10_03 Attabad

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Information about 10_03 Attabad

Published on May 16, 2010

Author: Dr_Dave

Source: authorstream.com

Slide 1: Landslide dam in Hunza David Petley Wilson Professor, Durham University Global fatal landslides : Global fatal landslides Global distribution of landslide damsErmini and Casagli 2003 : Global distribution of landslide damsErmini and Casagli 2003 Attabad landslide : Attabad landslide 1858 landslide dam : 1858 landslide dam Longevity of dams Ermini and Casagli 2003 : Longevity of dams Ermini and Casagli 2003 Dam failure - floods : Dam failure - floods Dam break flood, Peru : Dam break flood, Peru Wenchuan Earthquake experience : Wenchuan Earthquake experience 257 barrier lakes 32 had potential for outburst flooding 17 required emergency action All were successfully mitigated through spillway construction that initiated outburst floods No loss of life due to precautionary evacuations Water splash : Water splash Erosion from splash Probable second phase Slide 13: Landslide dam Scarp 800m 620m Landslide stats:- Length: 800 m Width: 620 m Potential lake volume: 315 m m3 Volume: 20 million m3 Lake Wenchuan Risk TableYang et al 2010 : Wenchuan Risk TableYang et al 2010 Current engineering works : Current engineering works Spillway construction is the correct approach Excavation rates are impressively high Plan to construct channel c. 30 m deep and 40 m wide Bed of channel will be lined with boulders to prevent erosion Intention is to ensure that collapse does not occur Future scenarios: 1. Successful spillway : Future scenarios: 1. Successful spillway Pros: No outburst Cons: Loss of large section of KKH Isolation of upstream communities Future potential instability through earthquake or flood event 2. Spillway erosion and failure : 2. Spillway erosion and failure Pros Hazard will reduce after flood If outburst is slow, limited downstream damage KKH can be reopened quickly Preparation for flood is possible Cons Potential for severe flooding downstream Sedimentation at Tarbela Likelihood of spillway failure? : Likelihood of spillway failure? Positives: Wide channel Boulders in lower materials Negatives Materials (especially the lacustrine deposit) >2000 cumecs summer floods Height of dam Potential for downstream scour 3. Landslide into lake triggers failure : 3. Landslide into lake triggers failure Comparatively rare, but this triggered failure of the 1858 landslide dam at Salmanabad Landslide into the lake → wave that overtops the dam → rapid collapse Probability of landslide increases as lake depth increases Probability of wave overtopping increases as lake depth increases 4. Piping / Seepage failure : 4. Piping / Seepage failure Comparatively rare Water passage through dam creates a pipe that causes rapid collapse Quite unlikely in this case due to presence of mudflow deposit 5. Earthquake-induced failure : 5. Earthquake-induced failure Vey rare but potentially catastrophic Seismic shaking causes mass movement of landslide material, triggering rapid release Location of landslide is a high seismic risk area – high potential in the long term Critical concern is failure by erosion of spillway : Critical concern is failure by erosion of spillway Current engineering works may be successful in stabilising the dam However, there is a significant risk of an outburst event at this point Failure could be rapid and trigger extensive flooding downstream How big might that flood be? Analysis from known landslide failures Flood modelling NB both flood modelling and data-driven analysis are associated with high levels of uncertainty Data driven flood estimation : Data driven flood estimation Typical values of maximum discharge using this approach: 14,000 cumecs (Costa and Schuster 1986) 12,000 - 26,000 cumecs (Walder and O’Connor 1997) Flood modelling : Flood modelling Two flood models have been run, based upon a 12,000 cumec peak discharge and a total flood duration of 24 hours BOKU (Austria) model uses Flo-3D University of Newcastle code uses a bespoke hydrodynamic code Base elevation model is SRTM NB this is very crude for this type of exercise Land-use analysed from Landsat data Suggested alert states : Suggested alert states Alternative approach: 1858 flood outcomes : Alternative approach: 1858 flood outcomes 1858 flood wave (From Prof. Ken Hewitt) : 1858 flood wave (From Prof. Ken Hewitt) NB the landslide dam was a little larger than Attabad 10 – 20 m above peak summer flows at Gilgit, 20 m at Chilas, 15 m at Attock, 10-15 m at Tarbela Induced extensive erosion of river terraces Reverse wave travelled 50 km up Kabul River NB this is not the flood that killed >1500 Sikh soldiers of the British Army That occurred in 1841 from a landslide dam from the Lichar Spur of Nanga Parbat Preparedness : Preparedness Sensible precaution would be to prepare for an outburst flood when the water reaches the spillway (March / April?) Precautionary evacuations for those in potential direct path of a wave in upstream sections (Attabad to Gilgit) Preparation to move for those from Gilgit to Tarbela Who to move? 60 m river terrace appears not to have been affected by 1858 flood : 60 m river terrace appears not to have been affected by 1858 flood The 60 m terrace : The 60 m terrace 60 m above current flow = 30-50 m above peak summer flood Terrace deposits indicate that it predates the 1858 flood event This could be considered to be the safe level (but NB erosion and landslides) Who might be affected by a flood? : Who might be affected by a flood? Those located within 50 m of the current flow level Those close to terrace edges Those living on or near to landslides Those located on bridges prone to overtopping or scour Known landslide sites : Known landslide sites Between Attabad and Gilgit there are a number of known active landslides Great care is needed to ensure that the populations are safe There are further landslide sites downstream of Gilgit Proposed actions 1: monitoring the current state : Proposed actions 1: monitoring the current state Monitoring is needed of: Seepage and movement of the downstream face of the landslide Is seepage developing? Is the water dirty (indicates scour)? Is there a downstream slope failure developing? The rate of rise of the lake level and the rate of inflow to allow forecasting of the date of flow in the spillway We should be careful to give realistic estimates The stability of the slope at Attabad Safety of the workers, integrity of the channel The state of the slopes above the lake Potential for failure Look for development of cracks, increased rockfall rate 2. Development of a series of alert states : 2. Development of a series of alert states Level 1: landslide aware (current state) Inform affected population of potential for a flood Evacuation plans developed and disseminated Monitoring of dam state Level 2: Landslide alert Lake level is approaching spillway Precautionary evacuations for most endangered populations to Gilgit 24 hour monitoring, rapid dissemination plan in place Slide 40: Level 3: Landslide warning Water flow on spillway Evacuation of potentially evacuated population to Gilgit Population downstream made aware of potential need to move at short notice Level 4: Severe landslide warning Landslide starts to develop rapid erosion or downstream face collapse Evacuation of all population below 60 m level Closure of KKH – all vehicles and people moved to higher levels Emergency relief plan initiated Staged evacuation plan : Staged evacuation plan At alert state 2: All population below 60 m above current river from Attabad to Gilgit All population on highly endangered terrace edges At alert state 3: All population on known landslides between Attabad and Gilgit All population below 60 m between Gilgit and Chilas? At alert state 4: All population below 60 m between Attabad and Tarbela All population on endangered terrace edges and known landslides 4. Warning system and communications : 4. Warning system and communications Active monitoring of dam state is essential Focus is locating a team with CCTV, but this needs government level assistance Need for active communication system to Tarbela Focus has undertaken considerable work between Attabad and Gilgit, but none below this point This does not mean that the risk downstream of Gilgit is low There is an urgent need to consider the population below Gilgit All clear and long term monitoring : All clear and long term monitoring Erosion often starts some days after initial flow Need an expert group with agreed criteria to give all clear signal Need to consider slope failures above the lake Need for long term monitoring and agreed evacuation plans, and engineering works Critical points after initial emergency will be: Peak summer flow 2010 Exceptional flow events (e.g. 1:10 year, 1:100 year floods) During a seismic event

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