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Flood handling and Flood handling and emergency action emergency action planning for damsplanning for dams

Dr.ing.thesis Dr.ing.thesis byby

Grethe Holm MidttømmeGrethe Holm Midttømme

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1. Evaluation of present practice for risk analysis and emergency action planning (as a tool for handling floods)

2. Development of method for creation of flood scenarios

3. Study of previous floods (preferably extreme floods)

4. Case study: create a modern flood scenario for the developed Vinstra River Basin based on the ”Storofsen” flood of 1789

Main activities

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Several challenges related to analysis of flood scenarios were recognised:

1. It may be difficult to assign probabilities to various events/failure modes

2. So far, “human errors” seem to have been underestimated as a contributing factor to failures and adverse incidents

3. The commonly used RA-methods are not suitable for analysis of complex systems such as a river basin with several dams and waterways (being hit by the same flood event)

Evaluation of present practice for RA Evaluation of present practice for RA & EAP& EAP

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Analyses and emergency exercises must be

based on realistic scenarios More focus on the complexity of a flood than on the peak discharge and water

level is needed.

Transfer of scenarios from one system to another must be considered thoroughly.

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Method for creation of an extreme flood scenario

A new scenario can be based on:

1. A recent flood

2. Reconstructed historical flood

3. Hypothetical flood (e.g. PMF)

4. Flood in actual river basin or in similar river basin

Adjustment of data in time or space must be done when necessary

Combination of data from several previous floods can be utilised

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The study of previous floods

Criteria used for selection of flood

cases:

• Floods should be relevant for the study river basin (Vinstra)

• Floods causing problems for dams should be emphasized

• Major/extreme floods should be emphasized

• When several similar floods are well documented, the best documented flood event is emphasized

Objective: to increase the knowledge on how severe and extreme floods will affect dam safety, i.e. provide a basis for creation of extreme flood scenarios

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Floods included in the study (1) - Glomma River Basin, Norway

•1789•1860•1938•1987•1995•2000

Severity: ranging from Q100 to Q1000

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Floods included in the study (2)

Year - River basins:

1979 – Jostedøla

1985 – Ore älv

1986 - Trysilelva

1997 - Fulufjäll 1996 – Saguenay, Canada

(Appr. Q10 - Q10000)

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The flood at Mt.Fulufjäll in 1997 – a true extreme event

(Lundquist, 2002)

(Specific peak discharges)

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Consequences of the Fulufjäll-flood,1997

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Some conclusions from the study of previous floods (1)

Deviations from ”normal” conditions must be expected and it is difficult to foresee all possible events that may occur during a flood

Examples:

Braskereidfoss Dam in 1987 and the propagation of the 2000-flood in South-eastern Norway

”Worst case”-scenario for Braskereidfoss Dam (from the 1987-RA): increase in waterlevel from HRWL to dam crest will take 13 hours (1.7 m).

During autumn flood in 1987 (Q100) WL rose as much as 1.4 m in 1 hour due to unexpected failure of both normal and auxiliary power supply

Flood in Oslo November 2000

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Some conclusions……… (2)

Several cases show that problems tend to pile up during a flood. Thereby, relatively moderate floods can cause significant problems to dam operation.

Typical problems:

•Damaged infrastructure

•Personnel unavailable due to vacations, sickness absence etc

•Technical equipment out of order

•Extreme weather hindering essential actions

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Case study: Vinstra River Basin

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Creation of extreme flood scenario for Vinstra River Basin using the Storofsen-flood of 1789 as

basis (1)

Consequences of the 1789-flood: extensive damages to roads and properties, 68 persons killed (3 in Vinstra River Basin)

Elements from the 1789-flood used in extreme flood scenario:

1. Constructed inflow hydrographs

2. Prevailing weather conditions

3. Landslide activity

Many people were convinced that they were witnessing the Day of Judgment. Many became mentally disordered after the disaster.

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Basis flood: Storofsen 1789

Hit SE-Norway 20th - 23rd July

Prior to the flood:

Large volumes of snow in the mountains, warm and wet weather for several weeks -> high discharges in the rivers before 20th July

Weather conditions causing the flood:

Persistent heavy rain (thunderstorms) from 20th to 23rd JulyGHM SEPT 2002

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Creation of extreme flood scenario…(2)

Elements from the 1938-flood:

1. More natural hazards

2. Secondary effects (e.g. broken access roads)

3. Observed meteorological and hydrological data from 1938-flood used for calibration of rainfall/runoff model when reconstructing the 1789 flood

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Creation of extreme flood scenario…(3)

Elements from 1987-flood and 1995-flood (for updating of scenario to present)

•More secondary effects

•Operational and administrative problems

•Attention from media and the general public

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The scenario was tested in the River Flood and Accident Simulator (RIFA)

RIFA = a computer-based training simulator for dam and hydro plant operation during floods (Alfredsen & Midttømme, Hydropower and Dams, no2,2002)

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Conclusion

1. Data from previous floods, even historic floods, can be utilised in a systematic way to create realistic flood-scenarios for analyses and exercises

2. There is a lot of interesting information available from a variety of sources; official reports, history books, observation series, water level registrations, dam operator’s log-books, eyewitnesses etc

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Recommendations for further studies on flood handling and dam safety

1. Expand the study of flood events

2. Emphasize ”human errors” in future analyses

3. Develop an analysis-method for systems including several dams based on the RIFA-model

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