The discharge dynamics of distributaries of the Ganges as determined with object-based image analysis

Abstract

In this thesis, insight is gained in the discharge distribution and channel evolution of meandering distributaries in the Ganges Delta by the development of a method that extracts channels from satellite imagery and that analyses the channel geometry. This focus is part of a project of the Utrecht University Physical Geography Department about the Ganges-Brahmaputra delta and is aimed at unravelling a small part of the delta to gain knowledge about the larger scale. To support the methodology and to compare and interpret the results of this study a comprehensive literature overview is presented additionally.

The image analysis is performed with eCognition Developer 8 software and comprises segmenting a Landsat 5 TM image using six different segmentation scales. Meander objects are delineated by classifying the objects based on shape characteristics. The small meander objects from the different segmentation scales are merged into one dataset containing objects that represent large parts of the channel network. The skeletons of the objects are processed to obtain channel centerlines. The geometry of the centerlines is analysed in MATLAB to obtain inflection points that are used to approximate meander wavelength. This is used to determine discharges and sinuosities of the channels. Larger scale meandering present in the general course of numerous rivers in the area is analysed by fitting a cubic spline through the inflection points and determining the inflections of the spline.

The methods that are developed in the study are successfully able to nearly automatically delineate meander objects, merge objects into large channel representations and obtain channel centerlines. Furthermore, it is proven that the used extraction methods yield the possibility to obtain river and discharge dynamics by analysing the centerline geometry.

The discharge data that results from the analysis seems relatively accurate, taking into account the many potential sources of error of the methods that are used. However, the lack of ground truth of the rivers in the study area causes the accuracy to be uncertain. Therefore further research is needed to calibrate the discharge calculation methods. In terms of the river evolution in the study area it is confirmed that the Hooghly and the Bhairab have been the main Ganges course. For the Jalangi and Gorai this could not be confirmed.