· · · Repository Hydraulic Engineering Reports

In most developed coastal areas, seawalls protect towns, road, rail and rural infrastructure against wave overtopping. Similar structures protect port installations worldwide, and may be used for cliff protection. When a large tidal excursion and severe environmental conditions concur to expose seawalls and vertical face breakwaters to wave impact loading, impulsive loads from breaking waves can be very large. Despite their magnitude, wave impact loads are seldom included in structural analysis of coastal structures and dynamic analysis is rare, leading to designers ignoring short-duration wave loads, perhaps contributing to damage to a range of breakwaters, seawalls and suspended decks. Over the last 10 years, improved awareness of wave-impact induced failures of breakwaters in Europe and Japan has focussed attention on the need to include wave impact loads in the loading assessment, and to conduct dynamic analysis when designing coastal structures. Recent experimental work has focused more strongly on recording and analyzing violent wave impacts. These new data are however only useful if methodologies are available to evaluate dynamic responses of maritime structures to short-duration loads. Improvements in these predictions require the development of more complete wave load models, based on new measurements and experiments. Moving from a brief review of documented structural failures of caisson breakwaters and existing design methods for wave impact loads, this paper reports advances in knowledge of impulsive wave loads on vertical and steeply battered walls, based on physical model tests in the large wave flume at Barcelona under the VOWS project (Violent Overtopping of Waves at Seawalls). These data are used to support a revised simple prediction formula for wave impact forces on vertical walls. The paper also discusses dynamic characteristics of linear single degree of freedom systems to non-stationary excitation. Responses are derived to pulse excitation similar to those induced by wave impacts. Response to short pulses is shown to be dominated by the ratio of impact rise time tr to the natural period of the structure Tn. A functional relation between impact maxima and rise-times is given for non-exceedance joint probability levels. The relation is integrated in a simplified method for the evaluation of the static-equivalent design load and the potential cumulative sliding distance of caisson breakwaters.

The erosion of sand beaches due to oscillatory water particle motion of non-breaking waves can be of importance, particularly where such a beach is fronted by a sea wall supported on spread foundation. Laboratory study was conducted with natural beach sand; waves were generated mechanically. Geometric variables included the inclination of sea walls front 15 to 90 degrees from the horizontal and dynamic variables included ratio of ''lave length to water depth and wave height to water depth. It has been determined that the "ultimate" depth of scour is a function of wave height and that the location of scour is a linear function of wave length.

Prediction of wave overtopping discharges for seawalls / breakwaters have improved significantly over the last 25 years, but processes associated with overtopping hazards to people on or close behind seawalls are not yet well understood. Despite research advances in recent years, there remain important gaps in knowledge and disagreements over safe levels of wave overtopping and the composition and spatial extent of overtopping. Similarly, there are few data on the direct effects of overtopping flows. This report summarises analysis developed within the EC CLASH project on the hazards arising from wave overtopping. It identifies sources of information on overtopping hazards, and discusses the basis for assessing the consequences of overtopping. The report reviews the state of guidance in Europe, describes instances of hazard, and draws potential guidance on limits to discharge, volume, velocity and depth. The report also draws supplementary data from parallel studies on overtopping and its effects.

Measurements of erosion and accretion in the neighborhood and in front of seawalls and beach walls.

Series of measurements with irregular waves were undertaken for model seawalls of vertical walls with and without protection by concrete block mounds. Experimental data were compared and supplemented with theoretical calculations by the combination of the author's random wave breaking model in shallow waters and the weir type overflow model. The results have been compiled as twelve diagrams for the estimation of overtopping rate for two sea bottom gradients (1/10 and 1/30), three values of equivalent deepwater significant wave steepness (0.012, 0.017, and 0.036), and two types of seawalls. Experiments employed irregular waves with H1/3= 15 cm, and T1/3 = 1. 7, 2.3, and 2.8 sec. Model seawalls with crest heights of 7.5 to 26.3 em were located at the water depths of 22.5 to -10 cm (above the waterline) on smooth slopes of uniform gradient. The overtopping rate was obtained as the average of three measurements, each for continuous two hundreds waves. Measurements were taken for 205 cases of vertical walls and 123 cases of concrete block mounds placed in front of vertical walls. Incident wave heights were estimated with a technique of resolving incident and reflected waves from two simultaneous records of irregular wave profiles. The results of experiments and calculations have clarified the effects of bottom gradient and wave steepness upon the overtopping rate of seawalls. Decrease of overtopping rate by concrete block mounds has been estimated quantitatively. Twelve diagrams with two supplementary figures for the effects of bottom gradient and wave steepness enable quick estimation of the overtopping rate of seawalls at any water depth from the offshore to the foreshore. The expected rate of wave overtopping with the data of regular wave experiments was reconfirmed to almost agree with the data of irregular waves except for the neighborhood of shoreline, where the phenomenon of surf beats is predominant. Tables of experimental data are attached as the appendix of the present report.

HR Wallingford was commissioned by The Rural Research Institute of the Korea Agricultural and Rural Infrastructure Corporation to carry out various studies relating to the final closure of the Saemangeum project offshore dikes currently under construction on the western coastline of South Korea. In addition to the numerical modelling of the complete closure works, reported separately, a schematised physical model of one of the closure gaps was constructed to investigate the flow regime during the final 100m of dike closure.

HR Wallingford has commenced an engineering review on the Final Closure of Saemangeum Dike. This draft final report represents the completion of the second phase of studies by HR Wallingford and is issued for review by KARICO and discussion with their representatives during meetings planned in Ansan during the second week of October 2005. The report contains some detailed matters which deserve consideration but the following overall conclusions are worthy of particular note: 1. Much of the work has been carried out by KARICO and RRI is of excellent quality and only deserves some small comments. However, there are a small number of issues that do require serious attention. 2. Scour either side of the existing bed protection will remain a problem and will become worse as velocities increase during the final phase of closure. We have considered the processes taking place and recommended that the bed protection be extended by a further 50 meters either side of the dike centre-line. 3. When estimating stable stone weights, the increases from estimated mind gap velocities to peak velocity, for example at the progressing ends of the closure bunds, has not been taken into account. We have applied appropriate speed up factors varying between 5% to 14% to allow for this, but the presence of flow asymmetry means that these increases may be exceeded. We have also allowed for high turbulence, which may be particularly evident in the vortex sheets emanating from the ends of the dikes. 4. We make recommendations for increases to the the stone weights and/or proportions of gabions to take account of these larger velocities. These changes are significant, requiring more heavy stone (up to 6t in weight) and higher proportions of gabions. In some cases modifications to the existing sill and bed protection will be necessary. Making appropriate modifications will require serious attention by KARICO in the following respects: i. To ensure that appropriate stability criteria have been adopted for all materials to be used. RRI have carried out very useful physical modelling, but not all material weights and combinations of gabions for bed protection, sill and closure bund were covered by this work. We have attempted to fill the gaps in understanding by the use of published stability formulae, but further physical modelling to confirm our results would be advisable. ii. To ensure that the financial and physical resources necessary to support these design and construction changes are put in place. 5. We have no particular recommendation to make on the issue of whether the March-April or April-May closure period is to be preferred. On the grounds of stability and wave overtopping, the later period is marginally more favorable, but this difference is not sufficient to require the use of the later period if the earlier period is preferred for construction or other reasons. 6. To the extent that information has been provided to us, procedures for construction appear to be satisfactory. 7. The problem of water leakage through the (extended) bed protection layer after final closure has been completed is significant. A strategy involving carefully timed pumping of gravel and sand into closure bund and bed protection layer is recommended.