Long-term Morphodynamic Evolution Research Articles

Reduced wave time series for long-term morphodynamic applications

Shoreline models have usually been recognized by professionals as the most appropriate tool for reproducing the long-term morphodynamic evolution of the shoreline of sandy beaches. Despite their underlying simplifications, the simulation of shoreline evolution at large temporal and spatial scales may imply significant computational efforts. Hence, to reduce computational costs, many approaches aimed to optimize the size of the input wave datasets have been proposed so far. A simplified novel method to reduce long-term offshore wave series is proposed herein. The rationale of the approach is to build reduced series that induce the same morphodynamic effects in the long-term as the ones induced by the whole, and more computationally expensive, original series. The method is conceived to define offshore reduced time series with the same chronological order of the complete series and is able to represent the bi-modal features of the wave climate. In-depth hydrodynamic and morphodynamic parametric analyses have been performed and it has been demonstrated that the method is capable to get reliable reduced offshore wave time series for reproducing the long-term evolution of sandy beaches with decreased computational costs.

Modelling shear stress distribution in ice-covered streams

Abstract Distribution of bed shear stress is the critical factor in regulating the meandering of single-thread rivers. However, the impact of ice cover on bed shear stress is largely unknown. In this study, we develop a theoretical model of cross-stream momentum balance to examine the distribution of bed shear stresses in ice-covered meandering rivers. To validate the theoretical model, field surveys were carried out in a river reach of the Red River in Fargo, North Dakota. Data monitoring was completed using an Acoustic Doppler Current Profiler to obtain time-averaged velocity profiles. Our theoretical model indicates that an ice covering develops high-shear zones near both the inner and outer banks, which might exacerbate sediment transport and enhance bank erosion. Velocity measurements confirm the results of the proposed model and demonstrate a clear impact of meandering river banks on velocity profiles and secondary flow patterns under ice cover. Based on our results, we hypothesize that ice cover increases turbulent stresses near banks, which in turn lead to the enhancement of the bed shear stress. Our work provides new insights into the impact of ice cover on bed shear stress distribution, which could play an important role in driving sediment-transport processes and the long-term morphodynamic evolution of meandering rivers seasonally covered by ice.

Long-term morphodynamic evolution in the Modaomen Estuary of the Pearl River Delta, South China

Anthropogenic activities and natural disturbances will cause large-scale changes in the morphology of estuarine systems. Studies of estuarine morphology tended to focus on a single driving factor, however, the intertwined effects (e.g., land reclamation, sand excavation, tides, waves, runoff) are rarely mentioned in the literature. This work characterizes and conceptualizes the morphodynamics of the Modaomen Estuary (ME), the largest estuary in the Pearl River Delta (PRD), China, based on Landsat satellite imagery, high-resolution bathymetry survey data, and in situ hydrographic measurements. The morphodynamic evolution of the ME since the 1970s–2010s is analyzed with respect to planform configuration and bathymetry. Quantification of estuarine hydrodynamic and sediment parameters for the most recent years indicates that the ME has transformed from a traditional fluvial- and tide-dominated estuary into a fluvial- and wave-dominated estuary. Ocean currents intrude along the eastern channel and draw down along the western channel, in which estuarine flow deflection is induced by the combined effect of upstream runoff, coastal waves, and littoral currents. Furthermore, a life-cycle model of the morphological migration of the ME is presented by synthesizing hydrodynamics and sediment dynamics. This conceptual model of three geomorphological states clearly shows that the main channel experiences a life cycle covering several decades, from the origin of the main channel to its bifurcation into two distributaries, and finally a recovery and westward deflection of the main channel. The cognition of the appearance and disappearance with one sandbar and two channels will be helpful in numerical modeling of estuary evolution, and it also has important implications for the regulation of estuarine regions, hazard prevention and channel navigability.

Long-term morphodynamic evolution of the Pearl River Delta from the perspective of energy flux and dissipation changes

The Pearl River Delta (PRD) is an extremely complex, large-scale estuary system in China. In this study, a long-term process-based morphodynamic model was developed to investigate the characteristics of variations in energy flux and dissipation and their evolution alongside the hydrodynamic and topographic evolution of the PRD since the Holocene transgression maximum. The results show that the evolution of the dynamics and topography of the PRD are largely influenced by the geological and geomorphological structures of the region. Energy flux and dissipation in the inner estuary exhibited a patchy distribution, with areas of high energy flux appearing in deep troughs, near headlands, and in gorges containing bedrock islands, and areas of relatively small energy flux appearing in the flow shadow zones of the islands, tidal fluctuation confluence area, and shoals. Corresponding to the energy flux distribution, sediment accumulation also exhibited a high degree of spatial and temporal variability. The evolution of the coastline of the PRD is not driven from north to south, or from the river outlet to the downstream area. Rather, multiple deposition bodies simultaneously developed in the north and south without overlapping. With the deposition of sediment in the estuary, the isoamplitude lines of tidal waves were gradually extrapolated and the spatial distribution gradually shifted from the northwest to the southwest. This indicates the mutual transformation between the estuarine river and tidal power, and also reflects the development characteristics of the river system.

Investigation on Factors of Influence on Long-term Morphodynamic Evolution of a Multi-outlets Estuarine-delta System: A Case Study of the Lingding Bay, Pearl River Delta

Su, M.; Gong, Z.; Yao, P.; Pu, J., and Lu, Y., 2020. Investigation on factors of influence on long-term morphodynamic evolution of a multi-outlets estuarine-deltaic system: A case study of the Lingding Bay, Pearl River Delta. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 664–668. Coconut Creek (Florida), ISSN 0749-0208.Deltas that formed by single river outlets have been widely studied, whereas there are also many deltas that formed by multiple river outlets, deserving more attentions. Compared with the delta which is developed by single river outlet with one or more branching channels, the influence factors and the interaction between outlets are more complicated. In this study, we aim to investigate the long-term morphodynamic evolution mechanism of a compound delta, formed by multi-river outlets simultaneously. This study takes the Lingding Bay of the Pearl River Delta as an example to explore evolution features and factors of influence of such compound deltas. Based on a schematized morphodynamic model, the simulated compound deltaic system shows two main features: dense channel network and asymmetry distribution between the eastern and western parts. The relative importance of several influencing factors has been assessed through a series of scenarios. Results suggest that tidal range is the most important factor on the evolution of compound deltaic system, while the influence of water and sediment discharge ratio of river outlets are ignorable. Pattern of the compound delta exhibits unique features compared to deltas formed by single river outlets. Furthermore, compound deltas developed by multi-river outlets is not the direct overlay of each sub-deltas. The interaction between sub-deltas is important for the development of compound delta and should not be ignored.

Morphologic modelling of tidal inlet on a barrier-lagoon coast: Case study of the Laolonggou tidal inlet in the Bohai Bay

Understanding the long-term morphodynamic evolution of tidal inlet systems and their control mechanisms has great significance in the management and sustainable development of coastal regions. This study addresses this issue by applying a process-based morphological model (Delft3D) to hindcast morphodynamic evolution of the Laolonggou tidal inlet (LTI) for a duration of 68 years and then analyze the mechanism of morphologic evolution and factors affecting it in tidal inlets. The morphodynamic model is validated against two periods, a natural stage (1938–1983), and a recent erosion stage with reservoir construction (1983–2006). Results show that the evolution pattern, sediment volume changes, and hypsometry curves simulated by the model agree well with field data. A series of scenarios were designed to analyze the mechanism of morphologic evolution and impacting factors. The model results reveal that the asymmetry of the tidal currents in the channels leads to tide residual sediment transport, which causes morphodynamic evolution. The inconsistency of tidal flow path, i.e., ebb dominance occurs in the west channel and flood dominance occurs in the east channel, is an important mechanism for the evolution of the whole system. In addition, the decrease of sediment supply caused by reservoirs in Luanhe River, and extension of the barrier island contribute to the evolution of LTI. Although this study is site-specific, the results are expected to provide valuable information for sustainable management of tidal inlet systems.

Hydrodynamic modelling of tidal-fluvial flows in a large river estuary

The transition between riverine and estuarine environments is characterised by a change from unidirectional to bidirectional flows, in a region referred to herein as the Tidally-Influenced Fluvial Zone (TIFZ). In order to improve our understanding of the hydrodynamics and morphodynamics of this zone, we present a combined field and numerical modelling study of the Columbia River Estuary (CRE), USA, tidally-influenced fluvial zone. The CRE is large measuring 40 km in length and between 5 and 10 km wide. A shallow water model (Delft3D) was applied in both 2D and 3D configurations and model sensitivity to the key process parameterizations was investigated. Our results indicate that a 2D model constrained within the estuary can sufficiently reproduce depth-averaged flow within the TIFZ of a stratified estuary.Model results highlight the interactions between tidal-, fluvial- and topographic-forcing that result in depth dependent tidal rectification, and thus zones of residual sediment transport that: i) may be flood-directed along shallow channel margins and in the lee of bars, and simultaneously ii) is ebb-directed within deeper channel thalwegs. This condition is enhanced at lower discharges, but increased fluvial discharge reduces the number and size of regions with net flood-directed sediment transport and flow. These sediment transport patterns provide a mechanism to extend the bar/island topography downstream, and generate flood-directed, ebb-directed, and symmetrical bedforms, all within the same channel. Analysis of the model data reveals flood-directed sediment transport is due to both tidal variability and mean flow. These results highlight the need to include the mean flow component (M0) when considering the long-term morphodynamic evolution in a TIFZ.

A numerical study of equilibrium states in tidal network morphodynamics

The long-term morphodynamic evolution of tidal networks on tidal flats is investigated using a two-dimensional numerical model. We explore the physical processes related to the development of the morphology and the presence of equilibrium configurations. Tidal networks are simulated over a rectangular domain representing a tidal platform, a different setting compared to estuaries (subject to riverine influence) and lagoons (offshore bars constricting the flow). In the early and middle phases of the tidal network evolution, large sediment patches with rhombus-like shape form and gradually migrate in the flood direction, even though the overall sediment flux is ebb-directed. A cross-section-averaged “equilibrium” state is asymptotically approached after about 500 years. The area and peak discharge of the lower flat cross-sections at year 500 approximately show a 1:1 relationship, which is in agreement with field observations. We also show that model results are consistent with the Q-A relationship (peak discharge Q versus cross-sectional area A), which is obtained under the assumption of a constant Chezy friction.

Two-phase flow modelling of sediment suspension in the Ems/Dollard estuary

Understanding and quantifying mud suspension and sediment transport processes are of great importance for effective exploitation and sustainable management of estuarine environments. Event-based predictive models are widely used to identify the key interactions and mechanisms that govern the dynamics involved and to provide the essential parameterisation for assessing the long-term morphodynamic evolution of the estuaries. This study develops a one-dimensional-vertical (1DV) Reynolds averaged two-phase model for cohesive sediments resuspension driven by tidal flows. To capture the time-dependent flocculation process more accurately, a new drag force closure which relates empirically to settling velocity of mud flocs with suspended sediment concentration (SSC) is incorporated into the two-phase model. The model is then applied to simulate mud suspension in the Ems/Dollard estuary during two periods (June and August 1996) of tidal forcing. Numerical predictions of bed shear stresses and sediment concentrations at different elevations above the bed are compared with measured variations. The results confirm the importance of including flocculation effects in calculating the settling velocity of mud flocs and demonstrates the sensitivity of prediction with the settling velocity in terms of flocs concentration. Although the two-phase modelling approach can in principle better capture the essential interactions between fluid and sediment phases, its practical advantages over the simpler single phase approach cannot be confirmed for the data periods simulated, partly because the overall suspended sediment concentration measured is rather low and the interaction between the two phases is weak and also because the uncertainties in the relationship between the settling velocity and flocs concentration.

One-dimensional modeling of a recent Ganga avulsion: Assessing the potential effect of tectonic subsidence on a large river

River avulsion as studied in small-sized and medium-sized rivers is partly explained by the water surface gradient advantage of a new channel course over the old course, caused by spatial differences in aggradation and compaction. Recently, the effect of meandering upstream of the avulsion node, or bifurcation, was shown to have an equally large effect on avulsion duration as gradient advantage. These effects remain poorly understood for the largest rivers on Earth, where gradients are very small, subtle gradient advantages are affected by tectonics, and often several anabranches remain active simultaneously. Our objective was to assess the relative importance of these factors in the River Ganga in determining the pacing of an avulsion. We used a combination of historical data, remote sensing, and one-dimensional modeling. The course of the Ganga in historical times was through the present Ganga–Bhagirathi system but then there was either a gradual or sudden shift to the present Ganga–Padma system. Historical evidence and remnant paleochannels, as observed in satellite sensor data, corroborate the changing pattern of the Ganga River system, but the exact causes of the shifting and of the short avulsion duration remain unclear. Based on generalized data, using a one-dimensional model we ran idealized scenarios bracketing different tectonic subsidence estimates for long-term morphodynamic evolution of the upstream channel and the two downstream bifurcates. The model predicts flow and sediment partitioning at the bifurcation node, and includes the effect of migrating meanders at the bifurcation and width adjustment of the bifurcates. Our modeling demonstrates that the old and the new branches can remain ‘open’ and morphologically active for a long time because of the large backwater effect and the high mobility of the sediment. The bifurcation stabilizes at an asymmetrical flow and sediment division, which in smaller rivers (such as the River Rhine) would be followed by residual channel filling but in the much larger Ganges results in morphologically active anabranches. The model results reveal that neither a gradient advantage nor a bend upstream of the bifurcation leads to an avulsion within centuries as has been observed in some large rivers in tectonically inactive regions. On the other hand, a realistic tectonic uplift of the old branch or subsidence of the new branch may force an avulsion to take place quickly, and historical data show that the study area is seismically active. The combination of these factors leads to a realistic modeled avulsion duration of less than three centuries. Historical data indicate that these general conclusions might also apply to other large rivers in this region, e.g. the Brahmaputra and the Teesta. We conclude that large rivers may avulse quickly in response to tectonics but attain an anabranching pattern because of the large dimension of the residual channel and backwater effects.

One-dimensional numerical modeling of the long-term morphodynamic evolution of a tidally-dominated estuary: The Lower Fly River (Papua New Guinea)

We use a one-dimensional morphodynamic model to analyze the long-term evolution of the lower reaches of the Fly River, Papua New Guinea, from the Everill Junction to the delta mouth. The model shows how the break in the exponential trend of river width triggers deposition, thus producing a tidal region characterized by a higher bed elevation with respect to the river-dominated one. Numerical simulations indicate that the river attains a dynamic equilibrium configuration in which the amount of sediment entering upstream is flushed seaward. A sensitivity analysis is performed, in which the effect of varying solid discharge, tidal harmonics, and initial conditions is discussed. The model shows that an equilibrium configuration results from a delicate balance between the aggrading effect associated with channel divergence (acting mainly during neap tide and at slack water) and the opposite effect of tidal flushing driven by residual water discharge. A physically meaningful morphodynamic equilibrium occurs only for a small range of values of sediment discharge prescribed at the upstream boundary. In particular, an increase in sediment discharge leads to aggradation, while a decrease triggers extensive scour and a deepening of the estuary.

A thirteen-year record of bathymetric changes in the North Passage, Changjiang (Yangtze) estuary

The mouths of major rivers in the world have always been important waterways and as a result, subject to significant human intervention. Therefore, it is necessary to understand the coupling of natural processes and human intervention in the sediment movement and deposition to determine long-term morphodynamic evolution in the mouth regions of major rivers. A multivariate technique was used to analyze high-resolution bathymetric data from the North Passage of Changjiang (Yangtze River), which is the vital shipping channel in the mouth region and for the entire Changjiang waterway. Our findings show that there are two modes of bathymetric changes. The first mode represents 85% of the variability, which includes the deposition in the peripheral groin fields along the shipping channel and deepening of the shipping channel, which is primarily due to the channel maintenance. The second mode represents 6% of the variability of the river-mouth shoal (seaward migration and size reduction), attributable to the declining sediment discharge of the Changjiang due to the Three Gorges Dam, and the enhancement of the ebb flow as the result of dredging.

Long-term morphodynamic evolution and the equilibrium mechanism of a flood channel in the Yangtze Estuary (China)

The stability of flood channels has attracted considerable attention because of their complicated interactions with the prevailing hydrodynamics and importance in ship navigation. This research examines long-term morphodynamic evolution in the Yangtze Estuary from 1861 to 2002 and the equilibrium mechanism of the Xinqiao Channel in the Yangtze Estuary by digitizing 15 selected maritime charts and calculating the volume of the channel. Although the total period of channel development is much longer than the historical data used in this paper, three stages are identified during the study period: the first embryonic stage (66 years), the second formation stage (33 years) and the third equilibrium stage (45 years). Variations in coastline location, channel volume, and hydrodynamics in the channel during the three stages indicate that the channel equilibrium was reached and maintained when the channel direction was aligned with the direction of offshore tidal wave propagation. Variations in river and sediment discharges affect erosion and deposition in the channel and thus channel geometry. However, future reduction in sediment supply by 10–33% due to the ongoing river engineering projects would increase the volume of the Xinqiao Channel only by 1–3%. It seems unlikely that the above change in sediment discharge will disrupt the equilibrium of the Xinqiao Channel.

Long-term morphodynamic evolution of Texel Inlet and its ebb-tidal delta (The Netherlands)

A series of regular bathymetric surveys of Texel Inlet and its ebb-tidal delta spanning a period of over 400 years forms a unique long-term morphodynamic dataset of this largest inlet of the Wadden Sea. The substantial changes in ebb-tidal delta evolution provide a clear example of the response of the inlet system to the cumulative effects of human intervention. Analysis of the evolution of the ebb-tidal delta morphology shows different stages, each characterized by specific orientations of the main channels and shoals. Prior to construction of extensive coastal defense works on the southern shore of the inlet in 1750 A.D., the ebb-tidal delta showed a downdrift asymmetry. Periodic shoal breaching and downdrift channel relocation were the dominant mechanisms for sediment by-passing (major shoal bypassing). After construction of the coastal defence works, a stable ebb-tidal delta with a westward stretching main ebb-channel developed over a period of ca. 60 years. Damming of the Zuiderzee, the major part of the back-barrier basin in 1932 A.D., distorted this stable state and over a period of about 40 years the main channel switched to a southward course, remaining stable ever since. During the pre- and post-damming stable states the sediment bypassing took place as minor shoal by-passing; the main channel remained in position and smaller parts of the swash platform (periodically) migrated landward over the ebb-tidal delta. The well-monitored large-scale changes on the ebb-tidal delta which were initiated by the construction of the coastal defence works and closure of the Zuiderzee show that incorporation of inlet modifications and back-barrier processes is vital for a correct description of the ebb-tidal delta dynamics and processes of Texel Inlet. A conceptual model is introduced to describe the process–response relation between intervention and ebb-tidal delta change.