7.9 Conclusion and Discussion
Coastal dunes alternate between phases of mobility and stability. A conceptual
model is presented in which the main factors influencing either mobility or stability and their complex interactions are described. Climatic factors impose their
influence through two of the driving forces: wind energy and vegetation.
Environmental factors, often influenced by man, affect vegetation growth and sand
availability in several ways. Both climate- and man-induced changes can alter the
state of the system. For managers, it is important to know how they can interfere
with these factors, to achieve their management goals.
The discussion of dune mobility and stability, and of environmental and
climatic control has brought us to the conclusion that dunes in general reflect two
different settings. In desert dunes (mostly continental dunes), there is an almost 1:1
link between dune mobility and climate. Vegetation growth is fully governed by
climate (and humans). With a deterioration in climate (or increased exploitation),
vegetation disappears and dunes become mobile. These dunes respond directly to
climatic change. In many of the coastal dunes in the world climate is not arid or
semi-arid, but temperate and/or humid. Although these dunes only can develop
under the guarantee of wind, the other decisive factor is sand supply. Without sand
supply, these dunes finally will stabilize. Sand supply can either be from the
shoreface to the beach, resulting in burial of vegetation that cannot cope with
deposition rates, or the release of previously deposited sand by erosion. Under
climatic conditions with limited possibilities for plant growth, small amounts of
sand suffice. Under climatic conditions favorable for vegetation growth (like ours),
large amounts of sand are needed. Bare foredune cliffs 15–25 m high, in combination with the right winds, can produce these amounts of sand. The response of
these systems to (delicate) climatic change is much more complicated.
In The Netherlands dune mobility is related to sand supply. Under the current
conditions, a huge supply is needed to overcome fixation by vegetation. Removing
vegetation alone is probably not sufficient to maintain dune mobility and consequently biodiversity. Some help from the sea will sustain a more durable development. In the case of foredune erosion, significant amounts of sand may be
released and transferred inland by the wind. Allowing foredune erosion followed
by dune remobilization may ‘‘restart the engine.’’ In other cases, where the
foredune cannot be incorporated, maintenance of mobility is necessary, at least
until all roots are removed from the system. However, this is no guarantee of
durable mobility either: dispersal of marram from seeds may under favorable
conditions stabilize bare surfaces rapidly.
A smart design of nourishments could give room for coastal erosion, creating
‘‘corridors’’ in the foredunes where sand can be transported inland. More
knowledge is needed on the transfer mechanisms. It is clear that wave erosion
releases sand for subsequent aeolian transportation, and that a very large supply of
122
S. M. Arens et al.
Coastal dunes alternate between phases of mobility and stability. A conceptual
model is presented in which the main factors influencing either mobility or stability and their complex interactions are described. Climatic factors impose their
influence through two of the driving forces: wind energy and vegetation.
Environmental factors, often influenced by man, affect vegetation growth and sand
availability in several ways. Both climate- and man-induced changes can alter the
state of the system. For managers, it is important to know how they can interfere
with these factors, to achieve their management goals.
The discussion of dune mobility and stability, and of environmental and
climatic control has brought us to the conclusion that dunes in general reflect two
different settings. In desert dunes (mostly continental dunes), there is an almost 1:1
link between dune mobility and climate. Vegetation growth is fully governed by
climate (and humans). With a deterioration in climate (or increased exploitation),
vegetation disappears and dunes become mobile. These dunes respond directly to
climatic change. In many of the coastal dunes in the world climate is not arid or
semi-arid, but temperate and/or humid. Although these dunes only can develop
under the guarantee of wind, the other decisive factor is sand supply. Without sand
supply, these dunes finally will stabilize. Sand supply can either be from the
shoreface to the beach, resulting in burial of vegetation that cannot cope with
deposition rates, or the release of previously deposited sand by erosion. Under
climatic conditions with limited possibilities for plant growth, small amounts of
sand suffice. Under climatic conditions favorable for vegetation growth (like ours),
large amounts of sand are needed. Bare foredune cliffs 15–25 m high, in combination with the right winds, can produce these amounts of sand. The response of
these systems to (delicate) climatic change is much more complicated.
In The Netherlands dune mobility is related to sand supply. Under the current
conditions, a huge supply is needed to overcome fixation by vegetation. Removing
vegetation alone is probably not sufficient to maintain dune mobility and consequently biodiversity. Some help from the sea will sustain a more durable development. In the case of foredune erosion, significant amounts of sand may be
released and transferred inland by the wind. Allowing foredune erosion followed
by dune remobilization may ‘‘restart the engine.’’ In other cases, where the
foredune cannot be incorporated, maintenance of mobility is necessary, at least
until all roots are removed from the system. However, this is no guarantee of
durable mobility either: dispersal of marram from seeds may under favorable
conditions stabilize bare surfaces rapidly.
A smart design of nourishments could give room for coastal erosion, creating
‘‘corridors’’ in the foredunes where sand can be transported inland. More
knowledge is needed on the transfer mechanisms. It is clear that wave erosion
releases sand for subsequent aeolian transportation, and that a very large supply of
122
S. M. Arens et al.