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DISCUSSIOI AID COICLUSIOI CHAPTER SEVEN Discussion and Conclusions

7.1. Discussion The present study has been aimed to carry out an assessment of landslides and zonation of landslide hazards in North region of State. It involves generation of database (landslide inventory) of slope failures, landslide hazard zonation and landslide risk assessment. Landslide inventory has been prepared using past records of slope failures from concerned government offices. The details of landslides such as location, site details and geometry parameters were confirmed during the field survey. Distribution analysis has been carried out for slope failures in the study area to study spatio-temporal patterns of distribution. The main focus of the study was to delineate landslide hazard zones using method developed by Bureau of hidian Standards (BIS, 1998) and Analytic Hierarchy Process (AHP) based model. Landslide Hazard zonation at regional scale has been carried out and validation is done using appropriate method. In addition to this, qualitative landslide risk assessment was also done to delineate the risk areas associated with slope failures in North Konkan.

7.1.1. Identiflcation and mapping of landslide prone areas The first objective of the study was to identify and map landslide prone areas and prepare landslide inventory for the study area. The landslide database of 1072 slope failure events has been generated using records of past slope failures for the period of 11 years from 2004-2014 collected from concerned government offices. This has been supplemented with the field survey wherein details of 181 slope failure events have been recorded. Parameters including location, month and year of the occurrence of slope failure, scar geometry, damage details, corresponding rainfall amount, etc. have been considered for distribution analysis. On the basis of both primary and secondary data sources, landslide distribution map has been prepared and preliminary concentration zones of slope failures in the study area have been identified. The specific findings of the first objective are given below. • The temporal patterns of slope failures in the study area indicate that maximum frequency of slope failures is observed in the year 2005 (251) followed by year 2007 (221) and year 2010 (137) which are the years of annual rainfall above average.

188 All the slope failure events in the study area occurred during monsoon period (June to September) with the maximum number of slope failure events in the month of July (48.7% of total events) followed by August (29.7%) and June (13%). This clearly indicates that slope failures in North Konkan are triggered by intense monsoon rainfall. The distribution of slope failures in the North Konkan region is attributed to the Western slopes of Western Ghat escarpment, plateau margins of dissected Jawhar plateau located in the North East part and slopes of isolated denudational hills in the coastal plains. The characteristics of slope failures vary greatly in terms of the type of movement, material and processes operating on these slopes. Figure 7.1 illustrates characteristics of slope failures in different parts of the study area. Rock fall and wedge failures are generally observed at the elevation above 550 m which are due to the development of wedges caused by fractures and joints in the rocks. Deep rotational landslides are mainly observed at the foot slopes of of the study area. A typical example of deep seated slump is observed along Jawhar - Vavar road (SH - 28) which is dominated by coarse to fine material. Slope failures in Ambenali Ghat area are shallow translational debris slides with elongated scar shape where displaced material in the slope failure events of this area hardly covers the entire road width. On the other hand, the slope failure sites around Mahad town exhibit unique characteristics with development of hydrothermal anomalies as studied by Thigale and Umarikar (2007). Translatoinal debris slide is a conmion slope failure type in the study area. However, debris slumps are also observed at places along the foot hill slopes of the Western Ghat escarpment and isolated hillocks. Near the crest of the Western Ghat escarpment of the study area rock fall, rock slide and wedge failures are also observed along major communication roads (Malshej ghat, Bhor ghat, Mumbai - Pune Expressway in Khandala section and Varandh ghat area. Landslide geometry parameters (such as landslide scar width, length, run out distance, scar slope angle, etc.) greatly vary within the study area. Geometry based indices such as L-W ratio, shape factor, form factor are also calculated.

189 The IVW ratio shows that most of the slope failures do not exceed 2. It means after the slope failure, many times, displaced material does not cover the entire road width hence partially blocks the road traffic. This information can effectively be used in prioritizing the landslide mitigation measures. Estimated area and volume of the displaced material shows variation within the study area. Most of the slope failures have volume of displaced material even less than 1000 m whereas maximum volume is about 0.55 million m at Dasgaon landslide site.

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Q ^ 7.1.2. Preliminary Landslide Prone Zones The second objective of the study was to identify and map potential landslide zones. To achieve this objective, the information about slope failures in the study area using past records from concerned government offices and field investigation has been mapped to delineate preliminary landslide prone zones. The specific findings related to the second objective are given below.

• Slope failures in the North Konkan region are concentrated in five major preliminary zones namely, Jawhar plateau area, Malshej ghat zone, Matheran hill complex, Parsik hills and Mahad - circle (Fig. 4.2). • The slope failures in the North Konkan are observed in small isolated pockets and are concentrated in the Western slopes of Western Ghat escarpment, plateau margins of dissected Jawhar plateau located in the North East part of the study area and slopes of isolated hillocks in the central uplands (Fig. 4.2).

7.1.3. Delineation of Landslide Hazard and Landslide Risk Mapping The third objective of the study was to delineate Landslide Hazard Zones and to map Landslide Risk prone areas in North Konkan region. To achieve this objective, two LHZ methods namely BIS based LHEF rating scheme (modified) and Analytic Hierarchy Process based LHZ model have been adopted to delineate landslide hazard zones. Ten input parameters including slope angle, slope aspect, elevation, lithology, structural discontinuities, land use and land cover, distance from drainage, distance from roads, NDVI and rainfall have been used for LHZ mapping. The LHZ maps prepared for both BIS based LHEF (modified) method and AHP based Multi - Criteria Decision Making (MCDM) approach have been evaluated and validated using thematic data layer of actual slope failures. The second part of this objective was to delineate landslide risk prone areas in North Konkan region. The qualitative landslide risk assessment has been carried out using three elements at risk namely, population density, built up areas and road traffic density combined with LH zones. All the thematic data layers have been integrated by multiplying hazard with vulnerability to each element of risk (R = H*V). Specific findings related to this objective are given below.

• The LHZ methodology developed by Bureau of India Standards (1998) is more applicable to Himalayan conditions. Modifications in the LHEF rating

192 scheme is required for deriving more reliable results in the unique geo- environment of North Konkan region. Therefore, modified form of LHEF rating scheme is used for LHZ in the study area. The LHZ map for North Konkan region derived from modified BIS based LHEF rating scheme shows that 26.55% of the total geographical area of the study area falls m high to very high hazard zones followed by 26.74% and 46.71% area under moderate and low to very low hazard zones respectively. The distribution of areas with high to very high hazard zones is confined to the dissected Jawhar plateau in the North East comer of the study area, western slopes of Western Ghats escarpment and rugged topography in the South East part of Raigad District. Besides this, few isolated hillocks in Palghar and Thane Districts also fall in high to very hazard zone(Fig. 5.14). The LHZ map based on AHP is produced by using Multi-Criteria Decision Model (MCDM). This method is developed by Saaty (1980). Analytic Hierarchy Process based LHZ map portray areas with varying degree of susceptibility to slope failures in North Konkan. The AHP based LHZ map shows that 15.34% of the total geographical area of North Konkan falls in high to very high hazard zone followed by 26.77% and 57.89% in Moderate to low hazard classes (Fig. 5.26). The distribution of LH zones delineated using AHP method shows concentration of high to very high hazard in plateau margins of Jawhar plateau area, Matheran hill complex, Kamala fort area, isolated hills in Vasai and Palghar tehsils, Parsik hill area, rugged terrain around Mahad and western slopes of Western Ghat escarpment throughout the study area. The dissected topography in the south and east part of Raigad District have been identified as moderate landslide hazard zone whereas major parts of the Thane and Palghar Districts are characterized by stable slopes except few isolated hills therein. LHZ is validated through the comparison of modified BIS based LHZ and AHP based LHZ with the actual evidences from the field and landslide events. Percentage of slope failures falling in high to very high hazard classes have been considered for comparing results. The results indicate that 75.69% and 73.84% of the slope failures falls in high to very high hazard zones in BIS

193 (modified) and AHP based LHZ maps respectively. This confirms the validity of both the approaches to LHZ (Fig. 5.15 and Fig. 5.27). • The qualitative landslide risk assessment is carried out using thematic data layers of population density, built up area, road traffic density and LHZ. The results show that 14.26% of the total North Konkan is identified as moderate to high risk zone whereas 54.5% and 31.24% area falls in low and no risk zones respectively. The concentration of moderate to high risk zones are confined mainly to the densely inhabited foot hill areas and Ghat sections along major road corridors passing across the Western Ghat escarpment (Fig. 6.11). • The distrbution of settlements at landslide risk is confiened to the inhabitated foot hill slopes along the isolated hills in the coastal low lands. It is observed that Parsik hill, coastal range near Palghar, hilly areas around Mahad are few of the risk prone zones in North Konkan region (Fig. 6.12 and Table no. 6.3).

7.1.4. Landslide Causative Factors Fourth objective of this study was to identify landslide causative factors responsible for slope instability in the North Konkan region. For this, six parameters including slope angle, aspect, elevation, lithology type, land use and land cover and rainfall have been extracted for each slope failure site. The characteristics of landslide causative factors are compared with frequency of slope failures in order to determine the role of each landslide causative factor in the initiation of slope failure. The specific observations related to this objective are discussed below.

• The results reveal that maximum (65.77%) slope failures are observed within the slope angle of 15° to 35°. However, few (12%) of the slope failures situated along the Western slopes of Western Ghat near its crest (Malshej ghat, Bhor ghat, Mumbai - Pune Expressway near Khandala section, Varandh ghat and Ambenali ghat section) where slope angle exceeds 35° and even near vertical slope at some places (Fig. 6.13). • 54.53% of the slope failures are observed along West, South West, South East and South aspect (Fig. 6.15). It reveals that slope direction (slope aspect) also play an important role in slope instability in Western Ghat and Konkan region which has been supported by Nagarajan et al. (2000) in the study of landslides along Mumbai - Goa highway.

194 • Relief plays major role in slope instability in the study area. It has been observed that over 58% of the slope failures are observed above the elevation of 300 m, few of which (Matheran hill, Malshej ghat, Ambenali ghat, etc.) are situated at the elevation exceeding 600 m. Translational debris slides are observed at the elevation between 300 - 500 m, slumps below 300 m and rock slide/falls are observed mainly between 500 and 600 m. elevation (Fig. 6.16). • Distribution of slope failures in North Konkan region is also influenced by lithology type. It is found that newer lava flows are more susceptible to landslides than older one. However, the distribution of observed slope failures indicates that most of the slope failure sites in the Northern parts of the study area are covered with Magacryst flows classified under Ratangarh formation whereas those in the Southern parts of Raigad District are dominated by Diveghat and Purandhargarh formations (Fig. 6.18). • The relationship between frequency of slope failures with the land use / land cover clearly indicates that maximum number of slope failures is located in the areas dominated by barren slopes, rock outcrop and agricultural land (Fig. 6.19). • Rainfall is a main triggering factor in the process of slope instability. Above 65% of the total slope failures identified in the field are situated in the areas with the average aimual rainfall exceeding 3250 mm whereas, few slope failure sites (Malshej ghat and Kasara ghat) are located in the areas with annual rainfall less than 2500 mm (Fig. 6.20). • Overall it can be concluded that slope angle, relief, lithology type, land use type and rainfall play a significant role in slope instability in North Konkan whereas little effect of aspect, proximity to major drainage and lineaments is observed.

7.2. Suggestions On the basis of the findings of this study related to the landslide inventory, landslide hazard zonation and risk assessment and causative factors, following suggestions are given for effective landslide hazard assessment, management planning measures and for landslide mitigation measures.

195 7.2.1. Regional Landslide Inventory System Accurate and reliable landslide hazard assessment requires complete landslide database with desired details. EMDAT is a main source of global level landslide database where only major slope failure events (Landslides with at least 10 fatalities and 100 affected people) are recorded. There is no separate landslide inventory system in India although few major events are recorded by NDMA (National Disaster Management Authority). Therefore, it is suggested to prepare and maintain regional landslide inventory system in concerned government offices with uniform inventory format throughout the country. This inventory system will not only provide basis for quantitative landslide hazard assessment but also prioritizing landslide mitigation strategies.

7.2.2. Application of revised methodology for LHZ Due to the limited scope for application of data driven methods for LHZ in hidia, heuristic approach like Bureau of hdian Standards (BIS) based LHEF rating scheme is largely adopted. However, this methodology is more useful in Himalayan conditions and needs to be modified before applying it to the Western Ghats and Konkan plains. The revised BIS based methodology for LHZ used in this study can be applied in other parts of the Western Ghat and coastal belt of India.

7.2.3. Micro-level Landslide Hazard Assessment Using Data driven Methods Besides regional scale landslide hazard assessment, micro level landslide hazard analysis should be carried out using field landslide information, geotechnical details and data driven methods which may be useful in predicting future slope failures accurately. The slope failures in Western Ghats and Konkan region are induced by intense monsoon rainfall. However, it is difficult to establish relationship between slope failures and rainfall amount due to limited availability of rain gauge stations. It is suggested to increase network of AWS (Automatic Weather Stations) so that the real time rainfall information associated with slope failures can be used for determining rainfall threshold.

7.2.4. Landslide Mitigation Measures Slope failures cause disruption to traffic flow along communication routes passing across the hilly terrain. Therefore, effective measures should be taken to minimize the impact of slope failures on human environment. Few landslide measures for North Konkan region are given below.

196 • Saturation of mass by infiltrated rainwater along road cut slopes often causes slope failures. To minimize the saturation of mass along such slopes pipes and culverts should be placed to allow water flow out of the road cut slopes. • In the view of future road development planning, it is recommended to choose the safer routes using LHZ maps. • Stabilization of road cut slopes by terracing in the upslope can also help in mitigating the effects of slope instability. • Besides, measures like retaining wall, metal nets, cementing the slopes, plantation along the slopes can also be taken up for effective landslide mitigation. • Using the preliminary landslide zones delineated for North Konkan region, the machineries like road graders, bulldozers, cranes, dumpers and blasting equipments should be placed and kept ready at the central locations during monsoon period so as to minimize delay in rescue operation and to regularize traffic flow immediately after the event. • People living in moderate to high risk zone should be warned before monsoon season about the risk. • Detailed data of risk prone settlements should be prepared and mitigation should be planned as per the settlement data.

7.3. Limitations of the study Although this study has attempted to analyze different dimensions of landslide hazards in North Konkan region of Maharashtra State, it came across few limitations. The limitations of this study are described below.

• Past landslide records maintained by concerned government offices is a proven database for landslide hazard and risk assessment (Jaiswal, 2010). However, records associated with slope failures in the study area found to be incomplete with lack of important details such as exact location and date and time of the landslide event. The incomplete landslide records led to limited scope for application of quantitative methods of Landslide Hazard Zonation. • Field survey for identification and characterization of slope failures in the study area was confined only to the major roads. Many other slope failures in the interior parts have not taken into consideration on account of non -

197 availability of records and only few of major slope failures were recorded in news papers. • The landslide hazard zonation mapping for North Konkan region has been carried out at the regional scale. Therefore, an outcome of this study may be used as base for the detailed micro - level landslide mitigation strategy. However, proper care should be taken while identifying landslide site characteristics. • Qualitative landslide risk mapping has been carried out in the study area using only three risk elements namely, population, built up areas and roads. Many other details such as number of commuters, actual traffic density, damage to vehicles, delay in traffic, type of building, number of households, etc.

7.4. Scope for Future Research On the basis of the outcomes of this study and recent advancements in the researches on landslide hazard assessment, there is a scope for future research. Few suggestions for continuation of this research work in future are given below.

• There is ample scope for micro level landslide hazard assessment with more details in Western Ghat and Konkan region. Availability of landslide inventory along with high resolution satellite data will be helpful for generating landslide database. • All the slope failures in Western Ghat and Konkan region are rain - induced. To establish relationship between rainfall and landslide occurrence, real time rainfall and landslide data can be generated and rainfall threshold analysis may be carried out for accurate prediction of future slope failures. • There is a need to identify and quantify various risk elements associated with slope failures in Konkan region in order to delineate risk prone areas accurately. • The methods adopted for LHZ and risk assessment can be applied to the rest of the areas of Western Ghat and South Konkan region.

7.5. Conclusions For effective landslide mitigation planning at regional scale, landslide hazard assessment is essential. Several studies have been carried out in different parts of the Himalayan region to assess different dimensions of slope failure hazards at varying

198 spatial scales. However, very little attempts have been made to develop appropriate methodology for landslide hazard and risk assessment in Western Ghats and rugged coastal belt.

The review of the existing literature on landslide hazard assessment reveal that landslide causative parameters including slope morphometry, relief, slope aspect, lithology, drainage, land use, land cover and rainfall are widely used as input parameters in landslide hazard assessment. It is also found that heuristic approaches like Bureau of Indian Standards and muticriteria decision making approaches are commonly used for LHZ.

The distribution analysis carried out for assessment of spatial patterns of landslide distribution in the study area reveals that slope failures in the study area are observed in small pockets and are mainly confined to the western slopes of Western Ghat, margins of dissected Jawhar plateau in the north east comer of the study area and slopes of isolated hills in the coastal plains.

The LHZ map prepared for the study area shows that one fourth (26.55%) of the total geographical area is identified as high to very high landslide hazard zones. The distribution of LH zones is associated with the western slopes of Western Ghat escarpment and isolated hillocks in the coastal plains.

The risk analysis carried out for the entire study area revealed that the areas with population concentration and other infrastructures are at high risk e.g. Kalwa - Mumbra area. In other geographical areas moderate to low risk is observed. Out of 4481 settlements, 434 settlements are in high risk areas while 815 settlements fall in moderate risk areas. It can be further concluded that risk analysis carried out may not resemble with LHZ area due to spatial distribution of risk elements such as settlements and roads.

The study of identification of landslide causative factors in the study area reveals that slope angle, lithology, relief and rainfall play important role in slope instability whereas drainage, land cover and human interventions along hill slopes are other factors responsible for slope failures in North Konkan and Western Ghat region.

199 The outcome of this study may be helpful in prioritizing landslide mitigation measures and also in policy framing for development of related infrastructure in landslide prone areas. For more detail landslide investigations, micro level landslide hazard assessment of different parts of the study area with detailed landslide database should be carried out.

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