Offenporige gebundene Deckwerke - Einfluss der Porosität auf Seegangsbelastung und hydraulische Wirksamkeit - Eine experimentelle Studie -

Porous revetments made of crushed stones durably bonded by polyurethane or by any other type of binding material are increasingly used for coastal protection. Such revetments have several advantages as compared to conventional revetments. The crucial effect of the revetment porosity on the wave loading and the response of the structure has been illustrated by Liebisch et al. (2012). Though the revetment porosity may substantially be reduced over design life time (e.g. marine growth), the porosity is not yet explicitly and adequately considered in the current design practice. Therefore, the primary objective of this study is to improve the knowledge related to the effect of the porosity of a porous bonded revetment for different slope steepnesses on the hydrodynamic processes on and beneath the revetment. To achieve the objectives, extensive scale model tests have been conducted and a systematic analysis of the processes involved in the interaction of irregular waves and the porous revetment and its soil foundation is performed: (a) processes in front of and on the revetment (wave reflection, wave set-up and wave run-up/run-down); (b) wave-induced pressures on the revetment, (c) processes beneath the revetment such as the damping of wave-induced pressures in the revetment, the pore pressure in the sand core beneath the revetment, the internal mean water level and the stability of the sand core. Overall, the results have revealed for the first time the relative importance of the effect of the revetment porosity and slope steepness on all the processes involved in the interaction of irregular waves with a bonded porous revetment, thus providing an improved understanding of both effects. More specifically, it was shown for the first time that an increasing porosity leads to a larger effect of the incident wave height on the reflection coefficient. With increasing wave heights the highly porous revetment behaves increasingly like an impermeable revetment. A larger porosity also leads to lower values of both relative wave run-up and run-down with a more significant effect of the porosity on wave run-up. The spatial development of the internal mean water level in the sand core, which is decisive for the structure stability, was investigated systematically for irregular waves for the first time, showing that for the flatter slope 1:6 the internal wave set-up near the shoreline of the sand core is less affected by the swash processes than for the steeper slope 1:3.Poröse Deckwerke aus gebrochenem Kies, der mittels Polyurethankleber oder anderen Bindemitteln verklebt wird, finden immer häufiger Anwendung im Küstenschutz, da sie einige Vorteile gegenüber herkömmlichen Deckwerken haben. Der erhebliche Einfluss der Porosität auf die Wellenbelastung und die Reaktion des Deckwerks wurde bereits in Liebisch et al. (2012) veranschaulicht. Obwohl die Porosität eines Deckwerks im Laufe der Zeit durch z.B. das Einwachsen mariner Organismen erheblich reduziert werden kann, ist diese bisher nicht explizit und ausreichend in der derzeitigen Bemessungspraxis berücksichtigt. Daher besteht das Hauptziel dieser Arbeit darin, den Wissensstand bezüglich der Auswirkungen der Porosität auf die hydrodynamischen Prozesse auf und unterhalb offenporiger, gebundener Deckwerke bei unterschiedlichen Böschungsneigungen zu verbessern. Um dieses Ziel zu erreichen, wurden umfangreiche kleinmaßstäbliche Modellversuche durchgeführt und die Prozesse bei Wechselwirkung von unregelmäßigem Seegang mit dem porösen Deckwerk und seinem Unterbau systematisch analysiert: (a) Prozesse vor und auf dem Deckwerk (Reflexion, Brandungsstau, Wellenauf- und -ablauf); (b) welleninduzierte Druckbelastung auf dem Deckwerk; (c) Prozesse unterhalb des Deckwerks einschließlich der Dämpfung der welleninduzierten Drücke durch das Deckwerk, des Porenwasserdruckabbaus im Sandkern, des internen, mittleren Wasserspiegels (IMWL) und der Stabilität des Sandkerns. Insgesamt haben die Ergebnisse zum ersten Mal die relative Bedeutung von Deckwerksporosität und -neigung gezeigt und zu einem verbesserten Verständnis ihres Einflusses auf alle vorher genannten Prozesse geführt. Im Einzelnen wurde erstmalig gezeigt, dass eine erhöhte Porosität zu einem größeren Einfluss der einlaufenden Wellenhöhe auf den Reflexionskoeffizienten führt. Mit ansteigender Wellenhöhe verhält sich das hochporöse Deckwerk zunehmend wie eine undurchlässige Struktur. Eine höhere Porosität führt außerdem zu reduzierten Wellenauf- und -ablaufhöhen, mit größerem Einfluss auf den Wellenauflauf. Die räumliche Verteilung des mittleren Wasserspiegels im Sand, die einen entscheidenden Einfluss auf die Bauwerksstabilität hat, wurde erstmalig systematisch für unregelmäßigen Seegang untersucht. Dabei hat sich gezeigt, dass der interne Brandungsstau nahe der Uferlinie bei flacherer Böschungsneigung (1:6) weniger von Auf- und Ablaufprozessen auf dem Deckwerk beeinflusst wird als bei steilerer Böschung (1:3)

Effects of wave load on the long-term vegetation development and their resistance as grass revetments on sea dikes

Construction and design processes of revetments, sea dikes and estuarine dikes along the German coastline adhere the paradigm to protect and safeguard reliably the coastal hinterland from wave attack and storm surges. Following these standards coastal protection structures provide only poor ecosystem services in any proper design or maintenance approach. As a result, the EcoDike-project has been started with the aim to quantify and enhance the ecosystem services of revetments, sea dikes and estuarine dikes while preserving or possibly even enhancing the existing safety standards. Therefore, a profound understanding of the complex long-term interactions between wave load and vegetation development on sea dikes is inevitable. To achieve these objectives a typical seadike in prototype scale is tested under realistic and long-term wave loading in the new outdoor wave basin at the Ludwig-Franzius-Institute in Hannover (Germany)

Development of an outdoor wave basin to conduct long-term model tests with real vegetation for green coastal infrastructures

The demand for physical model tests with real vegetation is increasing due to the current trend to elucidate the performance and durability of green coastal infrastructures to ensure and promote ecosystem services. To address this demand, a new outdoor wave basin (OWB) was built in August 2017 at the Ludwig-Franzius-Institute in Hannover, Germany. This paper reviews the general characteristics and the ongoing development of the new OWB. First insights into the long-term development of the ecosystem services of different grass revetments are discussed in terms of their ecological value and safety standards of sea dikes. Focus is placed on the resistance and ecological value of different grass mixtures that are typically applied on sea dikes situated along the North Sea. Further research concepts are briefly described to highlight how experiments in the new OWB may contribute to the current understanding and design recommendations of green coastal infrastructures. The operation of the OWB enables the performance of long-term experiments over seasonal growth stages of coastal vegetation using either fresh or seawater with wave load stresses and varying sea water levels. The first conducted experiments with different grass revetment combinations mimic typical storm surge conditions with a constant wave load (with a duration of up to 10 hours every second week) on a natural dik

Influence of convex and concave curvatures in a coastal dike line on wave run-up

Due to climatic change and the increased usage of coastal areas, there is an increasing risk of dike failures along the coasts worldwide. Wave run-up plays a key role in the planning and design of a coastal structure. Coastal engineers use empirical equations for the determination of wave run-up. These formulae generally include the influence of various hydraulic, geometrical and structural parameters, but neglect the effect of the curvature of coastal dikes on wave run-up and overtopping. The scope of this research is to find the effects of the dike curvature on wave run-up for regular wave attack by employing numerical model studies for various dike-opening angles and comparing it with physical model test results. A numerical simulation is carried out using DualSPHysics, a mesh-less model and OpenFOAM, a mesh-based model. A new influence factor is introduced to determine the influence of curvature along a dike line. For convexly curved dikes (ad = 210° to 270°) under perpendicular wave attack, a higher wave run-up was observed for larger opening angles at the center of curvature whereas for concavely curved dikes (ad = 90° to 150°) under perpendicular wave attack, wave run-up increases at the center of curvature as the opening angle decreases. This research aims to contribute a more precise analysis and understanding the influence of the curvature in a dike line and thus ensuring a higher level of protection in the future development of coastal structures.Peer ReviewedPostprint (published version