** EDIT 9/6/21, Seems the Jetty is unsafe, funny we remarked about some loose pylons when we were here setting this EC up, sadly it seems it will be a long time until it is repaired. I'm unable to visit and take new coords at the moment so I'm going to leave it upto cachers to choose thier own location based on the access available at the time.**
To successfully complete this earth cache be required to visit GZ and WP2 twice, complete a simple measurement, take a photograph, and answer some questions, please read this cache page before going to GZ so you are prepared.
A special thanks to Freddo for giving us a helping hand with this cache, couldn't have done it without him.
Spencer Gulf, is 100km wide at its mouth, and extends northward for almost 400km creating a tall triangular shape. The gulf is a shallow marine depression up to 50m deep in the south, shallowing into extensive sand and mud banks with a central channel around 20m deep in most of its northern half. This earthcache is the product of the unique shape of the Gulf.
Apart from the Broughton River, emptying into the Gulf through a delta just south of Port Pirie, there is no major river input. There are numerous tidal streams on both sides of the Gulf. These creeks only flow intermittently with freshwater in wet seasons and after storms. Given the arid climate and high evaporation rates it is noted that salinity of water in the gulf increases in the northern sections. As such the Gulf is recognised as an inverse estuary.
Along the Gulf coasts, there are rocky headlands, reefs, sandy flats, shelly beach ridges and accumulations of seagrass flotsam. Other parts, mostly north of the line Wallaroo to Arno Bay, contain intertidal mangrove swamps bordered landward by supra tidal evaporative lime- and dolomitic muds, with gypsum. Some elevated shingle beach ridges in the north indicate that up to 4.5 m of regional uplift has occurred over the last 5000 years.
At Port Germein specifically, the beach is fronted by sandflats almost 1 km wide, which contain numerous low sand ridges. South of the jetty, the beach extends for over 1.5 km to a tidal creek, which cuts through recurved beach ridges. South of the creek, a sandy beach extends for 7.5 km to the northwest of Weeroona Island. The beach is backed by a series of low beach ridges and fronted by a tidal flat with multiple sand ridges; the tidal flat is 1 km wide and abuts dense seagrass beds. There are numerous sand shoals and spits in the northern gulf; one of these, Cockle Spit in Germein Bay, is of interest in that it is used at low tides to host occasional cricket matches between the yacht clubs of Whyalla and Port Pirie.
For the purposes of this Earth Science lesson, it is important to note that Spencer Gulf’s geophysical shape and coastal geology creates a rare phenomenon called locally a Dodge Tide. While very flat neap tides occur in a number of locations around the world, the term 'dodge tide' is used only in South Australia. A Dodge Tide describes a long period of slack water occurring twice a month.
Tides are driven by interaction of the gravity of the Moon and Sun on the oceans, with the Moon typically having twice the effect of the Sun. In a few places in the world, due to the shape of a water body and how the tidal waves respond, the effects of the Sun and Moon are about equal. During the month the effects of the Moon and Sun can cancel each other out so that we get no tidal movement whatsoever for a day or two.
The unique low laying sand flats and extra long jetty (once over a mile long but sadly shortened by storms over the past century) at Point Germein are the perfect place to view this phenomenon, although not every day is a Dodge Tide, it is certain that each day the earth will interact with the Moon and Sun and the tides will change.
To log this Earth Cache we require you to consider the information presented to you, visit the area twice, conduct the experiment, take the photo and then perhaps you will need to do some research of your own, then message us with the following answers to the best of your ability.
** EDIT 9/6/21, Seems the Jetty is unsafe, funny we remarked about some loose pylons when we were here setting this EC up, sadly it seems it will be a long time until it is repaired. I'm unable to visit and take new coords at the moment so I'm going to leave it upto cachers to choose thier own location based on the access available at the time.**
Remember you will need to visit GZ twice, at least 1 hour apart, 6-7 hours apart would be fantastic. For your convenience we have linked the tide times here.
This experiment requires a ruler, long string or fishing line aprox 10 meters, a sinker (or something sinker like) and a pen/texta capable of marking the string/line.
How to complete the experiment:
Tie the sinker on the end of the string, lower your string so the sinker touches the top of the water.
Mark the string.
Measure this length and record your results.
Q1. / Visit 1. Record your results and send them through with the rest of your answers.
Note the tide position.
Wp1. Do the measuring.
WP2. Do the measuring.
Visit 2. Record your results and send them through with the rest of your answers.
Note the tide position.
Wp 1. Do the measuring.
Wp2. Do the measuring.
Q2. What is the difference in depth between visits? Is this representative of a dodge tide or a usual tide?
Q3. What is the difference in depth between Wp 1 and Wp 2. Why do you think this is so?
Q4. Is the water cloudy or clear? Why do you think it is like this?
Q5. WP3 post a photo of yourself with the measuring device once used by ships in the area.
You are welcome to log your answers straight away to keep your TB's and Stats in order but please message us with your answers within a couple of days. Cachers who do not fulfil this Earth Cache requirement will have their logs deleted without notice.
Source:
https://www.researchgate.net/publication/264936449_Spencer_Gulf_Geological_setting_and_evolution
https://www.adelaide.edu.au/press/system/files/media/documents/2019-04/uap-coast-sa-ebook.pdf
http://www.bom.gov.au/oceanography/proje
cts/ntc/dodge/dodge.shtml#:~:text=dodge%20tide,used%20only%20in%20South%20Australia.