"A regional land use planning can be started immediately in Kerala with the available macro-scale landslide geo database to reduce the risk to a great extent," says Dr. Saibal Ghosh, Director (Geology), Geohazard Research and Management Centre, Geological Survey of India (GSI), in an e-mail interview to The Hindu as the State continued to face the increasing threat of disasters induced by landslide events.
With landslides on the rise in Kerala following extreme weather events, is it not time for the State to make land use zoning regulation mandatory?
In hilly/ mountainous terrains, it is possible to understand or map areas with varying degrees/ likelihood of susceptibility to initiation of landslides or similar mass wasting processes, which can act as a vital geo information tool for use in land-use zoning regulations. However, the availability of that particular spatial geo information and its scale of mapping is very important to the planners and administrators for implementation.
GSI has just completed a macro scale (1:50,000) landslide susceptibility mapping in 2020 for the entire landslide-prone areas of 19,300 sq. km. in parts of 13 districts of Kerala. While preparing the landslide susceptibility map, GSI has also collected field-based data of 1395 nos. of landslide incidences. The above spatial geo information on a 1:50,000 scale can be used for regional planning purposes of land use zoning regulations. However, for the implementation of local-scale land-use zoning regulations in a more specific way on a smaller area, larger scale (at least 1:10,000 or larger) landslide susceptibility or hazard maps are required.
Now after completing the nation-wide baseline data generation task on the 1:50,000 scale, GSI, in consultation with the State governments concerned is stressing more on carrying out mesoscale (1:10,000) landslide susceptibility mapping in critical sectors in all the landslide-prone states in the country, including Kerala. GSI has a perspective plan to cover about 200 such critical sectors on a 1:10,000 scale by the next 4-5 years in India. I think with all the above tools together, feasible land-use zoning should be attempted in the fragile hilly/ mountainous terrains in the future in all the landslide-prone States.
What are the lessons learnt from the GSI's landslide susceptibility mapping programme carried out in Kerala?
Spatial areas with the varying likelihood of susceptibility to landslide initiation (Low, Moderate, and High) on a baseline macro-scale (1:50,000) are now delineated and known. The distribution of historic landslides is also known, which gives a proxy to the prevalent landslide susceptibility scenarios in an area. However, this inventory is getting updated every year with the occurrence of new/ fresh landslides in the State.
The macro-scale (1:50,000) landslide susceptibility mapping provides an excellent baseline geoinformation tool for understanding landslide scenarios over a regional area. This also facilitates the identification of critical areas for taking up upscaled (1:10,000/ 1000) studies further.
It is also important to note that, the conventional landslide susceptibility maps can only model the zone of initiation or source areas of any future landslide, and this is a systemic limitation of any landslide susceptibility model; however, like other areas, and in Kerala too, many damages have been noticed because of linear debris flows, where relatively stable slopes in the landslide runout and/ or accumulation zone, located much away from the source areas also become severely hazardous/ fatal, and thus need to be modeled too to comprehend the complete hazard scenario in an area, which is not only difficult but also is very challenging too. That is why, in mesoscale (1:10000) landslide susceptibility mapping, GSI is also attempting debris flow runout impact model outputs in the landslide susceptibility model to make it more inclusive, appropriate, and useful at that larger scale.
Landslide susceptibility models are developed using rated and weighted pre-disposing geofactor maps which are mostly static in nature, but landslides are ultimately triggered by some dynamic triggering factor (e.g., rainfall, earthquake). In India, landslides are predominantly triggered by rainfall, occurring mostly during the monsoon period. In any terrain, the triggering of a landslide depends on the terrain-specific rainfall threshold. If those specific areas suddenly receive an extremely high amount of intense rainfall, which is much higher than the local rainfall thresholds, there could be the chances of occurrence of extreme landslide events in that area, and that may heavily transgress into the moderate, and even low susceptible areas too. In India, in many landslide-prone States in recent times, the same has been experienced often, where the triggering rainfall was 5-10 times higher than the normal peak monsoon rainfall intensity of that specific area, and that triggered severe damages through rockfall, slides, and flows, even in low to moderate areas. This particular extreme situation is very difficult to model.
The short-term and long-term solutions for the State to avoid such disasters in the future?
Landslides are natural phenomena, and the same can rarely be avoided in hills/ mountains. What we can do is to reduce its risk to a great extent by implementing the following tasks in a planned phase-wise manner.
Land-use zoning regulations are required in Indian hills/ mountains, and for which all the available scientific data should be used. This should be implemented phase-wise by prioritising the most vulnerable districts and sectors in the State (both short and long term).
In each district of the State, all the critical sectors for mesoscale (1:10,000) landslide susceptibility and hazard mapping, and critical landslides for site-specific remediation need to be identified for planning and prioritising landslide remediation, mitigation, and management work (short term)
To take up comprehensive landslide mitigation and management, some pilot districts in the State are to be identified first; and a comprehensive mitigation strategy for those districts be planned and implemented by taking into account all the available scientific data/ information; if new data is to be generated, the same may be prioritized for use; arrangement for exclusive funding arrangement for such mitigation tasks be made beforehand in a DPR mode; execution of mitigation work also needs to be monitored closely by an expert group (both short and long term). The identified pilot district be made a model district for implementing comprehensive landslide mitigation and management tasks.
Simultaneous implementation of regional rainfall threshold-based landslide early warning system, and site-specific landslide early warning system using in-situ instruments for a few critical landslides in the pilot district be planned and implemented in the identified pilot district too (both short and long term).
Landslides severely affect the community, and they are the first-level responders. Any relevant endeavour for landslide disaster risk reduction (DRR) thus requires the involvement of the community right from the planning stage. Therefore, any measures planned to be implemented should be designed and executed by involving the local stakeholders, and the community from the very beginning so that their understanding of landslide risks, their capacity, and resilience are improved altogether, to achieve a suitable landslide DRR in that area.
