Landslides at Karwar: The Expert Committee Report

Executive Summary

The multidisciplinary expert committee, consisting of members from  ecology, geology, forestry, Government officials, etc., under the Chairmanship of Sri Ananth Hegde Ashisar, Chairman, Western Ghats Task Force (WGTF), on 14th Oct 2009, visited the locations where landslides occurred during October 2-3, 2009. Members of the expert committee are:

The committee met briefly at the Forest Guest House, Karwar and explored the possible reasons for landslides in Karwar and visited the following locations where landslides had occurred. Latitude and longitude of these locations were recorded using calibrated GPS (Global Positioning System) and Survey of India toposheets.

Background
Landslides occur when masses of rock, earth or debris move down a slope.  Mudslides, debris flows or mudflows, are common type of fast-moving landslides that tend to flow in channels. These are caused by disturbances in the natural stability of a slope, which is triggered high intensity rains. Mudslides usually begin on steep slopes and develop when water rapidly collects in the ground and results in a surge of water-soaked rock, earth and debris. Causes may be of two kinds: 1. Preparatory causes & 2: Triggering causes. Preparatory causes are factors which have made the slope potentially unstable.. The triggering cause is the single event that finally initiated the landslide. Thus, causes combine to make a slope vulnerable to failure, and the trigger finally initiates the movement. Thus a landslide is a complex dynamic system. An individual ‘landslide’ characteristically involves many different processes operating together, often with differing intensity during successive years.

The main trigger of landslides is heavy or prolonged rainfall. This could be either an exceptional short lived high intensity event, or of a long duration rainfall event with lower intensity, lasting several days, such as the cumulative effect of monsoon rainfall. In the former case it is usually necessary to have very high rainfall intensities, whereas in the latter the intensity of rainfall may be only moderate - it is the duration and existing pore water pressure conditions that are important.

Gravity caused slope instability occur naturally due to earth quakes, etc. Slope failures are also triggered by high degree of human impact  on the environment. Land use changes may lead to land degradation, and make hillslopes more susceptible to instability (or mass movement). The past several decades have witnessed intense disruption of forest cover, for alternative land uses (mining, quarrying, monoculture plantations, agriculture, human settlements, roads, railways  etc.) or for biomass needs, in the mountains and hills of India. In such events when extreme meteorological events, such as heavy rainfall happen landslide episodes, have increased causing human and cattle deaths and destruction of property and livelihoods.

Susceptibility to landslides may be considered as a natural, tangible process in a landscape, taking into account the rainfall history, as well as the geological and geomorphic setting of the area. Risk levels increase drastically in landslide prone areas whenever human pressures increase. In addition to rainfall, several factors probably contribute to slope failure. These include:

• Geological structures like shattered, fragmented and highly jointed rocks and topography like steep slopes, interacting to provide the geometry favouring landslides.
• Prolonged rainfall certainly produces abnormally high antecedent groundwater levels, more so in places where the ground water level is already high due to the nearness of rivers or estuaries or other water bodies. It is the collective rainfall of many days that enhances the pore water pressure.
• Poor drainage network in the hillocks – due to changes in land cover (removal of vegetation or obstruction of drainage channels) alters the drainage.
• Steep slopes cut for pathways and residential buildings near the base of the slope further destabilizes the slope by removing the basal support.

Landslide description

Kadwad (Zariwada) land slidev :  Major landslide was noticed at Kadvad village to the northeast of Karwar, where 8 to 9 houses were washed with earth flow causing 19 deaths of humans. The hill is about 65 m asl and the slope is < 20º.  First order streams from this hill drains to Kali river and the hill base bordering the Kali estuarine area, has density human habitation along with plantations of coconut, banana, fruit trees etc.  The cutting of slopes for construction of housing and home gardens was observed. Also, large scale soil removal in the recent years, from this mound has been reported.  The committee assessed the various factors responsible for the slide -  slope angle, human interventions in slopes,  apart from assessing the accumulated collapsed debris.

After observing the slip surface and other features on the slope, it was evident that it was a slope failure in the form of a mudslide or debris slide. The length of the slide is about 250 m, the width about 75 m, depth about 30 m in the deepest part and the shape of the slide is concave.

The instability in the slope was mainly due to removal of vegetation (deforestation), of the hill, alteration of slopes (base cutting for construction of houses and household gardens), large scale removal / excavation of soil at the toe of the hill (for developmental works in the region).  Deep rooted vegetation provides significant cohesive strength into slope soil, and facilitates good drainage, thereby reducing the chances of landslides.  Removal of vegetation or other changes in the natural land cover, such as forests, permanently reduces rooting strength, thus increasing chances of landslide.

The collective rainfall of many days, before the landslide has increased the pore water pressure, which along with lateral pressure due to swelling of oversaturated clay rich soil horizon (layer) have powered (or enhanced) the driving force for the slide resulting in the burial or displacement of houses and human deaths and loss of properties.

It was inferred after the assessment of the instability in the area, that the basement of the hill at Zariwada hamlet in Kadwad is a potential risk zone due to intense human activities. Hence, it is not advisable to rehabilitate this landslide-affected zone till the slope is stabilized to the best possible extent.

Implementation of remedial measures shall include - afforestation of hill tops with native tree species (preferably the ones with tap root systems); trees with specialized root systems to resist water-logging are to be considered especially for base of the hills.  Sealing of cracks, slope-grading, proper drainage measures and soil reinforcement (using geo-grid and biotechnical measures) are other measures. However, appropriate selection of these measures along with their design is only possible after an in-depth geological and geotechnical study of the slide area.

Landslides at National Highway (NH 17) : The antecedent high intensity rain has triggered the slides. Main causal factors are unscientific road cuts (without proper mitigation measures),  building of large compound wall (for naval base) altering/blocking the natural drainages and  large scale granite quarrying.  Rock and debris fall were noticed around Arga village limits.  In the Baitkolgudda, earth flow in larger quantity was seen, just missing the fishing harbour and dense human settlement narrowly. Weathered granites on the hills and weakening of soil due to excessive rains have triggered the landslides near Binaga, National Highway.

On the basis of prima facie observations and studies the committee suggests the following and requests the Government to implement these suggestions on priority.

Recommendations of the expert committee (based on the field visit on 14th Oct 2009 and subsequent studies and land use analysis)

1. Planting of native vegetation on hilltops and slopes. The roots of the native vegetation, especially of certain specially chosen tree species, can act as good soil binders, thereby providing slope stability. The removal of trees (with deep tap roots) and subsequent taking over of secondary vegetation, planting of cashew trees on the Zariwada hill by removing earlier natural tree cover, most of them with shallow roots has reduced the soil binding properties.

2. Restoration of natural drainage network. Alteration in hydrological regime due to changes in drainage network consequent to deforestation - Inappropriate locations of human habitations on the first and second order streams in Kadwad has also hindered the water movement. Heavy pressure of rain water within the hill removed all the blockages on the way – as evident from mudslide and collapse of houses (in Zariwada)

3. Discouraging monoculture plantations– the land given to cashew plantations in the Kadwad hills should be taken back and the afforestation has to be carried out  with the native species of flora according to suitably designed planting programmes.

4. Immediate banning of large scale illegal quarrying of granite stones and  mineral mining considering the hazards proneness of the region (Karwar) and presence of sensitive pockets (Naval base, Kaiga nuclear plant, eco sensitive Anshi-Dandeli tiger reserve). Weathered granites on the hills and weakening of soil due to excessive rains have triggered the landslides near Binaga, National Highway.

5. Banning soil and rock mining on the hills- Landslide prone areas should not be leased out for soil or stone removal. All illegal mining and quarrying have to be stopped  

6. Improving drainage connectivity. The collective rainfall of many days has enhanced the pore water pressure. This with lateral pressure due to swelling of oversaturated clay rich horizon has set the driving force resulting in burial of houses and humans displacement and destruction of houses and loss of properties (at Zariwada, Kadwad).

7. Need to investigate the suitability of human habitations in regions prone to landslides.

1. The Konkan Railway track connecting Karwar town with Goa and other places passes through this Zariwada paleo-river valley and also crosses the lineaments. The vibration generated by movement of trains is also suspected to have developed the cracks on hill tops resulting in the loosening of the soil (subsequent to soil mining reported to have taken place). Long spell of high intensity antecedent rainfall has triggered the mud slide.
2. Geomorphologically, the valley area was a paleo-river channel that formerly drained into the Kali River that flows in the north of Zariwada/Kadwad. The shift of the river course might have led to the formation of hills/mounds.  Composition of this mound which is mainly of laterite clay further confirms the existence of paleo stream in this region. The paleo-river channel is still connected to the Kali River and the groundwater seepage into the channel increases or decreases rhythmically according to high and low tides in the River.
3. Soil formation for centuries on the hillocks and heavy rain might have led to the swelling of clay inside the hillocks resulting in the landslips.

8. No large scale developmental projects particularly in Karwar taluk and ecologically fragile regions in Central western Ghats. Considering the implementation of large number of mega projects in Karwar, it appears that the region has exceeded the carrying capacity and further implementation of any mega projects would prove detrimental to the local population. The region has already prone to hazards and could be categorized as hazard hotspot. This also emphasizes the need for carrying capacity study for Uttara Kannada district.

9. Considering the hazard proneness of the region it is necessary to set up 'Disaster management centre (DMC)’ to assist in regional planning, management of disasters and also to assist the administration in rehabilitation measures in case of eventualities. Also, the region around 25 km radius of Karwar is seismically sensitive and also occurrence of lineaments further emphasizes the need for setting up a seismic monitoring cell (within DMC) in the district to assist the district administration in the predication and also mitigation measures. Disaster management centre shall house seismic monitoring cell, and shall have the state of the art gadgets to predict calamities due to natural as well as human induced causes.

10. Naval authorities should be asked to remodel the protection wall considering the natural drainages on priority. Construction of the protective wall by the naval authorities without any due consideration to natural drainage systems has resulted in large scale flooding of the region, which also has triggered series of landslides all along the Highway (NH 17). Removal of the protection walls at many locations during the floods, highlights the need for holistic approaches in planning and implementation of large scale projects. Many such projects have seriously impaired the ecology of the region, affected the livelihood of nature people and also have posed serious threats to the existence of local population.

11. Shifting of all affected families (located on the hill side) at Madibag/Zariwada to appropriate locations without affecting their current livelihood dependence.

12. Afforestation with native vegetation in the region considering the large scale deforestation in recent times. At present many hill tops are barren or with highly inadequate tree cover.

13. Setting up special Uttara Kannada package to restore ecosystems – The  funding shall be from all developmental projects in the district. Karwar taluk, in all probability, has transgressed its limits of growth, beyond the carrying capacity - Series of landslides consequent human tragedies and property loss is the indication of the lack of integrated approaches in planning and the region has crossed the thresholds of carrying capacity. Large scale land cover changes has resulted in alterations in hydrological regimes evident from the conversion of perennial streams to seasonal streams, enhanced siltation in the catchment evident from increased sedimentation in reservoirs in recent years.  The region being one among the global biodiversity hotspots call for immediate measures to restore the ecosystems. Large scale projects such as hydro power plants, project sea bird, Kaiga nuclear plants have played significant role in degradation the ecosystems which have also affected the livelihood of local people.  These projects shall also make provision to provide a recurring grant to sustain the proposed Disaster Management Centre at Karwar and also for Ecological Research in Karnataka part of Western Ghats.

14. Considering the level of devastations in Karwar, landslide susceptibility mapping should be carried out to delineate potential zones of instability, particularly in areas where human lives and properties are involved. If any indication of slope instability is noticed in an area which has major risk elements, the fact should be shared with society in a proper way. This suggestion has two major implications.

1. First, with the increasing large scale environmentally unsound development activities in the region, the risk posed by natural hazard must also be evaluated. This is well-exemplified by a series of natural hazards (landslides) in the region. Considering rainfall-intensity and changes in climate / hydrologic regimes (due to global warming) stochastic relationships have to be developed to assess high-risk areas.
2. Secondly, with the successful establishment of relationship between rainfall and landslide activity, analysis of palaeo landslides would provide insights based on the past variation in rainfall patterns. Equally, such relationships aid in predicting changes in mass movement activities based on modelled regional impacts of global climate change.

15. Apart from reforestation of barren hill slopes and hill tops, sealing of cracks, slope-grading, proper drainage measures, soil reinforcement using geo-grid and biotechnical measures have to be done. However, appropriate selection of these measures along with their design is only possible after an in-depth geological and geotechnical study of the slide area.