2289
2011 Seoul Debris Flow and Risk Analysis
Coulée de boue à Séoul en 2011 et analyse des risques
Yune C.-Y., Kim G., Lee S.W., Paik J.
Department of Civil Engineering, Gangneung-Wonju National University
ABSTRACT: A series of debris flow events occurred around 8:00 to 8:50 a.m. on July27, 2011 in the Umyeon Mountain area located
in central Seoul, Korea. Umyeon Mountain is a relatively small one with a height of 250m and slopes with the average angle of
around 15°. Due to the debris flows with runout distances ranging from 300m to 1,000m, 16 people were killed and more than 150
houses had been damaged in the area. In this work, we seek to understand the physical characteristics of the initiation and propagation
behavior of debris flows from field investigation in the area and the analysis of rainfall data collected by AWS (Automatic Weather
Stations) rain gauges. Field investigation shows that about 33 debris flows occurred in the Umyeon Mountain area and most of these
debris flows were initiated from small slope failures induced by high-intensity rains. The application of landslide hazard map which
has been recently developed by taking slope angle and direction, strength of soil, hazard record, rainfall condition and plantation into
account demonstrates its good performance to highlight areas that are vulnerable of heavy-rainfall-induced slope failure and the
resulting debris flow disasters.
RÉSUMÉ : Une série de coulées de boue s'est produite vers 8h00 ~ 8h50 le 27 Juillet 2011 dans la région du mont Umyeon situé dans
le centre de Séoul, en Corée. La montagne Umyeon est relativement petite avec une hauteur de 250m et des pentes à angle moyen
d'environ 15°. En raison de ces coulées de boue sur une distance allant de 300 à 1000m, 16 personnes ont été tuées et plus de 150
maisons ont été endommagées dans la région. Dans cette étude, nous chercherons à comprendre les caractéristiques physiques de
l’initiation et du la développement de la propagation des coulées de boue à partir d’une enquête sur place dans la zone concernée et de
l’analyse des informations de précipitations recueillies par les pluviomètres de l’AWS (« Automatic Weather Stations », Station
météorologique automatique). L’enquête sur place montre que près de 33 coulées de boue ont eu lieu dans la région du mont Umyeon
et la plupart de ces coulées de boue ont pris naissance à partir de petites ruptures de pente provoquées par des pluies très intenses.
L'application de la carte des risques de glissement de terrain qui a été récemment mise au point en prenant en compte l'angle et la
direction des pentes, la résistance du sol, l’historique des désordres, les conditions des précipitations et des plantations démontre sa
bonne performance pour mettre en évidence les zones qui sont vulnérables à des ruptures de pente provoquées par de fortes pluies et
des catastrophes résultant de ces coulées de boue.
KEYWORDS: debris flows, field investigation, rainfall, hazard map
1 INTRODUCTION
Most of the debris flow hazard has been concentrated during the
rainy season from June to August in Korea. In 2011, there were
Typhoon Meari (22 June to 27 June), Typhoon Muifa (28 July
to 9 August) and the seasonal rain front that stayed in the
middle part of the Korean peninsula. They poured much more
rainfall compared to the average value, and eventually led to
debris flows around the country (Yune and Jun, 2011). A series
of debris flow events occurred around 8:00 to 8:50 a.m. on
July27, 2011 in the Umyeon Mountain area located in southern
part of Seoul, Korea.
Debris flows which generally contain more than 50% of
granular materials larger than sand particles flow fast (Johnson
and Rahn, 1970; Hutchinson, 1988). Because of their fast
movement, it is very hard to take refuge, even though people
recognize beforehand the outbreak of the debris flow. Between
8:00 to 8:50 a.m. on July 27, 2011, many debris flows occurred
simultaneously at Umyeon Mountain in Seoul where there had
been a previous debris flow event in 2010 caused by Typhoon
Kompasu. Because the mountain is located at the center of a
dense residential area in Seoul, the hazard had a great impact on
the society compared to debris flows that occur in rural areas,
and it led to careful scrutiny of the hazard area and the causes of
debris flows.
In this research, a site survey on the debris flow at Umyeon
Mountain was carried out to analyze the causes of the event,
and rainfall records were also collected to investigate the
triggering characteristics of the rain. In addition, risk analysis
based on the hazard map of the landslide and debris flow was
performed.
2 2011 SEOUL DEBRIS FLOW
2.1.
Overview of Umyeon Mountain
Located in southern part of Seoul (Fig. 1) with the height of
312.6 m above the sea level, Umyeon Mountain includes lots of
eco-friendly facilities, such as natural ecological park and
mineral spring which can be easily accessed by local residents.
It has a main ridge formed from the northeast to the southwest,
including several valleys perpendicular to the main ridge. Most
of the mountain consists of highly-weathered banded gneiss
with subordinate granitic gneiss. Directions in ENE- and NW to
NNW-striking faults are predominant with strike- or normal-slip
sense. The groundwater level in the basin seems high because
six mineral springs are located side by side at the altitude of 220
to 250 meters on the slope. Field observation of Son et al.
(2012) also showed that groundwater level at locations where
debris flows initiated reaches to the surface so that surface
runoff occurs at the continuous rainfall condition of more than
250 mm, even if there is no significant antecedent rainfall. In
