Outer slopes of grass dikes under wave attack are likely to have residual strength, which is the strength after initial damage has occurred. This strength is not included in current design and assessment tools. To quantify the residual strength of grass under wave attack and implement this in design and assessment tools, a large research program is initiated within the Dutch WTI 2017 project. This project is financed by Rijkswaterstaat. In this research program an integrated approach, a combined use of a so-called wave impact generator and large-scale wave flume tests in the Delta Flume is applied. This approach contributes to a future strength model which includes residual strength of the outer slope of grass dikes under wave loads, primarily along large rivers. Grass cannot be scaled properly and many variations exist in grass covers (clay quality, grass quality, transitional structures, objects in or on the dike, et cetera). For this reason, testing with traditional physical wave flume models would lead to unacceptable high costs since many tests are required. Therefore, a wave impact generator is developed (Van Steeg et al, 2014). This machine can be placed easily on a prototype dike in the field and can generate wave impacts on a slope. During testing, the machine is continuously filled by a pump. By opening a pre-programmed valve irregularly, a mass of water is relieved leading to an impact that resembles impacts caused by natural waves. The developed wave impact generator is applied in an extensive measurement campaign on several grass dikes in the Netherlands. Variations of the thirteen different test sections were on grass and clay quality but also transition structures and objects (pole, open concrete blocks allowing grass growth, stairs). This leads to valuable erosion patterns as function of geometric properties of the outer slope of the dike. The hydraulic load during all tests was the same. Although wave run-up levels and wave impact pressures due to the wave impact generator are close to natural waves, there is a need to calibrate the results obtained with the wave impact generator. Therefore, large scale physical model tests in the Delta Flume, with a selection of the dikes tested with the wave impact generator, are performed. Blocks of 2 m x 2 m x 0.8 m were taken from dikes and were transported to the Delta Flume. In this flume (L x B x D = 235 m x 5 m x 7 m), waves can be generated up to a significant wave height of H s = 1.6 m. Erosion patterns obtained with the wave impact generator and erosion patterns obtained in the large scale flume were compared. Based on this comparison and based on impact pressure analysis it is concluded that the wave impact generator represents a load which is equivalent to a significant wave height of H s = 0.6-0.7 m, a wave steepness of s op 4-5%. The integrated use of the wave impact generator and a large-scale wave flume led to valuable data. This data will be used to improve the strength model for outer slopes of grass dikes under wave attack.

In order to protect the Netherlands against flooding, the resistance of dikes, hydraulic structures, dams and dunes has to be assessed. These assessment regulations are summarized in technical reports and software and are being developed in various research programs (in the Netherlands, such an example is the WTI-2017 program). Not only desk studies, but also practical experiences show that transitions and (nonwater retaining) objects (such as buildings and trees) are usually weak spots in flood defenses (e.g. Seed et al 2005). With the presence of transitions and objects on dikes it is expected that the probability of flooding increases. This paper discusses some computational results obtained from the PC-Ring model. 2 THEORETICAL BACKGROUND 2.1 General

A new Wave Run-up Simulator has been designed, constructed, calibrated and used for testing of the seaward face of dikes. The upper part of dikes or levees often have a clay layer with a grass cover. The new device is able to test the strength of the grass cover under simulation of up-rushing waves for pre-defined storm conditions. The cumulative overload method has been developed to describe the strength of grass covers on the crest and landward side of dikes, for overtopping wave volumes. In essence there is not a lot of difference between the hydraulic load from an overtopping wave volume or from an up-rushing wave. Therefore the hypothesis has been evaluated that the cumulative overload method should also be applicable for up-rushing waves. Tests on a real dike have been used to validate this hypothesis. The main conclusions are that the new Wave Overtopping Simulator works really well, but that the results on testing till so far has not yet been sufficient for a full validation of the method. More research is required. Furthermore, a new technique has been developed to measure the strength of a grass sod on a dike: the grass pulling device. Tests with this device showed that it is possible to measure the critical velocity (= strength) of a grass cover, which is much easier than performing tests with a Wave Run-up or Overtopping Simulator.

Recent events including the establishment of national levee committees in many countries as well as the recent establishment of a Technical Committee on Levees at the International Commission on Large Dams points to the growing interest in levee safety. Additionally, the use of information obtained from risk assessments for levee safety management is becoming more prevalent throughout the world. Several countries are routinely applying this information to support decisions and prioritize actions in all aspects of the life cycle of a levee. An initial effort was given to gather information on the determination of risk related to levees and the application of that risk in making risk management decisions. In November 2018, representatives from Canada (British Columbia), France, Netherlands, United Kingdom, and United States met to discuss risk-informed decisions for levees. During the forum, each country representative shared how risk assessment information is applied to levee management decisions in their respective country. A report of findings from this workshop was presented as an oral presentation at the 2019 ICOLD Conference, and a number of additional countries were queried to expand the existing information. This paper will provide a comprehensive report of information collected from numerous countries on the collection and application of risk information to inform levee management decisions. Preliminary findings suggest that many countries are performing risk assessments but the use of that information to inform decisions is inconsistent and in many instances only using a component of the risk. It is anticipated that the findings from this effort will serve as the impetus for the development of consistent international practices in the management of levee safety.