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The principal nature-based solution for offsetting relative sea-level rise in the Ganges-Brahmaputra delta is the unabated delivery, dispersal, and deposition of the rivers’ ~1 billion-tonne annual sediment load. Recent hydrological transport modeling suggests that strengthening monsoon precipitation in the 21st century could increase this sediment delivery 34-60%; yet other studies demonstrate that sediment could decline 15-80% if planned dams and river diversions are fully implemented. We validate these modeled ranges by developing a comprehensive field-based sediment budget that quantifies the supply of Ganges-Brahmaputra river sediment under varying Holocene climate conditions. Our data reveal natural responses in sediment supply comparable to previously modeled results and suggest that increased sediment delivery may be capable of offsetting accelerated sea-level rise. This prospect for a naturally sustained Ganges-Brahmaputra delta presents possibilities beyond the dystopian future often posed for this system, but the implementation of currently proposed dams and diversions would preclude such opportunities.

 

Understanding channel migration is essential in interpreting long‐term evolution of fluvial systems and their deposits. Using data from an experimental delta, we analyzed the kinematics of the upstream channel and assessed the relative dominance of continuous lateral channel migration versus abrupt changes (i.e., avulsions). Detailed investigation of channel centerline location at minute intervals reveals a short‐term correlation between the magnitude of migration rates measured at the same location and a spatial correlation that diminishes with distance between points. The main finding is that the channel migrates across the entire deltaic domain without large and abrupt lateral shifts but through continuous lateral migration at variable rates. Long periods of back and forth small moves are separated by short bursts of rapid lateral migration. This finding contradicts the default expectation that that aggrading systems are characterized by avulsions and suggests that highly mobile rivers tend to avulse less. We contrast this with another experiment conducted under similar conditions but with finer sediment supplied at a lower rate which shows drastically less lateral migration; the kinematics is instead dominated by periodic flow reconfiguration episodes akin to avulsions, an indication that channel migration‐style depends on the sediment load. The characteristics of these two experiments parallel two regions of the Mississippi River, the meandering and highly mobile alluvial plain and the less dynamic deltaic region, suggesting that bedload sediment deposition at the transition into backwater zone plays an important role in re‐shaping the river planform and migration style.

 

Abstract. River deltas are sites of sediment accumulation along thecoastline that form critical biological habitats, host megacities, andcontain significant quantities of hydrocarbons. Despite their importance, wedo not know which factors most significantly promote sediment accumulationand dominate delta formation. To investigate this issue, we present a globaldataset of 5399 coastal rivers and data on eight environmental variables.Of these rivers, 40 % (n=2174) have geomorphic deltas defined eitherby a protrusion from the regional shoreline, a distributary channel network,or both. Globally, coastlines average one delta forevery ∼300 km of shoreline, but there are hotspots of delta formation, for examplein Southeast Asia where there is one delta per 100 km of shoreline. Ouranalysis shows that the likelihood of a river to form a delta increases withincreasing water discharge, sediment discharge, and drainage basin area. Onthe other hand, delta likelihood decreases with increasing wave height andtidal range. Delta likelihood has a non-monotonic relationship withreceiving-basin slope: it decreases with steeper slopes, but for slopes >0.006 delta likelihood increases. This reflects differentcontrols on delta formation on active versus passive margins. Sedimentconcentration and recent sea level change do not affect delta likelihood. Alogistic regression shows that water discharge, sediment discharge, waveheight, and tidal range are most important for delta formation. The logisticregression correctly predicts delta formation 74 % of the time. Our globalanalysis illustrates that delta formation and morphology represent a balancebetween constructive and destructive forces, and this framework may helppredict tipping points at which deltas rapidly shift morphologies. 

Groundwater is the primary source of water in the Bengal Delta but contamination threatens this vital resource. In deltaic environments, heterogeneous sedimentary architecture controls groundwater flow; therefore, characterizing subsurface structure is a critical step in predicting groundwater contamination. Here, we show that surface information can improve the characterization of the nature and geometry of subsurface features, thus improving the predictions of groundwater flow. We selected three locations in the Bengal Delta with distinct surface river network characteristics—the lower delta with straighter tidal channels, the mid‐delta with meandering and braided channels, and the inactive delta with transitional sinuous channels. We used surface information, including channel widths, depths, and sinuosity, to create models of the subsurface with object‐based geostatistical simulations. We collected an extensive set of lithologic data and filled in gaps with newly drilled boreholes. Our results show that densely distributed lithologic data from active lower and mid‐delta are consistent with the object‐based models generated from surface information. In the inactive delta, metrics from object‐based models derived from surface geometries are not consistent with subsurface data. We further simulated groundwater flow and solute transport through the object‐based models and compared these with simulated flow through lithologic models based only on variograms. Substantial differences in flow and transport through the different geologic models show that geometric structure derived from surface information strongly influences groundwater flow and solute transport. Land surface features in active deltas are therefore a valuable source of information for improving the evaluation of groundwater vulnerability to contamination.

 

Coastal deltaic aquifers are vulnerable to degradation from seawater intrusion, geogenic and anthropogenic contamination, and groundwater abstraction. The distribution and transport of contaminants are highly dependent on the subsurface sedimentary architecture, such as the presence of channelized features that preferentially conduct flow. Surface deposition changes in response to sea‐level rise (SLR) and sediment supply, but it remains unclear how these surface changes affect the distribution and transport of groundwater solutes in aquifers. Here, we explore the influence of SLR and sediment supply on aquifer heterogeneity and resulting effects on contaminant transport. We use realizations of subsurface heterogeneity generated by a process‐based numerical model, DeltaRCM, which simulates the evolution of a deltaic aquifer with different input sand fractions and rates of SLR. We simulate groundwater flow and solute transport through these deposits in three contamination scenarios: (a) vertical transport from widespread contamination at the land surface, (b) vertical transport from river water infiltration, and (c) lateral seawater intrusion. The simulations show that the vulnerability of deltaic aquifers to seawater intrusion correlates to sand fraction, while vertical transport of contaminants, such as widespread shallow contamination and river water infiltration, is influenced by channel stacking patterns. This analysis provides new insights into the connection between the depositional system properties and vulnerability to different modes of groundwater contamination. It also illustrates how vulnerability may vary locally within a delta due to depositional differences. Results suggest that groundwater management strategies may be improved by considering surface features, location within the delta, and the external forcings during aquifer deposition.

 

Climate change is raising sea levels across the globe. On river deltas, sea‐level rise (SLR) may result in land loss, saline intrusion into groundwater aquifers, and other problems that adversely impact coastal communities. There is significant uncertainty surrounding future SLR trajectories and magnitudes, even over decadal timescales. Given this uncertainty, numerical modeling is needed to explore how different SLR projections may impact river delta evolution. In this work, we apply the pyDeltaRCM numerical model to simulate 350 years of deltaic evolution under three different SLR trajectories: steady rise, an abrupt change in SLR rate, and a gradual acceleration of SLR. For each SLR trajectory, we test a set of six final SLR magnitudes between 5 and 40 mm/yr, in addition to control runs with no SLR. We find that both surface channel dynamics as well as aspects of the subsurface change in response to higher rates of SLR, even over centennial timescales. In particular, increased channel mobility due to SLR corresponds to higher sand connectivity in the subsurface. Both the trajectory and magnitude of SLR change influence the evolution of the delta surface, which in turn modifies the structure of the subsurface. We identify correlations between surface and subsurface properties, and find that inferences of subsurface structure from the current surface configuration should be limited to time spans over which the sea level forcing is approximately steady. As a result, this work improves our ability to predict future delta evolution and subsurface connectivity as sea levels continue to rise.

 

Understanding subsurface structure and groundwater flow in deltaic aquifers is essential for evaluating the vulnerability of groundwater resources in delta systems. Deltaic aquifers contain coarse‐grained paleochannels that preserve a record of former surface river channels as well as fine‐grained floodplain deposits. The distribution of these deposits and how they are interconnected control groundwater flow and contaminant transport. In this work, we link depositional environments of deltaic aquifers to stratigraphic (static) and flow and transport (dynamic) connectivity metrics. Numerical models of deltaic stratigraphy were generated using a reduced‐complexity numerical model (DeltaRCM) with different input sand fractions (ISF) and rates of sea‐level rise (SLR). The groundwater flow and advective transport behavior of these deltas were simulated using MODFLOW and MODPATH. By comparing the static and dynamic metrics calculated from these numerical models, we show that groundwater behavior can be predicted by particular aspects of the subsurface architecture, and that horizontal and vertical connectivity display different characteristics. We also evaluate relationships between connectivity metrics and two environmental controls on delta evolution: ISF and SLR rate. The results show that geologic setting strongly influences both static and dynamic connectivity in different directions. These results provide insights into quantitatively differentiated subsurface hydraulic behavior between deltas formed under different external forcing (ISF and SLR rate) and they are a potential link in using information from delta surface networks and depositional history to predict vulnerability to aquifer contamination.

 

River deltas are densely populated regions of the world with vulnerable groundwater reserves. Contamination of these groundwater aquifers via saline water intrusion and pollutant transport is a growing threat due to both anthropogenic and climate changes. The arrangement and composition of subsurface sediment is known to have a significant impact on aquifer contamination; however, developing accurate depictions of the subsurface is challenging. In this work, we explore the relationship between surface and subsurface properties and identify the metrics most sensitive to different forcing conditions. To do so, we simulate river delta evolution with the rule‐based numerical model, DeltaRCM, and test the influence of input sand fraction and steady sea level rise (SLR) on delta evolution. From the model outputs, we measure a variety of surface and subsurface metrics chosen based on their applicability to imagery and modeling results. The Kullback‐Leibler (KL) divergence is then used to quantitatively gauge which metrics are most indicative of the imposed forcings. Both qualitative observations and the KL divergence analysis suggest that estimates of subsurface connectivity can be constrained using surface information. In particular, more variable shoreline roughness values and higher surface wetted fraction values correspond to increased subsurface connectivity. These findings complement traditional methods of estimating subsurface structure in river‐dominated delta systems and represent a step toward the identification of a direct link between surface observations and subsurface form.

 

A major thread of theoretical research on the response of shorelines to changing boundary conditions has adapted the moving‐boundary approach from heat transfer and solidification/melting. On sufficiently short time scales, shorelines respond to changes in relative sea level in a simple, geometrically predictable way. On longer time scales, their behaviour becomes far more complex and interesting, because the surface over which the shoreline moves is itself continually modified by morphodynamics that depend sensitively on shoreline location. This makes the shoreline the archetype of moving‐boundary problems in morphodynamics, and subject to potentially counterintuitive behaviours over time scales on which the sediment surface modifies itself as relative sea level changes. We review existing moving‐boundary theories and propose two significant extensions to allow inclusion of first‐order effects of waves and tides. First, we show how wave effects can be included via planform diffusion linked to river‐mouth location, which results in shoreline smoothing during delta‐lobe growth and localized transgression after channel abandonment. Tides produce a low‐gradient region in which the sea and land overlap; we show how this can be treated in a moving‐boundary framework by replacing the shoreline with a ‘mushy region' so that the handoff from land to water occurs over a zone rather than a line. We also propose that the moving‐boundary approach can be readily generalized to other dynamic moving boundaries, such as those separating different regimes of river transport. The shoreline thus serves as a prototype for modelling dynamic facies boundaries along the whole source–sink system. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.