The river atlas preparation for Assam has been initiated during 2017. In this project, all the major and minor rivers entering Assam has been mapped at a scale of 1:4,000. Mapping includes incorporation of left and right bank, sediments, embankments, hydro-meteorological observatories, sluice gates, P&RD bunds, major locations, roads, railway lines etc. The LULC map for all the rivers has been created with defined buffer. In addition to this, district wise river catchment maps are being prepared to show the origin of all the rivers entering the respective districts of Assam. At present the project is in progress and mapping is in completion stage for 20 districts of Assam. In an interim review of this project during January, 2018, Hon’ble Chief Minister, Assam, expressed satisfaction on progress of the project and hoped that this exercise will be of great help in river planning & development in Assam.
Management of Natural Resources, especially land resources, is the key to attain food, water and environmental security. This has special relevance since per capita availability of agricultural land in India is decreasing due to population growth, industrialization and Urban Expansion. Amongst the various options available for improving land productivity, development of wasteland/degraded land is one of the most viable options. Similarly conservation of the productivity of existing land is also another viable options.
The objective of the project is generation of GIS database for Land Resources development and management planning. The database are mapping of land use land cover map (LULC) at 1:50000, sheet and Gully erosion area at 1:25000 scale, detailed drainage at 1:10000 scale, water body map of Assam and identification of agriculture and built up area surrounding to the sheet erosion area and water bodies available in the state . The existing LULC and water body map which were generated under National Natural Resources Census project by Department of Space are used as a input for this project. The LULC and water body map were generated at 1:50000 scale using LISS III multitemporal data. The LULC map for Assam consist of eight major classes and 43 sub classes. The sheet erosion map was prepared based on the derived information of LULC, Slope and soil texture. The gully erosion map at 1:25000 scale was prepared using Kompsat data utilizing Bbuvan web portal developed by NRSC, Hyderabad along with extensive ground information provided by Soil Conservation Department of Assam. Selected sites were visited for ground verification and collection of spectral signatures for gully erosion. Sheet Erosion is a derived product of LULC, Soil texture and slope of the terrain. The study shows that 2,98,745 ha areas are under sheet erosion and 153 ha area under gully erosion. Study further shows that Golaghat (13.5%),Chirang (11.09%), Kokrajhar (8.14%), Hojai (8.08%), West Karbi Anglong (7.91%) district having higher percent of sheet erosion area and Biswanath (7.29%), Lakhimpur (4.43%), Baksa (3.21%), Chirang (2.62%), Dhemaji (2.49%) districts are having higher percent of Gully erosion areas.
The project envisages monitoring and evaluation of IWMP projects using Bhuvan web services and Mobile app for the sanctioned projects from 2009-10 to 2014- 15 (This may differ State to State) for entire India and NESAC is carrying out the project for North Eastern part of India. Each project has to be monitored for a period of 5 years from the date of implementation. The project is being implemented at NRSC and geo spatial tools have been developed (Srishti – a web GIS interface on Bhuvan and Drishti – a mobile based android application). The project duration is up to 2020. The scope of the work of this project include Processing of high resolution satellite data – LISS-IV and Cartosat; Correction / fine tuning of Watershed boundaries based on SIS-DP database; Generation of LULC maps, NDVI, evaluation and assessment based on Drishti photographs , preparation of maps showing change detection in projects supported by limited ground truth of representative sites. The approach includes report generation for each project area year wise in suggested format. NESAC is carrying out following activities in collaboration with State Remote Sensing Application Centers of NER:
- Processing of high resolution satellite data- LISS IV and Cartosat.
- Correction/ fine tuning of watershed boundaries based on SIS-DP Satellite Image.
- Generation of LULC maps, NDVI, evaluation and assessment based on Dristi photographs, change detections maps coupled with ground truth as well as year wise report generation for each project area.
- All the processed data will be made available for online analysis/interpretation.
The Brahmaputra River, a major trans-boundary river in the Eastern Himalayan region, originates in China flows through India and Bangladesh before draining into the Bay of Bengal. The waters of the Brahmaputra are shared by China, India and Bangladesh and any changes in the quality or quantity of the river at any point is likely to affect downstream population drastically as millions of people rely on this river for their survival. Towards the end of November 2017, people residing beside the river at Pasighat (Arunachal Pradesh), where the river just reaches the plains, noticed an abnormal increase in sediments on the river. The Brahmaputra usually carries a lot of sediments during the monsoon season but even after the monsoons had subsided, the sediments on the river had not decreased and there were many reports of fishes and other aquatic life dying and the quality of water not suitable for any use by the local dwellers. The news resulted in several debates and the issue turned into an international concern due to the trans-boundary nature of the river. Many government, research and academic institutions started investigating on the quality of the river as well as the possible genesis of this sudden increase in turbidity levels. Observations made by Central Water Commission, Govt. of India on turbidity levels of water samples taken from the Siang River at Pasighat on 27 November 2017 showed an abnormally high suspended and particulate matter concentration.
This study was taken up to investigate the possible reasons for the sudden increase in turbidity levels of the Siang River with the help of multi-temporal satellite datasets.
Multi-temporal 30m spatial resolution Landsat-8 and 10m spatial resolution Sentinel-2 datasets have been used for this study. The Landsat-8 is an American Earth observation satellite launched on 11 February 2013, which acquire images of the entire earth in every 16 days interval. Sentinel-2 is an Earth observation mission developed by European Space Agency (ESA) as part of Copernicus Program terrestrial observations in support of services such as forest monitoring, land cover changes detection, and natural disaster management. The Sentinel-2 mission comprises a constellation of two polar-orbiting satellites (Sentinel-2A launched on 23 June 2015 and Sentinel-2B launched on 7 March 2017) placed in the same orbit, phased at 180 ͦ to each other. This mission can provide high temporal resolution of 10 days at the equator with one satellite, and 5 days with 2 satellites under cloud-free conditions. Landsat-8 images acquired on 9 November 2017 and 21 December 2017 have been used for the present study. Sentinel-2 images acquired on 5 November 2017, 25 November 2017, 10 December 2017, 4 January 2018 and 20 March 2018 have been analyzed. Sentinel-2 and Landsat-8 have been used for identifying and monitoring the landslide area and landslide-dammed lakes surface area. These satellites provide near real-time earth observation images freely in support of research and development.
Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) of 30m spatial resolution has been used for estimating the volume of water stored in these landslide-dammed lakes. They can be downloaded freely over the internet (http://dwtkns. com/srtm30m/), and their file format (.hgt) is widely supported. The earthquake epicentres, magnitude and date of occurrence provided by United States Geological Survey (USGS) have been analyzed. Active faults and sutures over the study area provided by HimaTibetMap are also analyzed in this study.
MultiMate satellite data were analyzed to identify the possible source of the sudden change in water quality. Careful observation of temporal satellite images identified a zone of severe landslides on both sides of the river channel around 425 kms upstream of Pasighat lying within Tibet. It is also observed that this area is at a close proximity to the epicentre of the recorded 6.4 magnitude earthquake on 17 November 2017. hese analyses indicates that there was a massive landslide after the 6.4 magnitude earthquake that occurred on 17 November. Satellite images also revealed blockages of the river channel due to the debris of these landslides resulting in natural damming at a few locations. Four such locations of landslide occurrences were identified and shown in the figure and named as L1, L2, L3 and L4 from upstream to downstream.
Removal of vegetation and debris deposits at the base of the slope and on the river channel is clear on the Sentinel-2 images acquired on 10 December 2017. Earthquake triggered landslides are very common in the Himalayas due to its active tectonics but the location of these landslides are crucial as it has occurred on the upstream of the Brahmaputra main channel and partially blocked flow of water. Three of these four locations as given in Figure also show the formation of lakes due to the accumulated debris on the river channel around 429 kms, 435 kms and 459 kms upstream of Pasighat. The total area affected by landslides is around 28 sq kms. Drastic changes in the river course are also observed at all the locations with significant increase in sediments downstream of the landslide affected region.
These landslide-dammed lakes were continuously monitored with the help of satellite images acquired on 21 December 2017, 4 January 2018 and 20 March 2018. The sentinel-2 images of spatial resolution 10m acquired on 10 December 2017, 4 January 2018 and 20 March 2018 are given in Fig 8. Not much change in the surface area of the lakes was observed. All calculations are only indicative as they are based on DEM generated before the occurrence of the landslides and there might be major changes in volume after the occurrence of the landslides due to deposition of debris. landslide-dammed lakes were continuously monitored with the help of satellite images acquired on 21 December 2017, 4 January 2018 and 20 March 2018. The sentinel-2 images of spatial resolution 10m acquired on 10 December 2017, 4 January 2018 and 20 March 2018 are given in Fig 8. Not much change in the surface area of the lakes was observed. All calculations are only indicative as they are based on DEM generated before the occurrence of the landslides and there might be major changes in volume after the occurrence of the landslides due to deposition of debris.
The present analysis has confirmed the occurrence of landslides in upstream areas (Tibet) as the cause of sudden increase in sediment levels in the Siang and Brahmaputra River. The debris from these massive landslides has blocked the river channel at several locations creating lakes and resulted in channel shift. These landslides may have been triggered due to the 6.4 magnitude earthquake recorded in Indo-Tibetan border on 17 November 2017. Three lakes created due to blockage by landslide debris were observed with surface areas of about 12 ha, 55 ha and 49 ha. However no significant change in surface area is observed in multi-temporal images, the breaching of the water impounded in these lakes might induce floods downstream of these locations and cause destruction to life and property. Therefore, a continuous monitoring using near real-time remote sensing data is necessary so that forthcoming devastation due to breaching of these lakes can be minimized.
Secretary, Ministry of DoNER requested to study two river streams i.e. Langdang Kong and RonjalKhong in Ukhrul district, Manipur state for identifying source of water and proposal for check dam to supply drinking water to Ukhrul town. As a part of this exercise a preliminary study has been carried out using satellite imagery, watershed analysis using digital elevation model and ancillary data. Suitable check dam location, total discharge, supply and demand analysis for drinking water requirements have been worked out.
A remote sensing and GIS based methodology was followed in this study as mentioned below:
- Langdang Kong and RonjalKhong streams are identified using toposheets.
- Automatic catchment dileneation is carried out using CartoDEM v3 and drainage lines are derived for both the streams.
- Ground water prospect of Ukhrul town and its surrounding is studied using ground water prospect maps.
- With the help of satellite imagery and DEM the check dam locations are identified by taking into consideration of height of check dams.
- Discharge of both the catchments are computed using rational method.
- Once the check dam locations are identified then inundation area is delineated and the area of inundation and storage volume is calculated
- The water demand per person for Ukrul town is calculated.
- Map showing Langdang Kong and RonjalKhong catchment along with proposed check dam location
From the study following conclusions are drawn:
- Since the RonjalKhong dam is 20 km away from the Ukhrul town, LangdangKhong dam found to be most suitable in terms of distance, discharge and storage capacity.
- The storage volume of the check dam of 50 m height is 1,31,79,620 m3.
- The discharge from the Langdong Kong catchment is 1,23,622 m3/day (computed based on rainfall of 1763 mm (100 rainy days) annually).
- Analysis related to ground water shows that, the area is considered as highly weathered zone as a whole. Sometimes the yield is varying due to the presence of fractures/cracks which governs the occurrence/ movements of ground water. It may also be noted that few springs are also located in and around the town which may require immediate attention for conservation measures of these springs to have an alternate source of water supply during the lean season.