Book of Abstracts 2024

V. Vuik¹, M. Huis in 't Veld¹ , M. Benit¹, N. Tack²

¹ HKV, The Netherlands; ² HaskoningDHV Nederland BV , The Netherlands

^*Corresponding author: nynke.tack@rhdhv.com

Introduction

Due to climate change, larger and more powerful storms are anticipated on the IJsselmeer. Therefore, the safety requirements are elevated, resulting in the necessity to reinforce the IJsselmeer dyke. During the exploration phase of this dike reinforcement multiple design options were considered including a foreshore to reduce wave-overtopping.

Studies and other projects, like the Houtribdijk and the Prinshendrik Zanddijk, have shown that a sandy foreshore nourishment can be an effective solution to reduce the wave impact on the primary barrier. In addition, foreshores contribute greatly to strengthening local biodiversity, which is crucial for a sustainable, future ecosystem. For the reinforcement of the IJsselmeer dyke a combination of a dam and foreshore was shown to be the preferred solution.

Objective and Methods

The objective of the study is to quantify the extent to which a combination of a dam and a natural foreshore can reduce the wave impact on the structures behind it, specifically in the case of the traditional IJsselmeer dyke.

The methodology used to quantify this impact consists of two steps. In the first step, the foreshore has been recreated in miniature during a physical wave flume test at Deltares. Three different configurations of the dam were tested, altering the structure and the height above the water surface. Additionally, the length of the foreshore was varied to determine its impact on wave head height. For each configuration, stability and several hydraulic loads were measured.

In step 2, the parameters from the wave flume test were utilized to determine the correction factors and uncertainties of the SWASH-wave model. This model was then employed to create new HRD databases, storing information about the influence of different foreshore designs on water height, wave height, period, and direction. By inputting these databases, the erosion and stability (GEBU/GEKB) modules were run, resulting in the final failure probabilities of the foreshore design.

Results

Our study demonstrates that integrating physical tests with modeling offers an effective approach for assessing the influence of nature-based foreshores on wave loads affecting defense structures. Key takeaways include:

1. Stone Gradation and Height: Stone gradation and height significantly impact dam stability. We observed that the stability of stone gravels (ranging from 300-1,000 kg) in the topsoil depends on relative water depth compared to the crest level. Reducing stone gradation to 40-200 kg and 1-50 kg in the toe zone and inner foreshore, respectively, minimized damage. Dam height adjustments affected stability differently at the front and back.
2. Foreshore Length and Water Level Changes: Foreshore length directly correlates with water level variations. Our tests on foreshores of 40, 60, and 100 meters revealed distinct water level rises. These variations depended on foreshore length and water depths. Longer foreshores led to greater energy dissipation and improved wave reduction.
3. Failure Probability Assessment: Foreshore lengths between 40 and 120 meters meet water safety standards. Depending on dyke location, failure probabilities range from 1 in 61,224 years to 1 in 295,930 years.

These insights can inform future reinforcement projects, allowing informed comparisons between traditional and nature-based solutions.

Schematic representation of the model train from base loads to local hydraulic loads, through the foreshore, to a failure probability calculation with the GEBU/GEKB module.