OPINION ON THE 

‘INTERIM REPORT OF THE COMPLEMENTARY STUDIES ON THE CAUSE OF THE 2011 UMYEONSAN LANDSLIDES    

(CONDENSED VERSION) 12 OCTOBER                    2012 KOREAN SOCIETY OF CIVIL ENGINEERS’ 

            

 

  

  

 

                          

                         Professor Andrew Malone 

                      Department of Earth Sciences                       

                          University of Hong Kong  

 

                                15 October 2012 

 

Opinion by Professor Andrew Malone on the ‘Interim Report of the Complementary Studies on the Cause of the 2011 Umyeonsan Landslides’

  

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CONTENTS 

INTRODUCTION                                                      page 3                                                                         

COMMENTS ON THE INTERIM REPORT              pages 4 & 5                                                                   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Opinion by Professor Andrew Malone on the ‘Interim Report of the Complementary Studies on the Cause of the 2011 Umyeonsan Landslides’

  

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INTRODUCTION

In September 2012 Professor Malone was invited by Dr Jong-Seok Won of the Seoul Institute on behalf of the Chief of the Investigation Team of the Korean Society of Civil Engineers, Professor Myoung Mo Kim of Seoul National University, to provide an opinion on the study of the 2011 Umyeonsan landslides by the Investigation Team.

Three other international experts provided opinions:

Dr Hiroshi Fukuoda Associate Professor Research Centre on Landslides Disaster Prevention Research Institute Kyoto University

Dr Akihiko Ikeda Chief of Sabo System Laboratory Sabo Technology Research Institute Tokyo

Professor Ko-Fei Liu Department of Civil Engineering Taiwan National University

On 12 October 2012 in the afternoon the international experts attended a discussion forum held at the Seoul Institute at which members of the investigation team of the Korean Society of Civil Engineers made presentations and the international experts gave their observations.

On the morning of 12 October the international experts had been taken to see four of the 2011 landslide sites at Umyeon Mountain. The visit was lead by Dr Chan-Young Yune of the Department of Civil Engineering Gangneung-Wonju National University and attended by Dr Jong-Seok Won of the Seoul Institute and Dr Min Sik Kim of the Korean Association of Soil and Water Conservation. The four sites visited were: the Eco-Park Reservoir at Hyeongchon valley, Jeonwon village, Bo Deok Temple and the Ramian APT / Deok U Am (Dukwooam) site.

Professor Malone arrived in Seoul on 11 October and left on 13 October. He wishes to express his appreciation to all for the valuable presentations and discussions on 12 October and for providing effective support during his visit.

The Interim Report of the Complementary Studies on the Cause of the 2011 Umyeonsan Landslides

(condensed version) dated 12 October 2012 Korean Society of Civil Engineers was the only document

provided to Professor Malone.

The comments which follow are subject to revision in the light of new information which may become available later. There are other matters in the report on which comments might be made but the following appear to be the most salient as they relate to the conclusions of the report.

Opinion by Professor Andrew Malone on the ‘Interim Report of the Complementary Studies on the Cause of the 2011 Umyeonsan Landslides’

  

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COMMENTS ON THE INTERIM REPORT

1. In regard to Interim Report section 3 (1) (b) Rainfall Analysis, in Hong Kong (about 1100sqkm) we have rainfall datasets obtained at 110 automatic raingauges, including at about 60 gauges with records for more than 30 years. Based on the geographical variations in rainfall measured in Hong Kong, I think it possible that the rainfall intensities during rainstorms may be significantly higher at the top of Umyeon Mountain than at the two Automatic Weather Stations for which frequency analysis was carried out (Seocho and Namhyeon stations), which are at significantly lower elevation.

2. The Hong Kong datasets show significant differences in return period correlation for rainfall intensities between one gauge and another gauge not far away. I think it possible that the return period correlation for rainfall intensities at the top of Umyeon Mountain may differ significantly from that established at the Seoul Observatory, which I was told is several kilometers west of Umyeon Mountain and at significantly lower elevation.

3. The Hong Kong datasets also show significant differences in 30-year mean annual rainfall between one gauge and another gauge not far away but at significantly different elevation.

4. Considering items 1-3 above, some qualifying remarks might be added to the text of the Interim Report to say that there is a degree of uncertainty about the intensities of the rainfall which triggered the debris flows in 2011 and the associated Return Periods. The rainstorm of 26/27 July 2011 might not be as rare as it is represented to be in the Interim Report.

5. Interim Report section 4 Risk Analysis of Landslide/Debris Flow for Umyeonsan (Mt.) Area and section 6 Concluding Remarks (number 1) and also section 3 (2) Geology rely on the use of landslide hazard zoning methodology. In this regard, in Hong Kong we have good quality rich datasets but we have found the susceptibility and hazard zoning methodology to be of limited resolution and reliability and generally not adequate for direct application to landslide risk management. We have used these methods (since 1979) only for applications where high resolution and reliability are not required.

6. In regard to Interim Report section 5 (2) Effect of landslide and drained water from Air Force base and section 6 Concluding Remarks (number 3), in the social context it is the consequences of the landslides which matter, rather than the number of landslides. I understand that landslides originating within the Air Force base were responsible for 7 deaths out of a total of 16/17 deaths at the Umyeon Mountain area on 27 July 2011. The contribution of the landslides/debris flows related to the Air Force base to the total damage was therefore not small.

7. Interim Report section 5 (4) Effect of blasting executed for tunnelling underneath the Umyeonsan and section 6 Concluding Remarks (number 6). The assessment to be presented in the report should include consideration of the possibility of degradation of soil properties due to the vibrations caused by the blasting that was carried out in 2011. Degradation of soil properties at the sources of the debris flows should be considered and, if entrainment occurred, along the tracks of the debris flows.

Opinion by Professor Andrew Malone on the ‘Interim Report of the Complementary Studies on the Cause of the 2011 Umyeonsan Landslides’

  

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8. Section 6 Concluding Remarks (number 4) refers to countermeasures and offers a judgement about avoidability. Recognizing that physical countermeasures works to all of the valleys at Umyeon Mountain could not practically have been completed by 26 July 2011, if only started in response to the 2010 landslides, it seems relevant to attempt in the report to answer the question: did the Seoul Metropolitan Government consider taking or take any action to address the wider problem of landslide risk at Umyeon Mountain or elsewhere in Seoul following the landslides/debris flows in 2010 at Umyeon Mountain?

Professor Andrew Malone

15 October 2012