A fairly involved earthcache about dunes, their formation and propogation. It involves some measurements, some surveying, and some conclusions about the inland dunes near Te Henga (Bethells Beach) on the west coast of Auckland. There are a number of tasks to complete, but none of them should prove particularly difficult and kids should be able to complete this earthcache with a little guidance. Read through all of the tasks before you go so that you are equipped and prepared to complete all of them!
A dune is a hill of sand built either by wind or water flow. Dunes occur in different forms and sizes, formed by interaction with the flow of air or water. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind. The valley or trough between dunes is called a slack. A "dune field" is an area covered by extensive sand dunes. Large dune fields are known as ergs.
Dune Features
Sand dunes generally (although there are exceptions) take the form of a crested hill of sand with a rounded and/or sloping side facing the wind, and a straight slip face on the leeward side. There are different types of dunes that form under different circumstances, but this earthcache deals specifically with certain features and with the dunes specifically located at Te Henga.
Generally sand dunes will exhibit rippling on the surfaces where loose sand resides and where the wind hits or passes over the dune, but not on the slip face. In situations where there is a constant or predominant strong wind direction, ripples will form and straighten out in a direction perpendicular to the wind and may show well developed bifurcation of crests (a single ripple banching into two). The wavelength of ripples is a function of wind speed and sand grain size.
The slip face of a dune is easy to spot, as it will take the form of a straight slope down, usually from a sharp crest at the peak of the dune. The migration action of a dune (other than a reversing dune, which is hit by wind in multiple directions) is always in the direction of the slip face as the wind takes sand from the windward side of the dune and deposits it on the leeward slip face.
If there is sufficient continued wind strength, supply of sand, and no other opposing forces, dunes will continue to migrate in the direction of the wind. Often dunes can be stopped from further migration if they meet a barrier that prevents or inhibits the dune's progress, there is a significant permanent or prolonged change in the wind direction and/or strength, the sand deposited on the leeward slip face is eroded by some other force, or if the windward supply of sand runs out or is suffuciently stemmed, or any combination of the above.
This Earthcache
To claim a find on this earthcache you will need to visit the published coordinates and the surrounding sand dunes. You will need to make a number of measurements and observations and then draw some conclusions about the dunes. Your answers must be sent to the cache owner before you log your find online. Don't visit at night as you won't be able to see anything and therefore you won't learn anything!
The tasks you need to complete are listed below along with any additional waypoints that relate to each task/question. The coordinates of the waypoints are listed in the Additional Waypoints section, below.
Waypoint 1
At or near this waypoint, observe the ripples on the surface of the dune:
a. Please measure the typical wavelength of the ripples (crest to crest).
b. Describe if there is bifurcation of the crests.
c. From the direction of the ripples, what is the predominant wind direction (N/NE/E/SE/S/SW/W/NE)? (Remember that wind direction is always stated as the direction the wind is coming from)
d. Describe the wind strength and direction you experienced during your visit to the dunes.
e. If there was any wind, did you witness sand movement due to the wind? If so, please describe it.
Waypoints 2 to 8
While you are visiting the dunes, you will be required to undertake a small survey of the crest of the slip face. For each of the waypoints, from Waypoint 2 to Waypoint 8, you will be required to take a GPS reading of the coordinates of the crest of the slip face closest to each of those waypoints. This data will be used to establish whether, or by how much, the dunes are presently moving.
Remembering to use the "Waypoint Averaging" function on your GPS unit (if it has such a function - most modern GPS units do) please record and provide coordinate readings for the slip face crest at each waypoint:
a. Waypoint 2.
b. Waypoint 3.
c. Waypoint 4.
d. Waypoint 5.
e. Waypoint 6.
f. Waypoint 7.
g. Waypoint 8.
Published Coordinates
Between Waypoints 4 and 5, near this earthcache's published coordinates, you are required to make some measurements and observations of the slip face:
a. Measure the angle of the slip face (this should be the angle from horizontal). It is up to you how you measure it, so long as it is an accurate measurement and not an estimate.
b. Measure the height of the slip face here. You can do this by measuring the elevation at the top of the slip face and subtracting the elevation at the bottom. If you can't do that, then think of another way as estimates are not acceptable.
Kick or push some loose sand from the top of the dune onto the slip face and observe its behaviour:
c. How does the sand behave on the slip face?
d. Why do you believe it behaves this way?
Waypoint 9
At Waypoint 9, unlike the other waypoints you have been visiting and surveying along the slip face crest above the stream, there is no slip face here:
a. Why do you think there is no slip face here?
General
Throughout your visit to this area, and keeping in mind all of the observations you have made:
a. Do you think the dunes will continue to migrate?
b. Justify your answer above and make reference to the factors that promote or inhibit dune migration.
Logging this Earthcache
You can log this cache straight away after you have emailed your answers to the cache owner, no need to wait for confirmation. Please include the name of this earthcache in the email - you'd be surprised how many people forget. Also, when contacting us with answers, if you want a reply, please include your email address. Any problems with your answers we'll be in touch.
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Team Welsh & Pagenz |