Coastal erosion is a common feature along the Dutch sandy
shorelines. Since 1990 a policy has been adopted that aims
at controlling structural erosion mainly through sand nourishments.
Although this policy has proven to be successful to keep
the coastline at its 1990 position, there is increased concern
with regard to the fate of the strategic sediment reserves
in deeper water, in view of sea level rise, new claims for
sand mining and construction of new harbours (e.g. Maasvlakte
Description of the coastline
The Dutch coastline along the southeast part of the North
sea is about 350 km long. The coast can be divided into
three regions, viz. the Delta coast in the south, the Holland
coast in the centre and the Wadden coast in the north. The
coast consists of straight sandy beaches and various large-scale
tidal inlet coasts. Large stretches of the coast have dunes
that prevent the low lying hinterland (which at many places
is below sea level) from being regularly flooded. Where
dunes are lacking, sea dikes have been constructed as a
flood protection measure.
Application of the Frame of Reference
of reference approach provides a systematic framework
for the development and implementation of a policy for coastal
management. The application of the Frame of Reference to
the Dutch coast is illustrated in the figure below and contains
the following elements:
In order to stop any further structural recession of the
coastline, in 1990 the Dutch Government adopted the national
policy of Dynamic Preservation. The strategic objective
of this policy is: a sustainable safety level and sustainable
preservation of values and functions in the dune area. This
objective was translated into the tactical objective
to maintain the coast line at its 1990 position. Considering
that morphological developments at larger scales (e.g. sand
losses at larger depths and long term developments like
seal level rise) are neglected, in 1995 the Dutch Government
decided on an extended large-scale approach: additional
compensation of sand losses at deeper water. The recent
National Spatial Strategy (2004) reconfirmed the strategic
objective of the large-scale coastal policy in the Netherlands,
rephrasing it as: to guarantee safety against flooding and
to preserve spatial quality of the coastal zone. As an additional
large-scale tactical objective, the Strategy defined
the preservation and improvement of the Coastal Foundation:
the area between dunes and the –20 m depth contour.
The Coastal Foundation is a new large-scale indicator acknowledging
sand as ‘the carrier of all functions’.
In fact the coastal policy of Dynamic Preservation
aims to maintain morphological boundary conditions proportional
to changing hydrodynamic boundary conditions; as such the
objective has been described recently as “growing
with sea level”. In this way, safety against flooding
of the predominantly sandy coast - primary concern of coastal
policy in the Netherlands -, can be guarenteed in a sustainable
Strategic coastal policy objectives have been translated
into tactical management objectives at three different scales
(see Fig. 2).
- Guarantee residual dune strength
- Maintain coastline position of 1990 (basal coast line)
- Preserve and improve coastal foundation
The basic idea behind the distinction into different management
scales is that the large scale provides boundary conditions
for the smaller scales. The minimum requirement of the dune
rest strength creates boundary conditions for safety against
flooding at any place and any moment. The maintenance of
the Basal Coast Line (BCL) creates boundary conditions for
the assurance of the dune rest strength over a period of
(10) years and alongshore distances of kilometers. And the
preservation of the Coastal Foundation in turn creates boundary
conditions for maintenance of the BCL over decades to centuries
and over alongshore distances of 10’s to 100’s
|Fig. 2 Definition sketch of three different
management objectives: dune residual strength (days
– metres); Basal Coast Line (years – kilometers)
and Coastal foundation (decades to centuries –
10’s to 100’s of kilometers)
Quantitative State Concept
The first element of the decision recipe for coastline management
is an objective assessment of the state of the coastal system.
To guarantee safety against flooding, safety standards have
been defined in the Flood Defence Act (1996) : dunes must
be able to withstand a storm event with a probability of
exceedance of 1 in 10,000 years in the provinces of North-
and South Holland. For coastal provinces with less economic
value the probabilities are 1 in 4,000, respectively 1 in
2,000 years. A test procedure using a dune erosion model
during hydrodynamic design conditions, indicates the strength
of the dunes as a flood defence. At the tactical level the
objective of the safety policy is to preserve the residual
strength of the dunes, defined as the minimal
dune volume to withstand the design storm ( Fig. 3).
|Figure 3 – Cross section of a coastal profile defining erosion and deposition during design conditions, the resulting position of the erosion line (R) and the position of the residual dune volume
To guarantee a sustainable preservation
of safety, the concept of the Momentary Coastline
(MCL) has been developed, defining the coastline
position as a function of the volume of sand in the near
shore zone ( see Fig. 4).
|MKL = Momentary Coast Line
DV = dune foot
GLW = mean low water line
|A = Momentary Coast Line zone (m²)
RSP = reference line
x = cross shore distance
| h = height between dune foot and mean
low water line
|Fig. 4 Definition sketch of Momentary
Coast Line ( TAW, 2002).
The calculation of the MCL is based on data
from the Dutch annual coastal monitoring programme JARKUS,
which has been operational since 1963. JARKUS measures coastal
depth profiles from the first dunes up to 1 km in a seaward
direction, at alongshore intervals of 250m (Fig. 5).
|Fig. 5 Alongshore position of yearly coast line
observations (250 m intervals)
A benchmarking procedure was developed, aimed at an objective
assessment of erosion problems of a structural nature.
For this purpose a predefined reference state needs to
be compared with an observed (or predicted) system state.
For the decision of interventions the benchmarking
procedure uses two criteria: 1) the position and trend
in momentary coastline (MCL) and 2) the sand volume per
coastal cell of the coastal foundation (i.e. Delta-, Holland-
and Wadden-coast) (see figure 7).
With respect to the position and trend in
momentary coast line the following procedure applies:
As a standard of reference the Basal Coast Line (BCL),
i.e. the position of the coast in 1990, has been defined
for each coastal section of 250 m wide. The actual state
of the coastline is based on the Testing Coast Line (TCL).
The position of the TCL is determined, in a similar way
as the BCL, by linear extrapolating the trend of coastline
positions (MCL) of ten previous years (see figure 6).
The state of the system is compared with the reference
state, i.e. by comparing the TCL position with the BCL
position. This comparison provides an indication for the
need for intervention.
The advantage to use the TCL instead of
the MCL is that the decision for intervention is based
on a ten year trend instead of the actual position of
the coastline in a single year.
|Fig. 6 Definition of BCL ( Basal Coast Line)
and of TCL ( Testing Coast Line) by linear extrapolation
of a 10 year trend.
|Fig. 7 Coastal cells of the Dutch coastal foundation
The procedure for preservation of the sand
volume in the coastal foundation, is based on compensation
of the yearly sand losses in the coastal foundation.
Geological information indicates that on
a time scale of 50 – 200 years, the coastal foundation
may be considered a closed system. Due to this fact, sea
level rise has a major negative effect on the active sand
volume of the coastal system. This sea level rise effect
may be calculated as the product of the area of the active
coastal system and the observed average sea level rise
over the last century.
Other factors resulting in sand losses of
the coastal foundation are sand mining, subsidence due
to gas extraction and sediment redistribution towards
the tidal basins as a result of major engineering works
in the past ( e.g. closure of the Zuiderzee in 1932).
The sum of all losses makes up the total
of the required compensation.
As stipulated in the policy of Dynamic Preservation, the
principal (most preferred) intervention procedure is sand
|Fig. 8 Sand nourishments along the Dutch coast
between 1990 and 2003 (RWS – RIKZ)
|Fig. 9 Beach nourishment at Walcheren ( SW Netherlands).
The Dynamic Preservation policy since 1990, has been succesful
in stopping structural erosion maintaining the position
of the coast line. The number of locations where the BCL
is exceeded has dropped to below 10% per year. The position
of the dune foot along the Holland Coast on the average
has shifted seaward at a rate of 1 m per year (Koster,
2007; Arens, 2009). The total yearly nourishment volume,
6 Mm3 between 1990 and 2000, was raised to 12 Mm³/year
in 2001 in order to preserve the total volume of the Coastal
Besides the success of maintaining the Basal
Coast Line, the policy evaluation report (RWS, 2006) states
some critical remarks: (-) considerations regarding nourishment
of the coastal foundation need to be more transparant;
and (-) nourishments aimed at functions other than safety
against flooding ( e.g. recreation and ecological and
natural values) need to get more attention.
Latest insight in the state of the coastal
system (De Ronde, 2009) shows that at the present rate
of sea level rise ( 2 mm/year), the total sediment loss
from the coastal foundation amounts ca. 20 Mm³/year.
Apparently, the present 12 Mm³/year is insufficient
to compensate for all losses from the coastal foundation.
The scale of nourishment will need upscaling in the near
In a study on options for climate adaptation
of the Netherlands for the coming century, the Deltacommissie
(2008; see www.deltacommissie.com)
is promoting sand nourishment as the major method to guarantee
a climate proof coastal zone, even suggesting a raise
of the yearly nourishment volume up to 84 Mm³/year
untill 2050, to anticipate a SLR of 13 mm per year. Such
a proposal is feasible in light of the fact that no physical
limitations exist in availability of sand resources on
the North Sea. Spatial reservation for future sand mining
of a zone between minus 20 m and the outer limit of 12
miles, must be able guarantee this availability.