Research Trends and Results

Detection of Landslide Dam Formation byMonitoring Flow Decline
UCHIDA Taro, Senior Researcher; KANBARA Junichi, Head; NIWA Satoshi, Guest ResearchEngineerErosion and Sediment Control Division, Research Center for Disaster Management
Keywords: river channel blockage, flow monitoring
1. Background
When water flow overtops a landslide dam formed byan earthquake or heavy rain, the result may be a drastic
breach that seriously damages the downstream area
(photo). To mitigate damage caused by landslide dams, it
is essential to detect their formation and take action,
including evacuation, as quickly as possible.
2. Objective
If landslide dams form at the stream of a mountain
river channel, flow rate sharply decreases (compared withflow during normal flooding) downstream from thelandslide dam. It was, therefore, considered that theformation of a landslide dam could be detected byidentifying a drastic decrease in flow downstream from alandslide dam. However, there are few studies that haveaimed to determine a method and criteria for judging
landslide dam formation using flow variation (first issue)or evaluating sections where the landslide dam formation
can be detected using flow data (second issue). Therefore,detection and/or confirmation of landslide dam formation
using flow rate observation data has not actually beenconducted. Therefore, NILIM, in cooperation with theKanto Regional Development Bureau, proposed a new
approach for analyzing these two issues ("proposedapproach").
3. Outline of the proposed approach
Regarding the first issue, we proposed that theformation of a landslide dam can be judged by whether
the degree of flow decline was larger than given criterion.
However, setting a relatively small degree of flow decline
as the criterion might suggest that a landslide dam hadformed when it had not. To reduce this possibility, weused a flow dataset to clarify the degree of ordinal flow
decline, then set the criterion at a point larger than thedegree of ordinal flow decline.
Next, for the second issue, we proposed an approach inwhich we monitor a section of the river where a drastic
and large decrease in flow beyond that expected under theabove described normal conditions is considered likely if
a natural dam is formed. The proposed approach assumed
that the degree of flow decrease because a landslide dam
is dependent on the ratio of the area of the river basin at
the flow observatory to the area of the river basin
upstream from the landslide dam, and on the distancefrom the flow observatory to the landslide dam. Based onthese considerations, we proposed an approach in which
we monitor the landslide dam formed at the sections
where the catchment area is larger than a certain size (thesmallest detectable catchment area) and the distance fromthe flow observatory is less than a certain distance (thelongest detectable distance) (figure).

天然ダムによる湛水

Photo: Example of Landslide Dam Formed inTyphoon No. 12 of 2011

限界面積最遠点までの距離が限界距離より短い
流域面積から見た監
視可能区間
流量観測箇所
限界距離
流量観測点からの距離で見た監視可能区間
限界距離より遠く、除外された範囲
監視可能区間Figure: Diagram of Proposed Approach[Reference]“Manual on data analysis for detection of landslide dam
occurrence using discharge data,” NILIM Document No. 767,
Nov. 2013.