Modeling of Spatial and Morphological Changes in Bhairab-Atai-Rupsha River Confluence

Abstract

The Rupsha River, also known as Pasur, which is formed of the union of the Bhairab and Atai rivers. At present, much of its water is from the Gorai diverted through the Nabaganga. From near Batiaghata upazila the Rupsa changes its name to Kazibacha, which is given up near Chalna in favour of Pasur. Near the Mongla port, the Pasur receives Mongla river, and near the forest outpost at Chandpai it receives the Mirgamari cross-channel from the Bhola, both on the left-bank. On the right bank the Manki, Dhaki and Bhadra are linked to the Shibsa system. In the lower delta, the Rupsa-Pasur is second only to the Meghna in size. Formerly it was third, after the Gorai-Madhumati, but with the considerable diversion of the Ganges water at upstream and due to less flow in the Gorai, the Gorai-Modhumati flow dwindled and as a result the Pasur gradually become bigger. Notable Nabaganga also receives flow from Modhumati. The Bhairab-Atai-Rupsha confluence system is the life line at upstream, situated at the center of Khulna city. Bhairab and Atai are the only gateways to reach to the upstream through Rupsha and Pasur. Therefore, morphological and environmental pattern study, of this three rivers system and their catchment area is very important. In this study, morphological behavior of total length of Atai and Rupsa River and two of the confluences in between their accumulated length is accomplished. Moreover, environmental and land surface pattern of the catchment area are studied in different point of view, such as Land Surface Temperature (LST), vegetation cover, amount of water body, barren and urban area etc. Two types of solver, named Nays2DH and River2D of iRIC 2.3 version software package, are used for modeling flow pattern in channel meander and confluence respectively. ArcGIS 10.4.1 version software package is used to study the environmental and land surface pattern of the catchment area. All the morphological data are collected from Bangladesh Water development Board (BWDB). Necessary satellite data, for selecting catchment area and environmental study, are collected from different website of the National Aeronautics and Space Administration (NASA), as https://glovis.usgs.gov/, https://earthexplorer.usgs.gov/ etc. The catchment area is selected from SRTM 1 Arc-Second Global digital elevation model (DEM) data. Before morphological modeling, a previous experimental work and related numerical models are compared with present numerical model, to observe applicability of the present study. During morphological modeling of meander and confluence, both high and ebbtide condition is considered, as this river system is under tidal influence. For the simulation of high and ebb-tide condition, fixed discharge is considered and discharge data are collected from the Mongla Port Authority (MPA). Satellite image processing technique is used, during modeling of environmental and land surface pattern of the catchment area. From modeling of river meander, positions are recognized along the length of Atai and Rupsha River, where flow velocity is decreased. From flow velocity and elevation relationship of the model, it is observed that velocity is decreased significantly, where flow depth is lower. It happens mainly because of channel bed friction, which significantly affects the channel flow. Also, it is found that secondary current is strong at both banks of the river, which creates a three dimensional effect and reduce the primary flow velocity magnitude near the banks. This flow velocity reduction causes siltation to a particular position of the channel, which brings inefficient navigability. So from this model, position of necessary future dredging work can be determined. Determination of separation zone is an important task to understand the complex flow pattern near the estuary of the confluence. During modeling of the confluence, these separation zones are detected both for high- tide and ebb-tide condition. So, from the separation zones, place of siltation near the estuary also be detected. Formation of separation zone is largely dependent on the channel geometry near the estuary. A hypothesis is proposed to reduce the intensity of the separation zone, by changing the geometry near the estuary. After changing geometry, the confluences are remodeled and a partial success is observed as separation zone is reduced in size and in some case eliminated. Lastly the modeling of environmental and land surface pattern of the catchment area shows that, at present large vegetation areas like Sundarban is under drought condition, while at past it was totally different. The reason may be increase of salinity in the western part of Bangladesh. Also, it is observed that barren area is mostly replaced by small vegetation like crop field. Lastly Land Surface Temperature (LST) decreases recently rather than the past because of replacement of barren area by small vegetation. A morphological model developed in this study largely reflects the flow pattern in Bhairab, Atai and Rupsha river geometries. Also the environmental and land surface pattern model of the catchment area reflects the present land surface pattern as compared with past years. From these models, several decisions can be made to keep the channel and estuaries functional as well as the catchment area habitable. This type of model based study addressed in this research, really convenient, helpful, cheap and less time consuming for predicting and making an engineering decision for different morphological and environmental condition.