This document discusses the use of the Xbeach model in the formal assessment of flood defences in the Netherlands in 2023. Currently, a 1D volume balance model called Duros+ is used, but it has limitations for certain coastal environments. Xbeach can potentially handle more complex situations like curved coastlines and structures. The challenges to using Xbeach include making it fully probabilistic, more user-friendly, continued validation, improving the wave envelope representation, and harmonizing hydraulic load calculations. Rijkswaterstaat aims to start work in 2018 to use Xbeach as the main dune erosion model by 2023, and the Ministry of Infrastructure will provide goals for the new assessment tools that year.

1. Towards using
Xbeach in the Dutch
formal assessment of
flood defences in
2023
Robert Slomp
Quirijn Lodder

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Flood Prone
Netherlands
• 26 % below sea level
• 55% is susceptible to
flooding
• 60% of our population lives
below sea level
• > 60 % of our economic
value is earned in the
lowest-parts of the country
• 350 km of coastline, dunes
and hard sea defences

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Flood Risk
Standards for
significant flood
defences

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How do we do this in Coastal Management?

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How do we do this in Coastal Management?
And wat is the role of Xbeach?

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Daily
circumstances
Gradual
structural
erosion
Beach, dunes and
coastal functions
are preserved
Compensate
erosion through
nourishments
Extreme
stormsurges
Severe
instantaneous
erosion
Assessment of
Flood Risk,
reinforcements
when necessary
Flood Risk is at
societal
tolerable level
Coastal
Management
Policy
Coastal
Recession
Impact

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7 De kust van Vlieland7 Oktober 2011
Vlieland
1688
1795

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Nourishments
08-09-2015

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Daily
circumstances
Gradual
structural
erosion
Beach, dunes and
coastal functions
are preserved
Compensate
erosion through
nourishments
Extreme
stormsurges
Severe
instantaneous
erosion
Assessment of
Flood Risk,
reinforcements
when necessary
Flood Risk is at
societal
tolerable level
Coastal
Management
Policy
Coastal
Recession
Impact

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Example Texel:
assess if the
total flood
probability for
the green
segment is
smaller then
1/1000 per year

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Current method
• Duros+ as implemented in
MorphAn
• 1 dimensional volume balance
approach dune erosion model
• Based on multiple large scale
wave flume tests in the 1970 –
1990 timeframe for variations
on the so called reference
profile.
• Hydraulic loads for specified
frequency based on maximum
permissible flood probability,
length effect and how the
probability is divided over
different parts of the fault tree
Reference profile
Example: Retreat distance vs Time plot
Example: dune erosion for 2007 profile Vlieland

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Fault Tree / Event Tree and
standard failure probability budget
How the probability is divided over
different parts of the fault tree

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Current method
• Semi probabilistic when used for assessment. The method was
derived in a probabilistic approach.
• In quite some areas additional assessments have to be done
in order to give a verdict on the current probability of failure
and flood risk

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Examples
Vlieland
• Curved coastline
• Strong wave damping on
Ebb-tidal delta. Hydraulic
loads at -20m depth contour
are not representative for
dune erosion
• Tidal channel in front of the
beach
• Currently adapted hydraulic
loads are necessary for the
assessment

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Examples
Katwijk &
Noordwijk
• Hybrid flood defence: artificial
dune and dyke inside the dune.
• Overtopping of the artificial
dunes might occur during
“normative” conditions
• Currently DurosTa (Time
dependent Duros+) in
combination with overtopping
model PC-Overslag is needed

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Examples
Multiple dune rows
• Dutch dunes are mostly
much wider then just one
dune row due to geological
reasons
• When the first dune fails to
meet the criteria derived
from the maximum
permissible flood probability
a second or third dune row
might function as flood
defence
• Currently additional GIS and
”special” Duros+ analyses
have to be made

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Examples
Hard objects in the
dunes
• In urban area’s often hard
structures are present in the
dunes e.g. retaining walls for
boulevards and basements of
buildings.
• Currently Duros+ with additional
erosion added locally near the
hard structure based on a study
by Deltares and Arcadis (2013)

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Examples
Current position of
Xbeach
• The detailed assessment with
Duros is compulsory. If the
result is not satisfactory a
local approach may be
chosen. This is often an
advanced assessment.
• Xbeach may be used in the
advanced assessment in
these kind of examples but it
is not mandatory
• Often also other models and
tools are used and allowed

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Outlook
• Rijkswaterstaat sees Xbeach as the most promising model for
the future formal assessment of (coastal) dune flood defences
because Xbeach is in principle able to handle most if not all of
these examples: a curved coast, hybrid constructions and hard
elements in soft dunes.
• Aim is to use Xbeach (1D or 2D) as the formal assessment
model for (coastal) dunes from 2023 onwards
• Depending on progress mandatory for the “detailed”
assessment or optional for advanced assessments.
• Use as a design tool for sandy inland shores (lakes) to reduce
wave action on dikes

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Challenges to get there
• Probabilistic: The aim is to go to fully probabilistic assessment for all Dutch
flood defences, to comply with the new safety standards . So also the dunes.
Probabilistic Xbeach is needed (at least 1D in 2023)
• User friendly: The user-friendliness has to be improved. Non specialist
mode/settings are needed.
• Continued validation: the validation of Xbeach has to continue both on
flume tests as on real world cases (using international testbed).
• Wave envelope: special attention is needed for the wave envelope this
sometimes seems larger than expected based on engineering experiences.
This is important for the assessment of wave overtopping for hybrid
constructions.
• Harmonized hydraulic loads: the way hydraulic loads are derived differs
quite a bit over all different types of Dutch water bodies (North Sea, Rivers
and Lakes) and for dikes and dunes. Harmonization in approach and strategic
choices are needed.
• And probably more...

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Involvement of Ministry of Infrastructure and the
Environment
• At Rijkswaterstaat we would like to start in 2018
with the actions needed to use Xbeach as formal
dunes erosion model per 2023.
• We would like to uniform our approach for hydraulic
loads for dikes and dunes.
• The ministry will set the goals for the formal
Assessment Tools 2023 at the beginning of 2018
based on feedback from:
– Users of the tools
– Developers
– Researchers

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