Low cost methods for safety improvement of medium and small dams

F. Lempérière

Hydrocoop, Paris, France

P. Royet

Cemagref, Aix-en-Provence, France

P. Blanc

Hydrocoop, Paris, France

ABSTRACT: Most risk analysis and safety improvements are made by wealthy owners of rather safe dams. But most of the failures and relevant victims around the world have been caused by small or medium size dams for irrigation or water supply ; owners of such dams have usually reduced financial resources and few technical knowledge of risks. After an analysis of ruptures of European dams, the paper suggests realistic and low cost ways to help improving dam safety : identification of dams most at risk within an important portfolio of medium and small dams, solutions to improve structural safety and solutions to improve safety against floods.

INTRODUCTION

There are in Europe 4500 "large dams". 3000 have reservoir capacity in the range of 1 hm³ and their owners often have very limited financial resources and no technical knowledge. They may hardly appreciate the possibly high relevant risks and even their legal responsibility.

The present report tries to help improving this situation in three ways :

-Analysis of European dams history and failures, to identify main general risks;

-Presentation of a French experience to identify, at low cost, dams most at risk within 360 medium dams;

-Suggestion of low cost solutions to reduce quickly relevant risks and hazards.

1 ANALYSIS OF EUROPEAN DAMS HISTORY AND ACCIDENTS

34 reported failures are classified according to failure and construction year, height, reservoir volume and failure circumstance for five different dam types (see table):

1.1 Masonry gravity dams

Over 200 have been built, quite all between 1850 and 1930. Foundation was sometimes poor, and, before 1900, cross section often thin. Moreover, masonry quality could be unable to prevent extent of upstream cracks due to various reasons. 7 dams failed (3%). The high risk of these dams may be underlined by the fact that 8 failures (most during floods) happened for similar structures built at the same time in India or Algeria by European engineers.

During floods, extra load due to higher water level (and sometimes to muddy water) increases considerably the risk, specially for rather low dams. Failures were not caused by downstream toe erosion but by stiding in foundation or cracks in dam body: failure may happen before overtopping.

1.2 Concrete gravity dams

400 were built in Europe from 1900 to 1950 (3 failed), and 800 later (no failed); almost all have a classical cross section and were, after 1950, generally founded upon fair rock. Good performance of such dams is similar around the world : no failure has been caused because of the quality of traditional concrete.

1.3 Arch dams

Europe has built 500 arch dams (quite all in con­crete), essentially between 1920 and 1970. Malpasset dam failure in 1959 during first filling was caused by foundation weakness. Several arches (including Idbar) were abandoned due to cracks or strongly reinforced. Main ageing problems may be bound with foundation drainage, masonry quality, or very thin structures.

Global performance, as everywhere else around the world, is as good as the performance of concrete gravity dams.

1.4 Multiple arches or buttress dams

250 were built in Europe between 1910 and 1970, at the same time as the concrete gravity dams but some buttresses are in masonry. 5 failed (2%), including 2 failures in masonry buttresses and failures caused by poor foundations. Most happened at first filling but ageing of thin reinforced concrete or masonry may be a serious risk, as well as earthquakes. For example in France during the recent decades, 2 multiple arches dams, in the range 15-20 m high, were decommissioned or demolished.

Performance of similar dams in other parts of the world is similar (4 failures from 200 dams), much less satisfactory than for concrete gravity or arch dams.

1. 5 Fill dams

Half European dams are fill dams. They have caused half of failures:

-300 were built in 19th century : 6 failed (2%);

-200 were built from 1900 to 1950 : 2 failed (1 %);

-over 1.500 were built from 1950 to 2000 : 8 failed (0,5%).

12 failures from 16 were caused by floods although floods intensity in Northem Europe is much smaller than worldwide. Few failures were caused by piping, much less than in America. (In the 19th century most European fill dams were built in Great Britain often with a well compacted thin clay core and in the 20th century most were built after 1950 usually with suitable drains and filters).

This does not mean however than all existing fill dams were well designed and built. Some failures may also have been unreported.

1.6 Tailing dams

This special problem is not studied hereunder but should not be underestimated. Since 30 years relevant failures in Europe (U.K., Italy, Bulgaria) caused hundreds of victims, much more than failures of classical dams during this period.

1.7 General comment...

34 European dams failures have been reported. 4 were caused by war and did apply to dams higher than 40m closing very large reservoir (over 100 hm³). .

From the 30 other failures :

-23 reservoirs had a capacity under 10 hm³;

-20 dams were lower than 30 m.

Most of dam breaches were in the range of 20 m deep (only 2 deeper than 30 m) but width of breaches in masonry and concrete dams was usually in the range of 100 m.

At least 18 failures caused victims; their number varies greatly. Sudden failures (most often in masonry or concrete) are much more dangerous than progressive fill dams failures. Many European failures happened in winter and during night; and cold water increases considerably the risk of victims.

16 failures happened more than 5 years after first filling, including 6 since 1975 (i.e. a risk between 0,5 and 1 x 10^-4 per year).

2 OVERVIEW OF FRENCH DAMS UNDER MINISTRY OF ENVIRONMENT AUSPICES

2.1 Dam safety in France

Today three categories of dams exist in France each with its own particular management structure:

-35 state-owned canal feeder dams exist, dating from the 19th century. They are currently operated by a recently created public agency: V.N.F. ' (French Navigable Waterways) and controlled by Ministry of Equipment services.

-200 hydroelectric dams are operated by two well-established companies: Electricité de France and Compagnie Nationale du Rhône. Most of the large French dams are included in this category, including most concrete dams. The Ministry of Industry ensures safety of these works through regional outlets aided by a central technical service: BETCGB (Technical and Management Planning Office for Large Dams).

-all other dams (several hundreds), used for water supply, irrigation, flood control or leisure activities, are under the jurisdiction of the Environmental Ministry since the end of the 1980's. These very varied dams belong to numerous, generally untrained owners who rarely own more than two or three dams. These owners depend upon specialized Consulting Engineers for dam monitoring and follow-up studies. Unfortunately, not every owner use this type of contract. Dam safety is ensured by local representatives of the Environmental Ministry aided for the past ten years by the Cemagref (French Research Institute for Agricultural and Environmental Engineering).

Uniformity of France's dam network is ensured by general legislation (particularly the August 14, 1970 circular defining dam surveillance and management requirements for public safety) and by the CTPB (Permanent Technical Committee on Dams) which examines all construction and reinforcement projects for dams over 20 m high.

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