Bridging the gap or broadening the problem?

Abstract

A bridge is a structure built to span physical obstacles such as a body of water, valley or road for the purpose of providing passage over the obstacle. Bridges over rivers cause an alteration of flow and modify the flow characteristics because of the flow obstruction. Remote sensing and GIS techniques have been used to study the alterations in river meandering characteristics. The objective of this study is to analyse the changes in river characteristics downstream of a bridge using satellite images for the lower Ganges River at two bridge sites in Bhagalpur and Munger in Bihar, India. ARC-GIS is used to calculate the normalised difference water index (NDWI), which has the capability to represent water bodies adequately from space. Based on NDWI maps, the river channel length and downvalley length were measured for different spatio-temporal conditions. The above parameters (channel length and downvalley length) were used to estimate the Sinuosity Index under 14 spatio-temporal situations. The Sinuosity Index (i.e. an indicator of local river fluvial characteristics) varies significantly because of the effect of bridges and their associated structures. There was an increase in the Sinuosity Index of 30 % magnitude in an 8-year time period (2006–2014) after the bridge construction phase at both locations. The pre-construction phase of the bridges indicates minor changes in the Sinuosity Index during the 9-year period from 1987 to 1996. The NDWI analysis also reveals that the river characteristics changed considerably in the pre- and post-bridge scenarios while the change was less significant for pre- and post-flood events (May 2014 to October 2014).