INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 6, No 2, 2015

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1 INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 6, No 2, 2015 Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article ISSN Simulation and modeling of debris flows using satellite derived data: A case study from Kedarnath Area Shovan Lal Chattoraj 1, P.K. Champati ray 1 Indian Institute of Remote Sensing, 4-Kalidas road, Dehradun shovan.iitb@gmail.com ABSTRACT Comprehensive assessment of landslide hazard requires process based modeling using numerical simulation methods. The present study aims to focus on analysis of landslides/debris flow movements and simulate landslides that occurred in Kedarnath area in June Although not widely discussed, these landslides were the prime cause of river blockade that contributed significantly in glacial lake outburst flood (GLOF) related disaster in and around Kedarnath. In the present study, the simulation was attempted by numerical modeling using Rapid Mass Movement Software (RAMMS). The algorithm is based on Voellmy frictional (dry and turbulent frictional coefficients, µ and ξ) parameters of debris flow with release area that can be identified on high resolution satellite images and derived DEM like Cartosat-1 DEM. Once the event is simulated model provides information on flow 1) Velocity, 2) Height, 3) Momentum, and 4) Pressure along the entrainment path. The main emphasis was to understand on how closely numerical simulation predicted the attributes of the landslides/debris flow that contributed to the unprecedented disaster in Kedarnath. This study revealed that simulated flow height at base of the entrainment to vary from 1m to 6m and flow velocity varying from 3 to 7m/s. These vital output parameters can be used to provide insight of the event and extent of run out zone of future potential flows. Thus, this work bespeaks that numerical simulation modeling is capable of emulating natural events and outputs can be used for mitigation measures. Keywords: Debris flow, Modeling, Frictional coefficients, RAMMS, Kedarnath 1. Introduction 1.1 Kedarnath Disaster 2013 Landslides in Himalaya have become annually recurring phenomena which severely affect life, property and livelihood that is heavily dependent on pilgrimage, tourism and agriculture. Very high altitude (>1500m), rugged mountainous terrain (relative relief > 300m), less agricultural land, extreme environmental conditions and very limited industrial development show the overall limited opportunity for economic development of the region and therefore, frequent landslides have emerged as greatest threats to life and livelihood of the region (Paul and Bist, 1993; Champati ray et al., 2013a;). The newly formed state of Uttarakhand and particularly the Garhwal Himalaya has experienced devastating landslides in the past. In the year 1998 more than 300 people were killed including 60 pilgrims due to Malpa landslide. In the same year, Ukhimath area was severely affected by numerous landslides that took 103 lives (Naithani et al., 2002). On 20th October 1991, the Uttarkashi Earthquake had caused numerous landslides, particularly on a 42 km road stretch between Uttarkashi and Bhatwari (Jain et al., 1992). In 1999 landslides induced by earthquake shocks again spread devastation in Chamoli district of Uttarakhand Submitted on August 2015 published on November

2 (Kimothi et al., 2005). The Varunavat landslide of Uttarkashi that occurred in 2003 is one of the most spectacular landslides in Bhagirathi river valley (Gupta and Bist, 2004; Kumar et al., 2007; Sarkar et al., 2010). In 2012, Ukhimath region was again devastated by debris flow that killed 33 people and damaged houses, roads and agricultural land (Islam et al., 2013; Martha and Kumar, 2013). Besides this there are many isolated smaller events that have caused loss of life and property (Bist and Sah, 1999) However, the wide spread landslides and associated flash floods during th June 2013 surpassed all records in the past in terms of loss of human lives, property and infrastructure across five districts of Uttarakhand state viz., Bageshwar, Chamoli, Pithoragarh, Rudraprayag and Uttarkashi and neighbouring districts of Himachal Pradesh. As the extreme precipitation event of th June 2013 coincided with the peak tourist season, thousands of pilgrims and local population were stranded at many places due to landslides and river erosion. The worst happened in Kedarnath valley where due to series of landslides, river blockade and finally breaching of Chorabari Tal, a glacial lake in the upstream of Kedarnath town, the pilgrim centre was swept away. The trail of devastation in the form of bank erosion, landslides further continued downstream up to Gaurikund and finally it left 580 confirmed deaths and 5280 missing including 92 foreign nationals who are now believed to be dead (Source: Statesman dated September 17, 2013 and DMMC, Govt. of Uttarakhand). As a result of this, many localities in Kedarnath, Rambara, Gaurikund, Sonprayag, Agastmuni and further downstream villages were completely wiped off. Several kilometers of roads, trekking routes, bridges were washed off at many places due to severe bank erosion and landslides (Champati ray et al. 2013b; Figure 1). Although this is the first instance of glacial lake outburst flood (GLOF) in Garhwal Himalaya, such events have occurred in the past in in Nepal and Bhutan Himalaya (Komori et al., 2012). Therefore, compared to all landslide and flash flood related disasters, 2013 event was the most devastating and demands detailed investigation. Although preliminary investigations have attributed the devastation to the most obvious cause related to breaching of Chorabari Tal lake and subsequent flooding (Dhobal et al., 2013). Based on satellite image analysis and corroborated by ground observation, it is now known that at maximum capacity, the reservoir could have stored 0.7 million cubic meters of water and when breached due to over topping initiated by small landslides and avalanche in the lake catchment, the water would have flown through the existing channels. As revealed by the satellite images, water instead of flowing through the existing Mandakini channel, it started flowing through the new channel created at the centre on top of the end morainic mound and captured paleo channel of Saraswati river towards east or left side of the Kedarnath valley. Therefore, the question is what made the river abandoned its original channel as it is the prime cause of water flowing through the centre of the town which led to maximum devastation. It is crucial to know the possible causes of the river blockade and carrying capacity of existing channels, if any, in order to provide complete insight of the event. As the valley is surrounded by large number of landslides initiated in the old avalanche chutes during the th June, it is imperative to assess the flow dynamics of such debris flows and their possible contribution to fill up the channels which led to blockade and river shifting. Thus with this background an attempt is made to model the physical process of landslides or debris flows in Kedarnath valley. Earth observation data has been successfully used for monitoring, risk assessment, modeling and mitigation management (Gupta et al., 2008; Onagh et al., 2012; Martha and Kumar, 2013). Pertinently, precipitation triggered debris flow models have also been studied in similar tectonically disturbed regions (Scott, 2000). A numerical simulation technique has thus been envisaged to model the single largest debris 1499

3 flow run-out on the left bank of Mandakini at around 1.25 km upstream from Kedarnath Temple. Keeping in view of significant contribution to main flow of Mandakini, other smaller but prominent debris flows in and around the temple have also been studied. The outputs of such simulation of natural events is likely to provide the stake holders actual insight of the cause of this event and also provide important physical parameters of flow viz. velocity, height, momentum and pressure quantitatively which can be very useful for engineering intervention and remedial measures. Extensive landslide mapping at adequate scales complimented by 3-dimensional modeling of landslides will provide the complete information to understand the event and plan for the mitigation measures in future (Herva set al., 2003; Champati ray et al., 2013b). 1.2 Debris flow modeling Debris flow modelling is an active area of research and the underlying principle can be applied to a variety of processes including snow avalanche, debris flows, landslides, mud flows and even rock falls and has therefore found significant role in disaster management Cruden and Vernes 1996; Iverson et al. 1997). Debris flows are often described as multiphase gravity-driven flows consisting of randomly dispersed interacting phases that can lead to extremely hazards events. Although well tested empirical methods are available to determine dynamic characteristic of a flow, numerical simulation techniques are now applied to predict flow paths and characterize the entrainment process (Tsai et al., 2011; Quan Luna et al., 2011). In the present study RAMMS (Rapid Mass Movements Software) developed by WSL institute of Snow and Avalanche, Switzerland, has been used which is a state of the art numerical simulation model that predicts the motion of a naturally occurring mass from head (release area) to base (deposition area) in three dimensions. This takes cues from a basic high-resolution DEM and other ancillary ground data including geotechnical parameters. This software uses the Voellmy rheological inputs which act as proxy for the physical characteristics of entrained debris material to model the run out parameters i.e. flow velocity and height of a flow (Ayotte and Hunger, 2000; Rickenmann et al., 2005; Christen et al., 2010). These process-based 3-D models, additionally, provide information on dispersion of momentum and impact pressure throughout the torrent which can be utilized for better understanding of the process and designing of suitable engineering structures (Iverson et al., 2000). Temporal variation of physical parameters along of longitudinal profiles of a particular flow can be used for time series analysis and change detection in case of repeated flows. 2. Study area 2.1 Topography The study area is bounded between N to N latitude and E to E longitude (lies in Survey of India (SOI) Toposheet No. 53N/ 1-2) and forms a part of upper Mandakini valley in Rudraprayag district of Uttarakhand. Overall the Himalaya has emerged as one of the global hotspots of landslides occurrences. The source of Mandakini river lies at a distance of 2.0 km upstream of Kedarnath town near the terminal and lateral moraine of Chorabari glacier. It is also fed by Saraswati river originating from Companion glacier and meeting the main Mandakini river at the northern end of the Kedarnath town (Figure 1). The relative height of the area varies from 5000m to 3500m above MSL. The prominent nearby peak of Kedarnath, lying due north of the study area is at 1500

4 elevation to 6913m above MSL. The valley glaciers feeding the main hang at elevation of 4200 o 4600m. The Kedarnath valley is located at elevation of 3540m. Figure 1: Study area showing locations of post disaster active landslides/glacier mapped from temporal R2 LISS IV images acquired on 21 June, 2013, image displayed with RGB- 231 combination 2.2 Regional Geology and Geomorphology This area lies towards north of Main Central Thrust (MCT) zone and is underlain by rocks of Central Crystalline complex traversed by two major thrusts (Figure 2, Valdiya et al., 1999). The lower litho-unit is known as Munsiari Formation which is overlain by Vaikrita Group and both units are separated by the Vaikrita Thrust. Middle Proterozoic granites and their cataclastic or mylonised equivalents including sericite-chlorite schist and amphibolites dominate the lower unit whereas the upper unit consists of medium to high grade metamorphic and intrusive porphyritic gneissic granite. The north dipping Vaikrita Thrust passes near Gaurikund and subsequent Pindari Thrust separates the lower part of the Vaikrita Group from the upper part mainly consisting of migmatite and different types of acidic granites (Valdiya et al., 1999). The Kedarnath peak is composed principally of migmatites and granites of upper unit. Mandakini flows from north to south in a linear transverse valley which is characterized by a strike slip fault that offsets the Pindari Thrust. The whole area bears surface manifestations of polyphase deformation showing different grades of metamorphism and several shear zones. Polyphase deformation and presence of shear and thrust zones have rendered rocks weak and prone to slope failures on steep slopes. 1501

5 Geomorphologically, the area is dominated by two distinct types of landforms attributed to glacial and fluvial origin. The most prominent feature, Chorabari glacier gives rise to the main flow of Mandakini river and is later joined by eastern tributary, Saraswati river originating from Companion glacier. The river valley from Kedarnath to Rambara is flanked by mainly lateral moraines and beyond Rambara reworked moronic material and river terrace deposits form the valley sides, along which the foot track (trekking route) to Kedarnath is located (Dobhal et al., 2013). As the foot tract is mainly on moronic and fluvial deposits and due to continuous river erosion and steepening of the slopes, it is very prone to landslides. Figure 2: Schematic geological map of Kedarnath region (redrawn after Valdiya et al., 1999) 2.3 Debris flow/mass wasting The upper Mandakini valley is mostly marked by numerous landslides, whose frequency has gone up in recent times. Particularly the trekking route at many places has been identified as prone to landslides as marked as high and very high hazard zones based on multi-parametric AHP approach (NRSC, 2002). During extreme event of June 2013, excessive river discharge laden with morainic debris has caused bank erosion and landslides at many places up to Gaurikund. The single largest landslide was observed for a distance of around 1.25 km on the left bank of Mandakini (Fig. 1). From Kedarnath to Rambara, the landslides associated with bank failures have occurred mostly on glacio-fluvial deposits and beyond Rambara in the downstream direction, slope failures have been observed on fluvial deposits. 3. Methodology and input data In the present study, the aim is to simulate debris flows that have devastating impact in and around Kedarnath shrine. Multi-temporal and multi-resolution Earth Observation satellite data products and derived information have been used to get parameters for flow modeling. In the second step, flow modeling has been performed and validated against the actual events of Based on the study and results, an attempt is made to unravel the main causative factors and sequence of events that led to Kedarnath disaster Satellite Data used 1502

6 Indian Remote Sensing Satellite data products such as LISS-IV (Resourcesat 2) data sets acquired on 21 st June and Cartosat-2 data acquired on 20 th June were analysed for post disaster event analysis. Cartosat-1 data of 2008 along with derived DEM and LISS IV (Resourcesat-1), Google Earth images along with ancillary terrain information from SRTM DEM Version 4 were also referred. 3.2 Debris flow run-out modeling The essential dataset required for the physical based model are topographic data (digital elevation model), release area and release mass as well as information on friction for dry and liquid phases and other geo-engineering parameters (internal shear angle and density). Topographical data sets in the form of high resolution digital elevation model (DEM) and the location of release area are the two most important parameters for flow modelling. Preferably, DEM in the form of ESRI ASCII Grid and ASCII X, Y, Z format is required for implementation. In the absence of high resolution DEM, contours digitized from toposheets can be used to provide height information. The contour generated DEM and the Cartosat DEM have produced comparable flow modeling results. Therefore, in the present case, readily available Cartosat-1 DEM was used (Fig. 3). Figure 3: Subset of the Cartosat-1 DEM used in RAMMS (right) for debris flow modelling vis-à-vis LISS IV image of 21 June, 2013 (left) (image displayed with RGB-123 combination). Debris flow modeling for unchannelized flows (as observed in the present case) requires a known release area with a given initial height for block release (Rickenmann et al., 2006). Therefore, the release areas for three debris flows have been identified using high resolution satellite images (Cartosat-1 and LISS-IV) and derived DEM (Figure 1 and 4). The initiation zone in the study area is steeper with slope angle ranging between with height varying from 3500 to 5000m. The depth of the initiation zone (depletion zone) varies from 1m to 1.5m as revealed from Cartosat-1 DEM and satellite images. The field observations revealed that the modelled landslides were initiated like debris slide and then flowed downward which at times bifurcated into several channels (Chattoraj et al., 2014). In the present study, two debris flow zones have been identified: the larger one at around 1.25 km upstream side of the Kedarnath temple and the smaller one just downstream of temple location (Figure 4). 1503

7 Figure 4: 2D and 3D view of the DEM (in RAMMS) with three separate area of release (in violate) and area of influence (green boundary) 3.3 Frictional parameters The RAMMS numerical simulation model is based on rheological characters of the slope derived from shear strength parameters of the slope. This model divides the frictional resistance into two parts: a dry-coulomb type friction (coefficient, µ) that related with the applied normal stress and a velocity-squared drag or viscous-turbulent friction (coefficient, ξ). The frictional resistance S (Pa) is then defined as: S= µρhgcos (φ) + (ρgu 2 )/ξ where ρ is the density, g the gravitational acceleration, φ the slope angle, H the flow height and U the flow velocity (Salm et al., 1990). The two major frictional input parameters are µ and ξ. However, it is known from law of friction that µ= tan φ, where φ is angle of internal resistance that can be determined in the laboratory. In the present case, direct shear test instrument was used to determine c (cohesion) and angle of internal friction from soil samples collected from Kedarnath. However, the major difficulty in case of debris flow simulation is that it involves wide variety of materials with different frictional parameters. Therefore, calibration of the friction parameters requires several simulations with different values for each input parameters to match the expected run-out based on actual event data. Finally based on the validation of such results with field data, best simulation outputs can be used for analysis (Sosio et al., 2008). In the present case, a number of simulations were carried out using various combinations of friction parameters. A range of values were used to find out the optimal friction value. Coefficient of dry friction values varied from 0.05 to 0.4 and for viscous turbulent flow is varied from m/s 2. Meanwhile other parameters viz. density of materials, release height, earth pressure coefficient (lambda) and the percent of momentum were kept constant. Afterward, validation of simulation output was carried out using total run-out distance and 1504

8 the aerial extents of real flow path as observed on satellite images and verified by field observation (Fig. 1). As the simulated flow matched approximately 95% (pixel wise) with real event for µ (Mu) = 0.4 and ζ (Xi) = 400 m/s 2, these were used for final analysis. For dry friction, it was observed that an increase in the friction coefficient µ (Mu) causes a decrease in the run-out distance due to increase in the basal friction of the flow. On the other hand, when the value of ζ (Xi) was varied within a range of ms -2, it did not change the run-out distance significantly. But in general, an increase in ζ (Xi) value increases the run-out distance due to smoother flow. The representative samples collected from base of the flows were analyzed in electronic direct shear testing equipment (Model No. AIM 104-2kN, Make Aimil Ltd, New Delhi) at Indian Institute of Remote Sensing, Dehradun at different saturation levels. Samples were tested at 0.25, 0.50 and 1 kgf/cm 2 normal load and consequent shear strength parameters at failure were calculated. The input dry coefficient of friction fed in the model was thus was further crosschecked instrumentally. 4. Results and discussion Flow modeling using RAMMS provides four main output parameters viz. velocity, height, pressure and momentum. Other outputs like run-out longitudinal profile and point information of a specific location are also provided in 2D and 3D. In the present case, debris flow run out distance and flow height is considered as very important output as these provided information on aerial extent of the debris flow and its possible role on blockade of the existing river course. Debris flow movement in 3D has been produced and results are summarized in Table 1 and Figure 5. Debris flow-1 is eventually un-branched till it reached the base. The total run out length was 1.5 km. The 2nd flow i.e. slightly downstream of temple and has a total run out length of around 2 km. A small bifurcation into left and right branches observed only near the base. As for other outputs, the simulated maximum velocity at the base for debris flow-1 was 5-7m/s and flow height 4-6m. The debris flow-2 is slightly lengthier compared to debris flow-1 but with lower flow height (1-3m) and velocity (3-5m/s). Both way, debris flow-1 appears to be more vigorous. It was observed that momentum was concentrated symmetrically in the middle zone of both the flows (Figure 5). Variation of pressure along the entrainment path for both the flows has been observed to mimic height profile by and large and thus not presented separately here. Figure 6 depicts ternary plots showing velocity change in all the debris flows with varying run out length and altitude. This enables to calculate the variation of all four vital physical parameters along the run out of the flow. 1505

9 Figure 5: 2D view of the simulated maximum height, velocity and momentum (respectively from left to right) varying along the run out length of debris flow-1 (a-c) and 2 (e-f) Figure 6: Ternary plots showing velocity change in modelled debris flows with varying run out length and altitude for debris flow-1 (left) and debris flow-2 (Red: right and Blue: left branch) The debris flow modeling results were also used to verify the selected parameter µ with instrumentally derived φ value from the soil sample which is in its maximum saturated state. The output of direct shear instrument utilized in bivariate plot using Mohr-Coulomb equation i.e. τ = σ tan φ + c which is a straight line equation in normal vs. shear stress axes (Fig. 7). The cohesion of the wet debris embedded in semi consolidated clay sample was determined to be of 4.6 Kpa in the fully saturated state and average φ is in the range of This matches with the coefficient of dry friction value provided to the model in terms of µ in the range of 0.4. Thus the instrumentally derived output is similar to modeled input parameters (Table 1). Figure 7: Cross plot showing of normal vs shear stress showing Mohr-Coulomb relationship of the saturated samples collected from base of flows Table 1: Flow specific modelled outputs viz. maximum height and maximum velocity at base of the debris flows 1506

10 Overall the study has revealed that, the major landslide which was in the form of a debris flow could achieve run out distance of 1.5 to 2 km and resultant debris can reach the Mandakini river. The modelled results show that debris flows can be postulated to contribute substantial sediment load to the Saraswati channel. Additionally, satellite data additionally revealed that on the right bank of Mandakini, major streams such as Dudh Ganga and Madhu Ganga have also contributed debris to the Madakini although not modeled in the present study. As a result of which the carrying capacity of the Mandakini has been reduced significantly. The debris flow-1, which is the largest and biggest contributor of debris to both Mandakini and Saraswati was active on 16 th June and brought a lot of debris to the lower elevation. On 17 th morning when the lake breached, debris movement would subsequently have taken place on the Mandakini channel but it was eventually blocked by morainic material and water started flowing towards east and created one more channel at the centre. The eastern most channel of Mandakini along with the Saraswati flow occupied the paleo channel of Saraswati. However, as both the channels were filled up by debris flow deposits, it could not allow smooth flow of the lake discharge, which eventually found its way into the town area along with the contribution from the newly formed channel. Therefore, debris slides have contributed significantly in partial blocking of the channels, which eventually led to lake discharge to pass through the town area. After 17 th June, for almost a year, the western channel of Mandakini was abandoned and water was flowing through the eastern most paleo channel of Saraswati. 5. Conclusion Three dimensional modeling of natural debris flow events around Kedarnath aided by satellite image based analysis provided two most important results. First of all, the study provided successful simulation of selected debris flow events and generated output parameters such as velocity, height, pressure and momentum taking inputs from remotely sensed and ancillary earth observation data products. Out of two modeled flows, the debris flow-1 showed maximum run out of 1.5 km (avg.) and a velocity of 5-7m/s and height 4-6m at the base. Such information is very crucial for designing remedial measures. Secondly, it provided insight into the event. Based on the study, it is concluded that the modeled flows have provided debris that reduced the capacity of the main channel (Mandakini) as well as Saraswati and the debris reached up to the northern margin of the Kedarnath town. When Chorabari Tal was breached and the original course of Mandakini was blocked, the lake discharge found a new course on top of the moraine and followed a paleo channel of Saraswati river. Due to reduced capacity, partial blockade of channels and readily available debris, the lake discharge along with the debris had to pass through the centre of the town causing maximum devastation. It is postulated that had there been no landslide and debris flow, the breached lake water would have been mostly carried by both the channels with 1507

11 minimum damage to the town. Therefore, it is important to note that the overall geomorphic set up which has undergone significant changes in 2013 need to be considered for potential debris flow and blockade of streams. Secondly, the filling up of the moronic dam needs to be monitored and intermittently the lake water needs to be released to avoid such catastrophic effect in future. 6. Acknowledgement This paper has immensely benefited from contributions and consultations with several colleagues at Indian Institute of Remote Sensing and other organizations which include Dr. Ajanta Goswami, Mr. Kamal Pandey, Dr. Praveen Thakur and students/jrfs (Subhajit, Ashraful, Rohit, Shailja, Amit Anand, Vedika, Sashi Gaurav). Organizational support and overall guidance provided by Dr. A. Senthil Kumar, Director, IIRS and Dr. SPS Kushwaha, Dean (Academics) is also duly acknowledged. SLC is thankful to Indian Space Research Organization, Department of Space, Government of India for the financial support provided in TDP project. 7. References 1. Ayotte, D., and Hungr, O., (2000), Calibration of a runout prediction model for debris flows and avalanches. Proceedings of the second international conference on Debris Flows, ed. G.F. Wieczorek and N.D., Naeser, Taipei, Balkema, Rotterdam, pp Bist, K.S., and Sah, M.P., (1999), The devastating landslide of August 1998 in Ukhimath area, Rudraprayag district. Garhwal Himalaya, 76 (4), pp Champati ray, P.K., Chattoraj, S.L., Chand, D.S., and Kannaujiya, S., (2013a), Aftermath of Uttarakhand disaster 2013: an appraisal on risk assessment and remedial measures for Yamunotri shrine using satellite image interpretation. Indian Landslides 6 (2), pp Champati ray, P.K., Chattoraj, S.L., and Kannaujiya, S., (2013b), Uttarakhand Disaster 2013: Response and Mitigation measures using remote sensing and GIS. Pre workshop full publication in National work shop on Geology and Geo-heritage sites of Uttarakhand with special reference to geo-scientific development of the region organized by Indian Geological Congress, Roorkee jointly with L.S.M. Govt. PG College, Pithoragarh Nov 11 to 12, pp Chattoraj, S.L, Champati ray, P.K. and Bandopadhyay, S., (2014), Debris Flow Simulation and Modeling: A Case Study from Kedarnath Area. Proceedings of the Geo-Environmental Hazards and Neo-Tectonic Activities in Himalaya, being held at HNB Garhwal University Campus Badshahi Thaul, Tehri Garhwal, October 28-30, pp Christen, M.K., Walski, J., Bartelt, P., (2010), RAMMS: Numerical simulation of dense snow avalanches in three-dimensional terrain. Cold Regions Science and Technology, 63 (1/2),

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