A Synthetic Spring-Neap Tidal Cycle for Long-Term Morphodynamic Models

Existing tidal input reduction approaches applied in accelerated morphodynamic simulations aim to capture the dominant tidal forces in a single or double representative tidal cycle, often referred to as a “morphological tide.” These strongly simplified tidal signals fail to represent the tidal extremes and hence poorly allow to represent hydrodynamics in the intertidal areas. Here, a generic method is developed to construct a synthetic spring-neap tidal cycle that (a) represents the original signal; (b) is exactly periodic; and (c) is derived directly from tidal time series or harmonic constituents. The starting point is a fortnightly modulation of the semidiurnal tide to represent spring-neap variations, while conserving periodicity. Diurnal tides and higher harmonics of the semidiurnal tide are included to represent the asymmetry of the tide. The amplitudes and phases of the synthetic signal are then fitted to histograms of water levels and water level gradients derived from the original sea surface elevation time series. A depth-averaged model of the Ems estuary (The Netherlands) demonstrates the effects of alternative tidal input reduction techniques. Adopting the new approach, the along-estuary variation in tidal wave shape is well-represented, leading to an improved representation of extreme tidal conditions. Especially the more realistic representation of intertidal dynamics improves the overall hydrodynamics and residual sand transport patterns, approaching nonschematized tidal dynamics.</p

Land Reclamation Controls on an Estuarine Regime Shift from a Multichannel to Single Channel Configuration

Deltaic intertidal areas disappear worldwide. This impacts delta morphology, because the extent and physiological character of the tidal floodplains control the tidal regime and, as a result, residual sediment transport patterns. Extensive reclamation of former tidal flats, effectively changing the ratio of channel volume to intertidal storage volume (Vs/Vc), drastically changes the functioning of the estuarine system. This might result in morphodynamic feedback loops that reach a tipping point towards an alternative stable regime (Van Maren et al., 2023). Our capacity to predict the consequences of future land reclamation or depoldering methods, a measure frequently suggested to cope with the effects of sea level rise, is limited, because the conceptual framework describing estuarine response to tidal flat reclamation fails to predict such regime transitions.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Environmental Fluid Mechanic

From ripples to large-scale sand transport: the effects of bedform-related roughness on hydrodynamics and sediment transport patterns in Delft3D

Bedform-related roughness affects both water movement and sediment transport, so it is important that it is represented correctly in numerical morphodynamic models. The main objective of the present study is to quantify for the first time the importance of ripple- and megaripple-related roughness for modelled hydrodynamics and sediment transport on the wave- and tide-dominated Ameland ebb-tidal delta in the north of the Netherlands. To do so, a sensitivity analysis was performed, in which several types of bedform-related roughness predictors were evaluated using a Delft3D model. Also, modelled ripple roughness was compared to data of ripple heights observed in a six-week field campaign on the Ameland ebb-tidal delta. The present study improves our understanding of how choices in model set-up influence model results. By comparing the results of the model scenarios, it was found that the ripple and megaripple-related roughness affect the depth-averaged current velocity, mainly over the shallow areas of the delta. The small-scale ripples are also important for the suspended load sediment transport, both indirectly through the affected flow and directly. While the current magnitude changes by 10–20% through changes in bedform roughness, the sediment transport magnitude changes by more than 100%

Tidal amplification and river capture in response to land reclamation in the Ganges-Brahmaputra delta

At a global scale, intertidal areas are being reclaimed for agriculture as well as urban expansion, imposing high human pressure on the coastal zone. The Ganges-Brahmaputra Delta (GBD) is an exponent of this development. In this delta, land reclamation accelerated in the 1960's to 1980's, when polders were constructed in areas subject to regular marine flooding. A comprehensive analysis of tidal channel evolution in the southwest GBD reveals how land reclamation leads to tidal amplification, channel shoaling, bank erosion, and interaction between channels in which one tidal river captures the storage area of a neighbouring river. We identify-two positive feedback mechanisms that govern these morphological changes. First, reclaiming intertidal areas results in immediate loss of tidal storage, which leads to amplification and faster propagation of the tides. In systems with abundant sediment supply, the blind tidal channels progressively fill in with sediment, leading to a continued loss of tidal storage and therefore further distorting the tides. Secondly, when intertidal areas of parallel (and inter-connected) river delta distributaries are asynchronously or unevenly reclaimed, one channel distributary may expand its intertidal area at the expense of the other. This is initiated by an increasing propagation speed of the tidal wave in the partially reclaimed distributary, travelling into the non-reclaimed distributary through connecting channels. These connecting channels progressively expand while the pristine channel shoals, and potentially degenerates. Both positive feedback loops are very stable and are responsible for pluvial flooding of polders, large-scale bank erosion, and poorly navigable primary waterways, including the navigation channel accessing Bangladesh's second-largest port. Interventions aiming to solve these problems have to account for the complex positive feedback mechanisms identified in this paper and be nature-based and holistic.Environmental Fluid MechanicsCoastal Engineerin

Modelling of annual sand transports at the Dutch lower shoreface

Dutch coastal policy aims for a safe, economically strong and attractive coast. This is achieved by maintaining the part of the coast that support these functions; the coastal foundation. The coastal foundation is maintained by means of sand nourishments. Up to now, it has been assumed that net transports across the coastal foundation’s offshore boundary at the 20 m depth contour are negligibly small. In the framework of the Coastal Genesis 2.0 program we investigated sand transports across this boundary and across other depth contours at the lower shoreface. This paper presents a computationally efficient approach to compute the annual sand transport rates at the Dutch lower shoreface. It is based on the 3D Dutch Continental Shelf Model with Flexible Mesh (3D DCSM-FM), a wave transformation tool and a 1DV sand transport module. We validate the hydrodynamic input against field measurements and present flow, wave and sand transport computations for the years 2013–2017. Our computations show that the net annual sand transport rates along the Dutch coast are determined by peak tidal velocities (and asymmetry thereof), density driven residual flows, wind driven residual flows and waves. The annual mean alongshore transports vary along the continuous 20 m depth contour. The computed total cross-shore transports are onshore directed over the continuous 20 m, 18 m and 16 m depth contours and increase with decreasing water depth. The effect of density difference and wind on the 3D structure of the flow and on the sand transports cannot be neglected along the Dutch lower shoreface. Our computations show that excluding the effect of density results in a significant decrease of the onshore directed transports. Also switching off wind largely counteracts this effect. The net cross-shore transport is determined by a delicate balance between gross onshore and offshore transports, where wave conditions are important. We show an example for Scheveningen where the net cross-shore transport is onshore directed when including all wave conditions but would be offshore directed when excluding waves higher than 3.5 m. In contrast, at Callantsoog the highest waves contribute more to the offshore directed transports. These results suggest that storm conditions play an important role in the magnitude and direction of the net annual transport rates at the lower shoreface

From ripples to large-scale sand transport: the effects of bedform-related roughness on hydrodynamics and sediment transport patterns in Delft3D

Bedform-related roughness affects both water movement and sediment transport, so it is important that it is represented correctly in numerical morphodynamic models. The main objective of the present study is to quantify for the first time the importance of ripple- and megaripple-related roughness for modelled hydrodynamics and sediment transport on the wave- and tide-dominated Ameland ebb-tidal delta in the north of the Netherlands. To do so, a sensitivity analysis was performed, in which several types of bedform-related roughness predictors were evaluated using a Delft3D model. Also, modelled ripple roughness was compared to data of ripple heights observed in a six-week field campaign on the Ameland ebb-tidal delta. The present study improves our understanding of how choices in model set-up influence model results. By comparing the results of the model scenarios, it was found that the ripple and megaripple-related roughness affect the depth-averaged current velocity, mainly over the shallow areas of the delta. The small-scale ripples are also important for the suspended load sediment transport, both indirectly through the affected flow and directly. While the current magnitude changes by 10–20% through changes in bedform roughness, the sediment transport magnitude changes by more than 100%

An integral approach to design the Roggenplaat intertidal shoal nourishment

The Eastern Scheldt, a tidal basin in the southwest of The Netherlands, underwent large physical and ecological changes due to a system-wide human interference. The construction of a storm surge barrier at the seaward side and closure of the upstream branches in the 1980s resulted in intertidal flat erosion. This has far reaching consequences for the ecological functioning of these habitats, especially as foraging ground for many wader species. Therefore, a 1.3 million m 3 sand nourishment is foreseen on the Roggenplaat intertidal shoal to mitigate the erosion and preserve suitable foraging habitat for waders for the coming 25 years. This paper presents an integral nourishment design approach. It consists of the following steps: (i) understanding the morphology and ecology, (ii) translation of the nourishment objective into an evaluation framework, (iii) construction of a suitability map indicating potential nourishment locations, (iv) generation of nourishment designs, (v) short-term morphodynamic numerical model simulations, (vi) estimation of the long-term shoal development using a simplified approach, (vii) integral evaluation leading to the preferred design. This integral approach resulted in a design that is expected to fulfill the Roggenplaat nourishment objective, accounting for ecological, morphological, economical and technical aspects. This integrated approach could form a basis for future intertidal shoal nourishment designs worldwide

Modelling of annual sand transports at the Dutch lower shoreface

Dutch coastal policy aims for a safe, economically strong and attractive coast. This is achieved by maintaining the part of the coast that support these functions; the coastal foundation. The coastal foundation is maintained by means of sand nourishments. Up to now, it has been assumed that net transports across the coastal foundation's offshore boundary at the 20 m depth contour are negligibly small. In the framework of the Coastal Genesis 2.0 program we investigated sand transports across this boundary and across other depth contours at the lower shoreface. This paper presents a computationally efficient approach to compute the annual sand transport rates at the Dutch lower shoreface. It is based on the 3D Dutch Continental Shelf Model with Flexible Mesh (3D DCSM-FM), a wave transformation tool and a 1DV sand transport module. We validate the hydrodynamic input against field measurements and present flow, wave and sand transport computations for the years 2013–2017. Our computations show that the net annual sand transport rates along the Dutch coast are determined by peak tidal velocities (and asymmetry thereof), density driven residual flows, wind driven residual flows and waves. The annual mean alongshore transports vary along the continuous 20 m depth contour. The computed total cross-shore transports are onshore directed over the continuous 20 m, 18 m and 16 m depth contours and increase with decreasing water depth. The effect of density difference and wind on the 3D structure of the flow and on the sand transports cannot be neglected along the Dutch lower shoreface. Our computations show that excluding the effect of density results in a significant decrease of the onshore directed transports. Also switching off wind largely counteracts this effect. The net cross-shore transport is determined by a delicate balance between gross onshore and offshore transports, where wave conditions are important. We show an example for Scheveningen where the net cross-shore transport is onshore directed when including all wave conditions but would be offshore directed when excluding waves higher than 3.5 m. In contrast, at Callantsoog the highest waves contribute more to the offshore directed transports. These results suggest that storm conditions play an important role in the magnitude and direction of the net annual transport rates at the lower shoreface.Coastal Engineerin

An integral approach to design the Roggenplaat intertidal shoal nourishment

The Eastern Scheldt, a tidal basin in the southwest of The Netherlands, underwent large physical and ecological changes due to a system-wide human interference. The construction of a storm surge barrier at the seaward side and closure of the upstream branches in the 1980s resulted in intertidal flat erosion. This has far reaching consequences for the ecological functioning of these habitats, especially as foraging ground for many wader species. Therefore, a 1.3 million m 3 sand nourishment is foreseen on the Roggenplaat intertidal shoal to mitigate the erosion and preserve suitable foraging habitat for waders for the coming 25 years. This paper presents an integral nourishment design approach. It consists of the following steps: (i) understanding the morphology and ecology, (ii) translation of the nourishment objective into an evaluation framework, (iii) construction of a suitability map indicating potential nourishment locations, (iv) generation of nourishment designs, (v) short-term morphodynamic numerical model simulations, (vi) estimation of the long-term shoal development using a simplified approach, (vii) integral evaluation leading to the preferred design. This integral approach resulted in a design that is expected to fulfill the Roggenplaat nourishment objective, accounting for ecological, morphological, economical and technical aspects. This integrated approach could form a basis for future intertidal shoal nourishment designs worldwide

Data files for Numerical Delft3D model of the Roggenplaat

Necessariy files to run the Delft3D Roggenplaat model