Experiments by Invited Researchers


Wave dissipation and transformation over coastal vegetation under extreme hydrodynamic loading

Project acronym: HyIV-FZK-07
Name of Group Leader: Dr. Iris Möller
User-Project Title: Wave dissipation and transformation over coastal vegetation under extreme hydrodynamic loading
Facility: Large Wave Channel (GWK)
Data Management Report: Report

User-Project Objectives

This study will make a significant contribution to the pressing issues of coastal defence and protection in the context of sea level rise and potential increases in extreme wave events on many of the world’s shorelines. The overall aim is to obtain reliable data on wave damping across macrotidal coastal marshes, with and without an intact middle to high elevation vegetation cover. The study will focus on (a) storm loading under realistic wave conditions and (b) loading in a degraded marsh condition. The realistic and prototype scale data obtained will provide a sound basis for the development of new design and safety concepts for vegetated foreshores as storm buffers. Integration of the results into European efforts on ecological safety concepts should be seamless and is already planned. To achieve these aims, the following objectives will be met: 1. Quantify the relationship between water level and wave damping across a marsh platform with intact and degraded (50 percent original height) vegetation cover for a range of wave heights; 2. Quantify the relationship between wave height and wave damping across a marsh platform with intact and degraded vegetation for a range of water depths; Logistical constraints (relating to the acquisition of intact vegetated wetland units for insertion into the flume) mean that the experiment shall be constrained to salt-marsh vegetation. This also allows the widest range of inundation scenarios to be simulated (partial and full submergence up to inundation depths at which the vegetated layer occupies less than 20 percent of the water column).

Short description of the work carried out

The objectives of the project were largely achieved, although the ‘degraded’ vegetation tests were replaced by tests over the mud surface after vegetation had been fully removed by mowing. This was due to the difficulty of mowing the vegetation to 50% height and to time-constraints introduced both by a delay due to longer time required for construction and the limited time vegetation can survive in flume conditions. A wide range of wave conditions were generated with vegetation in place in 2m water depth (13 tests with regular and irregular wave respectively) and 1m depth (5 tests with regular and irregular waves respectively) above the marsh soil surface. After mowing of the vegetation 4 of the 2m water depth tests and 2 of the 1m water depth tests were repeated as ‘control runs’ (each with both irregular and regular waves). For all those tests, the test section remained in place and any damage that occurred to the test section as a result of interference of artificial structures with waves was minimal (and easily repaired by infilling with clay on days when the section was exposed to allow plants to acquire oxygen). Any lasting damage to the test setup was thus purely due to wave action upon the plants or the soil surface itself. All equipment functioned for most tests. Some of the force transducers supplied by the users failed after a certain number of tests, but data was still acquired from the remaining force transducers. Some of the video cameras failed to operate at times, but the critical underwater window cameras operated throughout. Difficulties were encountered with the Data Acquisition System on the last day of tests but these were rapidly resolved by the staff at the facility.

Highlights of important research results

Quantitative data analysis has not yet been completed. But visual inspection of preliminary results from the Data Acquisition System showed that dissipation thresholds has been detected and are water depth / wave dependent. Plant breakage thresholds were also crossed as wave energy was increased, and soil erosion eventually began during the very high wave energy tests. Overall, the very high observed stability of the marsh platform was unexpected, but the data are also expected to show that, during high wave energy events, dissipation across the marsh may be reduced or occurs largely as a result of wave breaking, depending on wave steepness.

Iris Moeller, Tom Spencer, Franziska Rupprecht, Matthias Kudella, Maike Paul, Bregje Van Wesenbeeck, Guido Wolters, Kai Jensen, Tjeerd Bouma Process thresholds in plant-wave interactions on saltmarsh surfaces: implications for wave dissipation and marsh stability , 2014