Apurba Kumar Das

Assessing and mapping flood hazard, vulnerability and risk in the Upper Brahmaputra River valley using stakeholders’ knowledge and Multi‐Criteria Evaluation (MCE)

Assessing flood hazard, vulnerability and integrated risk has long been recognised as an important input for the formulation of policies aiming at flood risk management. This investigation is an endeavour to assess hazard, vulnerability and risk due to flooding, using an indicator-based methodology incorporating stakeholders’ knowledge and multicriteria evaluation in geographic information system (GIS) to achieve community-based assessment. The framework developed in this work is illustrated for the district of Dhemaji, a chronically flood-affected area in the Upper Brahmaputra River valley. Results show spatial distribution of hotspots of flood hazard and vulnerability and locations at risk at regional and subregional level. The emerged risk pattern indicates that vulnerability indicators are more significant contributors than hazard indicators while calculating risk for the Upper Brahmaputra River valley. The methodology provides a dynamic platform where the flexibility in uses of hazard and vulnerability indicators, depending on variation in physical and socioeconomic setup, is possible.

Living in troubled waters: stakeholders’ perception, susceptibility and adaptations to flooding in the Upper Brahmaputra plain

This work attempts to present the complex human–flood interactions from the chronically flood-affected district of Dhemaji in the Upper Brahmaputra floodplain. The objectives are to analyse perceptions, susceptibilities, indigenous knowledge and adaptations that enable coexistence of floods with floodplain dwellers. In addition, data on flood damage, breaches and cuts to embankments are collected from government departments and analysed. Results revealed a realistic perception of flood hazard amongst the floodplain dwellers. Susceptibility is higher due to the lack of infrastructure. Indigenous knowledge and adaptation strategies exist in the area enabling the floodplain dwellers to live with floods. The results also underscore the paradoxical situation where flood-control measures themselves have become detrimental to the floodplain dwellers. This creates a situation where environmental challenges outstrip the evolution of indigenous adaptation strategies. Our work is based in the Upper Brahmaputra River plains and is a contribution towards the long-standing debate on the choices humans make in response to riverine hazards.

Drainage skeletonization from flow-accumulated area without the use of threshold

This study presents a method of automatic drainage skeletonization from flow-accumulated area without the use of threshold which conserves drainage geometry at chosen digital elevation model (DEM) scale. To get all possible drainage at the chosen scale, stream order raster is generated by incorporating flow accumulation and flow direction raster derived from corresponding DEM. This allows generation of drainage network without the use of threshold. Resultant stream order raster, termed as raw stream order raster (RSOR), is tested against threshold defined stream order raster to evaluate its efficiency. Use of RSOR allows extraction of stream heads to greater stream head extent. Again, DEM downscaling takes care of overestimation in number of streams. So, the proposed technique is effective in controlling two basic aspects of drainage characteristics – stream number and extent. In our case, drainage estimated from re-sampled medium-scale DEM has the closest matching with that of the reference topographical map.

Assessment of Urban Heat Island Effect in Guwahati—A Remote Sensing Based Study

The manifestation of effects of urbanization on the thermal environment of a city is often related to Urban Heat Island (UHI) by various studies. The presence of abnormal warmth in urban areas as compared to rural surroundings is defined as UHI. Satellite images in thermal infrared can be used for assessing thermal environment as well as for defining Urban Heat Islands (UHIs) in urban areas. In this chapter, Guwahati city and its surrounding areas have been studied to observe the changes in the thermal environment that has occurred between the years 1989 and 2010 by means of satellite images provided by Landsat Thematic Mapper (TM) onboard Landsat 5. A range of influences have been investigated in the form of relationship between Normalized Difference Vegetation Index (NDVI) and Radiant Surface Temperature (Ts) to see the influence of sensible heat flux and urban density over the urban vegetation as an indicator of modified temperature regime over the years. The study tried to quantify the UHI phenomenon in the city by measuring Urban Heat Island Intensity (UHII) using remote sensing technique. The objective of the study was to quantify changes in the urban thermal environment of Guwahati, a rapidly growing city of Northeast India, which will help in studying various effects of urbanization in future.

Morphological Dynamics of the Rivers of Brahmaputra

This study documents the sequential morphological dynamics of rivers of Brahmaputra. Landsat images for the year of 1976 and 1987–2016 (thirty consecutive years) are used in GIS environment to document the activities that have been shaping and reshaping the morphology of the rivers. The use of thirty year consecutive images facilitates an explicit assessment of morphological changes. Apparently, distributary activities, meandering activities, breaches and avulsions, and confluence migrations are the dominant geomorphic agents of morphological changes. A range of distributaries activities are readily noticed in Brahmaputra plain. The plain is densely dissected with numerous rivers and rivulets, and distributaries activities often lead to reorganization in channel network. Considering the frequency of incidence and preserved imprints of abandoned channels, majority of the rivers of Brahmaputra can be categorized as free meanders. Difference in fluvio-geomophic setups of rivers of both the banks is reflected palpably in meandering activities. The upper stretch of Brahmaputra encompassing the rivers Simen, Gai and Jiadhal is found to be the most active in terms of avulsions. High seasonal flow, large sedimentation and absence of distinct river bed in some stretches are presumably leading to these frequent avulsions. The river Simen, further, exhibited repeated confluence migration through avulsion near its confluence with Brahmaputra. The explicit picture of the morphological dynamics of rivers of Brahmaputra at basin scale presented in this work is a key aid in formulating riverine management programs which is conceivably among the most water-troubled region of the country.

Overcoming source limitations in drainage delineation by combining the streams of toposheet and DEM in river morphometric studies

This paper suggests an approach that combines the drainages of DEM and toposheets, by matching streams (number and length) and spatial coincidence, the critical aspects of such operations, for overcoming source limitation in river morphometric studies. We illustrate the approach taking Gaurang, a Himalayan river basin, as an example where the toposheets for the uppermost part is inaccessible. Stream matching in terms of number and length is controlled by a method of automatic stream generation from downscaled DEM and flow accumulated rasters. Stream estimated by this method are tested, verified and finally applied to the portion of the basin where the data is unavailable. The other critical aspect- spatial coincidence is controlled by commonly used method of GCPs. In the present case, use of the nodal points of the major tributaries generated from DEM as GCPs for georeferencing of toposheets have restricted the positional mismatch to much less than a pixel. With the consent of both the aspects i.e. stream matching and spatial coincidence, the DEM generated stream is considered to combine with the stream of toposheets to get the drainage of entire basin.