Editorial

3 pages, 165 KiB

Open AccessEditorial

Resilient Flood Defenses

by Suzanne J.M.H. Hulscher, Jord J. Warmink and Bas W. Borsje

J. Mar. Sci. Eng. 2021, 9(4), 371; https://doi.org/10.3390/jmse9040371 - 31 Mar 2021

Cited by 1 | Viewed by 1673

Flood risk in deltaic regions is increasing due to a combination of more economic activities and an increase in flooding probability [...] Full article

(This article belongs to the Special Issue Resilient Flood Defences)

Research

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18 pages, 8154 KiB

Open AccessArticle

Observations and Preliminary Vulnerability Assessment of a Hybrid Dune-Based Living Shoreline

by Maria A. Winters, Brian Leslie, Evyan Borgnis Sloane and Timu W. Gallien

J. Mar. Sci. Eng. 2020, 8(11), 920; https://doi.org/10.3390/jmse8110920 - 15 Nov 2020

Cited by 6 | Viewed by 3983

Abstract

A novel hybrid (e.g., vegetation, sand, cobble, rip-rap) nature-based dune structure was constructed at Cardiff State Beach in Encinitas, California, to protect a critical transportation artery from undermining and frequent flooding. A collaboration between regulators, funders, state agencies, professional practice and academia developed [...] Read more.

A novel hybrid (e.g., vegetation, sand, cobble, rip-rap) nature-based dune structure was constructed at Cardiff State Beach in Encinitas, California, to protect a critical transportation artery from undermining and frequent flooding. A collaboration between regulators, funders, state agencies, professional practice and academia developed a high resolution robust unmanned aerial vehicle (UAV) based monitoring strategy to observe dune construction and evolution. Fifteen construction surveys were conducted to observe each substrate element for future morphodynamic modeling efforts. Six post-construction surveys were conducted to observe seasonal and storm-by-storm dune evolution. Backshore vulnerability was assessed using a sixty-one year time series of tides and hindcast wave forcing fit to a general extreme value distribution. The dune crest is above calculated 100-year water levels; however, the dune remains vulnerable to mass wasting caused by swash interaction at the toe of the dune. Sea-level rise will substantially increase the probability of dune erosion, breaching, and overtopping. Full article

(This article belongs to the Special Issue Resilient Flood Defences)

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24 pages, 39732 KiB

Open AccessFeature PaperArticle

On the Relation between Beach-Dune Dynamics and Shoal Attachment Processes: A Case Study in Terschelling (NL)

by Filipe Galiforni-Silva, Kathelijne M. Wijnberg and Suzanne J. M. H. Hulscher

J. Mar. Sci. Eng. 2020, 8(7), 541; https://doi.org/10.3390/jmse8070541 - 20 Jul 2020

Cited by 7 | Viewed by 2452

Abstract

Inlet-driven processes are capable of modifying the adjacent shoreline. However, few studies have attempted to understand how these changes affect coastal dunes. The present study aims to understand how shoreline changes induced by shoal attachment affect coastal dunes. A barrier island in the [...] Read more.

Inlet-driven processes are capable of modifying the adjacent shoreline. However, few studies have attempted to understand how these changes affect coastal dunes. The present study aims to understand how shoreline changes induced by shoal attachment affect coastal dunes. A barrier island in the Netherlands is used as a case study. Both bathymetric and topographic annual data were analysed, together with the application of a cellular automata model for dune development. The objective of the model is to explore idealised scenarios of inlet-driven shoreline movements. With the model, ten different scenarios were examined regarding beach width increase and rate of alongshore spreading of the shoal. Field data showed that, for the case study, dune volume and shoal attachments could not be directly linked. Instead, rates of dune volume change differed significantly only due to long-term ebb-tidal delta evolution. Such morphological evolution oriented the beach towards the main wind direction, increasing overall aeolian transport potential. Modelling results showed that shoals significantly increased dune volumes only on three out of ten scenarios. This suggests that beach width increase, and rate of alongshore sediment spreading, determine whether the shoal will influence dune growth. Therefore, within the studied time-scale, local rates of dune growth are only increased if shoals are capable of increasing the beach width significantly and persistently. Full article

(This article belongs to the Special Issue Resilient Flood Defences)

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32 pages, 1564 KiB

Open AccessArticle

Modelling the Wave Overtopping Flow over the Crest and the Landward Slope of Grass-Covered Flood Defences

by Vera M. van Bergeijk, Jord J. Warmink and Suzanne J. M. H. Hulscher

J. Mar. Sci. Eng. 2020, 8(7), 489; https://doi.org/10.3390/jmse8070489 - 02 Jul 2020

Cited by 13 | Viewed by 3124

Abstract

The wave overtopping flow can exert high hydraulic loads on the grass cover of dikes leading to failure of the cover layer on the crest and the landward slope. Hydraulic variables such as the near bed velocity, pressure, shear stress and normal stress [...] Read more.

The wave overtopping flow can exert high hydraulic loads on the grass cover of dikes leading to failure of the cover layer on the crest and the landward slope. Hydraulic variables such as the near bed velocity, pressure, shear stress and normal stress are important to describe the forces that may lead to cover erosion. This paper presents a numerical model in the open source software OpenFOAM^® to simulate the overtopping flow on the grass-covered crest and slope of individual overtopping waves for a range of landward slope angles. The model provides insights on how the hydraulic forces change along the profile and how irregularities in the profile affect these forces. The effect of irregularities in the grass cover on the overtopping flow are captured in the Nikuradse roughness height calibrated in this study. The model was validated with two datasets of overtopping tests on existing grass-covered dikes in the Netherlands. The model results show good agreement with measurements of the flow velocity in the top layer of the wave, as well as the near bed velocity. The model application shows that the pressure, shear stress and normal stress are maximal at the wave front. High pressures occur at geometrical transitions such as the start and end of the dike crest and at the inner toe. The shear stress is maximal on the lower slope, and the normal stress is maximal halfway of the slope, making these locations vulnerable to cover failure due to high loads. The exact location of the maximum forces depends on the overtopping volume. Furthermore, the model shows that the maximum pressure and maximum normal stress are largely affected by the steepness of the landward slope, but the slope steepness only has a small effect on the maximum flow velocity and maximum shear stress compared to the overtopping volume. This new numerical model is a useful tool to determine the hydraulic forces along the profile to find vulnerable points for cover failure and improve the design of grass-covered flood defences. Full article

(This article belongs to the Special Issue Resilient Flood Defences)

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22 pages, 16885 KiB

Open AccessFeature PaperArticle

Experimental Study on the Influence of Berms and Roughness on Wave Overtopping at Rock-Armoured Dikes

by Weiqiu Chen, Alberto Marconi, Marcel R. A. van Gent, Jord J. Warmink and Suzanne J. M. H. Hulscher

J. Mar. Sci. Eng. 2020, 8(6), 446; https://doi.org/10.3390/jmse8060446 - 18 Jun 2020

Cited by 11 | Viewed by 3363

Abstract

The average overtopping discharge is an important parameter for the design and reinforcement of dikes. Rock armour on the waterside slopes and berms of dikes is widely used to reduce the wave overtopping discharge by introducing slope roughness and dissipation of energy in [...] Read more.

The average overtopping discharge is an important parameter for the design and reinforcement of dikes. Rock armour on the waterside slopes and berms of dikes is widely used to reduce the wave overtopping discharge by introducing slope roughness and dissipation of energy in the permeable armour layer. However, methods for estimating the influence of a rock berm and roughness of rock armour at dikes on the average overtopping discharge still need to be developed and/or validated. Therefore, this study aims to develop empirical equations to quantify the reductive influence of rock armour on wave overtopping at dikes. Empirical equations for estimating the effects of rock berms and roughness are derived based on the analysis of experimental data from new physical model tests. The influence of roughness of the rock armour applied on parts of waterside slopes is estimated by introducing the location weighting coefficients. Results show that the newly derived equations to predict the average overtopping discharge at dikes lead to a significantly better performance within the tested ranges compared to existing empirical equations. Full article

(This article belongs to the Special Issue Resilient Flood Defences)

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30 pages, 19079 KiB

Open AccessFeature PaperArticle

Computational Modelling of the Impacts of Saltmarsh Management Interventions on Hydrodynamics of a Small Macro-Tidal Estuary

by William G. Bennett, Thomas J. van Veelen, Tom P. Fairchild, John N. Griffin and Harshinie Karunarathna

J. Mar. Sci. Eng. 2020, 8(5), 373; https://doi.org/10.3390/jmse8050373 - 23 May 2020

Cited by 12 | Viewed by 4025

Abstract

Saltmarshes are considered as natural coastal defences. However, owing to the large context dependency, there is much discussion over their effectiveness in providing coastal protection and the necessity of additional coastal defence interventions. The macro-tidal Taf Estuary in south-west Wales was chosen as [...] Read more.

Saltmarshes are considered as natural coastal defences. However, owing to the large context dependency, there is much discussion over their effectiveness in providing coastal protection and the necessity of additional coastal defence interventions. The macro-tidal Taf Estuary in south-west Wales was chosen as the case study in this paper to investigate the effects of anthropogenic coastal defence interventions such as construction of hard defences, managed realignment, and altering land use of the saltmarshes on the complex hydrodynamics of the estuary. A coupled flow–wave–vegetation model, developed using the Delft3D coastal modelling software, was used. The wave and current attenuation role of saltmarshes during two contrasting storm conditions was modelled, with and without saltmarsh management interventions. The study reveals that certain saltmarsh management interventions can have widespread impacts on the hydrodynamics of the estuary. Altering the land use by allowing extensive grazing of saltmarsh by livestock was found to have the largest impact on wave attenuation, where wave heights on the marsh almost doubled when compared with the no-intervention scenario. On the other hand, managed realignment has a significant impact on tidal currents, where tidal currents reached 0.5 m/s at certain locations. Changes in estuarine hydrodynamics can lead to undesired impacts on flooding and erosion, which stresses the importance of understanding the effects of localized anthropogenic coastal management interventions on the entire estuarine system. Full article

(This article belongs to the Special Issue Resilient Flood Defences)

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22 pages, 14546 KiB

Open AccessFeature PaperArticle

Artificial Structures Steer Morphological Development of Salt Marshes: A Model Study

by Rutger W. A. Siemes, Bas W. Borsje, Roy J. Daggenvoorde and Suzanne J. M. H. Hulscher

J. Mar. Sci. Eng. 2020, 8(5), 326; https://doi.org/10.3390/jmse8050326 - 05 May 2020

Cited by 15 | Viewed by 3298

Abstract

Salt marshes are increasingly recognized as resilient and sustainable supplements to traditional engineering structures for protecting coasts against flooding. Nevertheless, many salt marshes face severe erosion. There is a consensus that providing structures that create sheltered conditions from high energetic conditions can improve [...] Read more.

Salt marshes are increasingly recognized as resilient and sustainable supplements to traditional engineering structures for protecting coasts against flooding. Nevertheless, many salt marshes face severe erosion. There is a consensus that providing structures that create sheltered conditions from high energetic conditions can improve the potential for salt marsh growth. However, little proof is provided on the explicit influence of structures to promote salt marsh growth. This paper investigates how artificial structures can be used to steer the morphological development of salt marshes. A morphological model (Delft3D Flexible Mesh) was applied, which enabled the analysis of various artificial structures with realistic representation. A salt marsh in the Wadden Sea which has seen heavy erosion (lateral retreat rate of 0.9 m/year) served as case study. We simulate both daily and storm conditions. Hereby, vegetation is represented by an increased bed roughness. The model is able to simulate the governing processes of salt marsh development. Results show that, without artificial structures, erosion of the salt marsh and tidal flat continues. With structures implemented, results indicate that there is potential for salt marsh growth in the study area. Moreover, traditional structures, which were widely implemented in the past, proved to be most effective to stimulate marsh growth. More broadly, the paper indicates how morphological development of a salt marsh can be steered by various configurations of artificial structures. Full article

(This article belongs to the Special Issue Resilient Flood Defences)

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21 pages, 10020 KiB

Open AccessArticle

Development of An Integrated Numerical Model for Simulating Wave Interaction with Permeable Submerged Breakwaters Using Extended Navier–Stokes Equations

by Paran Pourteimouri and Kourosh Hejazi

J. Mar. Sci. Eng. 2020, 8(2), 87; https://doi.org/10.3390/jmse8020087 - 01 Feb 2020

Cited by 6 | Viewed by 2482

Abstract

An integrated two-dimensional vertical (2DV) model was developed to investigate wave interactions with permeable submerged breakwaters. The integrated model is capable of predicting the flow field in both surface water and porous media on the basis of the extended volume-averaged Reynolds-averaged Navier–Stokes equations [...] Read more.

An integrated two-dimensional vertical (2DV) model was developed to investigate wave interactions with permeable submerged breakwaters. The integrated model is capable of predicting the flow field in both surface water and porous media on the basis of the extended volume-averaged Reynolds-averaged Navier–Stokes equations (VARANS). The impact of porous medium was considered by the inclusion of the additional terms of drag and inertia forces into conventional Navier–Stokes equations. Finite volume method (FVM) in an arbitrary Lagrangian–Eulerian (ALE) formulation was adopted for discretization of the governing equations. Projection method was utilized to solve the unsteady incompressible extended Navier–Stokes equations. The time-dependent volume and surface porosities were calculated at each time step using the fraction of a grid open to water and the total porosity of porous medium. The numerical model was first verified against analytical solutions of small amplitude progressive Stokes wave and solitary wave propagation in the absence of a bottom-mounted barrier. Comparisons showed pleasing agreements between the numerical predictions and analytical solutions. The model was then further validated by comparing the numerical model results with the experimental measurements of wave propagation over a permeable submerged breakwater reported in the literature. Good agreements were obtained for the free surface elevations at various spatial and temporal scales, velocity fields around and inside the obstacle, as well as the velocity profiles. Full article

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17 pages, 1111 KiB

Open AccessArticle

The Sensitivity of a Dike-Marsh System to Sea-Level Rise—A Model-Based Exploration

by Richard Marijnissen, Matthijs Kok, Carolien Kroeze and Jantsje van Loon-Steensma

J. Mar. Sci. Eng. 2020, 8(1), 42; https://doi.org/10.3390/jmse8010042 - 15 Jan 2020

Cited by 3 | Viewed by 4216

Abstract

Integrating natural components in flood defence infrastructure can add resilience to sea-level rise. Natural foreshores can keep pace with sea-level rise by accumulating sediment and attenuate waves before reaching the adjacent flood defences. In this study we address how natural foreshores affect the [...] Read more.

Integrating natural components in flood defence infrastructure can add resilience to sea-level rise. Natural foreshores can keep pace with sea-level rise by accumulating sediment and attenuate waves before reaching the adjacent flood defences. In this study we address how natural foreshores affect the future need for dike heightening. A simplified model of vertical marsh accretion was combined with a wave model and a probabilistic evaluation of dike failure by overtopping. The sensitivity of a marsh-dike system was evaluated in relation to a combination of processes: (1) sea-level rise, (2) changes in sediment concentration, (3) a retreat of the marsh edge, and (4) compaction of the marsh. Results indicate that foreshore processes considerably affect the need for dike heightening in the future. At a low sea-level rise rate, the marshes can accrete such that dike heightening is partially mitigated. But with sea-level rise accelerating, a threshold is reached where dike heightening needs to compensate for the loss of marshes, and for increasing water levels. The level of the threshold depends mostly on the delivery of sediment and degree of compaction on the marsh; with sufficient width of the marsh, lateral erosion only has a minor effect. The study shows how processes and practices that hamper or enhance marsh development today exacerbate or alleviate the challenge of flood protection posed by accelerated sea-level rise. Full article

(This article belongs to the Special Issue Resilient Flood Defences)

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