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

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.