Journal of Coastal and Hydraulic Structures

Evaluation and validation of the spectral linear wave theory and ‘traditional’ formulae for pulsating wave loads for unimodal and bimodal seas

Henry Tuin — 2022-11-23

For the design of vertical hydraulic structures pulsating wave forces need to be calculated. The total wave force is a result of every wave component (long waves and short waves) within a wave field. The common formulae are derived for regular or unimodal narrow sea states and use one characteristic wave height and period. Broad-banded spectra like bimodal sea states are present at many locations. Moreover, new hydraulic structures like Panamax or post-Panamax locks do have a large vertical surface exposed to pulsating wave loads. Swell components within the wave spectrum are disproportionally contributing to the total wave force compared to short waves. This depth effect for broad-banded or bimodal wave spectra is not considered by the traditional wave formulae which could result in significant underestimations of wave forces on hydraulic structures.

This paper aims to determine the wave loads of irregular non-breaking wave fields under any wave spectrum: narrow banded, broad-banded, or bimodal. Spectral linear wave theory (LWT) is used to transform any wave spectrum to a wave force spectrum. The wave force or wave pressure at any level can directly be evaluated from the wave force spectrum or wave pressure spectrum for any shape of the wave spectrum considered within this research. Spectral LWT is compared to the outcome of wave flume experiments with bimodal seas and other wave force formulae, like the Goda formula and quasi-regular LWT and the NewWave theory.

This paper gives a description and evaluation of the spectral LWT applied for bimodal wave spectra and a comparison of the accuracy and validity of other wave force formulae. The peak forces and peak pressures distribution obtained by spectral wave theory compare well to the measurements. It appears that the use of a spectral LWT to obtain characteristic extreme forces improves the accuracy of the extreme load more than the use of a second order wave model with a quasi-regular assumption (i.e. where the spectral shape is not considered). For the typical conditions that occur at hydraulic structures (horizontal bed, intermediate to deep water, non-breaking, and uni- and bimodal seas) the often-used Goda formula can both under of overestimate the peak loads. Goda is well applicable for conditions with (breaking) waves narrow wave spectra and values of k_ph <0.5.

Feasibility of pumped hydro energy storage in a river cascade - case study of the Meuse

Jan Willem Lambach — 2022-11-03

The Meuse river in the Netherlands has been made navigable by the construction of a cascade of seven low head weirs. Because of environmental regulations, hydropower facilities exist at only two weirs. This implies the full hydropower potential of the Meuse cascade is not utilized. By using pump-turbines the river sections upstream of the weirs could be additionally usable as energy storage reservoirs and could improve and ensure river navigability under changed climate conditions.

The main goal of this study is to assess the possible utilization of the full energy storage- and hydropower potential of the Meuse cascade within Dutch environmental regulations. The novelty of this study is the evaluation of the concept of using canalized river sections for pumped-storage purposes within conditions of fluctuating discharge and -water levels throughout the year.

In order to meet the goal of the study the relatively fish-friendly Archimedean screw has been selected as pump-turbine. Next a conceptual design of a pumped-storage hydropower plant equipped with screws has been compiled. By using this design, the assessment of utilizing the hydropower- and energy storage potential of the cascade has been carried out by constructing and applying a numerical model.

The study shows it is possible to utilize the full hydropower- and the majority of the energy storage potential of the Meuse cascade. The cumulative installed turbine capacity for the cascade turns out to be 81 MW. The Annual Energy Yield (AEY) from regular hydropower alone is 225 GWh. In addition, the yearly surplus power that can be processed for energy storage purposes is 137.2 GWh, of which 77.2 GWh is returned to the grid by a round-trip efficiency of 56.25 %. In total 302.2 GWh can be delivered to the grid which can power up to 75.000 households. The specific cost is relatively high: roughly 15,000 euro/kW.

The method developed here can be applied to evaluate the storage- and hydropower potential of other canalized rivers as well, such as the upper Mississippi.

Investigating Wave Transmission through Curtain Wall Breakwaters under Variable Conditions

Hoang Thai Duong Vu — 2022-10-01

Coastal erosion has become a pressing problem all over the world, especially in areas where the hinterland is only slightly elevated over the sea level. The ongoing progression reveals the urging need for engineered coastal protection measures like breakwaters. Amongst others, curtain wall breakwater types (CWB) have proven their potential to dissipate the wave energy in multiple studies. Their application is often considered in situations where only a partial protection of the coast is needed (e.g. to maintain a hydraulic connectivity or enable sediment transport for land reclamation). Due to their slender design, they are assumed to offer an economical alternative in comparison to massive breakwater constructions, while their pillar-based foundation shows advantages for applications under soft soil conditions. Within the development process of a detached breakwater to face coastal erosion in the Mekong Delta, different types of CWB configurations have been investigated under regular wave conditions. Several characteristics of CWB structures such as the inclination and thickness of the wall, the height of the structures, the rate of submergence and emergence were examined for different water depths and wave parameters. The wave-structure interaction was analyzed using FLOW3D software, which is capable of simulating wave transformation. It showed a high agreement in comparison with own experimental investigations and the wave theory. The results showed a continuous reduction of the wave transmission coefficient with increasing inclination from 90° to 135°, whilst the orientation of the inclination (e.g. 60° vs 120°) only showed a minor effect regarding the wave reduction. All CWB arrangements showed increasing performance with decreasing wave periods. Besides, the wave transmission was mainly impacted by the level of submergence together with the amount of supporting piers and the thickness of the structures. Water depth changes due to tidal influence revealed an increase in wave transmission coefficient once the wave started to overtop the structure.

Experimental study on a breaking-enforcing floating breakwater

Joep van der Zanden — 2022-08-19

Floating breakwaters are moored structures that attenuate wave energy through a combination of reflection and dissipation. Studies into floating breakwaters have been generally restricted to optimising the attenuation performance. This study presents a novel floating breakwater type that was developed to have good attenuation performance while keeping wave drift loads as small as possible. The floating breakwater was designed as a submerged parabolic beach that enforces wave energy dissipation through breaking. The design was tested in a 3D shallow-water wave basin in captive and moored setups for regular and irregular wave conditions. Results are presented in terms of attenuation performance, motions, and (mooring) loads. The results show that the breaking of waves improves the attenuation performance of the floater in captive setup. However, in moored setup, the attenuation performance was dominated by diffraction and radiation of the wave field, with breaking being of secondary importance. This shows that breaking-enforcing floating breakwaters have potential, but require a high vertical hydrostatic and/or mooring stiffness in order to enforce intense breaking. Mean wave drift loads on the object showed significant difference between breaking and non-breaking waves in both setups, with breaking waves leading to lower normalized loads. This is attributed to breaking-induced set-up and set-down of the water level. As a result, the new breakwater design has a more favourable balance between wave attenuation and drift loads than common (i.e., box-, pontoon-, or mat-type) floating breakwater designs. Tests with varying surface roughness showed that floating breakwaters may benefit from dual-use functions that naturally increase the roughness (e.g., shellfish, vegetation), which have a marginal effect on the attenuation performance, but increase the added mass and hydrodynamic damping and as such, reduce mooring line loads.

A novel design method for wave-induced fatigue of flood gates

Joachim Kleiberg — 2022-08-10

This paper presents a novel design method to predict fatigue of flood gates due to dynamic wave loading. The accumulation of fatigue damage is predicted probabilistically over the entire lifetime of the structure rather than with a set of normative events. Load events are defined using a joint probability distribution of historical wind and water level data. The random phase-amplitude model is employed to obtain realisations of the wave state for every combination of environmental conditions. Linear wave theory and pressure-impulse theory are used to predict both quasi-steady and highly dynamic wave pressures. The stress response of the structure is predicted with a hybrid semi-analytical and finite element model. By applying a mode matching technique the fluid-structure interaction is solved in a computationally efficient manner. This facilitates the large number of simulations required for a comprehensive fatigue analysis without making concessions in the physical modelling. The fatigue damage is then evaluated with the linear Palmgren-Miner method by applying a rainflow algorithm. A Monte Carlo analysis is performed to estimate the expected fatigue lifetime of the structure. The modular structure of the model routine allows for easy adaptation to other situations where fatigue due to hydrodynamic loading is of interest. The design method is applied to a case study of a flood gate with an overhang inspired by the situation at the Afsluitdijk. Non-fundamental modes are taken into account without simplification of the fluid-structure interaction process and found to be governing for the fatigue damage for the studied case. Moreover, the interference of vibrations due to consecutive wave impacts is shown to have a significant influence on the outcome of the fatigue assessment. For the case study, the design method leads to a 10-20% reduction of the governing fatigue damage compared to a method commonly used in practice. At specific locations on the flood gate fatigue damage is found to be underestimated by current design methods. The presented design method is therefore found to be a significant improvement.

Estimating the Influence of Sea Level Rise and Climate Change on Coastal Defences in Western Taiwan

Yuchia Chang — 2022-08-10

Situated in the Western Pacific Ocean, Taiwan is frequently affected by powerful typhoons. The present research evaluates the storm surges that could take place along the western coastline of Taiwan, and the design water level that will likely be required by coastal structures in the course of the 21^st century to protect coastal settlements. To do so, the intensity of a given case study typhoon (Soudelor, which caused great damage to Taiwan in 2015) that could affect the target case study area (Yunlin county) was modified by taking into account climate change and sea level rise (SLR), by changing the sea surface temperature (SST), atmospheric air temperature (AAT), and relative humidity (RH) in an ensemble of 14 GCMs in CMIP5 according to RCP4.5 and RCP8.5, targeting the year 2041~2060 and 2081~2100 time horizons. The Advanced Research Weather Research and Forecasting Model (WRF-ARW) and the Unstructured Finite Volume Community Ocean Model (FVCOM) were utilized to simulate the typhoons and storm surges. The hindcasting of the historical typhoon showed good agreement with the observed data provided by the Central Weather Bureau (CWB) in Taiwan, and the simulations under future climate change scenarios forecasted an increase in typhoon intensity, especially the maximum wind speed. However, the storm surge simulations indicated a limited increase in storm surge height, and even a decrease when considering also SLR. Nevertheless, the estimated maximum water level, including both effect of SLR and future storm surge height, can increase up to 3.53 m and 3.84 m relative to mean sea level at the tidal stations in Yunlin and Chiayi County, respectively. The results showed that storm surges in the study area, characterized by a shallow bathymetry with many sandbars and land reclamation projects, are highly influenced by the change in water depth due to SLR and tidal changes, and the existence of the Central Mountain Range, which can greatly affect the accuracy of the simulated typhoon wind fields.

Admissible post-wave overtopping flow for persons on a horizontal surface

Jentsje Van der Meer — 2022-07-05

Admissible wave overtopping is a key parameter in design specifications and also in safety assessments of the crest level of many coastal structures. This paper considers the hazard to people/pedestrians by post-wave overtopping flow over a horizontal surface, like a dike or breakwater crest, or a boulevard. Such flow is given by a flow velocity and a flow thickness. The most recent guideline is given in EurOtop (2018), where a maximum overtopping wave volume of 600 l/m is seen as the admissible or tolerable maximum. But no flow velocities or flow thicknesses are given.

Previous work has been summarised by Sandoval and Bruce (2017) who brought existing fluvial tests on people or human subjects together with data derived from videos of actual overtopping hazard events available from the internet. A graph was developed with stable and unstable combinations of flow velocity and flow depth or thickness.

The paper describes first tests in the Delta Flume of Deltares with a volunteer exposed to wave overtopping hazard on the crest of a dike with wave heights up to 1.8 m. Analysis determines flow velocities and flow thicknesses for stable and unstable situations. Additional tests with the wave overtopping simulator on the crest of a dike are described. In these tests, flow velocities and flow thicknesses were accurately recorded as well as the reaction of a volunteer, guarded by a safety line, on the crest of the dike as well as on the landward slope. These tests gave also stable and unstable situations with known flow velocities and flow thicknesses.

The new data were added to the work of Sandoval and Bruce (2017) and a physically based as well as a simple guideline has been proposed for the transition between stable and unstable situations for people/pedestrians. In general overtopping velocities are allowed of 4 m/s with a flow thickness of 0.2 m, but also a large velocity of 7 m/s with only a flow thickness of 0.1 m. Flow thicknesses are always given without air entrainment.

Assessment of maintenance efforts and probabilities of failure at German inland waterways to advance the design of bank revetments

Julia Sorgatz — 2022-06-23

Revetments protect waterways or flood defenses against erosion from waves and currents. In Germany a high percentage of about 7235 km of waterways is secured by revetments. Like many stakeholders of various infrastructures, the German Federal Waterways and Shipping Administration increasingly aims for a more economic and ecological design and maintenance strategy. Thus, new methodologies must be introduced that relate the structural condition of the revetment to resulting consequences such as required maintenance. In this paper we investigate the correlation of maintenance and revetment stability. Using the example of German inland waterways, maintenance measures conducted over at least six years are correlated with a deterministic and a probabilistic stability assessment. To account for realistic traffic loads, the stability assessment employs field measurements which provide data on ship-induced waves. It was found that at least a linear correlation between revetment stability and maintenance must be assumed. A comparison between the deterministic and the probabilistic stability assessment and thereby obtained correlations shows that less maintenance is predicted with the deterministic stability assessment. Particularly for small sample sizes and small probabilities of failure, the probabilistic approach should be favored over the deterministic approach to account for various uncertainties. In the case that only maintenance is of relevance for design considerations, the results of the probabilistic approach indicate that β = 1.3 (p_f ≈ 10^-1) may be a suitable annual target reliability.

Loads and effects of ship-generated, drawdown waves in confined waterways - A review of current knowledge and methods

León-Carlos Dempwolff — 2022-05-10

A ship in motion generates a complex wave field consisting of several superimposed wave systems. The relevance of the wave systems' components varies, depending on individual ship and waterway parameters. This review work is specifically concerned with the long-period, primary wave system, large-volume ships travelling through confined waterways, generate, as it may exert intensive wave and current loading on the banks, affecting local morphology, engineering structures and ecology

So far, the effect of ship-generated waves on waterway embankments has yet only routinely been considered for inland waterways with a constant cross-section. Less attention has been payed to the ship-induced wave and current loading in more complex bathymetries like coastal waterways and estuaries, as naturally occurring loads had been thought to dominate. However, the hydrodynamic loads induced by ships grow and become increasingly relevant in coastal waterways, due to continuously growing dimensions of sea-going ships.

At the same time, requirements rise to allow for restoring the ecological value of of inland and coastal waterways, leading to spatially more diverse bathymetries and embankment structures. Hence, the prediction of ship-generated primary wave magnitudes at banks becomes increasingly complex, due to deformation processes of the propagating waves in shallow water. Knowledge on ship-generated waves characteristics and methods to predict induced loads are thus essential for the assessment of bank stability and the dimensioning of engineering structures to resist present-day and prospective ship-induced loads.

This review paper compiles, analyzes and assesses the findings of previous research quantifying the relevance of primary waves for the surrounding waterways and shows interconnections to the questions studied within naval hydrodynamics for confined waterways. Commonly applied methods for wave prediction are reviewed, highlighting their relevance and limitations. Finally, a concept for coupled numerical model development is suggested, based on the success of different modelling approaches presented previously.

Magnification of Tsunami Risks Due to Sea Level Rise Along the Eastern Coastline of Japan

Kentaro Koyano — 2022-04-14

Sea level rise is likely to increase the risks of inundation due to coastal hazards in the course of the 21st century. To understand how different sea level rise (SLR) scenarios will affect the disaster risk management of tsunamis in Japan, the authors applied the Probabilistic Tsunami Hazard Assessment (PTHA), using a logic tree approach, to the eastern coastline of Japan. Considering a similar generation zone as the 2011 Tohoku Earthquake and Tsunami, a number of tsunami propagation simulations were conducted. In the logic tree construction, different branches of magnitude ranges, positions of asperity, recurrence intervals, standard deviations of log-normal distribution and truncations of log-normal distribution were set. The results indicate that the maximum water levels at output points increased according to the different SLR scenarios that were considered. It was also found that the effects that SLR has on expected tsunami heights and 90% confidence intervals are nonlinear and could vary according to location. Such results highlighted the importance of considering the effects of SLR to improve emergency response capacity.

Analytical Models for Determining the Propagation of Rectangular Surface Jets for Fishway Attraction Flow

Veronika Wiering — 2022-04-13

The present paper evaluates suitability of two analytical models to determine the propagation of rectangular surface jets as a tool to design fishway attraction flow. It focuses on rectangular orifices of vertical slot fishways with aspect ratios (width-to-height) for W/H < 1. Both models were rewritten to match boundary conditions for fishways because they were initially derived for horizontal orifices. As the basis for the evaluation, the output of the analytical models to RANS simulations for 12 geometries 1/16 <= W/H <= 4 is compared. Applied analytical equations for half-lengths for cases W/H >= 4 are within 5 % of RANS modeling results for all cases. The location of centerline transition locations from analytical models also agree reasonably well with RANS modeling. The findings support efficient design of optimum attraction flow propagation using simple, rapid analytical approaches.