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

Description of failure mechanism in placed riprap on steep slope with unsupported toe using smartstone probes

This article is aimed at investigating the influence of toe support conditions on stability aspects of placed ripraps on steep slopes exposed to overtopping flows. All past experimental model studies investigating placed riprap stability under overtopping conditions have been conducted with ripraps constrained at the toe section. However, ripraps constructed on the downstream slopes of rockfill dams are generally not provided with any form of toe support. Hence, it is of importance from stability and economical standpoints to understand the failure mechanism in placed ripraps with realistic toe support conditions. This article presents findings from experimental overtopping tests conducted on model placed ripraps unsupported at the toe section. Employing Smartstone probes, a new technology in stone movement monitoring, laser measurement techniques and Particle Image Velocimetry (PIV) techniques, detailed description of failure mechanism in placed ripraps under overtopping conditions is presented within this study. Study findings demonstrate sliding as the underlying failure mechanism in placed ripraps with unsupported toes. Further, comparison of experimental results with past findings revealed that placed ripraps with unrestrained toes experience a fivefold reduction in stability, characterized by the critical overtopping magnitude as compared with placed ripraps provided with fixed toe supports. Furthermore, toe support conditions were found to have no effects on either the failure mechanism nor the overall stability of dumped ripraps. Further research is recommended to arrive at well-defined methodologies for design and construction of toe supports for placed ripraps.

Modelling the Wave Overtopping Flow over the Crest and the Landward Slope of Grass-Covered Flood Defences

The wave overtopping flow can exert high hydraulic loads on the grass cover of dikes leading to failure of the cover layer on the crest and the landward slope. Hydraulic variables such as the near bed velocity, pressure, shear stress and normal stress are important to describe the forces that may lead to cover erosion. This paper presents a numerical model in the open source software OpenFOAM® to simulate the overtopping flow on the grass-covered crest and slope of individual overtopping waves for a range of landward slope angles. The model provides insights on how the hydraulic forces change along the profile and how irregularities in the profile affect these forces. The effect of irregularities in the grass cover on the overtopping flow are captured in the Nikuradse roughness height calibrated in this study. The model was validated with two datasets of overtopping tests on existing grass-covered dikes in the Netherlands. The model results show good agreement with measurements of the flow velocity in the top layer of the wave, as well as the near bed velocity. The model application shows that the pressure, shear stress and normal stress are maximal at the wave front. High pressures occur at geometrical transitions such as the start and end of the dike crest and at the inner toe. The shear stress is maximal on the lower slope, and the normal stress is maximal halfway of the slope, making these locations vulnerable to cover failure due to high loads. The exact location of the maximum forces depends on the overtopping volume. Furthermore, the model shows that the maximum pressure and maximum normal stress are largely affected by the steepness of the landward slope, but the slope steepness only has a small effect on the maximum flow velocity and maximum shear stress compared to the overtopping volume. This new numerical model is a useful tool to determine the hydraulic forces along the profile to find vulnerable points for cover failure and improve the design of grass-covered flood defences.

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.

Effect of Soil Types on the Development of Water Levels and Erosion Processes during Overtopping Test

The construction of dike materials is an essential parameters in controlling the safety of hydraulic engineering. The dike material is an earthfill material constructed from non-cohesive materials or mixed from cohesive and non-cohesive materials. Overtopping failure can affect the dike stability during water cross above dike crest and could threat people lives and property. It is reduced the matric suctions binds soil particles due to the increasing volumetric water content during the transition of water level from the upstream into downstream slopes. In this paper, two spatial overtopping tests are conducted in Hydraulic Geotechnical laboratories at the Universiti Sains of Malaysia to observe the evolution of horizontal and vertical water levels as well as the development of lateral and vertical erosion processes under constant inflow discharge of 30 L/min. The vertical and horizontal water levels as well as the vertical erosion process are measured using one digital camera installed in front of dike embankment, while the horizontal erosion process was measured using another digital camera installed in front of downstream slope. Two types of coarse sand and very silty sand soils are used to construct dike embankment in small flume channel. The small flume is constructed from transparent PVC material to observe the development of water distributions and erosion processes with sediment box to collect the eroded materials. A pilot channel is cut in dike crest along the side wall of small flume channel to initiate breach channel in the dike crest. The initiation of breach channel is crucial for the evolution breach channel failure in the downstream and upstream slopes. The results show that the vertical and horizontal water levels are distributed faster in coarse sand soil compared with those in the very silty sand soil while the horizontal water levels are distributed faster than vertical water levels for both soils. The permeability of coarse sand increases the velocity of water flow for occupation soil particles and beginning failure in dike crest faster. The presence of fine particles in the very silty sand has reduced the rate of erosion processes inside dike particles in horizontal and vertical directions. The fine particles absorb a large amount of water content and, thus reduce the easiest water infiltration into particles with higher matric suctions. The analyzing of the distributions of water levels and erosion process help to understand the behavior of dike embankment during overtopping failure and increase the maintenance for dikes parts to reduce the potential danger.

Influence of dike slope on the development of infiltration water and erosion processes during overtopping tests

The usage of dikes embankments has a great influence on the development of hydraulic engineering. It increases the suitability of water resources for humanity needs. Different Geotechnical and hydraulics powers affect on the stability of dike construction through transition of water flow in the upstream slope. The development of erosion process inside dike construction occurred as a result of an overtopping water flow cross above the dike crest. The dike erosion initiates breach channel inside dike and later widened into upstream and downstream slopes. In this study, a 2D and 3D vertical and horizontal erosions test is constructed in Hydraulic Geotechnical laboratories at the Universiti Sains Malaysia to observe the evolution of the breach channel during spatial overtopping failure. Two constant dike slopes of (1V:3H) and (1V:2.5H) for both upstream and downstream slopes are used as a parametric test during overtopping tests. The vertical and horizontal distributions of water levels are also highlighted prior to overtopping failure. The results indicate that the vertical and horizontal erosion widen the dike breach for slope of 1V:2.5H faster than those for 1V:3H while the spread of water infiltration in horizontal and vertical levels are higher for latter slope than for previous one.

Effect of Soil Types on The Development of Matric Suction and Volumetric Water Content for Dike Embankment During Overtopping Tests

The resistance of dike materials has a great effect on the development of hydraulic engineering around the world. It helps to understand the mechanism of dike failure occurred due to the influence of hydraulics and Geotechnical parameters. The overtopping moment is one of the main failures that reduces the stability of the dike embankment through initiating the breach channel inside dike crest as a result of water flow above the downstream slope of the dike. Two spatial overtopping tests were conducted at in Hydraulic Geotechnical laboratories at the University Sains of Malaysia to observe the evolution of matric suction and volumetric water content for two soil types of sand and very silty sand soils. A pilot channel was cut in dike crest along the side wall of the small flume channel to represent the transition water flow from upstream into downstream slopes during overtopping test. The results indicated that the matric suction decreases due to the increase of volumetric water content during the saturation of dike body. The proportion increasing and decreasing of volumetric water content and matric suction is lower in very silty sand than those in sand soil due to the presence of fine particles in previous soil.

First Tests on the Symmetrical Breakwater Armor Unit Crablock

Single layer concrete armor systems are being widely used nowadays in the design of rubble mound breakwaters. Recently, a new concrete armor unit has been developed and applied as single layer armor system in the repair works of one damaged breakwater at Al Fujeirah, UAE. It has a symmetrical shape, in contrast to most other units. Modern single layer concrete armor units that exist at this moment have design guidelines in terms of placement, stability and overtopping. However, because of lack of laboratory research and the little experience of using Crablock, no design guidance exists yet for this new single layer block compared to other existing one layer units. Being a new armor unit, the placement was investigated first. Then physical model tests were performed in a wave flume to come up with results on stability and wave overtopping. Furthermore, to determine the interlocking properties of armor units, pull tests were also conducted in this research. The placement tests showed that uniform placement was best achieved with a rectangular grid on a relatively small underlayer of rock. Test results on stability showed that longer waves affected the armor layer a little more, with more rocking and earlier start of damage. Packing density as well as placement pattern showed no influence on wave overtopping. The overtopping tests gave larger overtopping than expected, which might be due to the fairly steep 1:30 foreshore that gave a large ratio of significant wave-height from the time domain and the spectral wave-height.

Aspectos innovadores en el diseño y construcción del dique Oeste en el puerto exterior de A Coruña en Punta Langosteira (España)

This article describes the innovative aspects in design and construction of the western breakwater of the outer port at Punta Langosteira (A Coruña, Spain). The secondary breakwater is formed by the south breakwater (1.st phase) and the western breakwater (2.nd phase), currently under construction, and it is intended to complete the shelter provided by the main breakwater of the outer port of A Coruña and to ensure high operability of the new port facilities, as well as to contain the movement of the beaches to the west of the port. The bidding process, which permitted variations to the preliminary design, favors innovation, allowing for a variety of solutions based on the use of different armor units, and cubipods were finally selected for the main armor layer. The 1.35km long secondary breakwater is protected by a single-layer 25 t and 30 t cubipod armor in the trunk, and a double-layer 45 t cubipod armor in the roundhead. The alternative cubipod solution allows for a breakwater which withstands wave climates higher than the defined limit state (Hs=8.75 m in the trunk) and significantly reduces the economic cost and the concrete consumption. In order to optimize and validate the alternative solution, 3D hydraulic stability and overtopping test were performed with 1/51 scale, similar to those made to justify the preliminary design by the port authority. 1,360 cubipod units (15- and 25-tonne) from the southern and northern breakwaters, which are now sheltered by the new breakwater, have been re-used at the western breakwater. A transition single- to double-layer armor has been constructed, with opposite fitting, increasing progressively the thickness of the filter layer to maintain a homogeneous exterior surface of the armor when modifying the armor thickness.

Evaluation of temporary flood-fighting structures

Sandbags have traditionally been the product of choice for temporary, barrier-type, flood-fighting structures. However, sandbag structures are labor intensive and time consuming to construct and may not always be the best choice when selecting a flood-fighting barrier. To compare different barriers, the U.S. Army Engineer Research and Development Center (ERDC) developed a standard protocol to evaluate flood-fighting structures. Rapid Deployment Flood Wall (RDFW), Portadam, and Hesco Bastion were selected for testing based on technical merit from proposals submitted by companies that manufacture barrier-type, flood-fighting products. A standard sandbag structure was also tested.The evaluation included time, ease, and cost of construction, performance under hydrostatic and wave loading, overtopping, debris impact, possible repair, reuse, environmental impact, and cost. The results of the evaluation showed that the RDFW performed well but is more expensive than the other structures and also not easy to dis-assemble. The Portadam was efficient under hydraulic loading but did not test well in the debris impact tests. The Hesco levee performed well except for the amount of seepage that was observed. The Hesco levee was later retested after the company improved the product, and much less seepage was observed during the retest. The sandbag levee failed during the overtopping test. These results provide the flood-fighting community with results that will assist in the selection of the product that best fits their need.

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.

Uncertainties in the physical modelling of the wave overtopping over a rubble mound breakwater: The role of the seeding number and of the test duration

This paper presents an experimental study on the variability in the wave overtopping discharge on a simple rubble mound breakwater caused by different random starting phases and different lengths of incident wave sequences sharing the same energy density spectrum. A total of 153 small scale laboratory tests were carried out in the wave flume of the Roma Tre University (Rome, Italy) to simulate 8 different spectra producing different levels of overtopping. The seeding of the random number generator used for the starting phases distribution was changed a number of times for each repetition of the same wave condition. The experiments allowed the quantification of how the variability in the wave overtopping grows with the dimensionless freeboard. The experimental confidence intervals have been calculated and compared with those provided by the EurOtop Manual (EurOtop, 2007). The role of the length of the tested wave sequence was also investigated. A sensitivity analysis carried out on the partial overtopping time series has pointed out that shorter time series (at least 500 waves) can be used for overtopping tests, by obtaining a comparable accuracy in average overtopping discharge with respect to the recommended 1000 waves series.

Flume Tests on Fine Soil Reinforced with Geosynthetics: Walls of the Salt Pans (Aveiro Lagoon, Portugal)

This paper presents exploratory work on the use of geosynthetics for reinforcing fine soils, particularly for applications in the Aveiro lagoon, Portugal. The behaviour of local fine soil reinforced with geosynthetics under hydraulic actions was studied using flume tests. The case study was a typical cross section of the walls of the salt pans of the Aveiro lagoon. A preliminary design of a structure was done, for different reinforcements (geogrid, geocomposite, association of geogrid and geotextile). Local soil was collected and characterised using laboratory tests. The flume tests included performing permeability, erosion and overtopping tests, for actions typical of the lagoon environment. The models reinforced with geogrid GGR exhibited the highest global permeability, due to the difficulty of soil lumps to penetrate the geogrid openings. Although this type of reinforcement provides low resistance to erosion, promoting vegetation growth or including other elements can reduce surface erosion. The other reinforcements (sheets) enabled containing the soil. Non-uniformity of the soil compaction caused local differences of permeability. Thus, ensuring uniform compaction on site is necessary; however it can be challenging, particularly for fine soils. The results indicate that seepage is likely to induce some clogging of the reinforcements. The reinforced soil models tested exhibited higher permeability and lower resistance to erosion and overtopping than the traditional solution (soil matrix with vegetation). The results indicate that a possible alternative solution for the walls could use fibre reinforcement. Further work is necessary to ensure adequate (low) permeability of new solutions for these walls.

SPH Modeling of Hydraulics and Erosion of HPTRM Levee

Post-Katrina investigations revealed that most earthen levee damage occurred on the levee crest and landward-side slope as a result of either wave overtopping, storm surge overflow, or a combination of both. In this paper, combined wave overtopping and storm surge overflow of a levee embankment strengthened with high performance turf reinforcement mat (HPTRM) system was studied in a purely Lagrangian and meshless approach, two-dimensional smoothed particle hydrodynamics (SPH) model. After the SPH model is calibrated with full-scale overtopping test results, the overtopping discharge, flow thickness, flow velocity, average overtopping velocity, shear stress, and soil erosion rate are calculated. New equations are developed for average overtopping discharge. The shear stresses on landward-side slope are calculated and the characteristics of soil loss are given. Equations are also provided to estimate soil loss rate. The range of the application of these equations is discussed.

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.

ESTIMATING GRASS SLOPE RESILIENCY DURING WAVE OVERTOPPING: RESULTS FROM FULL-SCALE OVERTOPPING SIMULATOR TESTING

This paper summarizes full-scale laboratory wave overtopping simulation test results from three years of testing conducted at the Colorado State University wave overtopping test facility. These results are specific to the tested soils and grass cover, so direct application to coastal dikes and levees built using different soils and grass species, or to locations subjected to different wave overtopping conditions, is not advisable. For this reason, robust design guidance will require significantly more data than are presently available. Tentative design methods are proposed for extension of the relatively few full-scale test results to other situations having different levee geometry, different wave conditions, and different grass/soil parameters. The intention is to provide a viable framework that can be extended as more knowledge is gained from additional testing with different grass/soil combinations.

The role of offshore boundary conditions in the uncertainty of numerical prediction of wave overtopping using non-linear shallow water equations

The paper examines the variability of wave overtopping parameters predicted by numerical models based on non-linear shallow water equations, due to the boundary conditions obtained from wave energy density spectra. Free surface elevation time series at the boundary are generated using the principle of linear superposition of the spectral components. The components' phases are assumed to be random, making it possible to generate an infinite number of offshore boundary conditions from only one spectrum.A reference case was provided by carrying out overtopping tests on a simple concrete structure in a wave flume. Numerical tests using the measured free surface elevation at the toe of the structure were carried out. Three parameters were analysed throughout the paper: the overtopping discharge, the probability of overtopping and the maximum overtopping volume. These showed very good agreement between the numerical solver prediction and the overtopping measurements. Subsequently, the measured spectra at the toe were used to generate a population of reconstructed offshore boundary time series for each test, following a Monte Carlo approach. A sensitivity analysis determined that 500 tests were suitable to perform a statistical analysis on the predicted overtopping parameters. Results of these tests show that the variability in the predicted parameters is higher for the smaller number of overtopping waves in the modelled range and decreases significantly as overtopping becomes more frequent. The characteristics of the distributions of the predictions have been studied. The average value of the three parameters has been compared with the measurements. Although the accuracy is lower than that achieved by the model when the measured time series are used at the boundary, the prediction is still fairly accurate above all for the highest overtopping discharges. The distribution of the modelled probability of overtopping was found to follow a normal distribution, while the maximum value follows a GEV one. The overtopping discharge shows a more complex behaviour, values in the middle of the tested range follow a Weibull distribution, while a normal distribution describes the top end of the range better.Results indicate that when the probability of overtopping is smaller than 5%, a sensitivity analysis on the seeding of the offshore boundary conditions is recommended.