Hi nonni55,
I’m sure that the change you've seen is due primarily to the passage
of time and natural processes, rather than a change in water level or
some tectonic cataclysm.
You mentioned that the developer brought in tons of sand. That means
that there weren't tons of sand there naturally. The natural action
of wind and waves, called longshore transport, carries beach sand
miles and miles along coast lines from where there is a sand source,
like a large river or a series of canyons, to where there is a natural
obstacle, usually an offshore canyon where the sand can accumulate.
In places where there is no beach sand naturally, there is no natural
means of replenishing the sand that is carried away by long-shore
transport. No matter how many tons of sand a developer can bring in,
it won't take long before mother nature carries all of it away. This
necessitates the developer bringing in sand on a regular basis, which
is incredibly costly, or results in the coastline regaining its
natural, rocky appearance over time.
Beaches change their configuration from season to season, as well.
Storm patterns dictate swell direction in the ocean. This means that
swells come from a different direction in summer than they do in
winter. For example: If the direction the swells come from in summer
happens to be from a place where there is a sand source, sand is then
deposited on the beaches. When the swells come from the other
direction in the winter, if there does not happen to be a sand source,
sand is eroded from the beaches. If you happened to visit the resort
the first time during a period of sand deposition, and visited again
during a sand-depleting seasonal swell, this would account for the
difference as well, however I’m still betting on the cause being
longshore transport of the sand.
Storms also change beach configuration, but in an environment where
sand deposition occurs, the effects are easily erased in a few months.
On the other hand, if a storm battered a shoreline where sand had
been dropped by a developer, you can see that erosional processes and
longshore transport would only be intensified and sand erosion
quickened.
A "seismic shift" resulting in the loss of that much sand sounds like
a long shot to me, but the Puerto Vallarta area is very seismically
active and this shouldn't be all together ruled out. An earthquake,
which is what a seismic shift is, could result in a rupture in the
rocks off shore, and create a depositional environment for the sand
where none existed before, but this would be a big deal in the
geologic community, and there would be tons of reports on it if it had
happened. The last time I know of that there was highly significant
ground rupture after an earthquake on land was the Landers quake in
Southern California in 1992. This produced volumes of reports because
of its significance. The ground rupture at Landers was dramatic, but
it was more like a long, 3 foot tall "hump" snaking along the ground
for a mile or so, rather than an entire canyon that could engulf tons
of beach sand. This is not to say that the Puerto Vallarta region is
incapable of seismic activity; in fact, just the opposite is true.
However, there have been no large, significant earthquakes there
recently. As stated above, I’m still betting on the cause being
longshore transport.
The more research I do on your question, the clearer it becomes that
this is a case of ongoing sediment transport. The Geological Society
of America is holding their 99th annual meeting in Puerto Vallarta
next year, (2003), and there is no mention of any local tectonic
cataclysm in any of the information for that meeting. Also, the Third
National Earth Sciences Meeting was held there earlier this month,
(November), and again, there was no mention in all the papers of any
such event along the local coast.
Along the Southern California coast where I live, The natural
sand-transporting mechanism has been interrupted by the construction
of jetties and harbors, which results in the famous Southern
California beaches being depleted of their sand. Government agencies
spend millions of dollars dredging harbors and lagoons, then deposit
the dredged silt on the beaches via mammoth pumps and pipelines, but
within a year or two, all their efforts are eroded as the sand is
transported to its depositional locations offshore. There's not
enough money in anyone’s pockets to keep it all in one place.
If you would consider this an answer please let me know.
Longshore transport:
http://www.gvsu.edu/videticp/longshore.htm
http://ceres.ca.gov/ceres/calweb/coastal/beaches.html
A study by the Army Corps of Engineers on the effects of storms on
Longshore Transport:
http://www.frf.usace.army.mil/duck94/exp19.html
Beach replenishment and dredging, Southern California:
http://www.calcoast.org/news/beach112901.htm
Landers Earthquake ground rupture:
http://www.scecdc.scec.org/landersq.html
Third National Earth Sciences Meeting:
http://www.ugm.org.mx/IIIRNCT-I.html
Geological Society of America Cordillera Section, 99th annual meeting:
http://www.geosociety.org/sectdiv/cord/03cdmtg.htm |
