5.4 Case Study 3: Foredune Destruction, Transgressive
Dunefield Restoration, Stewart Island
The impact of marram grass (Ammophila arenaria) on the morphology of beaches
and foredunes on temperate coasts has been recognized for some time (e.g.,
Cooper 1958; Huiskes 1979; Carter et al. 1990; Hilton et al. 2005; Hilton 2006).
Much less is known about the development of foredunes and coastal barriers
following marram eradication. The potential to restore marram-dominated dunes,
by eradicating the marram, is gaining recognition in New Zealand and elsewhere.
There is, however, concern that marram eradication will initiate sand drift, to the
detriment of down-wind infrastructure and ecosystems. An opportunity to monitor
the geomorphology of a prograded bay-head barrier at Doughboy Bay, following a
sustained marram eradication operation over a 12-year period (1998–2010) arose
when the Department of Conservation (DoC) commenced a marram-control program in Rakiura National Park (Stewart Island, southernmost New Zealand) in
1999. Specifically, the DoC and the second author examined:
1. The response of marram to herbicide application
2. The rate of decay of marram following herbicide application
3. The nature of sedimentation following marram necrosis and decay
A parallel study of marram eradication in Mason Bay, a much larger and more
complex dune system 2 km to the north of Doughboy Bay, commenced in 2000.
5.4.1 Study Area
The beach systems of Stewart Island are moderate to high wave and wind energy
environments. Beach morphologies are of the intermediate type with a broad
intertidal terrace comprising either sand or mixed sand and gravel. The back-beach
morphology has probably narrowed and steepened following the introduction of
marram grass and the development of continuous, uniform (Type I) foredunes (i.e.,
continuous, morphologically simple, high vegetative cover—after Hesp 2002).
Hart et al. (2012) measured the wind regime at Mason Bay from June 2002 to
February 2004 using a permanent Vector 3-cup anemometer at a height of 2 m
located on the foredune crest. The predominant direction of sediment transport is
onshore; the wind regime is dominated by strong, episodic, west to southwest
(onshore) winds throughout the year. Annual average wind speed is 6.4 ms
-1 with
\1 % calm conditions. The dune systems of Doughboy Bay occur at the head of a
pronounced embayment. Headlands shelter the southern and northern margins of
the sandy shoreline, but the central dune system and adjoining barriers are highly
exposed to onshore winds (Fig. 5.9).
Stewart Island, and Fiordland, the southern South Island beach and dune systems, retained relatively high natural values well into the twentieth century.
76
P. A. Hesp and M. J. Hilton
Dunefield Restoration, Stewart Island
The impact of marram grass (Ammophila arenaria) on the morphology of beaches
and foredunes on temperate coasts has been recognized for some time (e.g.,
Cooper 1958; Huiskes 1979; Carter et al. 1990; Hilton et al. 2005; Hilton 2006).
Much less is known about the development of foredunes and coastal barriers
following marram eradication. The potential to restore marram-dominated dunes,
by eradicating the marram, is gaining recognition in New Zealand and elsewhere.
There is, however, concern that marram eradication will initiate sand drift, to the
detriment of down-wind infrastructure and ecosystems. An opportunity to monitor
the geomorphology of a prograded bay-head barrier at Doughboy Bay, following a
sustained marram eradication operation over a 12-year period (1998–2010) arose
when the Department of Conservation (DoC) commenced a marram-control program in Rakiura National Park (Stewart Island, southernmost New Zealand) in
1999. Specifically, the DoC and the second author examined:
1. The response of marram to herbicide application
2. The rate of decay of marram following herbicide application
3. The nature of sedimentation following marram necrosis and decay
A parallel study of marram eradication in Mason Bay, a much larger and more
complex dune system 2 km to the north of Doughboy Bay, commenced in 2000.
5.4.1 Study Area
The beach systems of Stewart Island are moderate to high wave and wind energy
environments. Beach morphologies are of the intermediate type with a broad
intertidal terrace comprising either sand or mixed sand and gravel. The back-beach
morphology has probably narrowed and steepened following the introduction of
marram grass and the development of continuous, uniform (Type I) foredunes (i.e.,
continuous, morphologically simple, high vegetative cover—after Hesp 2002).
Hart et al. (2012) measured the wind regime at Mason Bay from June 2002 to
February 2004 using a permanent Vector 3-cup anemometer at a height of 2 m
located on the foredune crest. The predominant direction of sediment transport is
onshore; the wind regime is dominated by strong, episodic, west to southwest
(onshore) winds throughout the year. Annual average wind speed is 6.4 ms
-1 with
\1 % calm conditions. The dune systems of Doughboy Bay occur at the head of a
pronounced embayment. Headlands shelter the southern and northern margins of
the sandy shoreline, but the central dune system and adjoining barriers are highly
exposed to onshore winds (Fig. 5.9).
Stewart Island, and Fiordland, the southern South Island beach and dune systems, retained relatively high natural values well into the twentieth century.
76
P. A. Hesp and M. J. Hilton