Debris Flows and Landslides - River Dynamics and Integrated River Management - page 211

Buller River dams formed during 1929 and 1968 earthquakes failed within days or weeks of formation
during floods or high normal flow (Hancox et al., 1997). If a landslide dam fails it induces catastrophic
outburst floods and debris flows following rapid dam failure. One of the earliest historic accounts of a
landslide dam-break flood has been described by Hegan et al. (2001) and IGNS (2003) for Waimatai
Stream near Waihi in New Zealand.
Nevertheless, Korup (2004b) indicated that the failure theory is in fact not true. Korup et al. (2006)
found large rockslides and rock avalanches that blocked rivers in the late Pleistocene to Holocene were
preserved in the Himalayas, the Tien Shan, and the New Zealand Southern Alps despite rates of uplift
and erosion of up to 10 mm/year. These natural dams controlled fluvial response on 10 1 -10 4 year
timescales and the rockslide dammed lakes had persisted up to ten thousand years before being drained
or filled. Nicoletti and Parise (2002) analyzed the stability of seven landslide dams in southeastern Sicily.
These landforms were part of a set of 146 landslides in this area, which were considered to be stable.
Southeastern Sicily consists of a plateau (the Hyblaean Mountains) incised by canyons and surrounded
by lower lands. The landslide dams involved six rock slides and one rock fall, which were triggered by
an earthquake. Landslide volume ranged from about 0.5 to 34 million m 3 . With reference to Crozier and
Pillans (1991) classification of landslide lakes, all cases show a main valley lake, and back and supra
lakes were sporadically present. One damming was attributable to the 1693 earthquake with certainty;
another of the dammings, to the same earthquake with high probability. Although the seven dams were
stable and no failures had occurred, five dams were reincised to some extent.
Humans manage landslide dams in two ways: removal or preservation. Landslide dams triggered by
earthquakes were regarded as dangerous because massive amounts of water were pooling up at a very
high rate behind the landslide dams, which might eventually fail and result in dam break flooding,
potentially endangering the lives of thousands of people in the downstream reaches. To eliminate the risk
of dam break flooding, humans removed the landslide dams using explosives. Thus, the water volume
stored in the quake lakes was released and the pool level was reduced to a minimum, so that the flood risk
downstream was minimized. This strategy has been applied to landslide dams where there was a high
population density in the downstream reaches. Humans are used to having everything under control. The
landslide dams are naturally formed structures. The composition, structure, and physical forces within
landslide dams are not well understood. Therefore, people are predisposed to remove the landslide dams.
Removal of landslide dams is not a good strategy for long term management of incised mountain
rivers. Only if the stability of a landslide dam is low and a dam failure flood poses a threat to the
downstream reaches, the removal strategy must be applied. On the other hand, preservation of a landslide
dam has the potential to inhibit channel incision and is the best management strategy in most cases.
Safran et al. (2008) used a 1-D finite difference model of longitudinal profile evolution to explore the
implications of such processes for long-term (10 6 yr) incision patterns and morphologic development,
and concluded that the morphologic signature of landslide dams is context-dependent but can be
significant; correct genetic interpretation of longitudinal profile morphology may hinge on recognizing
such effects. Ouimet et al. (2007) explored a probabilistic, numerical model to provide a quantitative
framework for evaluating how landslides influence bedrock river incision and landscape evolution within
the Dadu and Yalong river catchments. Stable, gradually eroding landslide dams create mixed bedrock
alluvial channels with spatial and temporal variations in incision, ultimately slowing long-term rates of
river incision, thereby reducing the total amount of incision occurring over a given length of river.
Therefore, preservation of landslide dams may reduce new landslide hazard on incised mountain streams
(Wang et al., 2009a).
Landslide dams and quake lakes are a feedback within the natural system. The landslide dams may
form knickpoints on the river bed profile and initiate extensive and prolonged aggradation upstream.