Loon-Steensma, J.M., van

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Wageningen Yield 

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Dr. ir. Jantsje van Loon-Steensma is a 
researcher and lecturer of Climate Change 
and Flood Protection in the department 
of Enviromental Sciences at Wageningen 
University & Research. In the MFFD Program 
she works as a Postdoc in the project ‘Con-
tribution of Multifunctional Flood Defenses 
to landscape values and spatial quality’. Her 
research focuses on climate adaptation by 
integrating functions in flood defenses. She 
combines hydraulic, ecological, geographi-
cal and economical aspects in the search for 
climate proof, robust flood defenses that in 
addition to flood protection, also favor nature, 
landscape, heritage, recreation or economic 
values.
Jantsje van Loon-Steensma
INTEGRATING SALT MARSH FORELAND INTO THE DIKE DESIGN
A WINDOW OF OPPORTUNITY FOR A SELF-MAINTAINING LEVEE
Integrating natural salt marsh foreland with a 
structural flood defense is increasingly seen 
as a promising approach to flood protection 
under changing climatic conditions and as a 
way to combine multiple functions and values 
in the coastal zone (see Van Loon-Steensma 
page 148 this volume). The potential of this 
multifunctional flood defense concept has 
been explored in the Dutch Wadden Sea 
region. 
Extensive salt marshes are present along 
the dikes of both the Wadden Sea mainland 
and the barrier islands (see figure 1). These 
marshes form a shallow transition zone that 
attenuates incoming waves before they reach 
the dike. When water depths in this zone di-
minish to less than the wave base, the wave’s 
shape is modified and it starts shoaling. Wave 
length and wave velocity both decrease, and 
wave height increases before breaking. After 
breaking, wave energy is further dissipated 
by drag induced by marsh vegetation and 
by bottom friction. Wave damping depends 
strongly on the profile of the coast, the water 
depth above the salt marsh, the width of the 
salt marsh zone, surface topography, and veg-
etation characteristics (Le Hir et al. 2000; see 
also studies cited in Anderson et al. 2011 and in 
Gedan et al. 2011).
Salt marshes are areas vegetated by salt-
tolerant plants and subject to periodic flood-
ing due to the fluctuating water levels of the 
adjoining saline water body (Adam 1990). 
They generally develop high in the intertidal 
zone in sheltered conditions where wave ac-
tion is limited, allowing fine sediment to settle 
and accumulate (Allen and Pye 1992; Allen 
2000). Once the upper part of the intertidal 
zone is no longer submerged with each tide, 
salt marsh plants can become established. 
By trapping sediment, pioneer vegetation 
contributes to accretion and development of 
creeks, rendering the environment suitable 
for species (forbs, grasses and low shrubs) 
that need more stable sediment and are less 
tolerant to flooding (in duration as well as 
frequency) (Adam 1990; Allen 2000). This 
results in zones, with pioneer species seaward 
and more mature vegetation in the higher 
landward zone. Because of the positive feed-
back effects of salt marsh vegetation and 
sedimentation, vegetation plays an important 
role in salt marsh formation (Allen 2000). 
Salt marsh plants can thus be understood as 
eco-engineers, as organisms that physically 
change the abiotic environment (Jones et al. 
1994; Hastings et al. 2007). 
However, like most coastal systems, salt 
marsh ecosystems are extremely sensitive to 
changing environmental conditions. Strong 
currents or wave attack may lead to lateral 
erosion. Generally, a moderate sea level rise 
creates conditions where marshes build up 
by accretion (Allen 2000) or shift landward. 
To keep pace with a rising sea level, however, 
a permanent supply of sediment needs to 
enter the tidal system. If sediment import is 
insufficient, flats and marshes will drown (Van 
Goor et al. 2003).
Various exploratory and field studies have 
been conducted in the Wadden Sea region 
(which is characterized by shallow water 
depths and moderate storm wave heights).  
These studies have shown that the salt 
marsh areas adjacent to the dikes signifi-
cantly affect wave impact on the dike (see 
e.g. Smaale, 2014; Vuik et al. 2016). Includ-
ing salt marsh foreland into the dike design 
would affect both the height and revetment 
requirements needed to meet the required 
safety level. Analysis of the effect of realistic 
vegetation characteristics on modeled wave 
heights also showed that wave damping is 
strongly related to the variety of vegetation 
Figure 1. Extensive
salt marshes along 
the Wadden Sea 
dikes (Image cour-
tesy Jantsje van 
Loon-Steensma).
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Extensive research on this subject can be 
found in the PhD thesis of Jantsje van Loon-
Steensma: ‘Salt marshes for flood protection. 
Long-term adaptation by combining func-
tions in flood defenses' (2014).
The thesis investigates if and how the same or 
an even higher level of safety can be achieved 
in the Wadden region by means of creating 
a flood defence zone that favours, besides 
flood protection, nature and landscape values 
or heritage, recreational, and even economic 
values. While all available innovative flood 
defenses are considered, special attention 
is given to the role of salt marshes in this 
context.
The thesis shows that integration of salt 
marshes into long-term adaptation strategies 
is very promising for the Wadden region, 
especially for dike sections where salt 
marshes are already present or developing. 
Vegetation is a major factor in the wave 
damping capacity of salt marsh forelands, 
therefore it is important to take into account 
the zonation of different plant. Furthermore, 
the thesis reveals that in salt marsh 
restoration, the goals of flood protection 
and nature and habitat conservation and 
enhancement can be mutually reinforcing.
and the specific zone of the salt marsh (Van 
Loon-Steensma et al. 2016). At the study 
site, a densely vegetated foreland 90 m wide 
would dampen the wave height more than 
80% under average storm conditions (with 
a frequency of 5–10 times/y), whereas under 
extreme conditions (1/2000 y) the same fore-
land would dampen the wave height up to 
50% (Van Loon-Steensma et al. 2016). These 
results emphasize the potential of a multi-
functional flood defence using salt marsh 
forelands, which integrates safety with nature 
and landscape values.  
However, flood protection imposes different 
requirements on the extent and features of 
salt marshes than nature conservation and 
development (Van Loon-Steensma & Vellinga, 
2013). Wave damping is most effective with 
a high, stable, and densely vegetated salt 
marsh, while nature thrives with dynamic 
processes and differences in elevation (Allen, 
2000). In practice, this means that the design 
of the flood defense must offer space for nat-
ural salt marsh processes, which require varia-
tions in height and depth developing over 
time in the foreshore zone. The design needs 
to combine hard coastal defense infrastruc-
ture with a dynamic ecological zone adjacent 
to it: The overall design will thus be charac-
terized by a broad zone that includes a hard 
engineered solution, rather than by a merely 
metered cross section commonly used in en-
gineering solutions (Figure 3). If this ecologi-
cal zone is able to adapt to changing condi-
tions, for example keeping pace with sea level 
rise, then such a broad flood protection zone 
can be seen as a self-maintaining levee. Of 
course, the vegetated foreland and adjacent 
mudflats must be managed and maintained 
in such a way that they can meet as far as 
possible both the ambitions of flood protec-
tion and those of nature conservation.
Figure 2.
Salt marshes along 
Galveston Island at 
the bay shore, Texas, 
USA (Image courtesy 
Baukje Kothuis).
Figure 3. (above). 
Coastal defense 
system combining 
hard engineered in-
frastructure (a dike) 
with an adjacent 
dynamic ecological 
zone (a salt marsh) 
(Source: Van Loon-
Steensma et al. 2014).


Integrating salt-marsh foreland into the dike design : A window of opportunity for a self-maintaining levee

van Loon-Steensma, J.M.

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Integrating salt-marsh foreland into the dike design : A window of opportunity for a self-maintaining levee

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