H.J. Verhagen

A practical method for design of coastal structures in shallow water

Modern design formula for coastal structures (like rock stability formula and overtopping formula use wave parameters (H2% and Tm-1,0) which are not readily available from standard boundary condition wave data. For transforming values like Hs and Tp to the parameters used in the new formulas, often-fixed conversion factors are used. However, this may lead to significant errors. Therefore, it is better to calculate these new parameters with an appropriate wave transformation model. The one-dimensional Graphical User Interface for SWAN (SwanOne) is presented as a simple tool to perform the required transformation.

WAVE OVERWASH AT LOW-CRESTED BEACH BARRIERS

To increase physical insight into wave overwash processes at low-crested beach barriers, wave overtopping discharge events rather than the conventional average overtopping discharge need to be quantified. Also, in order to make intelligent use of the many empirical formulations on wave overtopping discharge at breakwaters from literature, a single-valued appropriate slope for a natural beach needs to be derived. To resolve these issues, laboratory experiments of composite-slope low-crested barriers were carried out. The tests deal with overwash on a smooth non-uniform slope on shallow foreshores. The conventional average overtopping discharge concept does not represent the discontinuous character and associated strength of overtopping flow. Instead, e.g. for purposes of morphological modeling, wave overtopping should be treated as an event-based process. In this study, several new parameters such as the wave-averaged overtopping time, the relative total overtopping time, the overtopping asymmetry, the average maximum discharge and the average instantaneous discharge are defined and formulated. A new approach for defining an equivalent slope is proposed in the parameterization of the overtopping discharge that also takes into account effects of the wave period. It is experimentally shown to be an improvement over the conventional approach by Van der Meer [1998], especially eligible for low-crested sandy slopes such as barriers, dikes, dunes, etc on shallow foreshores.

Stone Stability in Non-uniform Flow

This paper presents the results of an experimental study on stone stability under nonuniform turbulent flow, in particular expanding flow. Detailed measurements of both flow and turbulence and the bed stability are described. Than various manners of quantifying the hydraulic loads exerted on the stones on a bed are extensively reviewed and extended. On the basis of the data, a new relationship between flow parameters and bed damage—expressed as a stone entrainment formula—has been established for nonuniform flow. As the present data is in line with existing data on other flows, the present relation seems applicable for other types of nonuniform flow as well. Such a relationship could provide more consistent design criteria and allow an estimate of the cumulative damage over time, which is important for making decisions regarding maintenance frequency and lifetime analysis of hydraulic structures.

Environmentally friendly coastal protection

Preface. Summary of the Advanced Research Workshop. 1: Key-Notes. 1. Environmental Friendly Coastal Protection Structures - The Endless Struggle of the Engineer against Water, Waves and Erosion - Opening Address C. Zimmermann. 2. Beach Nourishment: Benefits, Theory and Case Examples R.G. Dean. 3. Innovations in Coastal Protection J. Pope, W.R. Curtis. 4. Classical, Innovative and Unconventional Coastline Protection Methods H.J. Verhagen. 5. On the Stability of Rock Slopes M. van Gent. 6. Integrated Hydraulic-Environmental Modelling J.A. Roelvink. 7. Interaction of Waves and Reef Breakwaters V. Penchev. 8. Coastal Protection and Associated Impacts. Environment Friendly Approach G. Rozynski et al. 2: Selected Participant Presentations. 9. Numerical Simulations in Coastal Hydraulics and Sediment Transport S. Mai et al. 10. Rehabilitation of Highly Protected Beaches by Using Environment-Friendly Structures P.L. Aminti et al. 11. Low Crested Structures: Boussinesq Modeling of Waves Propagation P. Prinos et al. 12. Flow Measurements and Numerical Simulation on Low-Crested Structures for Coastal Protection P. Lomonaco et al. 13. Performance of Submerged Breakwaters as Environmental Friendly Coastal Structures S. Cokgor et al. 14. Scour Development in Front of Coastal Structures at Intermediate Phases of Construction T. Marcinkowski. 15. Cliff Erosion - How Much Do We Really Know about It P. Dong. 16. Environmental Design and Monitoring of Large Submarine Outfalls: An Integrated Approach for Coastal Protection J.A. Juanes et al. 17. Evaluation of Coastal Defense Strategies in Portugal R. Taborda et al. Conclusions and Recommendations of the Working Groups. Authors Index. Subject Index.

Damage to grass dikes due to wave overtopping

Grass covers have been applied as an effective measure for protecting river levees and sea dikes. We conducted experiments to show how roots considerably improve the shear strength of soil on dike slopes. Roots of 1-year-old Bermuda and Carpet grass may increase the total shear strength of up to 20 kPa. Exposed to severe overtopping flow, dike slopes may possibly fail in various manners including ‘head-cut’, ‘roll-up’ and ‘collapse’. The ratio between shear strength of the grass cover and its subsoil layer would get a value of two to distinguish the first two manners and would be zero for the last one. To some extent, the findings contribute to the basis for thoughtfully investigating the strength and failure mechanism of grass-covered slopes.

Wave overtopping simulator tests on Vietnamese sea dikes

Little is known about the strength of the land-side slopes of the Vietnamese sea dikes under overtopping attack. Some grass-covered slopes were tested by a wave overtopping simulator. The paper describes the simulator and presents the test results. The tested grass covers could withstand overtopping rates varying from 20 1/s to 100 1/s per meter of dike length in some hours. Damage usually started at bare spots, at the transition between the slope and the toe, at the transition between different materials, and around objects (e.g. big trees). These features reduce the strength of a grass cover and therefore should be avoided.

INFLUENCE OF THE DENSITY OF PLACEMENT ON THE STABILITY OF ARMOUR LAYERS ON BREAKWATERS

Studies on the stability of the amour layer (d’Angremond et. al. [1999] revealed the importance of density of placement. The current research focuses on the influence of the density of placement on the stability of cubes in a double armour layer and tetrapods and rocks in a single armour layer. The experiments were performed in the Laboratory of Fluid Mechanics of the Faculty of Civil Engineering and Geosciences at the Technical University Delft. A model of a breakwater was constructed in a wave flume. An increase in density of placement resulted in all cases in an increase of stability, except in the case of cubes. Cubes have the tendency to start behaving like a placed block revetment including the characteristic failure mechanisms like uplift and sliding. Tetrapods seem to be unsuitable for single layer armour layers due to the fact that the filter layer is easily attacked by the waves even when no tetrapod has been removed. Experiments on rock showed that vertically placed elements lead to a much more stable construction due to their self-repairing ability. Characteristic for rock is the piling up of elements under the waterline caused by the impact of collapsing waves. This lead to very low densities of placement higher on the slope. Existing damage criteria are not suitable for density of placement. In this research an effort has been done to create a damage criteria, which considers area of attack, density of placement and different failure mechanisms.

The Beneficial Effects of Mangrove Forest to Sea Defence Structures

At many locations, especially in deltaic areas, dikes and other flood defence structures are needed to protect the hinterland against flooding by surges (typhoons, hurricanes) and/or tsunamis. Dikes are quite costly, and mangroves in front of the dike will lower the costs of the dike. Mangroves in general allow dikes to be lower and narrower. In this chapter the hydrodynamic effects of mangroves on dikes are elaborated as well as the financial impact on the costs of dikes. The main advantage of a mangrove forest in front of a dike is that the forest decreases the wave height just in front of the dike, and therefore the freeboard needed to cope with these waves can be considerably less. Because the waves in front of the dike are small due to the mangrove forest, often no costly revetment structure is needed any more. In this chapter computational methods are presented to determine the direct benefit for dike construction.

Performance-Based Design

A method for making a design based on performance by utilizing a statistical performance evaluation envelope is provided to design a high performance concrete structure suitable for surroundings by determining a satisfactory design specification through performance evaluation required from the concrete structure. A plurality of measurement data is obtained for correlation of more than two characteristics of a concrete material. Each normal distribution curve is made by calculating an average and standard deviation of each characteristic from the measurement data, and the number of characteristic values having predetermined intervals is calculated. Mutual combination is performed by modifying the number of variables for each obtained characteristic value section to conform with a sum of the numbers. A combination of more than one characteristic values satisfying a reference point is determined by comparing mutually combined data with the reference point of the other material characteristic. The characteristics are two selected among a unit water quantity, a ratio of water to cement, and a slump value.

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.