L. Berga,
Professor of the Polytechnical University of Barcelona, Spain.
Chairman of ICOLD Committee on Dams and Floods.
1. FLOODS AS A NATURAL HAZARD.
The natural hazards suppose an important impact for human life and produce serious social effects and grave economic losses. The natural disasters constitute a curb on the sustainable development affecting its three basic mainstays: economics, social and environmental. In spite of the efforts made by the International Decade for Natural Hazard Reduction of the United Nations ( 1,2,3 ), the natural disasters in the world, have experienced an increasing evolution during the last decades of the XXth.Century, producing at present a mean of some 40,000 victims per year and mean economic losses of more than 50 Billion $ per year. The number of major natural disasters during the period between 1963 and 1992 have been multiplied by 3.5 with relation to the affected people, and by 2.3 in relation to the number of victims (4). Likewise, the economic losses are increasing with an exponential tendency, having duplicated during the last decade of the 1990`s, with an actual evaluation of more than 50 B$ per year, as is shown in Fig. 1 (5).
Fig.1. NATURAL DISASTERS. ECONOMIC AND INSURED LOSSES WITH
TRENDS.
The greatest natural disasters of the last decades which have produced economic damages greater than 10 B$ are shown in Fig 2. It can be observed that of the 18 disasters, 7 are big floods, such as those of 1993 in the United States, 1991, 1996 and 1998 in China, those of North Korea in 1995, and those of the year 1999 in Venezuela, with more than 20,000 victims.
Fig.2. MAJOR NATURAL DISASTERS.
Within the natural disasters, the greater number correspond to floods (Fig. 3) which suppose about 30% of the socio-economic impacts ( 32% in relation to the significant damage and to the affected people, and 26% in relation to the number of deaths) (4).
fig. 3. major disasters around the world 1963 - 1992. percentageof significant disasters by type, based on damage, personsaffected, deaths.
In the last decades the impacts caused by the floods have been very important and the
Table nº 1 refers to the most catastrophic floods that have occurred in the last ten years.
In order to analyse with more detail the significance and importance of the floods, ICOLD has carried out a survey on the social and economic impacts of the floods in the 20 most important countries in large dams, and which represent about 90% of the existing large dams (6). It was found that the floods are the most important natural hazard in 65% of the countries and that the floods constitute in 90% of the cases the first or second most important natural hazard. On the other hand, the floods present a high recurrence, with an average incidence of 7.2 years, and in the majority of cases the number of years between important floods ranges from 5 to 10 years. The “mean” number of victims per year produced by the floods is shown in the Table nº 2.
The majority of victims are produced in the Asian countries, 200 victims per year in Bangladesh ( without including cyclones, nor storm surges), 250 in South Korea, 1,500 in India and more than 2,500 victims per year in China. Nevertheless, the cases also stand out of U.S.A., with 94 victims per year and Japan with 120 victims per year, in which exist an intense occupation of the flood plains, which together with the presence of flash floods give rise to these values. In most of the countries the mean number of victims per year is lower than 20.
In relation to the economic impact produced by the floods, Table Nº 3 shows the evaluation of the “mean” annual damages.
TABLE Nº 3. MEAN ANNUAL DAMAGES PRODUCED BY FLOODS.
It can be observed that in South Korea they represent some 500M$, Spain 600M$, China 3,000M$, but the most important damages are produced in very developed countries, United States with 3,400M$, and Japan with 7,200M$ per year. Also, it is necessary to quote the importance of the impacts and economic damages produced by the floods in several developing countries, where the amount of damages and social disruptions which floods produce, could become the cause of limiting their development. So then, the experience shows that the damages caused by the floods continue to increase progressively, and in many countries constitute a veritable restraint to the economic and sustainable development. For this the UN decided in the year 1987 to create the International Decade for Natural Disaster Reduction ( IDNDR ) for the ten years 1990 - 2000 with the objective of reducing by way of concerted international action, especially in the developing countries, the loss of lives, material damages and the social and economic disorders caused by the natural hazards. Among the essential elements of the activities of the IDNDR the following points stand out (3,7):
1.- Greater emphasis in planification and preventive measures.
2.- Adoption of integrated actions (structural and non-structural) for the reduction of the disasters.
3.- Establishment of forecasting and alarm systems compatible with the technology and culture
of the countries.
4.- Development of a social conscience of the necessity of the reduction of the impacts.
2. EXTREME FLOODS IN THE WORLD.
The studies of extreme floods at world level date from the year 1984, when the International Association of Hydrological Sciences ( IAHS ), published the “ World catalogue of maximum observed floods “ (8). These studies show that the maximum floods observed, are limited by an envelope curve, which adapted well to the equation given by Francou-Rodier (9):
The studies of the IAHS of the year 1984, selected the data of 41 extreme floods, which had values of K comprised between 5.19 and 6.76, corresponding this last value to the flood of the year 1953 of the River Amazon, in the Obidos station gauge. These extreme floods present an envelope curve with a value of the coefficient of Francou- Rodier of about 6, which for the upper enveloped reached the maximum value of 6.4, except for the data of the River Amazon which reached an extreme value of 6.76.
Recently, the Committee on Dams and Floods of the International Commission on Large Dams (ICOLD) has carried out a new study of extreme floods at a world level, with the objective of updating the data of the extreme floods and analyse the maximum observed floods on the dams (6). The measures of peak floods on the dams has the advantage of greater precision, with which it lessens the errors of measures of big floods in the gauging stations, which can reach a ± 15%. The work of ICOLD collects 340 new data of the big floods, which have permitted the selection of 21 data of the extreme floods, which have a coefficient K of Francou-Rodier greater than 6, and which are shown in the Table nº 4.
TABLE Nº 4. EXTREME FLOODS IN THE WORLD.
It must be indicated that the data of the floods of the River Amazon have been revised which has permitted, with the experience of the floods of the year 1989, a better estimation of the flood of the year 1953, evaluating it in some 320,000m3/sec. Among the extreme floods, stand out those of New Caledonia, those of the River Ruhe in China in the year 1975, which gave rise to the catastrophic failures of the Banqiao and Shimantan Dams, those of the River Amazon in the years 1953 and 1985 and those of the River Narmada in India.
All the data of the extreme floods is well adapted to the envelope curve, the relationship between the peak flood and the catchment area, with a value of coefficient of Francou- Rodier of 6.4 ( Fig.4 ).
FIG. 4. EXTREME FLOODS IN THE WORLD ENVELOPE CURVE.
Equally the Fig. 5 shows the values of the specific flow, peak flow per unit of area, in relation with the catchment area.
FIG. 5. EXTREME FLOODS. SPECIFIC FLOW.
Also the study of the variation of the coefficient K during the last decades, shows that the value of the coefficient K of 6.4 has not been surpassed, and so no upward movement is observed of the extreme floods, with the available data (6). Nevertheless, the analysis of the data of the extreme floods and of the envelope curve
shows that there is a change of the general behaviour for the basins of area less than 300
km2., and the values of the peak flow are less than those obtained from the
extrapolation of the envelope curve. So, for catchment areas of less than 300 Km2. the
extreme floods adapt better to a new envelope curve, defined as:
The Fig. 6 shows the good adaptation of the new envelope curve for the small basins, and the Fig. 7 corresponds to the envelope curve of the specific flood, in which it can be observed that, for small basins the specific flows are less than 100 m3/sec/km2.
Fig. 6 ENVELOPE CURVES OF THE EXTREME FLOODS. (missed in article)
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REFERENCES
1. IDNHR. 1987.”Confronting natural disasters”. National Academy Press. Washington. DC.
2. HOUSNER, G.W. 1989 “An International Decade of Natural Disaster Reduction: 1900-2000”. Natural Hazards
2,45-75.
3. IDNHR, 1994. “Yokohama Strategy and Plan of action for a safer world”. World Conference
on Natural Disaster Reduction. Yokohama.
4. ZUPKA, D. 1988. “Economic impact of disasters”. Undro News. Jan-Feb.
5. MUNICH RE.1998. “Topics. Annual review of natural catastrophes”.
6. ICOLD. 2002.” Dams and Floods”. Icold Bulletin. Paris
7. UN. 1987. “General Assembly. International Decade for Natural Disaster Reduction” A/Res/42/169.
8. INTERNATIONAL ASSOCIATION OF HYDROLOGICAL SCIENCES. IAHS. RODIER, J.A., ROCHE, M.
1984. “World Catalogue of maximum observed floods”. IAHS Publication Nº.143.
9. FRANCOU, J., RODIER, J.A. 1967. “Essai de classification de crues maximales observées dams le mond”
Chaiers ORSTOM. Série Hydrologie. Vol IV, nº 3. pp 19-46. ORSTOM Bondy.
10. BERGA, L. 2000. “Benefits of dams in flood control”. R35. Q77. 20 Th.Int. Congress on Large Dams. Beijing.
11. MORGAN, A.E., 1951. “The Miami Conservancy District”. McGraw-Hill, First edition, New York.
12. BERGA, L. 1995. “Dams in river flood hazard reduction”. In: Reservoirs in River Basin
development”. L. Santbergen, C.J. Van Weston (Eds). Vol 1, 119-128. A.A.Balkema.
Rotterdam.
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Fig. 7 SPECIFIC FLOWS OF THE EXTREME FLOODS.
In conclusion, the extreme floods in the world have envelope curves, which limit the relation between peak flows and catchment areas. For basins with areas greater than 300 km2, the envelope curve of Francou-Rodier with a coefficient K = 6.4 is valid, but for areas less than 300 km2, it adapts better to the world data, a new envelope curve with a new coefficient R.
3. THE ROLE OF DAMS AND RESERVOIRS IN FLOOD MITIGATION.
The dams and the floods present a mutual interrelation. On the one hand the floods suppose a danger for the integrity of the dams and for their safety, and on the other hand the dams and reservoirs could play an important role in flood routing, and are one of the most efficient structural measures to mitigate the damages produced by the floods. Analysing the natural history of the floods, the measures to prevent and reduce the damages produced by the floods, could be classified in two large groups (10).
A) STRUCTURAL ACTIONS:
They are measures to interfere in the phenomena of flood formation and routing.
A-1 Soil conservation and correction of the basins.
A-2 Dams. Flood control and regulating reservoirs.
A-3 Hydraulic works in the rivers (Levees and dikes, diversions, channel improvements, etc..)
B) NON-STRUCTURAL ACTIONS.
They are measures to mitigate or reduce the damages produced by the floods:
B-1 Risk maps.
B-2 Flood plains: Zoning. Land-use patterns.
B-3 System of insurances.
B-4 General legal regulation. Building regulations.
C) Other types of NON-STRUCTURAL measures are actions in order to foresee, and thus be able to reduce the damages produced by the floods.
C-1 Flood forecasting and flood warning systems.
C-2 Emergency Action Plans
In order to reach a greater effectiveness in the reduction of damages produced by the floods it is necessary to assess the flood control by way of a holistic vision. With this the problem is posed with a more critical vision and less optimistic, and with an integral approach as regards the basin and of alternatives.
The planning of the flood hazard reduction measures should be carried out as regards the basin, with a vision of the whole of the basin, and analysing the incidence that each one of the measures has and the relations between them, as also their effects downstream on the flood routing. On the other hand, the actions as a whole should be considered as a system of integrated measures, developing in each case the implanting of combined measures which contemplate the joint application of structural and nonstructural measures, it being necessary in many cases the development of zonings and land-use patterns downstream of the dam, and also the implantation of flood forecasting and flood warning systems, which are essential for the emergency action plans (Fig. 8).
FIGURE 8. MEASURES IN THE FACE OF FLOODS.
Within the measures in the floods fighting, the role of dams and reservoirs should be emphasized, since the dams constitute a very efficient structural measure, as they are the only solution which permits the storage of large quantities of flood volumes, modifying significantly the flood routing, and being able to reduce the peak flood in an important manner.
Usually the dams and reservoirs that have the main or single purpose of the flood routing are referred to as flood control dams, a denomination which induces to think that they are capable of controlling all the floods and therefore avoid any damage downstream. Evidently, this is not possible and less still in the uncertain subject of floods, in which the absolute zero risk cannot be attained, with the actual physical and technical knowledge. For this reason, it would be better to refer to Flood Mitigation dams or reservoirs, in the sense of indicating the capacity of these structures in the reduction of the damages produced by floods. With all that it is necessary to learn to live with the floods, of course, reducing as much as possible their important impact.
The dams and the reservoirs can be classified in four categories according to their purpose of the flood mitigation:
1. Reservoirs with a single purpose of regulation ( water supply, irrigation or hydropower ), in
which the incidence in the flood mitigation usually is small.
2. Multipurpose - reservoirs with a principal purpose of water storage, but in those in which the
flood mitigation is also an important objective.
3. Multipurpose - reservoirs with a principal objective of flood mitigation, combined with other
objectives of regulation of water.
4. Reservoirs with a single purpose of flood mitigation and the reduction of downstream
damages. Flood mitigation dams.
Furthermore in diverse situations, principally in the cases of large dams on important rivers, the effect of reservoir routing is designed in order to be operative only in a seasonable manner, during the flood season, combining the flood seasonable control purpose with other multipurposes, generally irrigation, hydropower or water supply.
These dams could be referred to as “ Flood season mitigation dams”. In general , the effects of the regulating reservoirs on the floods are more notorious in the low and medium return period floods, in which the reduction in the peak floods and the volumes retained by the reservoirs could be very important and in consequence the mitigation of the downstream damages could be very significant. Their effects on the extreme floods can be less spectacular, although almost always positive. All dams, if they are well designed and operated correctly, present flood mitigation benefits, but the maximum benefits are obtained in the flood control dams in which the routing effects of the floods and the reduction of the downstream damages is the main purpose. It must also be taken into account that in many countries of the world, large populations and important cities have been established over the years on the banks of the rivers which form the backbones of the country. With this, the application of some nonstructural measures is non-viable (resettlement, land-use patterns, etc.) , and the only possible measure for the reduction of the damages of the floods is to reduce the frequency and flows of the constant and repeated floods, a role that can only be carried out by the flood mitigation dams, which although not with a total protection, reduce in a very significant manner the grave impacts due to the almost “annual “ floods.
In the studies of flood mitigation dams there arises on numerous occasions, the alternative of constructing a larger dam on the main river close to the area to be protected, or various small dams located in the headwaters or middle stretch of the basin and on the tributaries of the river. In general, the smaller dams scattered over the basin although numerous, give lesser protection than one single large dam situated immediately upstream of the zone to be protected. So, the Miami Conservatory District in the valley of Miami showed that the realization of five large retention reservoirs give a much greater protection and with less cost than the construction of numerous small dams on the tributaries, and the Corps of Engineers in the basin of the River Merrimak showed that the construction of 13 small dams presented an efficiency of only 52% in
comparison with two large dams located on the main river (11).
In Spain, on the River Onyar in Girona, it has also been seen the effectiveness in flood mitigation decreases in a very important manner as soon as the dams were moved away from the zone to be protected, or were situated on the tributaries (12). So, then, in general, technically a greater protection is obtained with reservoirs situated upstream of the area where flood damages have to be mitigated, but on several occasions the economic, social and environmental aspects present a problem for the construction of dams in the immediate area upstream of the township to be protected.
The dams in the world have supposed enormous benefits in flood mitigation, and their operation during flood events have reduced, in the greater part of the cases, in a significant manner the damages produced by the floods.
The ICOLD Committee on Dams and Floods has studied and analysed diverse significant cases, which show with quantative values, the important role played by dams in the flood mitigation (6). The cases analysed refer to the flood control in ample areas with important flood problems, in Japan, USA, Brazil, China, Korea, Norway, Spain, etc., in which, in general, are combined the effects of the reservoirs, dams, levees and river canalisations, together with the operation in real time of flood forecasting and warning systems.
The flood control plan of the Tone River in Japan began in the year 1900 for the control of the floods in the area of the Bay of Tokyo. After successive revisions its actual formulation is of the year 1980. In the plan are included 10 flood control dams with a reservoir capacity of 229 Hm3. Of these 6 dams are in operation, and the 4 remaining dams are in construction. The combined project flood discharge of dams and dikes is of 22,000 m3/sec, corresponding to a protection for the flood of 200 years return period. In recent years no serious floods in the Tone River have occurred, partially due to the flood control dams and the improvement of river channels. The greatest flood of the last forty years, was that of the typhoon of August of 1982, in which the dams were operated to reduce the flood damage. From among these the Shimokubo Dam greatly contributed to mitigate the damages with a peak discharge reduction of 62%.
In the USA there was in 1993 an extreme flood, the Great Midwest Flood, principally in the upper Mississippi River basin. Seventy-six reservoirs have been developed by the U.S. Army Corps of Engineers ( USACE) in the upper Mississippi for the purpose of flood damage reduction, with a large capacity to store floods with more than 49 billion cubic metres, and a controlled drainage area of 956.000Km2. The USACE estimates that flood damage reduction facilities, (reservoirs, flood walls and levees ) prevented 19.1 Billion$ in damages. Of this quantity 7.4 Billion$ were attributed to the effects of the utilization of the flood storage reservoirs.
A case also to be pointed out in the USA is the Flood Control Miami District, developed on the Miami River, a tributary of the Ohio River, after a catastrophic flood in 1913, in which were produced 360 victims and some damages superior to 100 M$. The plan was implemented with the construction of five detention dams and river channel improvements in nine urban areas. Since completion the dams have stored water on more than 1,000 times providing some substantial benefits, and in the flood of the year 1959, with rainfall close to that of 1913, detention storage utilized was only 32% of the total storage available.
In Spain there exist numerous real cases of the beneficial effects of the dams and reservoirs in the mitigation of damages due to floods. One very significant case corresponds to the flood of November 1982 in the Ebro basin. The global effects of the reservoirs on the mouth of the River Ebro was of 57% of the reduction, with a peak flow discharged in the Ribarroja Dam of 3,200m3/sec, almost of the limit of capacity of the river channel in order not to produce important damages in the downstream townships, as contrasted with the 7,400m3/sec, that were estimated without the existence of the dam in the basin.
In general, in the cases analysed by the ICOLD Committee on Dams and Floods, the effects in the flood mitigation were very significant, with values varying between 25% to 85% in the reduction of the peak flow, surpassing the reduction in numerous flood situations the figure of 50%. The reduction of the flood volume varied from 10% to 73%, with greater values in the cases in which the flood reservoir capacity was high in relation with the flood volume, and in the cases in which the main purpose of the dam was the flood mitigation.
The hydrologic criteria for the design of flood mitigation dams is based on two design floods:
1. An “ Inflow Design Flood “ or “ Safety Check Flood “ to assure the hydrologic dam safety.
2. The protection design flood, which is the flood that the dam is capable of routing without
producing damages downstream.
In general, and apart from the specific analysis in each case, the protection design flood recommended are :
· In rural areas return periods of between 20 and 50 years.
· In urban areas return periods of between 50 and 200 years. In cases of protection of important cities, and if the economic, social and environments aspects are favourable, return periods of 500 years or even 1,000 years may be considered.
In the real cases studied by ICOLD the design flood protection varied between 35 and 200 years of return period, being able to reach in singular situations, in which there exists an important occupation of the flood plains and large cities downstream, values as high as 500 or 1,000 years. At the present time close to 20% of the total of the existing large dams have as a purpose that of flood control, be it a single purpose ( 8%), or as one of its principal objectives.
In the future it has been indicated that due to the exponential growth of the damages produced by the floods, it will be necessary to increase the measures of prevention and reduction of damages, for which the implantation and construction of new flood control dams will be necessary, together with measures which control the progressive occupation of the flood plains and the improvement of the reliability of the flood forecasting systems. For this, an increase of the flood mitigation dams is to foreseen in the future, with extensive flood mitigation plans, as are the cases of Japan, China, Spain and some areas of the USA.
For example, in Japan, the Flood Control dams have a very relevant part to play in the reduction of the damages produced by the floods, protecting the population in a range between the 50 years and the 200 years of return period, for which they can count on some 500 large dams for flood control, providing a total flood control capacity of some 3,700 Hm3. In the future the construction is foreseen of some 400 new large dams, which will contribute some additional 2,400 Hm3, to the flood control reservoir capacity.
In China, the construction of large dams in which the role of flood control plays an important part is also experimenting a strong increase. In the last decade only the floods with a return period of between 10 and 20 years were relatively controlled in the major rivers, and the flood control standards for medium and small rivers are even lower.
Actually with the increase in population and with the economic development, there is a higher requirement for flood control. On the other hand, because of sediment deposition, the flow capacity of the rivers and the flood storage of reservoirs are gradually degrading. Also, the flood storage of the lakes used as additional inundation areas for the extreme flood is also decreasing, due to the deposition of sediments. So, the danger of inundation in extensive areas is increasing especially in the Yellow and Yangtze Rivers. From that, the large dams constructed on these rivers have an important part to play in the flood control, as are the cases of the dams of Longyangxia, Liujiaxia and Xialangdi, on the Yellow River, or the Three Gorges on the Yangtze River.
In Spain, due to serious impacts produced by floods, a considerable increase has been produced in the latter years in the number and importance of reservoirs dedicated exclusively to flood routing, or whose principal purpose is flood mitigation. Actually, today, there are 30 Flood Control dams, which represent about 3% of the existing dams, and in the Hydrologic Plan foresees the construction of some 40 new reservoirs for flood mitigation.
An interesting and illustrated case is that of the American River Basin in California, USA, with a grave problem of floods in the city of Sacramento. For this the U.S. Corps of Engineers carried out in the year 1991 a Feasibility Report and Environmental Impact Statement, with the proposal of a Detention Dam Plan, with the Auburn Dam. The theme has been very problematic and it is in continuous discussion, not having reached yet the implantation of the solution contemplated in the Plan. In similar cases it has not been until after catastrophic floods, when the actions foreseen have been carried out.
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