Mathematical study of linear morphodynamic acceleration and derivation of the MASSPEED approach

Carraro, F.; Vanzo, Davide; Caleffi, Valerio; Valiani, Alessandro; Siviglia, Annunziato

doi:10.1016/j.advwatres.2018.05.002

Cited by 19 publications

(17 citation statements)

References 22 publications

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“…To reduce computational costs, for example, mixed method approaches have been studied that combine analytical and process-based models [16]. Even though process-based models of physical systems have greatly benefited from the increasing computational power over the last decades, the use of morphodynamic acceleration techniques is still the only way to perform medium-to long-term morphodynamic predictions [17,18].…”

Section: Introductionmentioning

confidence: 99%

Morphodynamic Acceleration Techniques for Multi-Timescale Predictions of Complex Sandy Interventions

Luijendijk

Schipper

Ranasinghe

2019

JMSE

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Thirty one percent (31%) of the world’s coastline consists of sandy beaches and dunes that form a natural defense protecting the hinterland from flooding. A common measure to mitigate erosion along sandy beaches is the implementation of sand nourishments. The design and acceptance of such a mitigating measure require information on the expected evolution at time scales from storms to decades. Process-based morphodynamic models are increasingly applied, together with morphodynamic acceleration techniques, to obtain detailed information on this wide scale of ranges. This study shows that techniques for the acceleration of the morphological evolution can have a significant impact on the simulated evolution and dispersion of sandy interventions. A calibrated Delft3D model of the Sand Engine mega-nourishment is applied to compare different acceleration techniques, focusing on accuracy and computational times. Results show that acceleration techniques using representative (schematized) wave conditions are not capable of accurately reproducing the morphological response in the first two years. The best reproduction of the morphological behavior of the first five years is obtained by the brute force simulations. Applying input filtering and a compression factor provides similar accuracy yet with a factor five gain in computational cost. An attractive method for the medium to long time scales, which further reduces computational costs, is a method that uses representative wave conditions based on gross longshore transports, while showing similar results as the benchmark simulation. Erosional behavior is captured well in all considered techniques with variations in volumes of about 1 million m 3 after three decades. The spatio-temporal variability of the predicted alongshore and cross-shore distribution of the morphological evolution however have a strong dependency on the selected acceleration technique. A new technique, called ’brute force merged’, which incorporates the full variability of the wave climate, provides the optimal combination of phenomenological accuracy and computational efficiency (a factor of 20 faster than the benchmark brute force technique) at both the short and medium to long time scales. This approach, which combines realistic time series and the mormerge technique, provides an attractive and flexible method to efficiently predict the evolution of complex sandy interventions at time scales from hours to decades.

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Section: Introductionmentioning

confidence: 99%

Morphodynamic Acceleration Techniques for Multi-Timescale Predictions of Complex Sandy Interventions

Luijendijk

Schipper

Ranasinghe

2019

JMSE

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“…It should be noted that the Q / I ratios used in our experiments (about 10, 20, and 30) are aimed to create supply‐limited conditions; that is, the sediment supply rate I is less than the transport capacity associated with the flowrate Q , so that sediment will not deposit before reaching the aggrading delta. These Q / I ratios are smaller than typical values of Q / I (≫100) observed in many supply‐limited rivers (Carraro et al, ; Lanzoni et al, ). The smaller values of Q / I would only accelerate the buildup of delta without affecting the underlying physical processes because, even if the flows are transcritical (see Table for Froude numbers over topset and foreset), deltas are long diffusion waves of bedform so that propagation of hydrodynamic fronts is too fast to be affected by sediment transport (Lanzoni et al, ; Sun et al, ).…”

Section: Methodsmentioning

confidence: 53%

“…It should be noted that the Q/I ratios used in our experiments (about 10, 20, and 30) are aimed to create supply-limited conditions; that is, the sediment supply rate I is less than the transport capacity associated with the flowrate Q, so that sediment will not deposit before reaching the aggrading delta. These Q/I ratios are smaller than typical values of Q/I (≫100) observed in many supply-limited rivers (Carraro et al, 2018;Lanzoni et al, 2006). The smaller values of Q/I would only accelerate the buildup of delta without affecting…”

Section: Water Resources Researchmentioning

confidence: 60%

Self‐Similar Morphodynamics of Gilbert and Hyperpycnal Deltas Over Segmented Two‐Slope Bedrock Channels

Lai

Chiu

2019

Water Resources Research

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Delta progradation over segmented two‐slope bedrocks is prevalent in nature, where the bedrock slope upstream or downstream of the slope‐break knickpoint is steepened by continued tectonic uplift or subsidence. Understanding the morphodynamics of the most common types of delta in different slope settings is important for interpreting the observed delta progradation into reservoir and predicting delta evolutions under future tectonic/climate scenarios or anthropogenic interventions. Here, we present an experimental and analytical study demonstrating the morphological responses of Gilbert‐type and hyperpycnal deltas to variations of bedrock slopes. Steepening the upstream slope accelerates shoreline migration; steepening the downstream slope decelerates shoreline migration. In either case, the subaqueous volume is enhanced yet the subaerial volume is reduced. Both types of delta exhibit self‐similar morphologies when evolving over segmented bedrocks. Hyperpycnal deltas, through enhanced sediment fluxes driven by dense underflows, develop larger subaqueous volumes. We further demonstrate the underlying self‐similarities in sediment flux and bed growth rate that come into play for attaining the self‐similar morphologies. The combined effect of flowrate (Q) and sediment supply rate (I) may be characterized by a dimensionless Q/I ratio. Increasing Q/I advances the entire delta. For Gilbert deltas, shoreline migration accelerates with Q/I. For hyperpycnal deltas, shoreline migration exhibits a “first‐accelerate‐then‐decelerate” trend with Q/I in a limited range of slope combinations close to the single‐slope setting, indicating that the effect of Q/I emerges only when the two‐slope effect is weak or absent. Away from this near‐single‐slope range, the two‐slope effect becomes dominant, thus suppressing the effect of Q/I.

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“…We also tested the simulation by implementing the same values for Δt m and Δt h (which costs much more computational effort); the results are the same as those corresponding to different values for Δt m and Δt h (as applied in this study). A more elegant and advanced technique, involves the use of variable, adaptive time steps (Carraro et al, 2018a(Carraro et al, , 2018b. Our method, however, appears quite adequate for the problem at hand.…”

Section: Computational Conditionsmentioning

confidence: 99%

Grain Size‐Specific Engelund‐Hansen Type Relation for Bed Material Load in Sand‐Bed Rivers, With Application to the Mississippi River

Gong

Naito

et al. 2021

Water Resources Research

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Many sand‐bed rivers worldwide have been experiencing significant reductions in sediment load over the past several decades. This is one of the causes of river delta drowning worldwide. This problem, however, has not been studied in detail in the context of sediment grain sorting. Considering the good performance of the original Engelund‐Hansen relation (OEH) for uniform sediment, and the fact that bulk grain size‐specific relations for bed material load that allow for sorting are relatively rare in the case of sand‐bed rivers, a grain size‐specific Engelund‐Hansen type relation (SEH) is proposed in this study based on data from a large flume. We embed both the OEH and the SEH in a one‐dimensional river morphodynamic model to simulate the morphodynamic evolution of the middle Mississippi River in response to the upstream cutoff of sediment supply. Simulation results using a single characteristic grain size show that bed material load delivered to the delta reduces only gradually in response to the cutoff of sediment supply, in agreement with previous studies. However, implementing the full grain size distribution of sediment leads to a much faster reduction of bed material load delivered to the delta, because the bed surface coarsens in response to grain sorting. This armoring inhibits the bed degradation that would replenish sediment load along the channel. The bed material load of finer sediment declines more rapidly than that of coarser sediment. The results of this study have practical implications for river delta restoration.

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Mathematical study of linear morphodynamic acceleration and derivation of the MASSPEED approach

Cited by 19 publications

References 22 publications

Morphodynamic Acceleration Techniques for Multi-Timescale Predictions of Complex Sandy Interventions

Morphodynamic Acceleration Techniques for Multi-Timescale Predictions of Complex Sandy Interventions

Self‐Similar Morphodynamics of Gilbert and Hyperpycnal Deltas Over Segmented Two‐Slope Bedrock Channels

Grain Size‐Specific Engelund‐Hansen Type Relation for Bed Material Load in Sand‐Bed Rivers, With Application to the Mississippi River

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