Water Hazards, Resources and Management for Disaster Prevention:
A Review of the Asian Conditions
IDNDR 1991-1999

The key to effective reduction in the loss of life and damage to property resulting from tropical cyclones, storm surges, floods and other forms of water-based disaster lies in the development and adoption of improved disaster prevention and preparedness measures.

Disaster prevention may be described as the application of measures that seek to prevent natural phenomena from occurring or to limit the scope and intensity of their effects. It is concerned with the formulation and implementation of long range programmes and policies which are aimed at elimination of the occurrence of disasters or the reduction of their adverse effects on the basis of a careful assessment of vulnerability and risk. It must be emphasized that major natural disaster phenomena such as tropical cyclones and widespread floods cannot be prevented from occurring, but the probability and extent of serious damaging effects can be minimized. These measures may include both structural and non-structural approaches, including legislative and regulatory measures.

Disaster preparedness may be described as the application of measures which are designed to minimize loss of life and property damage and to organize and facilitate timely and effective rescue, relief and rehabilitation when disastrous disaster events do occur. Again, the assessment of these risks must be undertaken in terms of risk and vulnerability (see Chapter 7). Preparedness measures may include forecasting and warning systems, community education, and organization and management of disaster situations including the preparation of operational plans, training of relief groups, the stockpiling of supplies and the provision of the necessary funds. It is must be supported by appropriate emergency legislation which comes into force in disaster situations or similar emergencies which cannot be avoided.

Prevention and preparedness measures are not isolated activities since both can be undertaken together or one can be a continuation of the other. This is because not all disasters can be prevented, and preventative measures may fail to achieve their objectives. Thus, to limit or mitigate the effects of disasters which cannot be prevented, certain measures have to be undertaken to return the community to normality as soon as possible after the event.

A variety of prevention and preparedness measures has been applied in the countries of the ESCAP Region, albeit with varying degrees of success and often in an uncoordinated fashion. Details of these approaches are presented in Chapter 6.

As previously stated, disaster prevention and preparedness consist of a wide range of measures, some long term and others short-term, aimed at saving lives and minimizing the amount of damage that might otherwise be caused. Prevention covers the long-term aspects and is concerned with policies and programmes to prevent or eliminate the occurrence of disasters. Preparedness covers the short-term measures which are designed to cover the action necessary during the approach of a possible disaster, during the existence of a disaster situation and in the ensuing period devoted to relief and rehabilitation. Disaster prevention and preparedness is usually accomplished using two fundamental approaches. Firstly, it may be achieved using permanent controls, structural or non-structural, designed and developed in advance of the disaster. Secondly, it may be achieved by using temporary measures, planned in advance but only put into effect during the emergency.

The destructive power of tropical cyclones is manifested by strong winds, flooding and storm surges. Any disaster prevention and preparedness system must include warnings and protective measures against each of these effects. Winds are a fundamental property of tropical cyclones, whilst flooding and storm surges may be a consequence of tropical cyclones but also of other natural events.

The principal preventive measures employed to mitigate the destructive and injurious effects of tropical cyclones involve the introduction of building design and construction standards aimed at improved resistance to the damaging effects of wind and water.

Disaster prevention measures attempt to lessen the impact of flooding or storm surge on the social and economic conditions of human settlements in floodplains or low lying coastal areas. The range of preventative controls adopted to protect development on floodplains includes both structural measures such as channel modifications, flood detention storages and levees which arc designed to reduce the incidence or extent of flooding, and non-structural measures such as flood insurance, flood zoning restrictions, land-use management, economic incentives, public information and community education. Non-structural measures are intended to modify flood susceptibility and flood impact. The range of measures available to protect against the effects of flooding is much wider than that available to reduce the impact of tropical cyclones.

Preventative measures to protect low-lying coastal areas against damage from tidal inundation also include structural and non-structural measures. The principal structural measure involve the construction of embankments capable of withstanding the anticipated storm surge heights and forces. Non-structural measures employ land-use zoning and controls over occupation in high hazard areas. Building controls are also imposed to restrict building on vulnerable areas. These controls require that flood heights are set a safe elevation above a given datum.

The selection of the best mix of measures to prevent the occurrence of future flood or storm surge disasters will be based on the consideration of all the available structural and non-structural options. The optimal mix of measures will be based on risk analysis and the economic performance of the overall scheme. Consideration of social and environmental factors in addition to the legislative and legal constrictions should be included in the planning process.

Disaster preparedness is seen as that action taken when the occurrence of a tropical cyclone. flood or storm surge threatens to become a disaster. Preparedness activities are designed to reduce social disruption and losses to existing property and are an essential component of overall disaster planning. They can serve in the absence of more permanent measures to reduce the threat to loss of life and property.

The main types of disaster preparedness include:

• forecasting and warning systems;
• evacuation from affected areas;
• flood fighting;
• flood relief;
• cyclone shelters.

Depending on the size of the drainage basin, the length of river and the time of concentration of floodwater in the main channel, flood forecasts and warnings may be issued well in advance of the arrival of the flood crest on large rivers. Flash floods originating on small catchments present special problems and usually require some form of forecasting based on rainfall estimates.

Although the forecasts for cyclones and floods may be accurate and timely they may have little or no effects on the intended recipients if the warning system for dissemination of the forecast is inadequate. Each agency responsible for emergency operations should receive prompt forecasts and warnings of the changing circumstances so that action needed to meet the emergency can be achieved. Dissemination of forecasts requires an effective communications system based on radio broadcasts, television, newspapers, telephone and special warning systems.

The evacuation of people from a potential or actual disaster area is one of the most important elements of disaster mitigation. Careful planning is necessary for the efficient evacuation and relief of flood victims. To be effective the plan should define hazardous areas and potential dangers. However, the difficulty in evacuating victims and property can be increased if escape routes cannot cope with the traffic volume, if evacuation services cannot be contacted or suitable evacuation equipment such as trucks, boats and helicopters are not available.

Flood fighting can be defined as the taking of precautionary measures against disaster at times of flood or storm-surge. These measures should aim to prevent damage or to minimize its extent to protect life and property and in general, to ensure the safety of the population. Successful flood fighting depends upon good organization, thorough advance planning, well-trained personnel and the effective coordination of operations at local, provincial and national levels. The planning should cover all those who will be involved, from the flood-fighting corps, municipality, town or village officers, and the general public, to the regional and central government. It involves the construction of temporary controls to exclude floodwater from protected areas or the strengthening of existing structures to ensure protection.

The main aim of relief is to provide immediate assistance to overcome personal hardship and distress, including essential repairs to houses and the repair and replacement of essential items of furniture and personal effects. Relief should include the reception and care of evacuated victims, the provision of medical services and similar activities.

As explained in the preceding sections of this Chapter, a variety of both structural and non-structural measures is available for coping with water-based disasters. These measures are discussed in further detail below. As we have already indicated, most or all of these methods have been used in the countries of the ESCAP Region as part of their disaster mitigation programs. The extent to which individual countries have applied them, and the degree to which they have been successful, has been varied, as will be described in Chapter 6 and commented upon in Chapter 7.

Winds are a fundamental property of tropical cyclones, whilst flooding and storm surges may be a consequence of tropical cyclones but also of other natural events. A variety of structural measures has be taken to protect lives and property against these effects.

Tropical cyclones may produce wind velocities of 200 km per hour or more. Under these conditions, buildings are subject to air pressure variations which can produce strong outwards forces on roofs, ceilings and walls, leading to explosive lifting, bursting or collapse. High winds may also induce falling debris and airborne wreckage to be carried from adjacent buildings, and these can impact with such force as to penetrate or severely damage conventional wall and roof materials.

Structural precautions which have been or could be taken to minimize the damage caused by these effects include:

• siting or re-siting of buildings in locations with minimal exposure to high-velocity winds;
• use of special cyclone-resistant building materials;
• special forms of roof and wall construction designed to withstand extremely high wind velocities;
• construction of cyclone shelters within or adjacent to buildings.

In selecting a location with reduced exposure to hazardous wind velocities, it needs to be remembered that cyclones are generally associated with intense rainfall and possibly heavy flooding. Clearly, buildings should not be located in positions posing increased flood hazard.

Cyclone are also often associated with storm surges, leading to potential flooding of coastal areas. Where possible, buildings should not be located in surge prone locations. Alternatively, buildings may be raised above expected flood levels. This is also a possibility for locations which are likely to be subject to cyclone-induced flooding.

Many countries in the region have Government or University laboratories which conduct research into the structural effects of tropical cyclone damage and have developed structural and building designs and special materials to suit such conditions. Many countries have also now developed improved structural design guidelines and standards governing building construction in cyclone-prone areas and these should be identified, emphasized in building regulations or by-laws and widely publicized amongst engineers, architects, builders and the general public.

Cyclone shelters are specially strengthened and equipped rooms or chambers constructed inside or adjacent to individual buildings. For short-term emergency shelters, a floor space area of about 0.5 m2 per person is adequate. For longer duration cyclones, which might last for 12 hours or more, a floor space of about 1m2 per occupant is desirable.

(i) Levees and flood walls

The principal purpose of levees and floodwalls is to confine floodwaters to the stream channel and a selected portion of the floodplain. These barriers protect only the land area immediately behind them, and are effective only against flood depths up to the chosen level for which they were designed. However, they may create a false sense of security about the degree of protection provided. Floods exceeding the levels for which the levees and floodwalls are designed can cause disastrous losses of life and property.

The requirements for the design and construction of levees and floodwalls are governed by degree of hazard to life and property within the protected area and by site conditions. Levees are normally constructed of earth and require significant space to accommodate the required base width. Floodwalls are usually constructed of concrete or steel and take up far less room. They are more suitable for use in congested areas.

Because levees and floodwalls can fail by overtopping, undermining, slumping and excessive seepage, the design of these structures should attempt to reduce the possibility of failure from these causes. Ample freeboard, which takes into account the settlement of levees, wave action, sedimentation of the river channel and inaccuracies in estimation of flood levels, reduces the possibility of overtopping of levees or floodwalls. Undermining is minimized by locating levees or floodwalls far enough away from channels to eliminate exposure to high velocity or scour. Proper side slopes and construction methods minimize slumping of earth levees. Excessive seepage can be reduced by the provision of seepage protection works. Damage can also be caused by termites and burrowing animals. Regular inspections are necessary to locate and remedy the damage in an early stage of development.

Levees and floodwalls complicate the drainage of land they protect and provision must be made for the discharge of internal drainage water unless adequate storage is available. Discharge through levees or floodwalls can be achieved by gravity flow through pipes equipped with gates. When prolonged flood stages prevent gravity outflow, the internal drainage water must be stored temporarily, removed by pumping or disposed of using a combination of these methods.

To be effective, levees require proper maintenance. Such maintenance should include regular inspections as well as periodical patrols during and immediately after severe floods. Vegetation, grazing and traffic on earth levees should be controlled. Proper attention to any defects will help ensure against levee failure.

Normal natural watercourses have a river channel of limited capacity, which may be exceeded annually, with excess floodwater overflowing onto the floodplain. Hydraulic improvements to the watercourse or to the floodplain, and/or flood channels constructed within the floodplain, enable flood waters to be passed at a lower level than would occur naturally. In urban areas, such works also permit the optimization of land use through improved residual drainage.

The various types of channel modification include:

• straightening, deepening or widening of the channel;
• removing vegetation or debris;
• lining the channel;
• raising or enlarging bridges and culverts which restrict flow;
• removing barriers which interfere with flow;
• installing river training works.

Channel modifications are similar to levees and floodwalls in that they can be used to protect a specific site or region. They can also provide the community with other positive benefits, such as improved navigation and recreation.

Channel modifications are likely to be most effective on steeper, smaller streams with overgrown banks and narrow floodplains. Channel modifications are unlikely to have any significant effect in flooding situations where there are extensive areas of overbank flooding, or where flooding effects are dominated by tide levels.

River training works are structural measures of various kinds which are undertaken in order to provide a more effective channel for the passage of flood flows and sediment loads. Such works may be designed either to retard flow rates along a river bank, in order to reduce erosive velocities and increase the deposition of sediments, or to provide protection for the bank against erosion or scouring.

Permeable groynes and revetments, constructed of piling, rock, concrete, fencing materials, vegetation or other materials, are generally used for these purposes. Groynes protrude into the channel and are designed to divert flow away from the bank, whilst at the same time causing an accumulation of sediment along the toe of the bank and on the downstream side of the groyne structure. Revetments, on the other hand, are constructed along or parallel to the bank, where they serve to reduce the velocity of flow along the bank, thus reducing bank erosion and allowing the river bank to stabilize.

Which of these devices should be used in a given situation depends upon characteristics of the stream channel and the extent and nature of the existing erosion damage. Whichever kind of device is employed, its satisfactory long-term performance will be very much dependent upon its continuing maintenance.

Disadvantages which are related to the use of channel modifications include the costs of proper maintenance, the destruction of riverine habitat for fish and wildlife, and the potential for the aggravation of channel scouring and bank erosion if the structures are not intelligently designed, well constructed and carefully maintained.

These structures serve two functions in flood mitigation. Firstly they create large, shallow reservoirs which store a portion of the flood water and hence decrease the flow in the main channel below the diversion. Secondly, they provide an additional outlet for water from upstream, improving flow characteristics and decreasing water levels for some distance below the diversion. Opportunities for the construction of floodways are limited by the topography of the area and the availability of low-value land which can be used for the floodway.

There are two types of by-pass floodways, natural and constructed. A natural floodway follows the course of an existing cross-country depression and carries floodwaters that can no longer be carried within the river channel. The land in the floodway is generally not different from other farmland, except that it may be low-lying. Some floodways have control banks constructed across them, or may be bordered by levees, in order to control the spread of floodwater. Restrictions are usually placed on land development in floodways to ensure that future loss and damage from major floods is reduced to a minimum and to ensure that the floodway functions as designed.

When required, controls in the form of spillways and gates are provided at the entrance to a floodway. Spillways take the form of a lowered and protected section of levee which is designed to control the amount of floodwater diverted into the floodway from the river. As spillways can be overtopped for long periods by high velocity floodwater, they have to be specially designed to avoid failure. Protection can be provided by rock gabions or, where appropriate, by building the spillway with gentle backslopes which are well grassed.

If the floodway possesses comparatively steep bed-slopes, control banks may be built perpendicular to the direction of flow at intervals along the length of the floodway. These banks are similar in design to the entrance spillway, and form a series of basins which reduce the water velocity by dropping the floodwater in progressive steps down the floodway alignment.

Diversions are works constructed to intercept flood flows upstream of a damage-prone area and route them around the area through an artificial channel. Diversions may either completely re-rout a stream or collect and transport only those flows that would cause damage.

Diversions are particularly well suited for protecting developed areas, because they do not usually require land acquisition or construction within the protected area. However, opportunities for diversions are often limited by the nature of local land formations and soil conditions. There must also be a receiving water body or stream channel with sufficient capacity to carry the flow bypassed through the diversion without causing flooding.

Flood storage and retardation involves the deliberate, controlled flooding of designated areas in order to minimize overall flood losses. It permits floods exceeding a specified magnitude to spread over low-lying lands situated behind embankments in a controlled fashion, accomplished by the operation of gated structures or spillway sections incorporated in the embankments. The diversion of floodwater, when carefully controlled, will reduce the flood peak at downstream locations and confine flooding to within the flood control system.

Areas selected for flood storage and retardation are traditionally low-lying locations which have a history of flooding. By the formulation of proper controls it is possible to utilize these areas for habitation and agricultural purposes, on the understanding that they will be flooded periodically. This calls for the preparation of a comprehensive programme of flood operation, a knowledge of the depth and extent of area inundated, the imposition of controls to ensure predictable flood behaviour and the implementation of a reliable flood forecasting and warning system to ensure timely and safe evacuation. Special provisions are also required for the protection of emergency services and for flood refuge areas.

To reduce the damages associated with controlled flooding, it is necessary to provide drainage works capable of emptying the flood storage area as quickly as possible after the cessation of main river flooding.

Retarding basins reduce downstream flood flows in both mainstream and urban drainage situations. They allow small flows to pass unimpeded but trap a portion of larger flows. In urban areas, retarding basins are most suitable for small streams which respond quickly to rainfall and/or stormwater flooding. However, they introduce a number of inherent problems, which should be carefully evaluated for each particular situation. These may include the following:

• basins may require a substantial area to achieve the necessary storage;
• long duration or multi-peak storms (when the basin is filled from a previous peak) can increase the risk of overtopping or breaching;
• the impact on floods larger than those for which they are designed is limited.

Sites for retarding basins in developed urban areas are generally limited in number and extent. Available sites are usually restricted to established recreational areas, such as parks, playing fields and parking lots. In new urban developments or re-developments, the incorporation of a system of retarding basins at the planning stage can result in effective flood protection for those areas.

Retarding basins are sometimes constructed by building an earth embankment across the watercourse and providing outlet facilities to control releases appropriate for the capacity of the downstream channel. The outlet facility usually takes the form of a box or pipe culvert. If earthworks are used for the construction of the basin embankment, the provision of adequate spillway capacity is essential to protect the basin from failure by overtopping if flows exceed the design flood.

Land along the river and natural depressions on the floodplain can be utilized for the off-river storage of floodwaters. Flood flows are diverted into them in order to reduce flood peaks downstream. The efficiency of operation of such storages can usually be improved by providing them with suitable intake structures for controlled filling and outlet structures arranged to permit controlled releases when downstream conditions allow.

In appropriate circumstances dams can be constructed to create reservoirs which control major flood flows by temporarily storing flood waters and releasing them at a safe flow rate. Such devices may be used to control floods arising from existing catchment conditions or to offset the impact of proposed land use changes. The amount of storage required depends upon the degree of protection needed and the downstream channel capacity.

The degree of mitigation provided by a flood control reservoir depends on the combination of dam storage, spillway capacity and the pattern of flood inflows. The effect of storage is to decrease the flood peak without reducing the total volume of floodwater. The reduction of the flood peak is achieved at the expense of an increased duration of dam releases at lower rates. For dams equipped with gates or valves, the way in which these controls are operated will determine the rate of release and the degree of downstream mitigation.

The protection afforded by a surface reservoir is greatest in the area immediately downstream of the dam. Protection further downstream is reduced by tributary flows and by run-off from land adjacent to the river. Protection may also decrease over time if the reservoir capacity is diminished by siltation. Surface reservoirs have the greatest potential to mitigate floods when they are empty.

Flood mitigation reservoirs are mostly used on small and moderate-sized streams. The large areas of land required to store the flood flows of major rivers are generally no longer available, especially where they involve the flooding of valuable agricultural lands. Many sites that are geologically and topographically suitable may require very considerable and expensive land acquisition and the displacement of large populations. The cost of large reservoirs can generally only be justified where they protect heavily developed urban areas and are the only practical means for significantly reducing flood damages. It is usual practice to reserve a component of the available storage capacity in multi-purpose dams for flood mitigation purposes. In such cases, careful coordination is necessary to permit flood mitigation reservoirs to serve also for water supply or irrigation purposes.

A major disadvantage of flood mitigation reservoirs is that downstream residents often do not appreciate that they can only control floods up to the peak rate for which they were designed. Complementary land use controls need therefore to be enforced to prevent unsafe development and encroachment on the downstream floodplain.

Drainage water produced by storm runoff from within the protected area behind levees or floodwalls may be disposed of by various means, which include:

• gravity release through pipes fitted with gates during periods of low river flow;
• temporary accumulation of drainage flow in storage areas;
• pumping of interior drainage water during periods when gravity drainage outflow is restricted by backwater.

Pumping is usually required for the disposal of interior drainage water whenever sufficient discharge by gravity flow cannot be achieved, which may be because of limited outlet capacity, insufficient storage capacity or the effects of backwater caused by flooding.

The design of drainage works for the removal of flood waters accumulating within the low-lying areas behind levees or floodwalls requires consideration of the entire drainage network servicing the protected area. Coordinated use of storage areas, channels, pipe systems and gravity outlets is needed so that the pump capacity, size and period of operation can be optimized. The efficient planning and design of pumping plants will involve careful selection of the required water removal rate, the auxiliary drainage facilities needed to minimize the pumping requirements and the location of the pumping plant to provide an effective outlet to the entire drainage system.

The period of pumping may be reduced by increasing the amount of available storage. This may be achieved by excavation. Where this is not practical, adequate pumping capacity must be installed to safely discharge any drainage inflow volume in excess of the available storage capacity.

There is a variety of structural or mechanical measures which can be applied to reduce the potential for land instability in areas where occupation cannot be prohibited. These measures might include the following:

• preventing or diverting runoff flows around critical sites;
• de-watering sites using drainage systems;
• planting trees or shrubs which remove sub-surface water by transpiration;
• planting deep-rooted vegetation to bind sub-soil material;
• underpinning foundations to stable rock;
• battering slopes to stable grades;
• constructing retaining walls along the toes of critical slopes.

There is a variety of structural measures which can be taken to mitigate the effects of severe drought. These essentially revolve around the careful management and conservation of surface and groundwater water resources. They can be considered in two categories - large-scale measures and small scale or on-farm measures.

Large-scale surface-water conservation measures revolve around the provision of large water storage reservoirs for the regulation of natural streamflow and the delivery of this water to critical areas, sometimes over considerable distances, through irrigation, stock or domestic water supply systems.

The availability of suitable and economical sites for large dams is limited and new sites need to be chosen with care. Unfortunate experiences with very large storages in many developing countries, particularly in tropical regions, have shown that they can have serious adverse environmental, social and economic consequences and they need to be planned and designed with very considerable care.

Efficient utilization of available damsites and economic considerations suggest that where possible, large water storages should be designed and operated as multi-purpose structures, incorporating where possible and appropriate irrigation, flood mitigation, power generation and recreational functions. These may not be mutually consistent, so that multi-purpose design requires a comprise solution based on the best overall net benefits to all potential users.

Irrigation, stock and domestic water supply delivery and distribution systems also need care in their design and location. Increasingly, environmental considerations may impose special restraints where proposed channel or pipeline routes may traverse areas of natural significance, wildlife habitat or historical or cultural value.

Unfortunate experience in many countries, where large-scale irrigation districts have been developed on semi-arid lands on the flood plains of major rivers, has been the development of salinity and water-logging in irrigated soils. In some cases, this has led to the total devastation of irrigated land and made it unsuitable for any form of agricultural activity. Within the ESCAP Region, it has occurred extensively in Australia, China, India, Pakistan and Thailand.

To avoid the possibility of future degradation from this cause, new irrigation areas need to be carefully sited and selected on the basis of the soil type, the nature of the underlying strata, the quality of the irrigation water to be use, and the ability to provide an adequate drainage and disposal system. The build up of salts in the soil, and the potential for water-logging, can be substantially reduced or eliminated through the provision of an appropriate drainage infrastructure and this must be considered an essential component of any irrigation system. Drainage water may be too high in salinity for safe disposal into a major watercourse, in which case an effective disposal process, such as transpiration from an irrigated salt-tolerant woodland or evaporation from an evaporation basin, could provide an effective solution.

In the ESCAP Region, groundwater is used extensively for irrigation, domestic and stock water supply purposes. Groundwater required careful management if it is to be available in adequate quantity and quality on a long-term basis, and particularly through prolonged drought conditions. There are some structural devices that can be used to improve the availability of groundwater supplies.

On the larger scale, groundwater distribution systems need special construction measures to control losses and optimize delivery efficiency. Artesian bores should always be capped and provided with adequate control valves, whilst all bores should be fitted with flow meters. Substantial seepage and evaporation losses may be experienced when groundwater supplies are delivered over considerable distances through unlined earthen channel systems, and pipeline delivery is much to be preferred. At the delivery end, temporary storage in tanks and the installation of well-designed domestic, irrigation delivery or stockwater troughing systems is highly desirable.

Where possible, groundwater supplies should be managed in conjunction with surface water supplies on an integrated, conjunctive use basis. Where appropriate, groundwater resources might be able to be replenished using surface water, particularly when excess water flows are available during flood periods. Detention storages designed to hold back floodwaters for a sufficient period of time to enable infiltration into an underlying aquifer are generally used for this purpose. These recharge storages need to be carefully sited over foundations which are permeable and facilitate the infiltration process, by contrast with more normal dam construction where a site offering low seepage losses is desirable.

On the small of farm-level scale, a variety of solutions is also available. In arid areas with intermittent rainfall, or on higher rainfall areas with marked seasonal rainfall patterns, the construction of appropriately designed and sited surface reservoirs is a common practice. These are principally constructed by excavation and/or by building an earth embankment. If they are to be used for irrigation, they need to be as large as possible and located on sites which provide a maximum of storage capacity per unit of excavation required for their construction. For stock and domestic use, particularly in arid areas, they need to be as deep as possible, with minimal surface area, to reduce long-term evaporation losses.

Where farm storages are filled by surface runoff, they should be equipped with an emergency spillway of adequate capacity. Where runoff is low and intermittent, the catchment or watershed area needs to be as large as possible and might need to be extended by the construction of diversion and collection devices such as catch drains. Under extremely arid conditions, the use of sealed catchment areas, with paved or rolled earth surfaces, might be necessary where the high cost of doing so is not an important factor.

Where groundwater or intermittent streamflow is available, above-ground reservoirs may be constructed and filled by pumping. Large offstream storages, called ring tanks, filled by pumping from a sump adjacent to a watercourse during flood flow periods, are commonly used in inland Australia for irrigation purposes. Elsewhere, smaller circular reservoirs called turkey’s nest tanks, constructed by pushing soil from the outside around their perimeter and filled by windmills from an underlying aquifer, are widely used for stock water purposes.

The vulnerability of land and property to water-related natural disasters can be reduced by structural works. The potential impact of these events can be further reduced by the imposition of land use controls, designed to manage land degradation and minimize exposure to the risk of disasters which cannot be avoided. To achieve this objective, legislative controls which empower the relevant government authorities to direct land use planning policies and practices related to watershed management need to be adopted and implemented. Whilst most of these measures have been introduced by individual countries within the ESCAP Region, they have not always be adopted along with a comprehensive range of structural measures in an integrated and coordinated fashion.

Such controls should strive to ensure that an effective and comprehensive legal and administrative system is adopted which addresses the problems of land degradation, environmental protection, and disaster mitigation in an coordinated fashion and is consistent with the principles of sustainable resource development. Such a system requires an integrated approach to the management and protection of natural resources, including land, water, vegetation and human activity, undertaken on the basis of a "total watershed" approach. This approach recognized that changes to the natural environment in the upper watershed will influence conditions in downstream areas, and significantly increase their potential for damage by flooding and drought.

Legalization should establish national standards for watershed management and downstream land occupancy which relate to the use, development and protection of land in a way which will minimize the risk to populations during the occurrence of water related natural disasters, particularly when they are brought about by the degradation of natural resources. Activities within a watershed should be controlled and protected through a comprehensive watershed management plan which places restrictions on those activities which can increase the risk of damage. Under this type of legislation, consent would be required for:

• large-scale land clearing;
• rural land development and use;
• forestry, mining and extractive industries;
• rezoning of land for urban use;
• occupation of flood plain land, steep slopes and other hazardous areas.

Where it is economically and socially acceptable, and population pressures and the demand for additional productive land allow it, land use zoning may provide the most effective and least costly solution to the problems of disaster management. This requires the prohibition or restriction of agricultural development or urban settlement in locations which are particularly susceptible to flooding, cyclonic damage or land instability. Where this is not feasible, land use controls might still be employed to restrict the use of the land to activities which are compatible with potential instability or result in minimal damage and loss of life should disaster events occur. This might include such means as the prohibition or restriction of clearing or logging of watersheds or the prohibition of urban settlement from land areas at hazard.

The various categories of non-structural controls available for disaster management and mitigation comprise the following:

• Legislative and regulatory measures for controlling land occupancy, structural standards and emergency policies and services;
• and-use zoning;
• warning systems;
• emergency agencies, facilities and equipment;
• evacuation and flood relief services;
• community education.

All of these activities must be provided for in an integrated and coordinated fashion and supported by appropriate legislative requirements and administrative arrangements if they are to be successful.

Of these devices, all are essential but perhaps the most likely to contribute most to overall regional and local disaster protection and preparedness is the technique of land use control, effected through land zoning plans and regulations.

Carefully prepared zoning plans are the basis for effective land use control. A variety of modern techniques, including remote sensing, satellite imagery, global positioning equipment and geographical information systems (GIS) provide effective tools for the preparation of basic topographical and geographical information.

Geographical information systems utilize geographical data and information with respect to three components: spatial data, which pertain to the locational aspects of geographical features, along with their spatial dimensions; attribute data, which pertain to the description, measurement and classification of geographical features; and time, which is particularly important in natural hazard assessment because of the rapidity with which geographical features may alter during the occurrence of disaster events.

The collection of such data has been greatly facilitated by the availability of various kinds of remote sensing systems. Its incorporation into a computer-compatible format, and its ability to be manipulated within the computer for rapid data analysis, classification and presentation, has been further facilitated by the ready availability of digital mapping devices and software programmes, which allow the ready transformation of analogue data from maps or remote sensing images into computer-usable format.

A GIS has four functional components, which comprise:

• a data input subsystem, which collects and processes spatial data from sources such as existing maps and remote-sensing imagery;
• a data storage and retrieval sub-system, which organizes data in a structured form and allows it to be retrieved in various forms for subsequent manipulation, analysis or display;
• a data manipulation and analysis sub-system allowing the modification or reorganization of data according to given rules and providing a basis for the preparation and manipulation of models of the geographic area; and
• a data-reporting sub-system capable of displaying all or selected parts of the data base in chosen tabular or cartographic formats.

A key advantage of the GIS approach is that it permits the integration of a wide range of categories of data and the merging or overlaying of various groupings of data, which greatly facilitates the use of the data for design, planning or policy-implementation purposes. By way of example, plans of urban and industrial development can be superimposed on topographic maps and plans of communication systems and the whole overlain by maps of major flood level contours to provide a basis for floodplain zoning rules. A further key advantage is that the GIS system permits the aggregation of spatial and attribute data into models of the land or resource system under study and provides a basis for the simulated operation of such models according to a variety of scenarios as a basis for planning and design problem-solving. In integrated catchment management, as well as disaster management, the model-forming capabilities of GIS packages are of very substantial potential value for management purposes, particularly as a basis for optimizing models, decision support systems and expert systems.

There is also a number of non-structural techniques available for drought mitigation. These include a variety of farming and stock management measures, as well as a variety of government policy, legislative, administrative and fiscal measures.

At the farm level, effective drought management procedures may include the following:

• conservation farming practices designed to improve the infiltration and retention of soil moisture;
• pasture improvement;
• the application of fodder conservation techniques;
• the management of stocking rates to avoid overgrazing and fodder shortage;
• the introduction of more drought-resistant plant and livestock varieties.

At the government level, effect drought management practices may include the following;

• the provision of drought-relief funding;
• the use of taxation relief and other fiscal measures including long-term, low-interest rate loans to encourage conservation farming, good stock management practices and water conservation;
• the organization and coordination of government agencies for the provision of drought relief and assistance in drought management;
• the development and implementation of advisory and extension services to educate and assist the farming community;
• the development, in association with other nations when appropriate, for research into the factors causing drought conditions, the forecasting of drought events and the operation of drought warning systems.

Where relevant, these measures need to be backed by appropriate legislation, policy promulgation and agency response.

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