Dam Safety Aspects of

Reservoir-Triggered

Seismicity

Dr. Martin Wieland

Chairman, Committee on Seismic Aspects of Dam Design,

International Commission on Large Dams (ICOLD)

Poyry Energy Ltd., Zurich, Switzerland

Official ICOLD Terminology

Old (misleading) term:

Reservoir-induced seismicity (RIS)

New (correct) term:

Reservoir-triggered seismicity (RTS)

ICOLD Bulletin 137

Table of Contents (Bulletin 137)1. INTRODUCTION

2. RESERVOIR TRIGGERED SEISMICITY PHENOMENA AND DEVELOPMENT

OF THEIR EVALUATION AND INTERPRETATION

3. FREQUENCY OF RESERVOIR TRIGGERED SEISMICITY

4. CHARACTERISTICS OF RESERVOIR TRIGGERED SEISMICITY

5. MECHANISM OF RESERVOIR TRIGGERED SEISMICITY AND RHEOLOGY OF

EARTH CRUST MATERIALS

6. PORE PRESSURES DIFFUSION TIME

7. GENERAL STATEMENT ON UNDERSTANDING RTS PHENOMENA

8. RISK MANAGEMENT

9. CASE HISTORIES

9.1. Hsingfengkiang Dam Case History

9.2. Mratinje Dam Case History

9.3. Kurobe Dam Case History

9.4. Takase Dam Case History

9.5. Poechos Dam Case History

10. ASSESSING THE POTENTIAL AND MONITORING RTS

11. CLOSING CONSIDERATIONS

12. REFERENCES

Is RTS a safety

concern for large

dam projects?

Overview

• Introduction RTS

•Dams and RTS

•Seismic design criteria and RTS

•Effects of RTS

•Monitoring of RTS

•Conclusions

Basic requirements for RTS

existence of active faults

and/or

existence of faults near failure limit

Main features of RTS

•Seismic events during and after impounding are more frequent than background seismicitybefore impounding.

•With increase of reservoir level and variation of water level, number and magnitude of RTS events increase.

•Often RTS events decrease towards background activity after peaking.

Frequency of RTS

•Number of cases with M > 5.7: 6

•Number of dams with height h > 100 m: > 400

•Probability of RTS (M > 5.7 and h > 100 m):

6/400 = 0.015

This is not a negligible value!

Large dams subjected to RTS

Hsinfengkiang buttress dam (China), 105 m high

1962, M=6.1, dam damaged and strengthened

Koyna gravity dam (India), 103 m high

1967, M=6.3, dam damaged and strengthened

Hoover arch dam (USA), 220 m high, 1935, M=5.0

Kremasta embankment dam (Greece), M=6.2

Kariba arch dam (Zambia), M=6.3

Maximum magnitudes suspected of being

caused by RTS: M = 6.0 to 6.3

Upper bound for RTS

The maximum observed/suspected RTS

magnitude is about 6.3.

It is unlikely to trigger the Safety Evaluation

Earthquake (SEE) by a reservoir – this has

not yet happened.

RTS Wenchuan Earthquake 2008 (China)?

Was the May 12, 2008 Wenchuan (Magnitude 8.0)

earthquake triggered by the reservoir stored behind

the 156 m high Zipingpu concrete face rockfill dam?

The Wall Street Journal 9.2.2009

Scientists Link China's Dam to Earthquake, Renewing

Debate

Observed Seismicity at Zipingpu Dam (China)

Water Level in Zipinpu Reservoir (China)

Reservoirs and Wenchuan Earthquake

There exists no factual evidence that

supports the assumption that the devastating

Wenchuan earthquake of May 12, 2008 was

triggered by the Zipingpu reservoir!

(Note: Several earth scientists still believe that this

was the case.)

Example of RTS: Nurek dam, Tajikistan

World’s highest dam

Damage due to RTS

•Koyna dam, a 103 m high straight gravity dam,

M = 6.3 (1967)

•Hsinfengkiang dam, a 105 m high buttress dam,

M = 6.1 (1962)

•Earthquakes suspected of being caused by RTS.

•Both dams developed substantial longitudinal cracking near top.

•Damage attributed to design or construction details that would be avoided in modern structures.

•Both dams were strengthened and are still in service.

Reservoir water levels at Koyna 1961 to 1995 and M > 4.0 events

Three periods of M > 5.0: 1967, 1973 and 1980

Increase in water pressure at various depths

with time in days

Repair of Koyna dam, India

Strengthening of Koyna dam, India

Seismic hazard due to RTS

•ground shaking: vibrations in dams, appurtenant structures, equipment and foundations

•mass movements into reservoir and rockfalls at dam site: impulse waves in reservoir; blockage of intakes; damage of hydro-mechanical and electro-mechanical equipment, and other damage.

• fault movements in dam foundation

• fault displacement in reservoir bottom: water waves in reservoir or loss of freeboard.

•noise

Note: Fault movements will be small due to small RTS magnitudes

Rockfall Hazard at Dam Sites

Landslide at tailrace of hydropower plant, 2008 Wenchuan earthquake, Sichuan Province, China

Taipingyi Hydropower station overtopping

Damaged gate by rockfall at power intake, Wenchuan earthquake 2008

Penstock failure at expansion joint

Transmission tower failure due to rockfall, Sefid Rud dam

Micro-seismic networks for RTS monitoring

Comprehensive monitoring of RTS

before construction,

during construction,

duringreservoir impounding, and

during first years of operation

is strongly recommended for large storage dam

projects located in areas with faults and high tectonic

stresses in order to dispel any doubts about what is

actually happening.

RTS Karkeh dam, Iran 2000–2001, 3.4.2001 M=5.1

Effect of small magnitude earthquakes on poorly built dams: Sharredushk Dam, Albania, after 2009 earthquake,

M=4.1, PGA = 0.07 g

Is RTS a safety

concern for large

dams?

Integral Dam Safety Concept

Structural SafetyDesign of dam according to state-of-practice (codes,

regulations, guidelines, etc.) (earthquake design criteria, methods of seismic analysis etc.)

Dam Safety MonitoringDam instrumentation, visual inspections, data

analysis and interpretation, etc.

Operational SafetyGuidelines for reservoir operation, qualified staff,

safe software, maintenance, etc.

Emergency PlanningEmergency action plans, water alarm systems, dam breach

analysis, evacuation plans, Engineering back-up, etc.

Seismic design criteria

Dam and safety-relevant elements (spillway,

bottom outlet):

Operating basis earthquake, OBE (145 years)

(negotiable with owner)

Safety evaluation earthquake, SEE (ca. 10,000 years)

(non-negotiable)

Appurtenant structures (powerhouse etc.):

Design basis earthquake, DBE (ca. 475 years)

Temporary structures (coffer dams) and critical

construction stages:

Construction level earthquake, CE (> 50 years)

Seismic performance criteria for dam and safety-relevant elements

(i) Dam body:

OBE: fully functional, minor nonstructural damage

accepted

SEE: reservoir can be stored safely, structural damage

(cracks, deformations) accepted, stability of dam must be

ensured

(ii) Safety-relevant elements (spillway, bottom

outlet):

OBE: fully functional

SEE: functional so that reservoir can be

operated/controlled safely and moderate (200 year return

period) flood can be released after the earthquake

Ground shaking

Earthquakes affect all components of a dam project at the same time:

dam

foundation

safety devices

pressure system

underground works

appurtenant structures

hydro-mechanical equipment

electro-mechanical equipment etc.

Design Earthquake

Title Element / Component

CE DBE OBE/

SEE

Diversion Facilities

- Civil Intake/outlet structures X

Tunnel, tunnel liner X

- Geotechnical Rock slopes X

Underground facilities X

Cofferdams X

- Electrical/Mechanical Gate equipment X

Dam: Dam Body Dam body X

- Individual Blocks OBE

Crest bridge X

Crest spillway cantilevers X X

Bottom Outlet cantilevers X

Foundation/Abutments Abutment wedges X X

Bottom Outlet Main gates, Valves X X

Guard gate X

Operating equipment X X

Dam: Electrical/Mechanical Essential parts X

Comparison of RTS ground motion with seismic design criteria for dams and

buildings

Dam and safety-relevant elements:

RTS < SEE

Appurtenant structures and buildings in reservoir

area:

RTS > DBE or RTS < DBE

Assessing the Potential and Monitoring of RTS

Any large dam of greater height (h > 100 m) is a candidate for RTS. To assess its potential, the following data are needed:

• tectonic conditions and data on structural geology, supported by study of aerial photographs.

•macroseismic data for reservoir under study.

• information on active faults and all data on recent fault activity in dam and reservoir region.

•assessment of seismic capability of all faults in dam and reservoir region.

• regimes of underground water.

Case studies (ICOLD Bulletin 137)

•Hsingfengkiang Buttress Dam in China, as a representative of large triggered seismicity, causing a strong local earthquake, which significantly damaged the dam.

•Mratinje Arch Dam in Yugoslavia as a representative of moderate RTS. It is of particular interest as seismic monitoring was introduced prior and after impounding, witnessing re-appearance of RTS after 17 years of service.

Case studies

•Kurobe Arch Dam was monitored prior and after impounding. The dam was reported as an RTS case on the basis of microseismic monitoring after impounding. But later analyses led to the conclusion that Kurobe dam was not an RTS case.

•Takase Rockfill Dam is a large dam monitored prior and after impounding, where similar microseismic activity was present before and after impounding.

Case studies

•Poechos Embankment Dam in Northern Peru

as a case of seismically active environment

where RTS was absent or was masked by basic

background activity.

Summary on RTS•Probability of RTS increases with dam height and reservoir size. RTS potential needs to be considered for dams with h >100 m.

•Triggered earthquakes are seismic events, which needed effects of reservoir load and pore pressure build up, to be manifested.

•Maximum magnitude and maximum surface intensity of seismic events cannot be increased through effects of reservoir impounding.

•Earthquake safety of dam is covered by SEE.

•Earthquake safety of appurtenant structures and buildings in dam vicinity should be checked for RTS.

• In cases of low historical seismicity, the neotectonic studies undertaken to assess the RTS potential might lead to determination of controlling earthquake.

Conclusions

•There is no need to treat RTS separately as ground

motion due to SEE is larger than that of RTS.

•Maximum observed magnitude for RTS is about 6.3.

• Impossible to prove that strong earthquake is caused

by reservoir as focal depth is several kilometres.

•RTS may cause mass movements into reservoir,

resulting in water waves. overtopping or blockage of

intakes.

•RTS may cause rockfalls damaging appurtenant

structures and hydro-mechanical and electro-

mechanical equipment.

Conclusions

•Seismic safety of dams, where RTS has taken place,

has to be re-assessed.

•Buildings and structures in reservoir region are

designed for smaller seismic forces than SEE, thus

RTS may cause damage and loss of lives in these

regions.

•RTS, which can be felt or heard, creates safety

concerns in the population.

Conclusions

Monitoring of seismic activity

• Prior to construction,

• During impounding of reservoir, and

• During first years of reservoir operation

is highly recommended for

(i) large storage dams, and

(ii) dams located in tectonically stressed regions.