What You Need and
“Things To Be Aware Of”
You will need two metre rulers (mark out two flat sticks if you
don't have this) a level would be handy but not necessary. Access
either from the Seal Point Road car park or follow the mark trail
from the Sandymount car park. The area is accessible to the public
but you should take care to respect and keep a distance from
sea lions and yellow eye penguins. The walk will most likely
take longer and be harder than you think.
General Map
Background
Sand dunes occupy about 1,100km of the NZ coastline. Impressive
dunefields can be found on Stewart Island, Paekakariki to Patea
(130km long and 19km inland) and at this location.
Sandfly Bay (from Seal Point Road
end)
Sand dunes need the following to form: 1) Loose sand (dune
material) and little vegetation; 2) Wind to move the sand (hence
the name Sandfly bay - because the "sand flies"; 3) Obstructions to
cause the sand to settle (eg rocks, trees). Sandfly bay fulfils the
three criteria and sand dunes form here.
If you are here on a windy day observe the sand movement.
Typically the wind only carries the sand up to 5cm from the ground.
Watch it closely and try to observe the three ways that sand
moves.
1. Saltation: The sand grains bounce along in the wind. About 95
percent of sand grains move in this manner. 2. Creep: When sand
grains collide with other grains -- like clay or gravel -- causing
them to move. Creep accounts for about 4 percent of sand movement.
3. Suspension: Sand grains blow high in the air and then settle.
About 1 percent of sand moves this way.
The ways sand can move
There are different types of dunes. Sandfly bay represents a
good exampl of a coastal dune complex. The wind erodes sand
particles from the windward side and depositing them on the leeward
side. Gradually this action causes the dune to "migrate" inland, as
it does so it accumulates more and more sand. Blowouts inland can
occur, the road to Sandymount periodically suffers from small
blowouts of sand. Indeed, large blowouts of sand were partly blamed
for the collapse of Maori coastal gardening about 450 years ago in
the South Island.
Most kinds of dunes are longer (and less steep) on the windward
side where the sand is pushed up the dune and have a shorter,
steeper "slip face" in the lee of the wind. The valley or trough
between dunes is called a slack.
Typical Sand Dune Shape (source:
www.howstuffworks.com)
Logging
Requirements
GZ is roughly at the highest point of the largest exposed dune
(though this point changes over time) where you will need to go to
complete the tasks below.
0. Preface any email with GC23B2E Sandfly Bay Dunes in the
subject line and title and clearly note the question number and
answer. You don't need to wait for a confirmation email to log but
you might be asked for more info if your answers are slim.
1. Use metre rulers (mark out two flat sticks if you don't have
these) to determine the gradient (%) of the slip face AND the
gradient (%) of the windward slope. See diagram below and example
below on how to do this. Email this.
Finding the slope gradient
In the example above the slope rises 12cm over 100cm (or 0.12m
over 1m). The gradient as a decimal is 0.12 (calculation: 12 / 100)
and multiplying by 100 turns it into a percentage (12%). Windward
gradients can vary but slip face gradients have a typical value
which I'll keep to myself as a check.
2. Use your GPS to determine the current maximum height of the
dune. Email this.
3. Upload a photo of task one being carried out.
Bonus question (2 Points - awarded to first logger to
answer): What is the typical ANGLE of a slip face (either
research this or turn your calculated slip face gradient into
degrees).
Key
Sources
How Stuff Works entry on
Sand Dune Formation
Wikipedia article on Sand Dunes
Te Ara Encylopaedia entry on Coastal
Dunes
Further
Reading
National Parks Service webpage on the
geology of sand dunes
2010 Stegan’s
EarthCache Masters Programme
Since it is not very feasible for many NZers to easily get more
than two states for the earthcache.org masters programme and
therefore not get past Bronze I’m running my own one on a
number of Earthcaches that I plan to release this year (all things
being equal) with the following points system:
5 points: First to find
3 points: 2nd to find
1 point: Logging a find
1-5 points: Special bonus questions.
Shared points for joint finds.
At the end of 2010, I’ll issue certificates for 1st, 2nd
and 3rd and the first place getter will be awarded a geocoin. If I
feel richer and/or it looks like this is popular I’ll upgrade
the prizes.
Updated information and Leaderboard at: Stegan’s
Earthcache Programme