TERRAIN EVALUATION FOR HILL SLOPE INSTABILITY ALONG BHAGIRATHI RIVER VALLEY IN A PART OF UTTARKASHI DISTRICT, GARHWAL HIMALAYA, U.P. (INDIA)

Abstract

Himalaya is geodynamically youngest mountain belt in the world, with immature topography &complex geology. It owes its origin to collision between the Indian Plate and the Eurasian Plate during the early Cretaceous - Eocene times. The continued subduction of the Indian Plate under the Eurasian Plate has resulted in further rise of the Himalayan hills which induces severe problems of slope instability in the terrain. The Himalayan terrain experiences frequent slope failures, especially during the rainy seasons, causing immense loss of life, property and the environment. However, such slope failures are found to occur &recur in areas characterised by unique combinations of terrain factors like topographic features, lithology, structure, slope, drainage, vegetation, climate and seismic activity. These factors play important roles in changing the geomorphic architecture of the area, both on short &long term basis. The morphology of an area can thus, indicate the instability in a terrain. Hence, an attempt has been made to identify, evaluate and forecast the hazardous zones of instability mainly through the study of terrain attributes and geomorphological signatures in a part of Lesser Himalaya in Garhwal region. The area of study, forming a part of Uttarkashi district, U.P., in Garhwal Himalaya between Dunda village and Uttarkashi city along the Bhagirathi river valley, bounded by longitudes 78°20'E -78°30'E and latitudes 30°38'N -30°45'N, is very sensitive to hillslope failures. The recent earthquake (1991) together with intense rains during monsoons has worsened the problem of hillslope instability. Geologically, the study area is a part of Lesser Himalayan Zone and comprises Garhwal Group of rock formations belonging to Precambrian age. The main stratigraphic units are the Uttarkashi formation comprising Netala Quartzite, Lower Uttarkashi Limestone, Pokhri Slate, Upper Uttarkashi Limestone &Bareti Quartzite memebers; Dunda formation comprising Dunda Quartzite, Dhanari Slate, &Khattukhal Limestone members; and the Gamri Quartzite formation. The area has been traversed &bounded by many thrusts like Srinagar (Dharashu) Thrust, Singuni Thrust, Dunda Thrust, Uttarkashi Thrust and the Main Central Thrust, which generally trend WNW-ESE. The terrain under study is characterised by rugged topography and typical ridge-valley iii features. The topographic elevation varies from 800m to 2711m above m.s.l. Slope failures in the terrain are found to occur at any elevation. Failed slopes are prominently found to occur mostly at elevations above 2400m, 2000-2400m, 1200-1600m and 1600-2000m zones, closer to the upper part ofthe ridges, and also at elevations of less than 1200m along the hillslopes near the road running almost parallel to the Bhagirathi river. The slope inclination is found to be highly variable. It varies from less than 15° to more than 45°. However, the slopes between 25° and 35° are most common and form 41% of the total area. Slopewise also, the failed areas are not uniformly distributed in different slope facets. Maximum failed slopes are found in moderate (25°-35°) to steep (35°-45°) slopes. About 68% of the hillface is occupied by natural vegetation and 23% ofthe total area is covered by agricultural lands. About 4% of the area is barren. Although barren and sparsely forested lands show high fractional failed slope area, the terrain is characterised by conspicuously high fractional failed slope area in thick and moderately thick forests. This may be attributed to overloading of slopes by the trees, wedging action of tree roots and the rapid infiltration ofwater along the root channels. Another possible cause of instability could be unplanned deforestation in these zones by the local people for want of wood for fuel, furniture and building purposes. Lithologically, out of five major rock types (quartzites, epidiorites, metabasic, slates and limestones), quartzites, epidiorites and metabasics are found as more susceptible to slope failures in comparison to other rock types under similar conditions. These rocks are generally found to be highly fractured and jointed as compared to slates and limestones. The slope stability has also been found to be affected by the occurrence, orientation and magnitude of structural &tectonic features like discontinuity planes or deformation/tectonic structures. Failed slopes are mostly found in the vicinity of thrust planes. Slopes are particularly unstable, when the discontinuity plane daylights on and runs parallel to the hillslopes. The sum effect of the terrain factors is found in morphology of the terrain. Despite their wide use, the morphometric parameters have little been used as instability indicators. In the present study, an effort has been made in this direction. The basins of third order which formed good working units, were the targets of study on morphometry vis-a-vis instability of hillslopes. The correlation coefficient between fractional failed slope area and IV 19 geomorphometric parameters derived from 20 third order drainage basins &sub-basins, indicates that the fractional failed slope area in a basin has a positive statistically significant corr. coeff. of 0.52 with the basin slope. Basin slope has a positive correlation coefficients of 0.77 and 0.68 with ruggedness number and relative relief respectively. Ruggedness number has significant correlation coefficient of 0.77, 0.68, 0.52, 0.46 and 0.46 with basin relief, relative relief, stream length, stream number, and basin length respectively. Basin relief shows significant correlation coefficients of 0.72, 0.51, 0.78, -0.48, -0.47, 0.63, -0.48 and 0.57 with basin perimeter, basin area, basin length, stream density, drainage texture, relative relief, basin circularity and basin form respectively. Relative relief has a positive correlation coefficients of 0.63 and 0.68 with basin relief and ruggedness number respectively. Most of the geomorphometric parameters are interrelated and are found to be have some relationship with basin slope directly or indirectly. Thus, basin slope appears as the single most important morphometric parameter which has in it the influence of many other morphometric parameters, and thus, refelects the cumulative effect of all the terrain factors responsible for slope instability. Based on regression analysis, a relationship has been worked out between fractional failed slope area and the basin slope as under: FFSA = 0.00146 BS - 0.0227 where FFSA is the fractional failed slope area and BS is the basin slope in degrees. It appears that basins with slope angles less than 15° will have virtually no failed slope areas. The stability of slopes in a basin decreases as the basin slope increases. Though the basin slope could be used as an indicator of slope instability in the terrain yet it does not indicate the nature of failure i.e. the processes of slope movements. Crozier (1973) showed that significant morphometric indices can be obtained from hillslope sections as indicators ofslope movement processes. In the present investigation, a similar approach has been made on the basis of 107 landslips studied in parts of Garhwal Himalaya. The morphometric indices namely classification index, dilation index, tenuity index, flowage index, and viscous flow index have been determined to identify the likely processes of slope movements viz. rotational slide, planar slide, slide flow, viscous flow and fluid flow. However, on the basis of field investigations, it has been found that the Crozier's morphometric indices for identifying a slope movement process are not directly applicable to the Garhwal Himalaya. This may be possibly due to marked differences in terrain conditions. Crozier studied a terrain made up largely of concavo-convex topography with very few bedrock outcrops, free faces and thick soil mantle in Newzealand. However, the present area exhibits rugged topography, high relief, steeply sloping valley faces, many bedrock outcrops, poor soil and regolith mantle cover. Hence, on the basis of present studies, a modified criteria for using morphometric indices as indicators of slope instability has been suggested. Further, displacement index has been useful as an indicator of the extent of instability in the terrain. The displacement index ranges from 35.59 to 61.49 for rotational slides, 59.24 to 86.02 for planar slides, 29.25 to 66.51 for slide flows, 30.51 to 60.87 for viscous flows and 29.62 to 60.87 for fluid flows. In general, the instability of a zone is high if the displacement index is low. Criteria for instability rating based on displacement index for different processes of slope -movements has been proposed. This criteria can be used to classify an area into six zones of instability, namely very high, high, moderately high, moderately low, low and very low instability zones. The efficacy of these modified indices as indicators of instability of hillslopes has been demonstrated when this approach was applied successfully on two different failed sites i.e Matli site at about 10 km downstream of Uttarkashi and Chilla site at about 200 km south of the study area in the Himalayan foothills. The developmental activities, specially along the road sections have affected the stability of hill slopes. Various types of slope movements, particularly flows, slides and falls, -have been observed along the road. Mostly7the slope material involved in the movement are rock blocks and rock fragments in the slides; soil, debris and rock pieces in flows and boulders &rock fragments in falls. Generally, the unstable slopes are devoid of any vegetation and highly dissected by numerous drains. In most of the failed slopes, the attitude of discontinuity planes (mainly the joints) is unfavorable and daylights in the valley. The major causes of slope failure are heavy rains, toe erosion by the river, road construction, deforestation and seismicity. Among the existing failed slopes along the state highway, Matli slope failure is the most problematic. It is located at km. 139.6 from Rishikesh, along the state highway 53, on -a sharp bend of the right bank of Bhagirathi river. Geologically,the site is characterised by highly fractured, jointed &weathered metabasics, phyllites and quartzites covered with

overburden of soil, debris &rock fragments. It is a complex failure where sliding, flowing, and slumping have been observed. Evaluation of geotechnical properties of slope materials shows that rocks have a uniaxial compressive strength of 250-500 kg/sq.cm. indicating an average rock quality. Soils present at the site are classified as SM-SC type, and have a liquid limit of 22, which ranges from 18 to 26. The strength of soils indicated that the cohesion varied from 14kN/sq.cm to 23kN/sq.cm. The angle of friction is 30°-38°, with an average 34°. The geotechnical data has been used for stability evaluation using a computer program. The factor of safety at different pore pressure ratios and seismic coefficients have been determined. The factor of safety varies from 1.875 to 0.275. It is found that the site is stable only under dry conditions and becomes unstable even at a pore pressure ratio of 0.10. An attempt has been made to study the relation between factor of safety and pore pressure ratios at different seismic coefficients. It is observed that the factor of safety drops directly with increase in pore pressure ratio under all conditions. The slope movements monitored through electronic distance meter (EDM) indicated that the higher rates of movement occur in highly unstable zones, predicted on the basis of morphometric approach. In a period of 2.5 years, the extent of movement has been found to vary from 87mm to 537mm. As per Varne's classification (1978), such rates ofmovement fall in the very slow rates ofmovement category. However, the area has been locally classified into six zones, viz. very high, high, moderately high, moderately low, low and very low movement zones to have an idea about the degree ofinstability in different parts. The map, thus, prepared onthe basis ofmovement data has been compared with the one prepared from morphometric analysis. It has been found that the various zones of instability correspond well with the various movement zones. The zones of very high and high instability have shown very high (>180mm/year) and high (160-180mm/year) rates of movement. Similarly zones of moderately high, moderately low, low and very low instability have shown moderately high (140-160mm/year), moderately low (120-140mm/year), low (100-120mm/year) and very low (<100mm/year) movement. The field instrumentation and monitoring studies have thus, corroborated the importance, efficacy and utility of morphometric approach in understanding the behaviour of slopes in hilly terrains. Based on above studies, remedial /control measures like retaining walls using slope debris, vegetative turfing, sealing of cracks, drainage modifications at appropriate locations VII have been suggested for road maintenance at appropriate locations. The kinematic analysis of discontinuity planes in the rockslopes have been used to identify the potential direction of failure as well as the type of potential failure along the probable plane. It has been found that the dominant mode of failures in rock slopes are planar and wedge sliding. Romana's Slope Mass Rating (SMR) technique has also been applied on rockslopes along the highway for designating the slopes in various categories of slope stability. Out of the 8 locations studied, 4 (S3, S4, S6 &S7) are completely unstable, 2(S1 &S8) unstable, and 2(S2 &S5) stable. The study indicates that the SMR technique is useful in assessing the stability of rock slopes in the study area except in few cases where it did not represent the actual field conditions. However, kinematic analysis alongwith it is found to be more useful. Finally, a relative slope instability zonation (RSIZ) map has been prepared for planning future development in the study area on the basis of National Methodology for Landslide Hazard Zonation using facet technique. It has been found that the total land hazard rating (TLHR) in the study area varied from 15.6 to 34.35. The total land hazard rating were classified into six zones viz. very high (TLHR-30.6 to 34.35), high (TLHR-26.85 to 30.6), moderate (TLHR-23.1 to 26.85), low (TLHR-19.35 to 23.1) and very low (TLHR-15.6 to 19.35) relative slope instability zones. The map thus, prepared indicates that 16% of the total area falls under very high or high instability zones. Moderately unstable zones occupy approximately 30% of the study area and the remaining 47% of the terrain is covered by low and very low instablity zones. The very high to high instability zones occur in a definite pattern. There is a linear ridge zone which runs SE-NW. This ridge is characterised by moderate to steep slopes at higher elevations. The other very high and high instability zone is arcuate in the northwestern part. This arcuate zone is conspicuously found to occur along the slopes of Bhagirathi river valley bend. The high instability of slopes in this arcuate zone may be possibly due to the presence of Baragadi fault in this zone and its truncation near the bend where the river takes a southern turn. This map may be used as first hand information for planning future landuse and developmental activities in this terrain. The study in short can be summarised as below: i). Slope instability in the area is related to actions &interactions of terrain factors like VIII topography, relief, slope, lithology, structure &tectonics, landuse, drainage besides rainfall &seismicity. ii). The sum offoct ofterrain factors is rofolocfod in morphomolric signnturos of the terrain. Among the 19 geomorphometric parameters studied, basin slope is the single most important parameter which has in it the influence of many other morphometric parameters and can be used as instability indicators, iii). Landslip morphometry can be successfully used in Himalayan terrain with some modifications to indicate the processes of slope mevements, namely slides, flows, falls, slumps etc. in the area. The study also suggests the use ofdisplacement index for the various processes ofslope movements as indicator of the degree of instability in different zones, iv). Developmental activities have aggravated the problem of hill slope instability resulting in complex failures like Matli. Such sites should be studied in details to design or plan suitable control or remedial measures at appropriate locations, v). Kinematic analysis of rock slopes can help in indicating the potential type of failure and its possible direction. Slope mass rating technique has been found applicable in this terrain except at few locations where it did not represent the actual field conditions, vi). The relative slope instability zonation map of the area can be used for future landuse planning and development in the study area.