The Disappearing Sands of Waikiki - A Coastal Erosion Earthcache
Waikiki Beach is located on the south shores of Oahu; famed for its white sand, shopping opportunities and Aloha spirit, this prominent beach has been a favorite with tourists and locals for generations. Not everything is as it seems at Waikiki; this famous beach is actually man-made and the famous white sands are disappearing at an alarming rate1! Please remember that this is an Earthcache, as such there is no container to find; instead you will need to take the time to make some observations on scene and answer the questions at the bottom of this page. If your answers to the questions are not received in a timely manner your found log may be deleted. Enjoy this Earthcache and your time in Waikiki!
A Beach Made by Men
Hawaii's most famous beach is completely man-made. According to the State of Hawaii's Department of Land and Natural Resources, since 1939 more than 300,000 cubic yards of pristine white sand (typically originating from the nearby island of Molokai) has been deposited on the beach1. The beaches of Waikiki are in desperate need of repair and maintenance. If you must repair a beach, this is probably the most gentle approach. Replenishment of sand consists of pumping sand onto the beach and building up the former dunes and upper areas of the beach that have been affected by erosion2. This means of beach preservation is rarely feasible in the long run. In 2012 the State of Hawaii spent 2.9 million dollars to add an additional 27,000 cubic yards of sand to the beach3. Beach replenishment, then, upsets the natural system, is costly, and temporary, requiring subsequent replenishment projects in order to remain effective. The Corps of Engineers refers to beach replenishment as an “ongoing” project, but “eternal” project is perhaps a better term. Nevertheless, beach replenishment is usually less harmful to the total dynamic equilibrium than the following methods2.
Stopping Mother Nature
In a desperate attempt to thwart the littoral transport of sand away from this tourist haven, several groins/jettys were built.
By the year 1950 more than 80 jetties, groins and breakwaters had been installed on Oahu's southern shores3. These man-made structures are designed to stymie the movement of sand away from the beach. As can be seen in the image to the right, these groins and jetties capture sands on the side where currents are coming from and remove sand on the side where currents are moving to. The interruption of the current flow helps keep sand deposits on the beach instead of being swept out to sea.
The jetty where you currently stand is the only object standing in the way of the total removal of sand for much of Waikiki Beach3. As you can see along the nearby boardwalk, much of the sand has already been eroded from this spot. This is the result of the removal of several groins in 2012 that were dilapidated and falling apart. The removal of the groins quickly lead to most of the erosion that you see to the west of GZ.
What Exactly is a Groin
Groins are an example of a hard shoreline structure designed as a so-called "permanent solution" to beach erosion. A groin is a shoreline structure that is perpendicular to the beach. It is usually made of large boulders, but it can be made of concrete, steel, wood or even sandbags! 
It is designed to interrupt and trap the longshore flow of sand. Sand builds up on one side of the groin (updrift accretion) at the expense of the other side (downdrift erosion). If the current direction is 
constant all year long, a groin "steals" sand that would normally be deposited on the downdrift end of the beach4. The amount of sand on the beach stays the same. A groin merely transfers erosion from one place to another further down the beach. As soon as one groin is built, property owners downdrift of it may start clamoring for the government to build groins to save "their" beach. Eventually, the beach may become lined with groins. Since no new sand is added to the system, groins simply "steal" sand from one part of the beach so that it will build up on another part. There will always be beach erosion downdrift of the last groin4. This is precisely the case with Waikiki.
How Do We Save Waikiki?
Beach replenishment is a costly and neverending project. Groins rob sand from one part of the beach to save another. What is the best way to save Waikiki? Unfortunately the only permanent solution is to retreat from the beach4. 
By moving properties, structures and traffic farther back from the shoreline we allow the beach to naturally form while interacting with the tides and currents. Ironically it's by intervening in natural processes that we cause the very erosion we're striving to avoid. This solution is not ever likely to be used. Waikiki produces more than 2 Billion dollars in tourist revenue each year for the state of Hawaii and moving away from the beach will only cut into that pot of money.
Logging your Earthcache
In order to log your Earthcache you must message the CO (using the link at the top of the cache page) with the answers to the following questions. If I fail to receive your answers in a timely manner, then your log will be deleted. I will always give you a warning prior to deleting a log.
- You are currently standing at the base of a groin. Based on the build up of sand and significant signs of erosion nearby, which direction is the predominant longshore current?
- Based on what you have read and what you have observed here on Waikiki, do you think the groins are an effective tool at stopping erosion? Why or why not?
- Longshore currents remove a majority of sand from the beach, but it's not the only source of erosion. Based on your observations on Waikiki Beach, what is another prominent form of erosion?
- (Optional) Post a photo of yourself, your GPS or your group at GZ.
Sources
- http://www.staradvertiser.com/2015/01/05/hawaii-news/sand-erosion-in-waikiki-poses-potential-dangers/
- http://coastalcare.org/educate/shoreline-engineering/
- http://www.toptraveldestinationdeals.com/hawaiis-coastal-erosion-predicted-to-double-by-2050-new-study-says/
- http://www.beachapedia.org/Shoreline_Structures