Loads (flow, waves, ships, porous flow), Erosion (flow, remainder) Load reduction, stability (flow, waves, porous flow), Protections, construction and maintenance, design.

Introduction to bed, bank and shore protection.

Sea dikes protect low-lying hinterlands along many coasts all around the world. Commonly, they are designed as embankments with grass covers or grey revetments accounting for the prevailing hydraulic loads. So far, incorporation of ecological aspects in the dike design is limited. With regard to increasing environmental awareness and climate change adaptation needs, the present study reviews methods for ecological enhancement of sea dikes and discusses limitations and challenges related to these methods. In doing so, one key aspect is to maintain dike safety while increasing the ecological value. Potential for ecological enhancement of sea dikes has been found regarding natural or nature-based solutions in the foreshore, dike surface protection measures (vegetated dike covers, hard revetments and dike roads) and the dike geometry. While natural and nature-based solutions in the foreland are investigated thoroughly, so far only few experiences with ecological enhancements of the dike structure itself were gained resulting in uncertainties and knowledge gaps concerning the implementation and efficiency. Additional to technical uncertainties, engineers and ecologists meet the challenge of interdisciplinary collaboration under consideration of societal needs and expectations.

https://www.mdpi.com/2073-4441/11/8/1617/pdf

Enhancing the Ecological Value of Sea Dikes

Dense grass covers are generally recommended for surface protection of sea dikes against mild and moderate hydraulic loads. The standard seeding mixtures were composed to meet the technical requirements and ensure dike safety. These mixtures are, however, limited in their species diversity. In the present study, four differently vegetated surfaces were tested regarding their erosion resistance against wave impacts and overflow. The test vegetations ranged from a species-poor grass-dominated reference mixture to species-rich herb-dominated mixtures. Two vegetations were reinforced with a three-dimensional geogrid. For the unreinforced vegetations, the erosion rate due to wave impacts decreased exponentially with increasing root density and root length density. The geogrid reinforcements functioned as additional protection when the upper vegetation layer was eroded and led to slightly decreasing erosion rate with depth. In overflow simulations, the relatively densely-vegetated grass-dominated mixture experienced least erosion. Erosion was mainly initiated at bare spots emphasizing the major role of a closed vegetation cover and dike maintenance. The present results give new insights into erosion patterns of unreinforced and reinforced vegetated dike covers and the relation between vegetation parameters and hydraulic resistance to wave impacts and overflow.

Investigating the Erosion Resistance of Different Vegetated Surfaces for Ecological Enhancement of Sea Dikes

Transitions in the dike revetment or in the grass cover can significantly affect the wave overtopping discharge and the dike cover erosion. At the University of Twente, two PhD students recently started on the challenge of quantifying the effect of (1) waterside transition on the wave overtopping discharge and (2) transitions in grass covered dikes on dike erosion. In this paper we present their preliminary results and outline their future plans. Firstly, new laboratory experiments show that the existing wave overtopping formulas are not able to accurately predict the overtopping discharge in case of transitions on the waterside slope. Secondly, the analytical dike cover erosion model shows that transitions in grass covers significantly affect the location of maximum flow velocity and potential dike cover erosion. In future work, detailed numerical models will be developed for both the waterside slope and the landward slope to further increase our understanding of the effects of transitions on the wave overtopping discharge and the dike cover erosion.

/pdf/modelling-wave-overtopping-for-grass-covers-and-transitions-41fwdt9vz1.pdf

Modelling wave overtopping for grass covers and transitions in Dike Revetments

Calibration and preparation of field measurements of oblique wave run-up and overtopping on dikes using laser scanners

Outer slopes of grass dikes under wave attack are likely to have residual strength, which is the strength after initial damage has occurred. This strength is not included in current design and assessment tools. To quantify the residual strength of grass under wave attack and implement this in design and assessment tools, a large research program is initiated within the Dutch WTI 2017 project. This project is financed by Rijkswaterstaat. In this research program an integrated approach, a combined use of a so-called wave impact generator and large-scale wave flume tests in the Delta Flume is applied. This approach contributes to a future strength model which includes residual strength of the outer slope of grass dikes under wave loads, primarily along large rivers.

Grass cannot be scaled properly and many variations exist in grass covers (clay quality, grass quality, transitional structures, objects in or on the dike, et cetera). For this reason, testing with traditional physical wave flume models would lead to unacceptable high costs since many tests are required. Therefore, a wave impact generator is developed (Van Steeg et al, 2014). This machine can be placed easily on a prototype dike in the field and can generate wave impacts on a slope. During testing, the machine is continuously filled by a pump. By opening a pre-programmed valve irregularly, a mass of water is relieved leading to an impact that resembles impacts caused by natural waves.

The developed wave impact generator is applied in an extensive measurement campaign on several grass dikes in the Netherlands. Variations of the thirteen different test sections were on grass and clay quality but also transition structures and objects (pole, open concrete blocks allowing grass growth, stairs). This leads to valuable erosion patterns as function of geometric properties of the outer slope of the dike. The hydraulic load during all tests was the same.
Although wave run-up levels and wave impact pressures due to the wave impact generator are close to natural waves, there is a need to calibrate the results obtained with the wave impact generator. Therefore, large scale physical model tests in the Delta Flume, with a selection of the dikes tested with the wave impact generator, are performed. Blocks of 2 m x 2 m x 0.8 m were taken from dikes and were transported to the Delta Flume. In this flume (L x B x D = 235 m x 5 m x 7 m), waves can be generated up to a significant wave height of Hs = 1.6 m.

Erosion patterns obtained with the wave impact generator and erosion patterns obtained in the large scale flume were compared. Based on this comparison and based on impact pressure analysis it is concluded that the wave impact generator represents a load which is equivalent to a significant wave height of Hs = 0.6 – 0.7 m, a wave steepness of sop 4-5%.

The integrated use of the wave impact generator and a large-scale wave flume led to valuable data. This data will be used to improve the strength model for outer slopes of grass dikes under wave attack.

/pdf/use-of-wave-impact-generator-and-wave-flume-to-determine-q979apa9fu.pdf

Use of wave impact generator and wave flume to determine strength of outer slopes of grass dikes under wave loads

When calculating the Carbon Footprint for a product or service, a direct link is made between the total amount of consumed energy and the produced amount of carbon dioxide during production. For that reason calculating the carbon footprint of various alternatives is a very straightforward method to compare energy consumption and more importantly environmental pollution. Applying this method to large hydraulic engineering projects is not being done frequently. In this study the possibilities to apply the Carbon Footprint method to coastal protection systems have been explored and analyzed. The analyses are based on a case study: A reinforcement work at the Dutch coast

/pdf/analysis-of-the-carbon-footprint-of-coastal-protection-6t9ehpktmx.pdf

Analysis of the carbon footprint of coastal protection systems

A new Wave Run-up Simulator has been designed, constructed, calibrated and used for testing of the seaward face of dikes. The upper part of dikes or levees often have a clay layer with a grass cover. The new device is able to test the strength of the grass cover under simulation of up-rushing waves for pre-defined storm conditions. The cumulative overload method has been developed to describe the strength of grass covers on the crest and landward side of dikes, for overtopping wave volumes. In essence there is not a lot of difference between the hydraulic load from an overtopping wave volume or from an up-rushing wave. Therefore the hypothesis has been evaluated that the cumulative overload method should also be applicable for up-rushing waves. Tests on a real dike have been used to validate this hypothesis. The main conclusions are that the new Wave Overtopping Simulator works really well, but that the results on testing till so far has not yet been sufficient for a full validation of the method. More research is required. Furthermore, a new technique has been developed to measure the strength of a grass sod on a dike: the grass pulling device. Tests with this device showed that it is possible to measure the critical velocity (= strength) of a grass cover, which is much easier than performing tests with a Wave Run-up or Overtopping Simulator.

/pdf/wave-run-up-simulations-on-real-dikes-1gb3ok043r.pdf

Wave run-up simulations on real dikes

This study deals with the effects of transitions and/or objects such as roads and trees in grass revetments on dikes/levees under (wave) overtopping conditions. Three types of impact categories are defined, that is 1) little impact, 2) moderate impact and 3) large impact. Based on the grass erosion model (i.e., the cumulative overload method) a relation is deduced that describes the critical overtopping discharge as function of the load–increase factor for both a sea and a river regime. This impact factor represents the load increase near transitions and objects which is here expressed by a decrease of the critical overtopping discharge. Lognormal distributions for the critical overtopping discharge are used to calculate the failure probability due to overtopping with and without transitions and objects. For several dike–sections in the Netherlands the differences are calculated with the model PC-Ring. The study shows that the differences are largest for dikes along the coast and lakes. For dikes in a river regime where the significant wave height is usually low the erosion differences are marginal.

/pdf/effects-of-transitions-and-objects-on-the-erodibility-of-1r1m9vv6nj.pdf

Astrid Labrujere

Papers

Use of wave impact generator and wave flume to determine strength of outer slopes of grass dikes under wave loads

Analysis of the carbon footprint of coastal protection systems

Wave run-up simulations on real dikes

Effects of transitions and objects on the erodibility of grass revetments on dikes