meters (Van Boxel et al. 1997). There are few data on the areas influenced around
blowouts, but measurements in The Netherlands indicate that one blowout can affect
several hectares with its sand spray. The result is a landscape with a mosaic of
younger vegetation types surrounded by older, even climax stages. The percentage
of younger types depends on the number of blowouts.
In both mobile dunes and blowouts, several stages of vegetation succession are
present, but their arrangement depends on the geomorphological dynamics, and is
completely different for the two systems.
7.3 Dune Mobility in The Netherlands
What does the Dutch system look like? What changes have affected the coastal
dunes in the past few centuries and what was their impact on dune mobility?
The Dutch coast is a sandy coast that has developed since the last Ice Age. With
a rising sea level a system of beach barriers has developed. Around 5000 BC,
despite a further rising sea level, the beach barriers reached their seaward-most
extension (Zagwijn 1986). Small dune systems, now referred to as the Older
Dunes, existed on the barriers. Since 4000 BC the barriers have gradually eroded.
Around AD 800 a new phase in coastal development started, when vast quantities
of sand, probably released by increased erosion rates, invaded the older barrier
systems and started to form the Younger Dunes. These dunes developed in several
phases of major dune mobility, roughly between AD 800 and 1600 (Jelgersma
et al. 1970; Klijn 1990). The exact mechanisms are not well understood. Klijn
(1990) and Jelgersma et al. (1970) mention storm activity, coastal erosion, and
exploitation by man as probable causes. But the main question remains about the
origin of the sand that built up the Younger Dunes: which process triggered the
invasion of these massive amounts of sand? A study by Pool and van der Valk
(1988) pointed out that during development of the Younger Dunes as much as
50 m
3 /m/year of sand was stored in the dunes, over several hundred years and
along several tens of kilometers. These amounts exceed the present day transport
by an order of magnitude. Probably, the input of sand was related to processes of
unknown origin on the shoreface, resulting in a steepening of the shoreface,
thereby providing the beach with large quantities of sand (Zagwijn 1986). Storm
surges in the tenth to sixteenth centuries caused large-scale coastal erosion and a
probable remobilization of sand. Whether there is a connection to the supply of
sediment by the large rivers is unclear. Climate variation may have played a role,
with periods of increased storminess and severe droughts as triggers for dune
mobilization. A recent study in the Dutch Delta area (Beekman 2007) relates
massive phases of dune building to coastal erosion, triggered by changes in coastal
inlets due to human interference.
The present morphology of the dunes reflects the extent and vigor of dune
mobility in the past. Most of our dunes consist of large series of parabolic dunes,
increasing in dimension from west (coastline) to east, alternating with large
7 Restoration of Dune Mobility in The Netherlands
111
blowouts, but measurements in The Netherlands indicate that one blowout can affect
several hectares with its sand spray. The result is a landscape with a mosaic of
younger vegetation types surrounded by older, even climax stages. The percentage
of younger types depends on the number of blowouts.
In both mobile dunes and blowouts, several stages of vegetation succession are
present, but their arrangement depends on the geomorphological dynamics, and is
completely different for the two systems.
7.3 Dune Mobility in The Netherlands
What does the Dutch system look like? What changes have affected the coastal
dunes in the past few centuries and what was their impact on dune mobility?
The Dutch coast is a sandy coast that has developed since the last Ice Age. With
a rising sea level a system of beach barriers has developed. Around 5000 BC,
despite a further rising sea level, the beach barriers reached their seaward-most
extension (Zagwijn 1986). Small dune systems, now referred to as the Older
Dunes, existed on the barriers. Since 4000 BC the barriers have gradually eroded.
Around AD 800 a new phase in coastal development started, when vast quantities
of sand, probably released by increased erosion rates, invaded the older barrier
systems and started to form the Younger Dunes. These dunes developed in several
phases of major dune mobility, roughly between AD 800 and 1600 (Jelgersma
et al. 1970; Klijn 1990). The exact mechanisms are not well understood. Klijn
(1990) and Jelgersma et al. (1970) mention storm activity, coastal erosion, and
exploitation by man as probable causes. But the main question remains about the
origin of the sand that built up the Younger Dunes: which process triggered the
invasion of these massive amounts of sand? A study by Pool and van der Valk
(1988) pointed out that during development of the Younger Dunes as much as
50 m
3 /m/year of sand was stored in the dunes, over several hundred years and
along several tens of kilometers. These amounts exceed the present day transport
by an order of magnitude. Probably, the input of sand was related to processes of
unknown origin on the shoreface, resulting in a steepening of the shoreface,
thereby providing the beach with large quantities of sand (Zagwijn 1986). Storm
surges in the tenth to sixteenth centuries caused large-scale coastal erosion and a
probable remobilization of sand. Whether there is a connection to the supply of
sediment by the large rivers is unclear. Climate variation may have played a role,
with periods of increased storminess and severe droughts as triggers for dune
mobilization. A recent study in the Dutch Delta area (Beekman 2007) relates
massive phases of dune building to coastal erosion, triggered by changes in coastal
inlets due to human interference.
The present morphology of the dunes reflects the extent and vigor of dune
mobility in the past. Most of our dunes consist of large series of parabolic dunes,
increasing in dimension from west (coastline) to east, alternating with large
7 Restoration of Dune Mobility in The Netherlands
111