Reliability of willows for wave load reduction on river dikes

Abstract

Flood defenses are important to keep large parts of the Netherlands dry. These primary flood defences
are every twelve years assessed on safety during the Dutch national safety assessment. In 2017, the new
safety standards (WBI2017) were introduced. These stricter standards caused that in total 251 km of the
primary flood defences was rejected on height, which includes the failure mechanisms wave overtopping
and overflow. Traditionally, dike strengthening is required. However, the new Delta Plan gives also spe-
cial attention to the implementation of innovative nature-based solutions, for example the integration of
shallow foreshores without or with vegetation in the safety assessment. These foreshores can reduce the
incoming wave load by dissipating energy through wave breaking, bottom friction and vegetation attenu-
ation.
(Vegetated) foreshores are also present in front of some river dikes, called floodplains. Probably, 5 to 15
procent of the foreshores in the current reinforcement program (HWBP) is not or conservatively consid-
ered in the safety assessment, which entails unnecessary costs (approximately 0.5 - 1 billion euros). In a
quickscan of the riverine area several locations are indicated which meet the requirements as promising
locations for wave reduction by vegetation. However, despite a renewed focus on building with nature,
implementation of nature-based flood protections is hindered by te dike managers because of a knowl-
edge gap in the effects of the variable vegetation on wave energy dissipation. The state and functioning of
vegetation during design conditions is uncertain. This uncertainty is the most important reason for dike
managers to hamper the implementation of a nature-based flood protection. The main objective of this
thesis is to determine the reliability of willow vegetation as wave load reduction for river dikes.
In order to assess the variability of vegetation on the wave damping, a field test and simulation models
are used. The measurement set-up of Deltares consists of a plot of 7 x 7 meters with young willow
branches and pressure transducers perpendicular to the river, which measure the (by ships produced)
waves in the river Noord. Based on the field measurements, a model is developed and applied to determine
the drag coefficient of willow branches. This model is an one-dimensional SWASH model which contains
the bathymetry and the willow vegetation, characterised by a density, branch diameter and drag coefficient
for three height ranges. These parameters provide the obtained wave attenuation by vegetation. The
diameter and density were measured in the field. Therefore, the drag coefficient can be determined by
calibrating the outcomes of the numerical model with the field measurements.
From the field measurements by Deltares, it can be concluded that willow branches reduce incoming
waves with a height higher than 0.20 meter with approximately 4 % per meter willow vegetation width.
The wave attenuation depends mainly on the water depth and shape of the willows. The stiff stem of old
willows damps waves less than high dense, more flexible branches above the stem, and the wave damping
decreases with increasing water depth mainly when branches submerge. The field measurements are used
to calibrate the drag coefficient. The 25th and 75th-percent value of the calibrated drag coefficient were
calculated on -1 and 5, a large range. A negative value may not be possible, because this means an increase
of the wave height by the willow vegetation. This large range in calibrated values and the existence of
negative values are caused by the measurement set-up. The plot with willows was too small for accurate
measurements and the crates, in which the willow branches were planted, caused distortion of the waves
due to the sudden bottom elevation. Because of this, the first sensor at the edge of the crate (at the river
side) has probably encouter measurement errors. Third, the angle of the incoming ship waves caused an
underestimation of the calculated wave damping capacity of willows. The set-up should be improved to
give proper results and conclusions about the drag coefficient of willows. The drag coefficient remains
therefore uncertain and gets a large standard deviation in the second part of the research.

In this second part, a study is carried out to the uncertainties of implementing wave damping wil-
low vegetation. A one dimensional wave model based on the wave energy balance, is created for a dike
section at the Heesseltsche Uiterwaarden, alongside the river Waal. Waterboard Rivierenland suggest
this floodplain as opportunity for planting wave damping willow vegetation to compensate the required
heightening of the dike.
In the wave model, various input parameters are required to calculate the wave damping: boundary con-
ditions, dike-foreshore parameters and the vegetation parameters. These parameters have all certain stan-
dard deviations or variations. First, the effect of these standard deviations on the wave damping is deter-
mined, and so the overtopping discharge and required crest level. Second, a thorough analysis is made
to exogenous threats which affect the vegetation parameters. For willows this could be diseases, insects,
ice, animals, drought, flood, fire and storm. The effects of these threats on the vegetation parameters are
estimated by the knowledge and expertise of willow experts and a literature study.
It is concluded that from all input parameters with standard deviations, the variations of the vegetation pa-
rameters has the largest effect on the obtained wave overtopping. The wave reduction depends mostly on
the vegetation height and water depth, which was also concluded by the field measurements. Second im-
portant vegetation parameter is the density of the branches or the drag coefficient, if the drag for willows
can be lower than 0.4. The most relevant influential threats with a high risk and significant consequence
are storms, floodings and droughts.
These effects of the standard deviations of input parameters, exogenous threats and annual maintenance
on the overtopping discharge (and required crest level) are used for a recommendation about the design
and maintenance strategy of willow vegetation as solution. The willow vegetation should ensure enough
wave damping during the total lifespan of the reinforcement to comply the safety assessment. Therefore
it is recommended to extend the width of the willow vegetation with a buffer zone in which all the vegeta-
tion uncertainties can be accommodated. The width of the vegetation plot is therefore the main parameter
that should be assessed during inspection. Note that the uncertainties and probabilities of the exogenous
threats are assumed. Further research is required to validate these assumed values.
To conclude, willows can be used as wave damping vegetation at floodplains along rivers. The
branches of willows will reduce the incoming waves significantly if they are not submerged. The height
of the willows is therefore the most important vegetation factor and should be assessed during inspections
just like the required vegetation width to ensure the wave damping capacity during the total lifespan of
the dike reinforcement. The reliability of willows for wave load reduction on river dikes is sufficient if
the maintainer apply the design and maintenance principles as recommended.