On the hydraulic design of granular filters for maritime structures under oscillatory flow orthogonal to the layer interface

Deckwerksaufbauten von Sohlsicherungen in Offshore-, Küsten- und Hafenbauwerken unterliegen der Einwirkung aus welleninduzierten Strömungen und wirken Erosionsprozessen entgegen. In diesem Zusammenhang werden Kornfilter, bestehend aus rundkörnigem oder gebrochenem Gesteinsmaterial, eingesetzt, um (i) Sohl- und Böschungssicherungen vor dem Ausspülen des Gründungsmaterials zu schützen, (ii) die effektiven Spannungen durch zusätzliche Auflast zu erhöhen und (iii) somit das Einsinken von Deckwerkselementen zu verhindern. Die hydraulische Stabilität von Kornfiltern ist abhängig von der hydraulischen Einwirkung aus welleninduzierten Strömungen sowie vom Widerstand des Filtermaterials gegenüber Erosionsprozessen. Es ist daher die Zielsetzung dieser Studie, die Grundlagen zur Interaktion zwischen geotechnischen und hydraulischen Prozessen für die hydraulische Bemessung von Kornfiltern mariner Bauwerke zu schaffen. Durch eine systematische Analyse des Wissensstands zur Stabilität von Kornfiltern unter welleninduzierten Strömungen wurden die maßgebenden Prozesse und die wichtigsten Wissenslücken identifiziert. Darauf aufbauend wird ein Versuchsaufbau in einer neuen Wechseldurchströmungsanlage entwickelt und optimiert, um systematische Laboruntersuchungen durchzuführen. Aufgrund der Analyse der experimentellen Daten wird die Stabilität von Kornfiltern in Abhängigkeit der maßgebenden hydraulischen Gradienten bei oszillierender und gleichgerichteter Strömung klassifiziert. Eines der wichtigsten Ergebnisse ist die deutlich geringere Stabilität von Kornfiltern unter oszillierender Strömung gegenüber gleichgerichteter Strömung. Weiterhin wird die Interaktion zwischen den Erosionsprozessen und dem hydraulischen Verhalten des Filteraufbaus als Grundlage für die dynamische Stabilität von Kornfiltern beschrieben. Darüber hinaus werden die Widerstandskoeffizienten der Forchheimer-Gleichung als Beitrag zur Modellierung der oszillierenden Strömung in Kornfiltern analysiert. Zur Erweiterung der Labordaten wird ein numerisches Modell optimiert, validiert und für eine systematische Parameterstudie zu hydraulischen Prozessen angewendet. Dabei wird die hydraulische Einwirkung in Abhängigkeit der Wellenbedingungen beschrieben, wodurch die hydraulische Filterbemessung mit dem maßgebenden und kritischen hydraulischen Gradienten ermöglicht wird. Zur Bemessung der erforderlichen geometrischen Filtereigenschaften werden Formeln unter Berücksichtigung der Wellenbedingungen entwickelt.Bed protections for offshore, coastal and harbour structures are subject to the impact of wave-induced currents and are supposed to resist erosion processes. In this context, granular filters consisting of round or crushed rock materials are used to (i) preserve bed and bank protections against erosion of the seabed material, (ii) increase the effective stresses by additional load, and (iii) prevent sinking of cover layer elements into the seabed. The hydraulic stability of grain filters is dependent on the hydraulic load of wave-induced currents as well as the resistance of the filter material to erosion processes (e.g. contact erosion and internal erosion). The aim of this study is, therefore, the assessment of the interaction between geotechnical and hydraulic processes for the hydraulic design of granular filters in marine structures. The current scientific knowledge on the stability of granular filters under wave-induced currents is analysed, in order to identify the most relevant knowledge gaps. By analysing the laboratory data of the oscillatory flow facility, the critical hydraulic gradient under unidirectional and oscillatory flow is determined. One of the most important obtained results is the significantly lower resistance of grain filters under oscillating flow compared to unidirectional flow. Furthermore, the interaction between the erosion processes and the hydraulic behaviour of the filter setup is observed and established for the dynamic stability of grain filters. In addition, the further analysis of the laboratory experiments provides the resistance coefficient of the Forchheimer equation as an important contribution to the modelling of oscillating flows in wide-graded filter materials. A numerical model is optimised, validated and applied for the extension of the laboratory data. The numerical simulations are used for a systematic parameter study to investigate further effects on the hydraulic processes in the filter structure. The hydraulic effect can be described as a function of the wave conditions for the determination of the decisive hydraulic gradients at the layer boundary between the base and the filter material. The hydraulic design criteria for granular filters are based on the relevant hydraulic gradient and the critical gradient as a function of the filter properties. New design formulae are developed for the determination of the required geometric filter characteristics taking into account the wave conditions

Erosion resistance of vegetation-covered soils: Impact of different grazing conditions in salt marshes and analysis of soil-vegetation interactions by the novel DiCoastar method

The analysis of soil-vegetation interactions in erosion processes along coastlines requires accurate information about various factors influencing the upper soil layer. Yet, some of these parameters were previously determined by simplified, often hand held devices, and these were often biased by the skill and experience of the operator. This study thus investigates the erosion resistance of salt marsh soils influenced by grazing conditions for providing crucial findings for policy making and land management decisions; to that end, we present and use a novel shear resistance measuring device. This measuring device, called DiCoastar, was developed with a controllable step-motor and now allows us for the first time the determination of time histories of shear resistance by repeatable in-situ measurements, gaining information about the interaction between soil and root systems. A field study was conducted in salt marshes at Cäciliengroden and at Sönke-Nissen-Koog, both foreland salt marshes at the German North Sea coast. The two sites had been chosen due to their difference in grazing intensities, featuring semi-natural/ungrazed, moderately grazed and intensively grazed salt marshes. This was to enable the investigation of influences on soil shear strength and vegetation cover. Measurements of shear resistance were conducted with the DiCoastar in the chosen sites in the vicinity of the dike toes; it is found that the new device now provides consistent and repeatable measurements, irrespective of the operator, and only based on the pre-set control parameters. Results of the field study demonstrate that a marked increase of shear strength is only found in sites with high intensity grazing, but this is accompanied by a strong reduction in the vegetation cover and plant diversity, especially with regard to the vertical density distribution of the vegetation cover. As the reduction in vegetation cover leads to reduced wave attenuation over salt marshes and increased flow velocities, an increased shear stress on the soil surface, which potentially exceeds the increased shear strength, is expected. Based on this, the results obtained lead to the assumption that an increase in the erosion potential of these foreland marshes by high grazing pressure is more likely as well as a reduction in dike stability. © 2022 The Author

Proposing a novel classification of growth periods based on biomechanical properties and seasonal changes of Spartina anglica

Salt marshes are a valuable ecosystem with coastal protection potential, for example by absorbing hydrodynamic energy, increasing sedimentation and stabilizing the soil. This study investigated biomechanical properties of Spartina anglica to improve future models of wave-vegetation interaction. To fully understand the correlations between hydro- and biomechanics, the biomechanical vegetation properties from December 2021 to July 2022 are investigated with specimens collected from the field monthly. 551 specimens were used to determine the vegetation properties during storm surge season with high hydrodynamic forces. Additional geometrical properties were determined for 1265 specimens. Three-point bending tests measured the stiffness S (N/mm) and maximum forces Fmax (N). Different phenological states were observed over time and separated for analysis. These states provide a novel classification of growth periods for evaluating the coastal protection potential of Spartina anglica. Especially during storm season, most specimen were identified as broken shoots with a mean stiffness of 1.92N/mm (using 304 samples) compared to the bottom part of flowering shoots in December and January with a mean stiffness of 2.98N/mm (using 61 samples). The classification of plant properties recognizing phenological differences, based on plant state and seasonality, can be used to explain and reduce variability of biomechanical properties obtained during field campaigns. Additionally, this study shows that March to April is recommended for future investigations focusing on shoot properties during storm surge season, which is the important season for coastal engineers considering vegetation state

Wellenbelastung und Stabilität hydraulisch gebundener Deckwerke

Many sea dikes along the coast of the North Sea are protected against wave loading and currents by riprap revetments that are grouted with mortar. The mortar bonds the individual stones of the top layer, thereby forming a coherent structure that is able to withstand normal forces and shear forces as well as momentums, thus leading to a planar load distribution. While this kind of revetment has been built for decades, its design is yet solely based on empirical knowledge. On the one hand, the current design practice of mortar-grouted riprap revetments may therefore potentially lead to an uneconomic design exceeding the load and safety criterion for a particular site. On the other hand, it is also possible that the current design practice will lead to a weaker revetment than is required for the load and safety criterion for a particular site. Therefore, the objective of the project "Wave Toad and Stability of Mortar-Grouted Riprap Revetments" is to derive a scientific basis for the design of mortar-grouted riprap revetments. In order to describe the structural integrity of mortar-grouted riprap revetments, the results of an assessment of the condition of revetments in the field and a literature research are used to describe mechanisms leading to damage of the revetments. For the mechanism "crack development in the top layer" a structural model is set up in order to describe the load and resistance in the limit state. The hydraulic load due to wave action was measured duringfull-scale model tests in the Targe Wave Flume in Hannover, Germany. The resistance and structural parameters were determined using mechanical and fracture mechanical tests with the individual components as well as with the compound material of mortar and stone. Furthermore, pull-out tests to determine the force for debonding of an individual stone were carried out in the field and under laboratory conditions. The models for the structural stability of mortar-grouted riprap revetments presented in this study describe the processes relevant for designing a mortar-grouted riprap revetment, namely "crack development in the top layer" and "debonding of an individual stone". For the functional dimensioning, the wave run-up height was determined and reduction coefficients for the EurOtop wave run-up formula have been established.Team Sander Wahl

Tools for the Improvement of the Efficiency and Sustainability of Shore Nourishments – Results of the research project STENCIL

Online First, geplanter Druck 202

EcoDike – Grüne Seedeiche und Deckwerke für den Küstenschutz

Online First, geplanter Druck 202

Wave Run-Up on Mortar-Grouted Riprap Revetments

The wave run-up height is a crucial design parameter that determines the crest height of a sea dike and is used for estimating the number of overtopping waves. Therefore, a reduction of the wave run-up height is generally aspired in the design of dikes, which can be achieved by mortar-grouted riprap revetments (MGRR). Although MGRRs are widely utilized revetments along the German North Sea coast, no investigations into the wave run-up height on this revetment type are available to date. Full-scale hydraulic model tests were hence conducted to investigate wave run-up heights on partially grouted and fully grouted MGRRs. The wave run-up was determined using 2D-LIDAR measurements, which were validated by video data. Partially grouted MGRRs, due to their roughness, porosity, and permeability, reduce wave run-up heights from 21% to 28%, and fully grouted MGRRs due to their roughness reduce wave run-up heights from 12% to 14% compared to smooth impermeable revetments. Influence factors have been determined for four widely used revetment configurations, which can now be used for design purposes. A comparison and subsequent discussion about the representation of the physics of wave run-up by different parameters is carried out with the results presented