Simulating dam - breach flood scenarios of the Tangjiashan landslide dam induced by the Wenchuan earthquake

Xuanmei Fan, C. Tang, C.J. van Westen, D. Alkema

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

Floods from failures of landslide dams can pose a hazard to people and property downstream, which have to be rapidly assessed and mitigated in order to reduce the potential risk. The Tangjiashan landslide dam induced by the Mw=7.9 2008 Wenchuan earthquake had impounded the largest lake in the earthquake affected area with an estimated volume of 3×10^8m^3, and the potential catastrophic dam breach posed a serious threat to more than 2.5 million people in downstream towns and Mianyang city, located 85 km downstream. Chinese authorities had to evacuate parts of the city until the Tangjiashan landslide dam was artificially breached by a spillway, and the lake was drained. We propose an integrated approach to simulate the dam-breach
floods for a number of possible scenarios, to evaluate the severity of the threat to Mianyang city. Firstly, the physicallybased BREACH model was applied to predict the flood hydrographs at the dam location, which were calibrated with
observational data of the flood resulting from the artificial
breaching. The output hydrographs from this model were in-
putted into the 1-D–2-D SOBEK hydrodynamic model to
simulate the spatial variations in flood parameters. The simu-
lated flood hydrograph, peak discharge and peak arrival time
at the downstream towns fit the observations. Thus this ap-
proach is capable of providing reliable predictions for the de-
cision makers to determine the mitigation plans. The sensitiv-
ity analysis of the BREACH model input parameters reveals
that the average grain size, the unit weight and porosity of the
dam materials are the most sensitive parameters. The vari-
ability of the dam material properties causes a large uncer-
tainty in the estimation of the peak flood discharge and peak
arrival time, but has little influence on the flood inundation
area and flood depth downstream. The effect of cascading
breaches of smaller dams downstream of the Tangjiashan
dam was insignificant, due to their rather small volumes,
which were only 2 % of the volume of the Tangjiashan lake.
The construction of the spillway was proven to have played a
crucial role in reducing the dam-breach flood, because all the
other natural breach scenarios would have caused the flood-
ing of the downstream towns and parts of Mianyang city.
However, in retrospect improvements on the spillway design
and the evacuation planning would have been possible. The
dam-break flood risk will be better controlled by reducing the
spillway channel gradient and the porosity of the coating of
the channel bottom. The experience and lessons we learned
from the Tangjiashan case will contribute to improving the
hazard mitigation and risk management planning of similar
events in future.
Original languageEnglish
Pages (from-to)3031-3044
JournalNatural hazards and earth system sciences
Publication statusPublished - 2012

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landslide
dam
earthquake
spillway
hydrograph
lake
mitigation
porosity
Sichuan earthquake 2008
peak discharge
integrated approach
coating
spatial variation
grain size
hydrodynamics
hazard
city
prediction

Keywords

  • ITC-ISI-JOURNAL-ARTICLE

Cite this

@article{b36878214d104d18a885b17ddd48661d,
title = "Simulating dam - breach flood scenarios of the Tangjiashan landslide dam induced by the Wenchuan earthquake",
abstract = "Floods from failures of landslide dams can pose a hazard to people and property downstream, which have to be rapidly assessed and mitigated in order to reduce the potential risk. The Tangjiashan landslide dam induced by the Mw=7.9 2008 Wenchuan earthquake had impounded the largest lake in the earthquake affected area with an estimated volume of 3×10^8m^3, and the potential catastrophic dam breach posed a serious threat to more than 2.5 million people in downstream towns and Mianyang city, located 85 km downstream. Chinese authorities had to evacuate parts of the city until the Tangjiashan landslide dam was artificially breached by a spillway, and the lake was drained. We propose an integrated approach to simulate the dam-breachfloods for a number of possible scenarios, to evaluate the severity of the threat to Mianyang city. Firstly, the physicallybased BREACH model was applied to predict the flood hydrographs at the dam location, which were calibrated withobservational data of the flood resulting from the artificialbreaching. The output hydrographs from this model were in-putted into the 1-D–2-D SOBEK hydrodynamic model tosimulate the spatial variations in flood parameters. The simu-lated flood hydrograph, peak discharge and peak arrival timeat the downstream towns fit the observations. Thus this ap-proach is capable of providing reliable predictions for the de-cision makers to determine the mitigation plans. The sensitiv-ity analysis of the BREACH model input parameters revealsthat the average grain size, the unit weight and porosity of thedam materials are the most sensitive parameters. The vari-ability of the dam material properties causes a large uncer-tainty in the estimation of the peak flood discharge and peakarrival time, but has little influence on the flood inundationarea and flood depth downstream. The effect of cascadingbreaches of smaller dams downstream of the Tangjiashandam was insignificant, due to their rather small volumes,which were only 2 {\%} of the volume of the Tangjiashan lake.The construction of the spillway was proven to have played acrucial role in reducing the dam-breach flood, because all theother natural breach scenarios would have caused the flood-ing of the downstream towns and parts of Mianyang city.However, in retrospect improvements on the spillway designand the evacuation planning would have been possible. Thedam-break flood risk will be better controlled by reducing thespillway channel gradient and the porosity of the coating ofthe channel bottom. The experience and lessons we learnedfrom the Tangjiashan case will contribute to improving thehazard mitigation and risk management planning of similarevents in future.",
keywords = "ITC-ISI-JOURNAL-ARTICLE",
author = "Xuanmei Fan and C. Tang and {van Westen}, C.J. and D. Alkema",
year = "2012",
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}

Simulating dam - breach flood scenarios of the Tangjiashan landslide dam induced by the Wenchuan earthquake. / Fan, Xuanmei; Tang, C.; van Westen, C.J.; Alkema, D.

In: Natural hazards and earth system sciences, 2012, p. 3031-3044.

Research output: Contribution to journalArticleAcademicpeer-review

TY - JOUR

T1 - Simulating dam - breach flood scenarios of the Tangjiashan landslide dam induced by the Wenchuan earthquake

AU - Fan, Xuanmei

AU - Tang, C.

AU - van Westen, C.J.

AU - Alkema, D.

PY - 2012

Y1 - 2012

N2 - Floods from failures of landslide dams can pose a hazard to people and property downstream, which have to be rapidly assessed and mitigated in order to reduce the potential risk. The Tangjiashan landslide dam induced by the Mw=7.9 2008 Wenchuan earthquake had impounded the largest lake in the earthquake affected area with an estimated volume of 3×10^8m^3, and the potential catastrophic dam breach posed a serious threat to more than 2.5 million people in downstream towns and Mianyang city, located 85 km downstream. Chinese authorities had to evacuate parts of the city until the Tangjiashan landslide dam was artificially breached by a spillway, and the lake was drained. We propose an integrated approach to simulate the dam-breachfloods for a number of possible scenarios, to evaluate the severity of the threat to Mianyang city. Firstly, the physicallybased BREACH model was applied to predict the flood hydrographs at the dam location, which were calibrated withobservational data of the flood resulting from the artificialbreaching. The output hydrographs from this model were in-putted into the 1-D–2-D SOBEK hydrodynamic model tosimulate the spatial variations in flood parameters. The simu-lated flood hydrograph, peak discharge and peak arrival timeat the downstream towns fit the observations. Thus this ap-proach is capable of providing reliable predictions for the de-cision makers to determine the mitigation plans. The sensitiv-ity analysis of the BREACH model input parameters revealsthat the average grain size, the unit weight and porosity of thedam materials are the most sensitive parameters. The vari-ability of the dam material properties causes a large uncer-tainty in the estimation of the peak flood discharge and peakarrival time, but has little influence on the flood inundationarea and flood depth downstream. The effect of cascadingbreaches of smaller dams downstream of the Tangjiashandam was insignificant, due to their rather small volumes,which were only 2 % of the volume of the Tangjiashan lake.The construction of the spillway was proven to have played acrucial role in reducing the dam-breach flood, because all theother natural breach scenarios would have caused the flood-ing of the downstream towns and parts of Mianyang city.However, in retrospect improvements on the spillway designand the evacuation planning would have been possible. Thedam-break flood risk will be better controlled by reducing thespillway channel gradient and the porosity of the coating ofthe channel bottom. The experience and lessons we learnedfrom the Tangjiashan case will contribute to improving thehazard mitigation and risk management planning of similarevents in future.

AB - Floods from failures of landslide dams can pose a hazard to people and property downstream, which have to be rapidly assessed and mitigated in order to reduce the potential risk. The Tangjiashan landslide dam induced by the Mw=7.9 2008 Wenchuan earthquake had impounded the largest lake in the earthquake affected area with an estimated volume of 3×10^8m^3, and the potential catastrophic dam breach posed a serious threat to more than 2.5 million people in downstream towns and Mianyang city, located 85 km downstream. Chinese authorities had to evacuate parts of the city until the Tangjiashan landslide dam was artificially breached by a spillway, and the lake was drained. We propose an integrated approach to simulate the dam-breachfloods for a number of possible scenarios, to evaluate the severity of the threat to Mianyang city. Firstly, the physicallybased BREACH model was applied to predict the flood hydrographs at the dam location, which were calibrated withobservational data of the flood resulting from the artificialbreaching. The output hydrographs from this model were in-putted into the 1-D–2-D SOBEK hydrodynamic model tosimulate the spatial variations in flood parameters. The simu-lated flood hydrograph, peak discharge and peak arrival timeat the downstream towns fit the observations. Thus this ap-proach is capable of providing reliable predictions for the de-cision makers to determine the mitigation plans. The sensitiv-ity analysis of the BREACH model input parameters revealsthat the average grain size, the unit weight and porosity of thedam materials are the most sensitive parameters. The vari-ability of the dam material properties causes a large uncer-tainty in the estimation of the peak flood discharge and peakarrival time, but has little influence on the flood inundationarea and flood depth downstream. The effect of cascadingbreaches of smaller dams downstream of the Tangjiashandam was insignificant, due to their rather small volumes,which were only 2 % of the volume of the Tangjiashan lake.The construction of the spillway was proven to have played acrucial role in reducing the dam-breach flood, because all theother natural breach scenarios would have caused the flood-ing of the downstream towns and parts of Mianyang city.However, in retrospect improvements on the spillway designand the evacuation planning would have been possible. Thedam-break flood risk will be better controlled by reducing thespillway channel gradient and the porosity of the coating ofthe channel bottom. The experience and lessons we learnedfrom the Tangjiashan case will contribute to improving thehazard mitigation and risk management planning of similarevents in future.

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