Combined Events

A Systems Perspective 

Patrick Regan

Federal Energy Regulatory Commission

Division of Dam Safety and Inspections
Failures are seldom the result of a single “root 

cause” 

Most failures result from an unforeseen 

combination of events that include natural events 

(i.e. rainfall, landslides, etc.), man-caused events 

(i.e. failure of other man-made structures), and 

human/organizational factors (i.e. maintenance 

decisions, operational errors, staffing decisions, 

etc.)
If we focus on just the extreme storm, we may 

miss the more likely system failures that may have 

a similar or larger impact.
Noppikoski Failure Picture
Map
Causes

The Simple Answer

− Too much rain

− Too little spillway capacity

− Mechanical hoist failure
Causes

A more complete list 

1) Extreme precipitation and a high ground water 

level.

2) Mechanical fault in the hoisting equipment.

3) Great difficulties using roads, since ordinary 

brooks in the entire area cut off all roads. This 

excludes personnel reinforcements and 

availability of lifting equipment and tools.

4) The telephone went dead.

Mr. Bengt Skog, Korsnäs AB

The Dam Failure at Noppikoski
5) Transmission poles fell, resulting in blackouts.

6) It all occurred during the night – it is dark and 

rain is falling.

7) Problems getting hold of a mobile crane and 

not least crane operators – it could be Friday 

evening.

8) Helicopters are unable to fly when it is dark.

Mr. Bengt Skog, Korsnäs AB

The Dam Failure at Noppikoski
9) The helicopter radio could only communicate 

with an airforce base – but not on Saturdays, 

when it is closed.

10) At Vässinkoski there were difficulties 

providing large capacity pumps.

11) Exhausted personnel.

12) Unforeseen high downstream water level, 

preventing normal opening of the gate in the 

diversion tunnel.

Mr. Bengt Skog, Korsnäs AB

The Dam Failure at Noppikoski
13) Complications when trying to drain the 

tunnels, since the raised upper water level 

surpassed the intake gate, filling the tunnels.

14) Staff problems – getting hold of qualified extra 

personnel to work 24-hour shifts during a 

weekend.

Mr. Bengt Skog, Korsnäs AB

The Dam Failure at Noppikoski
Southfork Dam Post Failure
 Southfork Original Design
 Southfork Rebuild
Causes of Failure

Overtopped and Failed

 2’ crest lowering to allow carriages to pass 

(spillway capacity reduced to ~72% of design)

 3.5’ settlement due to poor re-build 

(spillway capacity reduced to ~35% of design)

 Fish screen constructed in front of spillway 

(spillway capacity reduced to ~21% of design)
Southfork Post Failure
Debris at Stone Bridge
Kerckhoff 1997
Finally I would like to convey a thought of mine. 

In the dam safety branch we are now at a point 

when we are to realize what has previously been 

analyzed scientifically – how precipitation could 

result in an influx unthought of.

Mr. Bengt Skog, Korsnäs AB

The Dam Failure at Noppikoski
In my opinion, the important question of how to 

adapt the plants – with the exception of 

augmented discharge functions – to practical 

operation, in view of the complications of the kind 

previously listed, has not been considered or 

documented to the same extent.

Mr. Bengt Skog, Korsnäs AB

The Dam Failure at Noppikoski