PROJECT SUMMARY REPORT
Project acronym:
ame of Group
Leader:
User-Project Title:
Facility:
HyIV-FZK-07
Dr. Iris Möller
Wave dissipation and transformation over coastal vegetation
under extreme hydrodynamic loading
Large Wave Channel (GWK)
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).
User-Project Achievements and difficulties encountered:
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 wet and
largely flattened vegetation to 50% of the original height and to time-constraints
introduced both by (i) the late start of the experimental runs (a two week delay due
to longer time required for construction) and (ii) the limited amount of time the
vegetation can survive in the conditions to which it was exposed in the flume (wave
tests meant lack of oxygen and large degree of physical stress over time). Objective
2. thus changed to the quantification of the relationship between wave
characteristics and wave damping across a vegetated and non-vegetated marsh
platform. 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 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.