Accelerated river avulsion frequency on lowland deltas due to sea-level rise

Chadwick, A. J.; Lamb, Michael P.; Ganti, Vamsi

doi:10.1073/pnas.1912351117

Cited by 50 publications

(137 citation statements)

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“…Models also show a regime at low sea‐level rise rates, where avulsion frequency is insensitive to sea‐level rise because the rate of topset aggradation that drives avulsion is primarily controlled by sediment supply and delta progradation (Chadwick et al., 2019, 2020; Ratliff et al., 2018). At very high rise rates, avulsion frequency is expected to either reach an upper limit where nearly the entire sediment supply is deposited on the active delta topset (Chadwick et al., 2020) or avulsions do not occur because the entire delta is drowned (Muto, 2001; Muto et al., 2007; Parker et al., 2008; Tomer et al., 2011). Enhanced aggradation during sea‐level rise has been linked to more frequent avulsions on the Rhine‐Meuse delta (Stouthamer & Berendsen, 2001; Törnqvist, 1994) and in delta laboratory experiments (Martin et al., 2009).…”

Section: Introductionmentioning

confidence: 99%

“…Avulsion nodes on lowland deltas have been documented to shift with movement of the shoreline (Ganti, Chadwick, Hassenruck‐Gudipati, Fuller, & Lamb, 2016; Ganti et al., 2014) to maintain a constant avulsion length

L_{A}

that scales with the backwater length‐scale

L_{b}

(Figure 1e) (Chatanantavet et al., 2012; Jerolmack & Swenson, 2007),

L_{A} \propto L_{b}

where

L_{b} = H_{c} / S

is the ratio of bankfull river channel depth,

H_{c}

, to channel‐bed slope

S

(Lamb et al., 2012; Paola & Mohrig, 1996). Therefore, deltas with backwater‐influenced avulsions might translate upstream, rather than reduce in size, with rising sea level (Chadwick et al., 2020; Moran et al., 2017).…”

Section: Introductionmentioning

confidence: 99%

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Climate‐Change Controls on River Delta Avulsion Location and Frequency

Chadwick

Lamb

2021

JGR Earth Surface

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Coastal rivers that build deltas undergo repeated avulsion events—that is, abrupt changes in river course—which we need to understand to predict land building and flood hazards in coastal landscapes. Climate change can impact water discharge, flood frequency, sediment supply, and sea level, all of which could impact avulsion location and frequency. Here we present results from quasi‐2D morphodynamic simulations of repeated delta‐lobe construction and avulsion to explore how avulsion location and frequency are affected by changes in relative sea level, sediment supply, and flood regime. Model results indicate that relative sea‐level rise drives more frequent avulsions that occur at a distance from the shoreline set by backwater hydrodynamics. Reducing the sediment supply relative to transport capacity has little impact on deltaic avulsions, because, despite incision in the upstream trunk channel, deltas can still aggrade as a result of progradation. However, increasing the sediment supply relative to transport capacity can shift avulsions upstream of the backwater zone because aggradation in the trunk channel outpaces progradation‐induced delta aggradation. Increasing frequency of overbank floods causes less frequent avulsions because floods scour the riverbed within the backwater zone, slowing net aggradation rates. Results provide a framework to assess upstream and downstream controls on avulsion patterns over glacial‐interglacial cycles, and the impact of land use and anthropogenic climate change on deltas.

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Section: Introductionmentioning

confidence: 99%

L_{A}

that scales with the backwater length‐scale

L_{b}

(Figure 1e) (Chatanantavet et al., 2012; Jerolmack & Swenson, 2007),

L_{A} \propto L_{b}

where

L_{b} = H_{c} / S

is the ratio of bankfull river channel depth,

H_{c}

, to channel‐bed slope

S

Section: Introductionmentioning

confidence: 99%

Climate‐Change Controls on River Delta Avulsion Location and Frequency

Chadwick

Lamb

2021

JGR Earth Surface

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“…A mechanistic understanding of the processes that culminate in lobe‐scale river avulsions has wide‐ranging implications for flood risk management, coastal sustainability, and the interpretation of fluvial stratigraphy (Bull, 1977; Ganti et al, 2019; Jones & Hajek, 2007; Mohrig et al, 2000; Syvitski et al, 2009; Törnqvist, 1994; Trower et al, 2018). Further, climate change and human activity are forecasted to alter flood frequency, relative sea level, and the amount and caliber of sediment supply (Best & Darby, 2020; Hirabayashi et al, 2013); however, their impact on river avulsions is unclear (Chadwick et al, 2020).…”

Section: Introductionmentioning

confidence: 99%

Flood Variability Determines the Location of Lobe‐Scale Avulsions on Deltas: Madagascar

Brooke

Ganti

Chadwick

et al. 2020

Geophysical Research Letters

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River deltas grow through repeated lobe-scale avulsions, which often occur at a location that correlates with the backwater lengthscale. Competing hypotheses attribute the avulsion node origin to either the morphodynamic feedbacks caused by natural flood discharge variability (backwater hypothesis) or to the prograding delta lobe geometry (geometric hypothesis). Here, using theory, historical flood records, and remotely sensed elevation data, we analyzed five lobe-scale delta avulsions in Madagascar, captured by Landsat imagery. Avulsion lengths were 5-55 km, distances significantly longer than the backwater lengthscale and inconsistent with the geometric hypothesis. We show that the steep, silt-bedded rivers of Madagascar have flood-induced bed scour, driven by backwater hydrodynamics, that propagates farther upstream than the backwater lengthscale. The avulsion lengths are 3.1 ± 1.5 times the predicted flood scour lengths, similar to low-gradient deltas, and consistent with backwater hypothesis. Results demonstrate that erosion initiated by nonuniform flows in the backwater zone is a primary control on delta avulsion locations. Plain Language Summary River deltas grow through abrupt channel shifts, called avulsions, which pose a threat to life and property, but we do not understand why avulsions occur where they do. One hypothesis is that flood discharge variability creates a preferential zone of sediment accumulation within the so-called backwater zone of coastal rivers, which becomes the locus of avulsion. The rationale is that the flow acceleration and deceleration within the backwater zone during different sized floods creates nonoverlapping patterns of erosion and deposition resulting in a peak in sediment accumulation. If this hypothesis is correct, then avulsion sites should be farther upstream, relative to the backwater lengthscale, in steep rivers with fine-grained sediment beds that have more significant bed scour during large floods. By analyzing historical avulsions on Madagascar, we found that avulsions occurred far upstream due to the large floods and fine-grained sediment, in support of the flood discharge variability hypothesis. Our work highlights that changes in flood regime and sediment grain size of coastal rivers due to land use and climate changes can influence avulsion location.

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“…The flux partitioning in the network can change over time through the process of avulsion, which may result in the abandonment of old channels and creation of new ones or merely a shift in the flux partitioning without modification of the number of channels (termed "soft avulsion") (6). A wide body of research has aimed to better understand avulsions in order to better predict their timing and location and to explore how deltas record themselves in stratigraphy (7)(8)(9)(10)(11)(12)(13)(14).…”

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Chaos in a simple model of a delta network

Salter

Voller

Paola

2020

Proc. Natl. Acad. Sci. U.S.A.

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The flux partitioning in delta networks controls how deltas build land and generate stratigraphy. Here, we study flux-partitioning dynamics in a delta network using a simple numerical model consisting of two orders of bifurcations. Previous work on single bifurcations has shown periodic behavior arising due to the interplay between channel deepening and downstream deposition. We find that coupling between upstream and downstream bifurcations can lead to chaos; despite its simplicity, our model generates surprisingly complex aperiodic yet bounded dynamics. Our model exhibits sensitive dependence on initial conditions, the hallmark signature of chaos, implying long-term unpredictability of delta networks. However, estimates of the predictability horizon suggest substantial room for improvement in delta-network modeling before fundamental limits on predictability are encountered. We also observe periodic windows, implying that a change in forcing (e.g., due to climate change) could cause a delta to switch from predictable to unpredictable or vice versa. We test our model by using it to generate stratigraphy; converting the temporal Lyapunov exponent to vertical distance using the mean sedimentation rate, we observe qualitatively realistic patterns such as upwards fining and scale-dependent compensation statistics, consistent with ancient and experimental systems. We suggest that chaotic behavior may be common in geomorphic systems and that it implies fundamental bounds on their predictability. We conclude that while delta “weather” (precise configuration) is unpredictable in the long-term, delta “climate” (statistical behavior) is predictable.

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Accelerated river avulsion frequency on lowland deltas due to sea-level rise

Cited by 50 publications

References 67 publications

Climate‐Change Controls on River Delta Avulsion Location and Frequency

Climate‐Change Controls on River Delta Avulsion Location and Frequency

Flood Variability Determines the Location of Lobe‐Scale Avulsions on Deltas: Madagascar

Chaos in a simple model of a delta network

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