Wave Overtopping Tests Research Articles

GRASS SOD PULLING TESTS TO DETERMINE RESISTANCE AGAINST EROSION BY WAVE OVERTOPPING

From 2007 wave overtopping tests with the Wave Overtopping Simulator (Van der Meer et al, 2008) have been performed on real dikes in order to determine the strength of grass covers against loads from wave overtopping. Over 15 different locations and over 50 different test sections have been tested. Most of these large scale tests have been performed on grass covers on a clay subsoil. As part of the tests since 2014, grass pull tests with a grass sod pulling device have been performed in order to estimate the critical velocity, the strength of the grass cover, on a specific subsoil with a small and easy to perform test. The critical velocity is derived from the critical normal stresses which are calculated from the force needed to pull out the grass sod and the dimensions of the pulled sod. The method of determining the critical velocity from grass pull tests has been validated with the results of tests with the Wave Overtopping Simulator. The practical, mobile and easy to perform grass pull tests can be used to evaluate the strength of grass covers over time after seeding and for an estimation of strength of existing grass covers on different subsoils.

Simulation of random wave overtopping by a WCSPH model

In this work the Weakly Compressible SPH-based (WCSPH) model DualSPHysics has been validated and applied to study the random wave overtopping of dike-promenade layout in shallow water conditions. Data from physical model tests carried out in a small-scale wave flume have been used for model validation. The results have been compared in terms of water surface elevation, mean discharges and individual overtopping volumes distribution. The selected geometrical layout is representative of the coastal area of Premià de Mar, in Catalonia (Spain). This stretch of the coast presents both railways and a bike path very close to the shore and therefore exposed to possible sea storms. For the first time an SPH-based model has been employed to reproduce long-lasting wave overtopping tests, made of time series of 1000 irregular waves, which are representative of real sea states. The density diffusion scheme and the modified Dynamic Boundary Conditions have been applied in the present simulations. By employing standard setup for SPH modelling of wave-structure interaction problems of a very long duration, stable simulations and accurate results have been attained.

WAVE OVERTOPPING TESTS TO DETERMINE TROPICAL GRASS SPECIES AND TOPSOILS FOR POLDER DIKES IN A TROPICAL COUNTRY

A dike or levee will protect a polder to build in a tropical country against coastal flooding. To ensure that the performance of the dike is in accordance with the safety standard, wave overtopping tests with a wave overtopping simulator have been performed on a mock-up dike. These wave overtopping tests will guide the selection of the grass species and topsoil for the grass cover of the landward side of the dike. The paper describes the design of a new dedicated wave overtopping simulator, the construction of the mock-up dike, the results of the eight tests on the mock-up dike and the critical velocities (strength indicator of the grass cover) established with the cumulative overload method.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/MCuKojNyNyQ

HYDRAULIC STABILITY AND OVERTOPPING PERFORMANCE OF A NEW TYPE OF REGULAR PLACED ARMOR UNIT

The XblocPlus is a new type of interlocking single layer armour units that is placed with uniform orientation. This is novel and different from all other single layer, interlocking armouring systems. The hydraulic stability of the XblocPlus breakwater armour unit was tested in 2D and 3D hydraulic model tests. Wave overtopping tests were performed to determine the roughness coefficients of the EurOtop overtopping formula for the XblocPlus. Model tests on a rubble mound breakwater with XblocPlus armour included 2D tests with a 1:30 seabed slope and with 1:2 and 3:4 breakwater slopes and 3D model tests with a flat seabed and with a 3:4 breakwater slope. Wave heights up to 150% of the design wave height were tested in the 2D tests and up to 200% with wave directions 0° to 60° in the 3D tests. No armour unit displacements were observed in 2D tests with 1:2 slope. In the 2D tests with 3:4 slope one armour unit was displaced when the wave height reached 159% of the design wave height. No damage to the XblocPlus armour layer was observed in the 3D tests. A roughness coefficient of 0.45 was deduced from overtopping tests with wave heights of 60% to 100% of the design wave height. The model test results indicate little or no influence of wave steepness on XblocPlus stability and no adverse influence of wave obliquity while the seabed slope in front of the breakwater may have some impact on the XblocPlus armour layer stability.

VERTICAL SEA WALL CREST MODIFICATIONS FOR OVERTOPPING

Crest modifications such as a storm wall, parapet or a bullnose are widely used to reduce the wave overtopping over coastal structures where spatial and visual demands restrict the crest heights, especially in urban areas. Although reduction factors of these modifications have been studied for sloped structures in EurOtop Manual (2016), there is limited information regarding the vertical structures. This paper presents the experimental set-up and first results of wave overtopping tests for a vertical wall with several different super structure types: a) seaward storm wall, b) sloping promenade, c) landward storm wall, d) stilling wave basin (SWB), e) seaward storm wall with parapet, f) landward storm wall on the horizontal promenade with parapet, g) landward storm wall with parapet, h) stilling wave basin (SWB) with parapet, under breaking wave conditions. The SWB is made up of a seaward storm wall (may be a double shifted rows) , a sloping promenade (basin) and a landward storm wall. The seaward storm wall is partially permeable to allow the evacuation of the water in the basin.

충격파 및 비충격파 조건에서 직립식구조물의 월파량산정식

In this study, two dimensional wave overtopping tests for vertical wall were performed and overtopping formulas were suggested for impulsive and non-impulsive wave conditions. The test results from this study were compared with those from EurOtop(2007). The wave overtopping formulas were derived and suggested considering the recent research trends, while the existing method used the diagram. The wave overtopping formulas have the form of exponential and power functions using non-dimensional variables for wave overtopping and freeboard heights for non-impulsive and impulsive condition, respectively. The wave overtopping formula and effective parameters for inclined superstructure were also suggested. It is analyzed that the locations of inclined superstructure do not have the significant effects on wave overtopping, that is, the wave overtopping rate were almost same for each locations.

WAVE OVERTOPPING SIMULATOR TESTS ON TRANSITIONS AND OBSTACLES AT GRASS COVERED SLOPES OF DIKES

Previous tests performed with the Wave Overtopping Simulator showed that objects on dikes may impose a weak spot. To investigate this conclusion further in 2013, within the Dutch program SBW (Strength and Loads on Water Defences), new wave overtopping tests have been performed. These tests were particularly aimed at the influence of transitions between the soft cover layer of clay and grass on dikes with hard structures like asphalt and concrete. In these tests research was performed on the behaviour of the grass cover by wave overtopping at the direct interaction between grass cover and the hard obstacle or cover. Specific tests were performed to determine the critical velocity of the grass covered slope, which is used in the cumulative overload method, and the influence of objects on that. Grass pull tests were also performed with a new developed device and further analysis may give a relationship with the critical velocity.

Application of Grass Cover Failure Models Following in Situ Wave Overtopping Experiments in Belgium

In situ wave overtopping tests were performed on typical dike structures along the Schelde in Flanders (Belgium) with the Dutch Wave Overtopping Simulator. For a storm duration of 2 hours, wave overtopping characteristics were derived for significant wave heights ranging from 0,75 to 1 m and peak periods between 3 and 4 s. Prior to the in situ experiments, soil investigations were executed together with a detailed study of the quality of the grass cover. During the tests, video recordings were made and detailed pictures of the damage were taken. Water content and evolution of the grass cover were monitored. At one location, no failure was experienced up to 50 l/s per m, even after applying initial damage to the surface protection. Other locations failed after facing 30 l/s per m. Besides the cumulative overload method, available failure models are derived for steady overtopping conditions. Assuming erosion equivalence with unsteady overtopping conditions, different erosion resistance as well as (superficial) slip models are applied. Outcomes of the models are compared to the results of the in situ wave overtopping tests in Flanders (Belgium). Different approaches for describing the load term are discussed. Whether for unsteady overtopping conditions, erosion is predominant determined by the front velocity or an approximation of the average velocity of consecutive waves still needs further investigation. However, this discussion is maybe rather philosophical, because one's choice has a direct influence on the threshold values under which waves are not accounted for. For all methods, the determination of threshold values given different qualities of covering is at-the-time still questionable.

INTEGRATED DESIGN OF COASTAL PROTECTION WORKS FOR WENDUINE, BELGIUM

Wave overtopping tests were performed with a 1:25 physical scale model to determine the optimal geometry for design of new wave return walls at Wenduine, Belgium. Wave overtopping on the shallow foreshore at Wenduine was found to be dominated by low-frequency infragravity waves (f < 0.04 Hz at prototype scale). Mean wave overtopping discharge measured with the physical model compared well with the Van Gent (1999) empirical overtopping equations for shallow foreshores. Physical model tests confirmed that the stilling wave basin concept proposed by Geerearts, et al. (2006) and wave wall parapet concepts of van Doorslaer & De Rouck (2010) were effective at reducing the wave wall height required to meet the tolerable discharge overtopping standards.

DESTRUCTIVE WAVE OVERTOPPING AND WAVE RUN-UP TESTS ON GRASS COVERED SLOPES OF REAL DIKES

In March 2011 new wave overtopping tests have been performed in the Netherlands. In contrast to previous tests the grass cover of this dike was not maintained well, which had significant effect on erosion stability. Additionally, for the first time a pilot test has been made on wave run-up from an asphalt berm onto the grass covered upper slope of the dike. The tested dike sections had a sand core covered by a layer of clay and a grass cover. The objective was to test the erosion stability of seaward and landward slopes for wave overtopping as well as wave run-up. For the wave overtopping also the influence on erodibility of the grass cover caused by obstacles or other elements, which may be present at dikes (stairs, fences), was investigated.

Wave Transformation and Runup on Dikes and Gentle Slopes

ABSTRACT Kobayashi, N.; Pietropaolo, J.A., and Melby, J.A., 2013. Wave transformation and runup on dikes and gentle slopes. Coastal flood-risk mapping requires the prediction of wave runup and overtopping of dikes and beaches. The cross-shore numerical model CSHORE is adjusted to predict irregular wave runup on impermeable dikes. The model is compared with 137 wave runup tests and 97 wave overtopping tests. The seaward boundary of the computation is taken at the location where wave setup is negligible. The spectral period and peak period at this boundary location are specified as input. The model predicts the measured cross-shore variation of the spectral significant wave height on the barred and sloping beaches in front of the dikes. The measured 2% and 1% exceedence runup heights are predicted within errors of about 20% for the spectral period used as input to CSHORE. The model predicts the threshold of wave overtopping, but the minor wave overtopping rates can be predicted only within a factor of 10. T...

SLIDING STABILITY OF LANDWARD SLOPE CLAY COVER LAYERS OF SEA DIKES SUBJECT TO WAVE OVERTOPPING

Sea dikes with landward slopes covered by grass and clay cover layers, subject to wave overtopping, can become unstable and slide. Sliding stability of the cover layer is caused by a decrease in shear strength due to an increase in pore pressure in and underneath the clay cover layer. This holds for both clay dikes and sand dikes with a clay cover layer. A method is presented to determine the potential pore pressure build up due to a storm event with wave overtopping. The method combines of the shelf knowledge and is supported by laboratory measurements and field measurements during prototype scale wave overtopping tests and sliding test on Dutch sea dikes. The method contains three steps: 1) determine the infiltration time, depending on the storm duration and the sea state; 2) determine the infiltration capacity of the dike slope, either by choosing a safe value or field measurement and 3) determine the potential pore pressure build up, determined by step 1) and 2) and the dike structure and geometry. The potential pore pressure build up can be used in the standard stability analyses tools published in Dutch guidelines on dike design and dike safety assessment.

A performance test of nearshore wave height prediction with CLASH datasets

The CLASH database was analyzed for extraction of a set of data having the measured wave heights at both the deep station and the toe of the structure for wave overtopping tests, yielding 1214 data from 29 datasets. Wave heights in front of the toe of the structure were estimated with the Goda formulas and compared with the measured ones. Comparison yielded the overall mean of 1.106 with the standard deviation of 0.155 for the ratio of the estimated to the measured heights, which support the use of the Goda formulas for prediction of nearshore wave heights. Another set of 1215 data having the measured wave heights at the deep station and the wave heights calculated with the SWAN model was also extracted from the CLASH database. A comparative test of the SWAN model using the wave height estimated with the Goda formulas in lieu of the measured height indicated the performance of the SWAN model being similar to that of the Goda formulas, but a tendency of underestimation was noticed in shallow water on a beach of very gentle slope.

The new wave overtopping database for coastal structures

Significant effort has been made to generate a homogeneous database on wave overtopping consisting of more than 10,000 irregular wave overtopping tests from more than 160 independent projects or test series, each described by means of 31 parameters. Many coastal structures, including dikes, rubble mound breakwaters, berm breakwaters, caisson structures and combinations have been considered and have been schematised for inclusion in the database. All these overtopping tests are represented by over 300,000 numbers in the database. Within the framework of the CLASH-project specific overtopping tests on missing parameters and structures (white spots) in the database have been performed, as well as prototype overtopping measurements and small scale overtopping simulations. The results of these tests have enlarged and homogenized the database. White spot tests concentrated on the roughness of all kind of armour units and on the influence of oblique wave attack. These tests were performed by the University of Edinburgh (UK) and Aalborg University (DK), respectively. Prototype measurements were performed at Ostia (IT), Samphire Hoe (UK) and Zeebrugge (BE). The small scale simulation tests were performed by most of the partners in CLASH. The database has been created in such a way that particular and confidential information on actual projects is not present. This creates the opportunity to finally release the database to all interested people. The database was created as a first step and as basis for the generic prediction method on wave overtopping. The latter has been described in Van Gent [Van Gent, M.R.A., Van den Boogaard, H.F.P., Pozueta, B., Medina, J.R., 2007. Neural network modelling of wave overtopping at coastal structures, Coastal Engineering, Vol. 54, Issue 8, pp. 586–593]. But the database can also be applied on its own. Furthermore, the minimum set of hydraulic and structural parameters to describe a test is very useful in creating other databases and could be considered as essential parameters to be described in each test report on physical model investigation of coastal structures. The paper describes: • the gathering of the data • the screening of the data • the development of parameters to describe each individual test • the parameters required for an artificial neural network prediction method • a parameter analysis of the total database • the application of the database for design purposes