Hydraulic performance of elastomeric bonded permeable revetments and subsoil response to wave loads

Elastomeric bonded permeable revetments, also called PBA (Polyurethane bonded aggregate) revetments, are highly porous structures made of mineral aggregates (e.g. crushed stones) which are durably and elastically bonded by polyurethane (PU). Despite their numerous advantages as compared to conventional revetments and the large experience available from more than 25 pilot projects, physically-based design formulae to predict their hydraulic performance, wave loading and response are still lacking. Therefore, the present study aims at improving the understanding of the processes involved in the interaction between wave, revetment and foundation, based on large-scale model tests performed in the Coastal Research Centre (FZK), Hannover/Germany, and to provide prediction formulae/diagrams. This paper is focused on the prediction of the hydraulic performance (wave reflection, wave runup and run-down) and the response of the sand core (pore pressure) beneath the revetment for a wide range of wave conditions, including the analysis of an observed failure due to transient soil liquefaction.BASF-Elastogran Gmb

Wave attenuation over coastal salt marshes under storm surge conditions

Coastal communities around the world face increasing risk from flooding as a result of rising sea level, increasing storminess, and land subsidence1–2. Salt marshes can act as natural buffer zones, providing protection from waves during storms3–7. However, the effectiveness of marshes in protecting the coastline during extreme events when water levels are at a maximum and waves are highest is poorly understood8,9. Here, we experimentally assess wave dissipation under storm surge conditions in a 300-meter-long wave flume tank that contains a transplanted section of natural salt marsh. We find that the presence of marsh vegetation causes considerable wave attenuation, even when water levels and waves are highest. From a comparison with experiments without vegetation, we estimate that up to 60% of observed wave reduction is attributed to vegetation. We also find that although waves progressively flatten and break vegetation stems and thereby reduce dissipation, the marsh substrate remained stable and resistant to surface erosion under all conditions. The effectiveness of storm wave dissipation and the resilience of tidal marshes even at extreme conditions suggests that salt marsh ecosystems can be a valuable component of coastal protection schemes.This is the author's accepted manuscript and will be under embargo until the 29th of March 2015. The final version has been published by NPG in Nature Geoscience here: http://www.nature.com/ngeo/journal/v7/n10/full/ngeo2251.html

Wave overtopping at near-vertical seawalls: Influence of foreshore evolution during storms

This work presents the results of an investigation on how wave overtopping at a near-vertical seawall at the back of a sandy foreshore is influenced by sequences of erosive storms. The experiments were carried out in the Large Wave Flume (GWK) at Leibniz University, Hannover (Germany). The tested layout consisted of a near-vertical 10/1 seawall and a sandy foreshore with an initial 1/15 slope. Three sequences of idealised erosive storms were simulated. Within each storm both the incident wave conditions and still water level were varied in time to represent high and low tide conditions. Each sequence started from a 1/15 configuration and the beach was not restored in between storms. The measurements included waves, beach profile, wave overtopping volumes. The profile of the beach was measured after each sea state tested. Wave overtopping at each stage of the tested storms was significantly influenced by bed changes. This was linked to the measured evolution of the beach. Measurements showed that a barred profile developed quickly at the start of each sequence, and scour developed at the toe of the structure during high water level conditions, while accretion or partial backfilling developed during low water level conditions. Due to these processes, the position of a sea state in the tested sequence is shown to be an important factor in determining the wave overtopping volume. Remarkably, when a weaker idealised storm followed a more energetic one, nearly the same level of overtopping was recorded. This is explained by the foreshore erosion, leading to increased water depths and wave heights at the toe of the structure. This finding allows to quantify and to explain the variability of wave overtopping in storms following one another at intervals shorter than the recovery time of the foreshore

Erfahrungen mit Sandersatz im Küstenschutz - Eine allgemeine Entscheidungsunterstützung für die Praxis mit aktuellen Erkenntnissen aus der Wissenschaft

Die SWOT-Analyse zeigt, dass jedes Verfahren für die Materialentnahme und die Aufspülung eine Reihe von Vor- und Nachteilen mit sich bringt, die der Anwender gegeneinander abwägen muss. Mit weiteren Forschungsergebnissen, v.a. hinsichtlich der langfristigen ökologischen und morphologischen Auswirkungen der Verfahren, könnten einige der potenziellen Chancen und Risiken zu bekannten und absehbaren Stärken bzw. Schwächen werden. Bis dahin muss der Anwender unter Berücksichtigung aller Faktoren ein Verfahren auswählen, das für den geplanten Anwendungsfall geeignet ist und bei dem die Risiken für die Region vertretbar sind. Dabei muss jedoch beachtet werden, dass viele bisher unbekannte biologische Prozesse möglicherweise weitere Auswirkungen auf das Küstenökosystem haben könnten. Daher sind Eingriffe in die Küstenumwelt immer mit gewissen Risiken für das Ökosystem und den Menschen verbunden und sollten nur dann erfolgen, wenn sie unter Abwägung der wesentlich beeinflussten marinen und terrestrischen Kompartimente und nach Diskussion und Priorisierung der primären Schutzziele (Lebens-, Natur- und Wirtschaftsraum) unbedingt erforderlich sind

Plant stiffness and biomass as drivers for drag forces under extreme wave loading: A flume study on mimics

© 2016Moving water exerts drag forces on vegetation. The susceptibility of vegetation to bending and breakage determines its flow resistance, and chances of survival, under hydrodynamic loading. To evaluate the role of individual vegetation parameters in this water-vegetation interaction, we conducted drag force measurements under a wide range of wave loadings in a large wave flume. Artificial vegetation elements were used to manipulate stiffness, frontal area in still water and material volume as a proxy for biomass. The aim was to compare: (i) identical volume but different still frontal area, (ii) identical stiffness but different still frontal area, and (iii) identical still frontal area but different volume. Comparison of mimic arrangements showed that stiffness and the dynamic frontal area (i.e., frontal area resulting from bending which depends on stiffness and hydrodynamic forcing) determine drag forces. Only at low orbital-flow velocities did the still frontal area dominate the force-velocity relationship and it is hypothesised that no mimic bending took place under these conditions. Mimic arrangements with identical stiffness but different overall material volume and still frontal area showed that forces do not increase linearly with increasing material volume and it is proposed that short distances between mimics cause their interaction and result in additional drag forces. A model, based on effective leaf length and characteristic plant width developed for unidirectional flow, performed well for the force time series under both regular and irregular waves. However, its uncertainty increased with increasing interaction of neighbouring mimics

Salt marsh surface survives true-to-scale simulated storm surges

A full-scale controlled experiment was conducted on an excavated and re-assembled coastal wetland surface, typical of floristically diverse NW European saltmarsh. The experiment was undertaken with true-to-scale water depths and waves in a large wave flume, in order to assess the impact of storm surge conditions on marsh surface soils, initially with three different plant species and then when this marsh canopy had been mowed. The data presented suggests a high bio-geomorphological resilience of salt marshes to vertical sediment removal, with less than 0.6 cm average vertical lowering in response to a sequence of simulated storm surge conditions. Both organic matter content and plant species exerted an important influence on both the variability and degree of soil surface stability, with surfaces covered by a flattened canopy of the salt marsh grass Puccinellia experiencing a lower and less variable elevation loss than those characterized by Elymus or Atriplex that exhibited considerable physical damage through stem folding and breakage.We thank all of the support staff at the Grosser Wellenkanal; Ben Evans, James Tempest, Kostas Milonidis, Chris Rolfe and Colin Edwards, Cambridge University; and Dennis Schulze, Hamburg University, for their in valuable logistical assistance. Fitzwilliam College, Cambridge supported the research time of IM. The work described in this publication was supported by the European Community’s 7th Framework Programme (Integrating Activity HYDRALAB IV, Contract No. 261529) and by a grant from The Isaac Newton Trust, Trinity College, Cambridge. We thank Mark Schuerch, Kiel University, for helpful insights into storm surge flooding on Sylt, Germany Wadden Sea. The authors have no conflicts of interest to declare.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1002/esp.386

Vegetation-wave interactions in salt marshes under storm surge conditions

Vegetation-wave interactions are critical in determining the capacity of coastal salt marshes to reduce wave energy (wave dissipation), enhance sedimentation and protect the shoreline from erosion. While vegetation-induced wave dissipation is increasingly recognized in low wave energy environments, little is known about: (i) the effect of vegetation on wave dissipation during storms when wave heights and water levels are highest; and (ii) the ability of different plant species to dissipate waves and to maintain their integrity under storm surge conditions. Experiments undertaken in one of the world’s largest wave flumes allowed, for the first time, the study of vegetation-wave interactions at near-field scale, under wave heights ranging from 0.1–0.9 m (corresponding to orbital velocities of 2–91 cm s−1) and water depths up to 2 m, in canopies of two typical NW European salt marsh grasses: Puccinellia maritima (Puccinellia) and Elymus athericus (Elymus). Results indicate that plant flexibility and height, as well as wave conditions and water depth, play an important role in determining how salt marsh vegetation interacts with waves. Under medium conditions (orbital velocity 42–63 cm s−1), the effect of Puccinellia and Elymus on wave orbital velocities varied with water depth and wave period. Under high water levels (2 m) and long wave periods (4.1 s), within the flexible, low-growing Puccinellia canopy orbital velocity was reduced by 35% while in the more rigid, tall Elymus canopy deflection and folding of stems occurred and no significant effect on orbital velocity was found. Under low water levels (1 m) and short wave periods (2.9 s) by contrast, Elymus reduced near-bed velocity more than Puccinellia. Under high orbital velocities (≥74 cm s−1), flattening of the canopy and an increase of orbital velocity was observed for both Puccinellia and Elymus. Stem folding and breakage in Elymus at a threshold orbital velocity ≥ 42 cm s−1 coincided with a levelling-off in the marsh wave dissipation capacity, while Puccinellia survived even extreme wave forces without physical damage. These findings suggest a species-specific control of wave dissipation by salt marshes which can potentially inform predictions of the wave dissipation capacity of marshes and their resilience to storm surge conditions.M.P. acknowledges funding by the German Science Foundation (grant no. PA 2547/1-1). The work described in this publication was supported by the European Community’s 7th Framework Programme through the grant to the budget of the Integrating Activity HYDRALAB IV, Contract no. 261529 and by a grant from The Isaac Newton Trust, Trinity College, Cambridge

Salt marsh surface survives true-to-scale simulated storm surges

A full-scale controlled experiment was conducted on an excavated and re-assembled coastal wetland surface, typical of floristically diverse NW European saltmarsh. The experiment was undertaken with true-to-scale water depths and waves in a large wave flume, in order to assess the impact of storm surge conditions on marsh surface soils, initially with three different plant species and then when this marsh canopy had been mowed. The data presented suggests a high bio-geomorphological resilience of salt marshes to vertical sediment removal, with less than 0.6 cm average vertical lowering in response to a sequence of simulated storm surge conditions. Both organic matter content and plant species exerted an important influence on both the variability and degree of soil surface stability, with surfaces covered by a flattened canopy of the salt marsh grass Puccinellia experiencing a lower and less variable elevation loss than those characterized by Elymus or Atriplex that exhibited considerable physical damage through stem folding and breakage.We thank all of the support staff at the Grosser Wellenkanal; Ben Evans, James Tempest, Kostas Milonidis, Chris Rolfe and Colin Edwards, Cambridge University; and Dennis Schulze, Hamburg University, for their in valuable logistical assistance. Fitzwilliam College, Cambridge supported the research time of IM. The work described in this publication was supported by the European Community’s 7th Framework Programme (Integrating Activity HYDRALAB IV, Contract No. 261529) and by a grant from The Isaac Newton Trust, Trinity College, Cambridge. We thank Mark Schuerch, Kiel University, for helpful insights into storm surge flooding on Sylt, Germany Wadden Sea. The authors have no conflicts of interest to declare.This is the author accepted manuscript. The final version is available from Wiley via http://dx.doi.org/10.1002/esp.386