Search for: All records

Arising from the non‐uniform dispersal of sediment and water that build deltaic landscapes, morphological change is a fundamental characteristic of river delta behavior. Thus, sustainable deltas require mobility of their channel networks and attendant shifts in landforms. Both behaviors can be misrepresented as degradation, particularly in context of the “stability” that is generally necessitated by human infrastructure and economies. Taking the Ganges‐Brahmaputra‐Meghna Delta as an example, contrary to public perception, this delta system appears to be sustainable at a system scale with high sediment delivery and long‐term net gain in land area. However, many areas of the delta exhibit local dynamics and instability at the scale at which households and communities experience environmental change. Such local landscape “instability” is often cited as evidence that the delta is in decline, whereas much of this change simply reflects the morphodynamics typical of an energetic fluvial‐delta system and do not provide an accurate reflection of overall system health. Here we argue that this disparity between unit‐scale sustainability and local morphodynamic change may be typical of deltaic systems with well‐developed distributary networks and strong spatial gradients in sediment supply and transport energy. Such non‐uniformity and the important connections between network sub‐units (i.e., fluvial, tidal, shelf) suggest that delta risk assessments must integrate local dynamics and sub‐unit connections with unit‐scale behaviors. Structure and dynamics of an integrated deltaic network control the dispersal of water, solids, and solutes to the delta sub‐environment and thus the local to unit‐scale sustainability of the system over time.

We examine variations in discharge exchange between two parallel, 1‐ to 2‐km‐wide tidal channels (the Shibsa and the Pussur) in southwestern Bangladesh over spring‐neap, and historical timescales. Our objective is to evaluate how large‐scale, interconnected tidal channel networks respond to anthropogenic perturbation. The study area spans the boundary between the pristine Sundarbans Reserved Forest, where regular inundation of the intertidal platform maintains the fluvially abandoned delta plain, and the anthropogenically modified region to the north, where earthen embankments sequester large areas of formerly intertidal landscape. Estimates of tidal response to the embankment‐driven reduction in basin volume, and hence tidal prism, predict a corresponding decrease in size of the mainstem Shibsa channel, yet the Shibsa is widening and locally scouring even as the interconnected Pussur channel faces rapid shoaling. Rather, the Shibsa has maintained or even increased its pre‐polder tidal prism by capturing a large portion of the Pussur's basin via several “transverse” channels that are themselves widening and deepening. We propose that an enhanced tidal setup in the Pussur and the elimination of an effective Shibsa‐Pussur flow barrier are driving this basin capture event. These results illustrate previously unrecognized channel interactions and emphasize the importance of flow reorganization in response to perturbations of interconnected, multichannel tidal networks that characterize several large tidal delta plains worldwide.

Quantitative interrogation of grain sizes in sedimentary systems has the potential to improve predictions of stratigraphic architecture, facies distributions, and downstream reservoir characteristics. To quantify these relationships, downstream fining data are coupled with rates of mass extraction, with input grain‐size distribution, accommodation, and sediment input from multiple transport pathways providing primary controls on resulting sediment dispersal patterns. We spatially apportioned mass distribution along three sediment delivery pathways with distinct accommodation characteristics within the Ganges‐Brahmaputra‐Meghna Delta to calculate chi (χ), the total fraction of supplied sediment flux lost to deposition at any given point. Low rates of downstream fining and sand‐rich channel facies characterize a bypass‐dominant pathway along the western margin of Sylhet basin, whereas two splay deposits that prograde into the underfilled basin interior exhibit higher rates of fining and preservation of mud‐rich facies. Both splay deposits show a shift from sand‐dominated to mixed sand and mud facies and increased mud preservation (above 30%) at aχvalue of ∼0.8. No comparable increase in mud preservation occurs along the bypass‐dominated pathway, suggesting that this course operated in an inherently different extraction mode due to limited mid‐Holocene accommodation. A similarity solution model effectively reproduces most of the spatial patterns of mass extraction observed in Sylhet basin, except in one location receiving lateral sediment input from a distributary channel. These field and modeling results indicate that grain‐size data and sediment volume measurements can be used to not only reconstruct paleodynamics of transport networks and resulting stratigraphy but also lead to predictive insights on subsurface heterogeneity, and thus improved reservoir and aquifer characterization.