are modified by near-shore bathymetry and coastal configuration and where the effects of inland hazards such as flash flooding and debris flows often reach coastal and near-shore areas due to the presence of steep and relatively short watersheds.

This concept of watershed can be used to illustrate an area's vulnerability to hazardous events caused by human intervention in the system. Such interventions may alter the landscape upstream, for example. But, because of the integrating characteristics of water and gravity, these alterations are not only important on-site, but are also important downstream, including near-shore areas where a sediment plume caused by upstream erosion may cover and suffocate a reef or sea-grass bed. Development activities of any kind (i.e., the use, improvement, or conservation of system services, including those that mitigate hazardous events) also require "integration." This kind of integration implies planning and, as a result, watersheds are often a basic unit of development planning. Even more importantly, however, it is necessary to understand the characteristics of watersheds if a concern for natural hazards is to be included in development planning.

Given the range of natural events affecting this broadly defined hypothetical watershed, the "boundary" of its coastal or lowlands portion should remain flexible. Offshore, the boundaries can be placed at a well-defined isobath located below the depth of any bottom features capable of influencing seaborne hazards. In contrast, the watershed's uplands boundaries are readily defined in physical terms (drainage areas) but are often quite porous in biotic, social, and economic terms.

2. Ecosystems and Associated Hazards

a. Uplands and Volcanic Activity (U1)
b. Uplands and Earthquakes (U2)
c. Uplands and Landslides (U3)
d. Uplands and Hurricanes (U4)
e. Uplands and Land/Sea-Borne Floods (U5)
f. Uplands and Desertification (U6)
g. Lowlands and Land/Sea-Borne Floods (L5)
h. Lowlands and Desertification (L6)
i. Estuary and Hurricanes (E4)
j. Estuary and Land/Sea-Borne Floods (E5)
k. Reef and Hurricanes (R4)
l. Reef and Land/Sea-Borne Floods (R5)
m. Open Sea and Hurricanes (S4)
n. Open Sea and Land/Sea-Borne Floods (S5)

The subsystems of our imaginary expanded watershed offer a surprisingly large number of natural services which can mitigate the effects of many of these natural hazards. Equally important, however, are attributes of these subsystems which can intensify the effects of natural hazard events.

Figures 3-10 and 3-11 indicate which subsystems of the expanded watershed contain attributes that influence the hazards summarized here. The paragraphs below describe how the natural services of these systems mitigate or intensify each natural hazard risk; interestingly, they are not all intuitively obvious. In the early planning phases these and other services are looked at fairly broadly, and in later iterations their roles are further and more explicitly defined. For example, the diagnosis may say only "The natural structure and processes of the upland ecosystem in this region play a role in the control of erosion and of flooding." At later stages the specific ecosystem function responsible for a given service would be cited and discussed. These might be that the "high soil water storage capacity of 'Uplands sandy loam' soil type, the transpiration from the deeply rooted species, and the high infiltration rates due to the strongly fractured structure of the sub-watershed's parent rock decrease the flood potential in storms of short duration." This gives the planner a better idea of what should be done in an ecosystem if natural flood control services are to be used, improved, and/or conserved rather than go unused or deteriorate or be destroyed.

Figure 3-8 - Map showing differences in complexity between a river basin and its watersheds

Figure 3-9 - Hypotethical Watersheed on a small volcanic island

Diagram of a small island showing various ecosystems (open sea, reef, estuary, lowlands, uplands) and indicators of potential natural hazards (rain, wind, and waves indicate hurricanes and flooding; volcano indicates eruptions; faults indicate earthquakes; faults and gullies indicate mass wasting).

a. Uplands and Volcanic Activity (U1)

The structures and functions of upland ecosystems that can influence the effects of volcanic eruptions are few. However, included in what does exist are:

- Relief (including valley depth, slope direction and steepness), which may orient the flow of lava, ash, mud, etc.

- Location and extent of the rift, which may absorb volcanic material and move it away from (or toward) populated areas.

These may either intensify or mitigate the effects of a volcanic eruption depending on the location of the development activity with reference to the event. In terms of the services provided, "storage of volcanic outflow material" could be possible depending on the relief of the watershed. The "location and extent of the rift" might intensify the hazard if development activities were sited without considering the numerous hazards that accompany volcanic activities.

b. Uplands and Earthquakes (U2)

Upland ecosystems do little to mitigate the consequences of earthquakes. They may, however, intensify the consequences because of landslides caused by groundshaking. One of the more dangerous aspects of this relationship occurs in areas of current and past glacial activity and concerns the natural damming of watercourses by terminal or lateral moraines and the consequent creation of lakes. Such dams are often quite weak and are easily breached if landslide material fills the lake. An unfortunate example of this phenomenon, of course, is the 1970 earthquake in Peru that jarred loose a large piece of the Huascarán mountain, which fell into a natural lake of this type. This material together with the water from the lake covered several villages as it moved down the narrow valley, causing the loss of over 10,000 lives.
















Uplands (U)







Lowlands (l)







Estuary (E)a/







Reef (R)







Open sea (S)







a/ Land/sea-borne flood includes the hazards of tsunamis, storm surges, and storm water run-off.

b/ Desertification in this discussion is concerned with erosion, sedimentation, and salinization in areas of dry-land farming and livestock grazing.

c/Estuary consists of mangrove, salt-ponds, sea-grass beds, and beaches.

Because many upland areas do not have much level space for construction, till material is often used to create some, and the buildings put up on this unstable ground can be destroyed when the earth shakes.

c. Uplands and Landslides (U3)

The structure and function of upland ecosystems can both intensify and mitigate landslide hazards. Landslides often occur naturally in these areas owing to very steep slopes, the nature of the bedrock and overburden, the amount and regimen of precipitation, other disturbances such as natural fires which clear soil-holding vegetation, and ground shaking. Any vegetation on the slopes of the upland system is a natural part of the soil stabilizing services, although this can only ameliorate landslides and will not stop them completely on the steeper slopes. If loose mantle overlays rock (especially sedimentary rock) that has been tilted off the horizontal plane, landslides will be intensified on the slope parallel to the sediment plain. On the other hand, there are fewer and less severe landslides on the slope that runs across the sedimentary strata. Landslides occur on the parallel slopes especially if high rainfall saturates and increases the weight of the soil and lubricates the interface between mantle and base rock. In these cases even vegetation may act as extra weight and intensify a landslide.

d. Uplands and Hurricanes (U4)

Upland areas, if extensive, can serve to reduce the energy level of hurricanes, since these storms receive their energy from warm open seas. On the other hand, the heavy rainfall, in terms of both intensity and amount, can cause high runoff levels from steeply sloping landscapes. It can also saturate the soil mantle and create conditions for substantial slope failure, especially where the holding capacity of tree and shrub roots has been disturbed. A major example of this phenomenon occurred along the north coast of Honduras in 1974 when landslides caused by Hurricane Fifi killed thousands of people.

e. Uplands and Land/Sea-Borne Floods (U5)

Upland ecosystems can indeed help mitigate the effects of "land-borne flooding" through the services of storage and slow release of water. Water is stored in lakes, ponds, streams, rivers, wetlands, soil, and snow or ice, and in aquifers when the service of groundwater recharge is also present. Further, there are services (evaporation, transpiration) which reduce the total amount of water available for flooding. The infiltration rate also has an influence, and this can change according to a number of physical, chemical, and biotic characteristics of the soil. Even the physical layout and size of the watershed or river basin can make a difference. And, depending on the nature and timing of each precipitation event, these also can mitigate flooding.

Many of these same ecosystem attributes can intensify land-borne flooding. If precipitation is heavy and infiltration slow or if the soil is already saturated because of previous storms, flooding can be more frequent and its consequences more grave. Lack of flooding storage capacity and the size and configuration of drainage can combine to increase the speed and amount of runoff. There are numerous combinations of characteristics that can influence flooding, each of which is further influenced by human activities.

Figure 3-11 - Attributes which can influence the effects of Natural Hazards

Diagram of a small island showing major ecosystems and associated natural hazards. The text explains the potential impact of natural hazards on an ecosystem and how natural services of the ecosystem can mitigate the effect of natural hazards.



Uplands and volcanic activity


Uplands and earthquakes


Uplands and landslides


Uplands and hurricanes


Uplands and land-borne flooding


Uplands and desertification


Lowlands and land/sea-borne flooding


Lowlands and desertification


Estuary and hurricanes


Estuary and land/sea-borne flooding


Reef and hurricanes


Reef and land/sea-borne flooding


Open sea and hurricanes


Open sea and land/sea-borne

f. Uplands and Desertification (U6)

Upland areas are related to desertification in both positive and negative ways. Indeed, over much of the earth's surface, it is the presence of uplands that create the conditions for deserts because of the rain-shadow effect. That is, if upland areas force moisture-laden winds upward, two important phenomena take place: (a) the rising air mass cools and its moisture is released on the windward side of the uplands; and (b) on the leeward side the air mass loses altitude and becomes warmer in the process, and this creates desert conditions because the moisture is tightly held and precipitation is reduced (Figure 3-11). In Latin America the prevailing winds are generally from east to west, so that the western slopes of mountains are drier. Exceptions occur, as in southern Chile, Argentina, northern Ecuador and Columbia, where the phenomenon is reversed: the western slopes of the Andes receive higher precipitation and the eastern slopes receive less, becoming drier as one moves eastward. Often, the dry areas occur fairly close to relatively wet areas, from which their populations and development activities can be supplied with water.

g. Lowlands and Land/Sea-Borne Floods (L5)

Coastal areas receive the brunt of heavy seas and high tides as well as tsunamis and storm surge. The combined effect can be that high tides act as barriers damming river and stream outlets to the sea, so that any heavier than normal flow caused by upstream runoff will overflow banks. The normal flow from uplands tends to spread out upon reaching lowlands, where slope is less pronounced and valleys are wider. Furthermore, water flow from the uplands loses some of its energy on reaching the lowlands, causing much of the sediment load of the river or stream to be dropped. This fills in the river bed with sediment and may even raise its level above the surrounding lands. If high water breaches the natural levees built up through this process, extensive areas may be flooded.

On the positive side, coastal areas, especially those having substantial estuaries, reefs, or wetlands, can absorb significant quantities of water and the wave energy accompanying sea-borne events which cause flooding (see below E4, E5, R4, and R5).

h. Lowlands and Desertification (L6)

As was noted in U6 above, lowland areas are often in a rain shadow of an upland area, which means that they can easily succumb to desertification. However, being downstream from areas that will normally have much higher precipitation rates, they potentially have a large degree of control over the distribution of water, both in space and in time, because they receive it from only a few sources of accumulation, whereas the more dispersed form in which upland areas receive water makes its control for potable, irrigation, and industrial purposes much harder. On the other hand, since water in lowland areas within a rain shadow is not generally dispersed, areas having less frequent and less sure access to a water source will generally suffer most from desertification processes (except for salinization).

i. Estuary and Hurricanes (E4)

Estuaries in their natural state are well known for their capacity to mitigate the effects of hurricanes. They can absorb the energy front of the storm with little damage, and to a large degree they can control or at least slow beach and sand erosion and distribute the effects of the accompanying storm surge over a wide area. Indeed, in some ways, such as through the flushing action of the storm, hurricanes are necessary for estuarine operations. Likewise, storm water runoff from upstream can be buffered by estuarine systems without damage if the general increase in fresh water is not too long-lasting.

j. Estuary and Land/Sea-Borne Floods (E5)

Estuaries over much of the tropics and sub-tropics are also important for buffering storm water runoff from the upland areas and the high water resulting from tsunamis and storm surges. The orientation and configuration of the estuary influence the amount and extent of flooding. The natural defenses of estuarine vegetation such as mangroves and sea grass beds (see Figure 3-12) can absorb much of the energy associated with storm surge and tsunamis. If the estuary is shallow and extensive, these characteristics can reduce wave height, and therefore can also reduce flooding.

k. Reef and Hurricanes (R4)

Many of the characteristics of estuaries that ameliorate the effects of hurricanes are shared by reefs. They can absorb much of the wave energy (see Figure 3-13), and coasts that are surrounded by barrier reefs suffer significantly less damage to beaches and shoreline infrastructure than do coastal areas that are exposed to the open sea without reefs. Again, the damage done to shore areas and infrastructure by wave energy will depend on the shape, depth, extension, width, and distance from shore of the reef system.

Figure 3-12 - Beneficial roles on mangrove forest in costal ecosystem

Mangrove forests can serve as a buffer against storm waves and thus protect human lives and man-made infrastructure in coastal regions.

Figure 3-13 - Beneficial roles of coral reefs in costal ecosystem

Coral reefs can serve as a buffer against storm waves thus protecting the shoreline and coastal lands, crops, houses and human life.

Source: Both figures adapted from Snedaker, S.C., and Getter, C.D. Coasts: Coastal Resources Management Guidelines. Coastal Publication No. 2, Renewable Resources Information Series. (Washington, D.C.: U.S. Agency for International Development, 1985).

l. Reef and Land/Sea-Borne Floods (R5)

Since the major problems suffered from hurricanes result from flooding, the same characteristics of reefs that help mitigate damage from a hurricane also mitigate sea-borne flooding. Land-borne flooding, however, may be a bit different in that, if the reef acts as a dam restricting the outflow of fresh water, it may intensify lowland floods. These effects, however, will not be as severe as in the case of estuary configurations that impede river outlets to the sea.

m. Open Sea and Hurricanes (S4)

Hurricanes are spawned in the open sea, and their energy is gained by passing over open seas of relatively high temperatures. Although the configuration of the sea bottom at some distance from a coast will not necessarily influence the height of the accompanying storm surge, bottom configurations nearer the coast will significantly affect the height and energy of the surge and can direct it either away from the shore or toward the shore.

n. Open Sea and Land/Sea-Borne Floods (S5)

The influence here is generally to cause rather than to mitigate flooding. Tsunamis may be generated by large undersea mass movements of "land," underwater eruptions of volcanoes, and earthquakes in the open sea. And storms other than hurricane-force winds are born at sea. Global phenomena like "El Niño" can change weather patterns over lengthy periods from little precipitation to heavy precipitation, as in the desert areas of northern Peru, which were severely flooded in 1982-1983 as a result of the related ENSO phenomenon.


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