System wide channel network analysis reveals hotspots of morphological change in anthropogenically modified regions of the Ganges Delta

Abstract

the Ganges Brahmaputra Meghna Delta (GBMD) is a large and complex coastal system whose channel network is vulnerable to morphological changes caused by sea level rise, subsidence, anthropogenic modifications, and changes to water and sediment loads. Locating and characterizing change is particularly challenging because of the wide range of forcings acting on the GBMD and because of the large range of scales over which these forcings act. in this study, we examine the spatial variability of change in the GBMD channel network. We quantify the relative magnitudes and directions of change across multiple scales and relate the spatial distribution of change to the spatial distribution of a variety of known system forcings. We quantify how the channelization varies by computing the Channelized Response Variance (CRV) on 30 years of remotely sensed imagery of the entire delta extent. The CRV analysis reveals hotspots of morphological change across the delta. We find that the magnitude of these hotspots are related to the spatial distribution of the dominant physiographic forcings in the system (tidal and fluvial influence levels, channel connectivity, and anthropogenic interference levels). We find that the anthropogenically modified embanked regions have much higher levels of geomorphic change than the adjacent natural Sundarban forest and that this change is primarily due to channel infilling and increased rates of channel migration. Having a better understanding of how anthropogenic changes affect delta channel networks over human timescales will help to inform policy decisions affecting the human and ecological presences on deltas around the world. Many river deltas are large and complex systems whose channels evolve in response to changes in internal and external forcings. Changes to water inflow, sediment load, land use, subsidence, modern climate, and anthropogenic fluvial modification structures such as dams, levees, embankments, and dredging operations can all play a role in delta morphodynamics. With more than 500 million people living on river deltas around the world 1 , detecting the magnitudes of changes to deltas is critical for managing these systems, and it is especially important to detect the portions of the system that are changing the most. The goal of this study is to examine a large delta channel network to identify if change is occurring, where hotspots of change are occurring, what the rates of change are, and what may be causing the changes in certain areas to be higher than others. This information will help us better understand morphodynamic patterns in delta systems and make predictions to inform policy decisions affecting the human and ecological presences on deltas. Deltas are currently threatened by a variety of factors. As inflow conditions of water and sediment change, channel movement such as channel widening, narrowing, lateral migrations, meandering, and avulsions all act to rework delta landscapes. In addition to the natural morphological evolution of delta systems, changes can also be

Stable ≠ Sustainable: Delta Dynamics Versus the Human Need for Stability

“…In contrast, rapid channel-bank migration is the primary cause of severe land losses in the active, fluvially-dominated parts of the delta (Jarriel et al, 2020). The GBMD is thus a clear example of how system-wide vs. local-scale assessments of mass balance and landscape dynamics lead to different conclusions on the sustainability of the system.…”

“…In all, the areal extent of land in the tidal delta plain is relatively constant, but intense morphodynamics cause areas of acute local change (Auerbach et al, 2015;Jarriel et al, 2020) (Fig. 3), largely promoted by feedbacks with prior embankment construction (Pethick & Orford, 2013;Bain et al, 2019).…”

“…Remotely sensed data can be employed to quantify the magnitude of change and hotspots, locations where the rivers are most active (Jarriel et al, 2020), and this information should be accounted for when designing delta management strategies. With the current remotely sensed data, timescales associated with river dynamics can be estimated over an interval of time, but more frequent observations will be needed to distinguish between abrupt changes and more gradual channel migration.…”

“…With increasing recognition of rapid global sea level rise, in the 1990s and 2000s the GBMD was often depicted as a passive coastal landmass at peril of drowning, but that perception neglected large sediment fluxes originating from its Himalayan hinterland. Now, it is often cited for its widespread local instabilities (i.e., dynamics) (Auerbach et al., 2015; Bain et al., 2019; Jarriel et al., 2020; Rogers & Overeem, 2017) while simultaneously being recognized for system‐scale sustainability with its large sediment supply and persistent net land growth (Allison, 1998; Wilson & Goodbred, 2015; Wilson et al., 2017). Typical for complex systems, instability reflects network dynamics and feedbacks among discrete but connected parts of the delta system, such as the upper and lower delta plain as well as the inner shelf.…”

Dominant process zones in a mixed fluvial–tidal delta are morphologically distinct

“…By looking at the evolution of channel planform overlap, morphological metrics have been used to quantify growth (Wolinsky et al, 2010) and channel network dynamics (Cazanacli et al, 2002;Liang et al, 2016b) over time. Channel networks can also preserve information about their evolution; in fluvially dominated deltas, Jerolmack and Swenson (2007) identified differences in channel and network morphology for distributary systems that evolve through mouth-bar deposition and those formed by avulsions.…”