Date of Award

Summer 2019

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Ocean & Earth Sciences

Program/Concentration

Ocean & Earth Sciences

Committee Director

Richard P. Hale

Committee Member

G. Richard Whittecar

Committee Member

Thomas R. Allen

Abstract

The Ganges-Brahmaputra-Meghna (GBM) Delta is one of the largest deltas in the world, covering more than 100,000 km2. The GBM River system transports over one billion tons of sediment annually and delivers ~750 million tons to the Bay of Bengal, ~25% of which is advected by tides into the fluvially abandoned western delta plain. The Sundarbans National Forest (SNF) is located within the GBM Delta and, covering more than 10,000 km2, is the world’s largest continuous mangrove stand. The present rate of sediment delivery allows the SNF platform elevation to keep pace with regional sea level rise, but India’s National River Linking Project (NRLP) could decrease the suspended sediment loads of Ganges and Brahmaputra Rivers by 39-75% and 9-25% respectively, and thus may change future sedimentation. In this study, we examine the idea that sediment stored in the tidal channels throughout the year is being resuspended and delivered to the mangrove platform during the dry season when sediment input to the system is at a minimum. Over two field seasons 70 shallow (cm) sediment cores were collected from the channel banks, imaged using x-radiography to observe fine scale changes in depositional characteristics on tidal channel banks, and subsampled for subsequent textural analysis. In more than half of the cores examined there is no change in the style of the laminations, suggesting that the processes controlling deposition remain constant throughout the year, despite a change in the external sediment supply. Cores where the depth of laminations decreases from wet to dry season demonstrate that sediment is being eroded from the banks and advected elsewhere, with the mangrove platform a likely sink. Over 70% of cores showed surface (0-2 cm) grain sizes coarsening between the monsoon to the dry season, which could be the product of the winnowing of fine sediments as overall sediment supply decreases and material with lower bulk density is preferentially taken away. Total organic carbon (TOC) results indicate constant reworking of sediment on the banks throughout the year. These results demonstrate that sediment in this system is constantly being reworked, and a portion of the sediment delivered to the platform during the dry season can be sourced from the banks.

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DOI

10.25777/khab-gp69

ISBN

9781088395615

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