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Using Multi-scale Sediment Monitoring Techniques to Evaluate Remediation
Effectiveness of the Tsengwen Reservoir Watershed after Sediment Disasters
Induced by Typhoon Morakot
Lin B.-S., Ho H.-C., Hsiao C.-Y., Keck J., Chen C.-Y., Chi S.-Y.
Disaster Prevention Technology Research Center, Sinotech Engineering Consultants, Taiwan
Chien Y.-D., Tsai M.-F.
Soil and Water Conservation Bureau, Council of Agriculture, Taiwan
ABSTRACT: The 2009 typhoon Morakot dumped more than 3,005 mm of rain in mountain areas of the Tsengwen reservoir
watershed and caused unprecedented landslide and sediment-related disasters. Subsequently, the storage capacity of the Tsengwen
reservoir was drastically reduced. In order to increase the longevity of the reservoir and also protect ecosystems and the peoples living
in the upper portions of the watershed, the Taiwan Executive Yuan implemented the "Tsengwen, Nanhua, Wushantou Reservoir
Remediation and Water Resources Protection Act". This study aims to use multi-scale sediment monitoring techniques including field
investigations and multi-stage remote sensing data to identify sediment migration patterns associated with remediated areas of the
Tsengwen reservoir watershed after typhoon Morakot and to guarantee the effectiveness of remediation efforts. A case study of the
Longjiao creek in Tsengwen Reservoir watershed shows that remediation works can not only reduced sediment production due to
erosion and landslides, but future sediment production will also be suppressed. The reduction of sediments carried by the Tsengwen
river will also lead to an increase in the service life of the Tsengwen reservoir.
RÉSUMÉ : En 2009, le typhon Morakot a déversé plus de 3 005 mm de pluie dans les régions montagneuses du bassin versant du
réservoir et provoqué des glissements de terrain Tsengwen sans précédent et les catastrophes sédiments. Par la suite, la capacité de
stockage du réservoir Tsengwen a été considérablement réduit. Afin d'augmenter la longévité du réservoir et aussi de protéger les
écosystèmes et les populations vivant dans les parties supérieures du bassin versant, le Taiwan Yuan exécutif mis en place le
"Tsengwen, Nanhua, Wushantou réservoir d'assainissement et de protection des eaux Loi sur les ressources". Cette étude vise à
utiliser multi-échelle des techniques de surveillance des sédiments, y compris les enquêtes sur le terrain et sur plusieurs périodes
données de télédétection pour identifier les schémas de migration des sédiments associés à des zones assainies du bassin versant du
réservoir Tsengwen après le typhon Morokot et de garantir l'efficacité des efforts d'assainissement. Une étude de cas du flux Longjiao
dans le bassin versant du réservoir Tsengwen montre que les travaux de réhabilitation peuvent non seulement réduit la production de
sédiments à cause de l'érosion et des glissements de terrain, mais la production de sédiments avenir seront également supprimés. La
réduction des sédiments charriés par le fleuve Tsengwen conduira également à une augmentation de la durée de vie du réservoir
Tsengwen.
KEYWORDS: Tsengwen reservoir watershed, typhoon Morakot, sediment disasters, remediation effectiveness.
1 INTRODUCTION
In 2009, typhoon Morakot brought heavy rainfall up to 3,005
mm, which was recorded at Alishan rainfall guage station of the
Tsengwen reservoir watershed over a five day period. Also, the
consecutive 72 hour accumulated rainfall exceeded historical
records in Taiwan (SWCB, 2011). This typhoon event induced
massive sediment-related disasters within the watershed, which
caused about 91,080,000 m
3
of sediment in the reservoir and
exceeded the original design level (5,610,000 m
3
/yr).
Afterward, Taiwan
government passed “Tsengwen, Nanhua,
Wushantou Reservoir Remediation and Water Resources
Protection Act”
and planned a project for managing and
remediating sediment problems. The primary goals of the
proposed project are to reduce reservoir turbidity levels, extend
the service life of the dam and protect security of the upstream
residents.
Sediment transport and deposit within the watershed is an
unavoidable natural process. It is very important to do field
survey and monitor periodically especially in major sediment
source areas including old debris flow, large-scale landslide and
massive alluvial soil or river terrace deposits. Many researches
has pointed out that the sedimentation of Tsengwen Reservoir
has been serious in flood season due to intense geological
activity. Recently, under the effect of global climate change, the
probability of extreme weather occurrence has increased. In the
mountain area, it can be observed that the magnitude of
disasters caused by water-sediment flows, induced by high
intensity and long duration rainfall events, has increased (Lo
et
al
., 2012; Lin
et al
., 2012). The mode of sediment transport can
be classified in different ways, according to the mechanics of
sediment transport process, from suspended load to debris flow.
Therefore, the sediment deposited in the Tsengwen reservoir
watershed comes from long-term deposits of the trunk river and
soil erosion as well as slope landslides due to 2009 typhoon
Morakot. It significantly affects water supply to residents and
rapidly reduced storage capacity of reservoir. For validating and
proving the effectiveness of remediation efforts after typhoon
Morakot event, the study integrates multi-scale sediment
monitoring techniques to collect time-dependent monitoring
data and spatiotemporal remote sensing information including
watershed scale, high-resolution airborne LiDAR DTMs. Then,
using the data obtained from the remediated environmental area,
remediation effectiveness of the Tsengwen reservoir watershed
with regard to suppression of soil erosion, vegetative recovery
rate, variation in amount of landslide and sediment trapping
efficiency are quantified. Finally, the proposed procedure of this
study will assist us to track remediation effectiveness, and
reduce sediment yield entering a reservoir or trap eroded
sediment for effective watershed management.
