per year. The groundwater level is probably the dominant factor here. If the
surface is dry for a certain period in a year, this apparently is enough to keep
erosion going, while wet circumstances during the year prevent the establishment
of marram. However, this shallow erosion only delivers a limited amount of sand
to the surrounding dunes, too limited to bury or kill vegetation. On the slopes of
the reactivated (parabolic) dunes, regrowth from marram roots is a major problem.
The roots extend to several meters in the soil, therefore, complete removal is
unrealistic. Despite higher wind speeds than in the dune slacks, the remaining roots
prevent erosion and easily restabilize the surface. Only locally, serious erosion
occurs, resulting in the development of blowouts. In contrast, removing vegetation
from the crest of the dune also results in huge erosion. Apparently, near the crest
the eroding capacity of the wind is high enough to kill the marram grass. However,
as a consequence, the crest erodes, loses sand, and the dune form may disintegrate.
Apparently, a single restoration intervention is not sufficient to restore the largescale, landscape-forming process. A certain form of maintenance, removal of
blown out and regrowing roots, for a number of years, is necessary to get the dune
rolling. New projects are started where regrowth of (mainly marram) roots is
prevented by volunteers, who regularly remove all visible roots by hand. Our
hypothesis is that continuous removal of roots for a number of years will result in a
smooth and truly bare surface at the end, which might then be the start of
autonomous mobility. Further monitoring in the coming decades will reveal if
these experiments are successful in the long run.
7.6 Foredune Projects
The situation near the foredunes is quite different from that of the inner dunes.
Stress factors are much larger (wind speed, exposure, salt spray, blowing sand
from the beach, coastal erosion, less biomass, no soil development). Earlier
experiments with foredune remobilization proved that by means of simple interventions (removal of marram grass) the foredunes turned into transgressive dunes.
In the province of North-Holland removal of vegetation and the digging of some
trenches in the direction of the wind caused the foredune to move inland, with
measured sand transport rates of about 45 m
3 /m/year, mainly derived from
redistribution of foredune sand. Similar results were obtained on the Wadden
Island of Terschelling (Fig. 7.9), but here the sand input is derived from the beach
as well. These examples suggest that in the foredune situation, the removal of
vegetation might be enough to release huge quantities of sand that blow inland.
Also, regular removal of vegetation is easy owing to the accessibility of the beach
for heavy equipment.
7 Restoration of Dune Mobility in The Netherlands
119
surface is dry for a certain period in a year, this apparently is enough to keep
erosion going, while wet circumstances during the year prevent the establishment
of marram. However, this shallow erosion only delivers a limited amount of sand
to the surrounding dunes, too limited to bury or kill vegetation. On the slopes of
the reactivated (parabolic) dunes, regrowth from marram roots is a major problem.
The roots extend to several meters in the soil, therefore, complete removal is
unrealistic. Despite higher wind speeds than in the dune slacks, the remaining roots
prevent erosion and easily restabilize the surface. Only locally, serious erosion
occurs, resulting in the development of blowouts. In contrast, removing vegetation
from the crest of the dune also results in huge erosion. Apparently, near the crest
the eroding capacity of the wind is high enough to kill the marram grass. However,
as a consequence, the crest erodes, loses sand, and the dune form may disintegrate.
Apparently, a single restoration intervention is not sufficient to restore the largescale, landscape-forming process. A certain form of maintenance, removal of
blown out and regrowing roots, for a number of years, is necessary to get the dune
rolling. New projects are started where regrowth of (mainly marram) roots is
prevented by volunteers, who regularly remove all visible roots by hand. Our
hypothesis is that continuous removal of roots for a number of years will result in a
smooth and truly bare surface at the end, which might then be the start of
autonomous mobility. Further monitoring in the coming decades will reveal if
these experiments are successful in the long run.
7.6 Foredune Projects
The situation near the foredunes is quite different from that of the inner dunes.
Stress factors are much larger (wind speed, exposure, salt spray, blowing sand
from the beach, coastal erosion, less biomass, no soil development). Earlier
experiments with foredune remobilization proved that by means of simple interventions (removal of marram grass) the foredunes turned into transgressive dunes.
In the province of North-Holland removal of vegetation and the digging of some
trenches in the direction of the wind caused the foredune to move inland, with
measured sand transport rates of about 45 m
3 /m/year, mainly derived from
redistribution of foredune sand. Similar results were obtained on the Wadden
Island of Terschelling (Fig. 7.9), but here the sand input is derived from the beach
as well. These examples suggest that in the foredune situation, the removal of
vegetation might be enough to release huge quantities of sand that blow inland.
Also, regular removal of vegetation is easy owing to the accessibility of the beach
for heavy equipment.
7 Restoration of Dune Mobility in The Netherlands
119