Shoreline on northern Alboran sea

Posted: November 11, 2015 by tchannon in Tides, weather

This article is of general interest without declaring any particular position. I hope it is interesting.

A few days ago Roger reblogged an article from MalagaBay about the sea level stand near Almayate, a small southern Spanish town 150km east of Gibraltar, 15km east of Malaga port, close to Velez-Malaga, a near coastal town on the Velez river. The most western Medeterrainin is called the Alboran Sea.

The Med is landlocked, has a very small tidal range but in consequence is prone to air pressure and wind modulation of stand, as well as fresh water incursion from rainfall. Moreover there much volcanic activity with severe crustal movement, sea bed change. In a way related the region is seismic with major tectonic faults also able to alter crustal stand.

There are in effect two Malaga’s

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Figure 1, Malaga port tide gauge, this is the place commonly known as Malaga.

PMSL carry no other useful tide gauge data in the region, all other records are very brief, although eg. Gibraltar must have a very long naval record but at the entrance to a large sea from an ocean the data would be strange.

This record is suspicious as though something has changed ~1990. In my experience this sort of station change is likely to be ground subsidence. A good case was found for Perth, Australia where deep aquifer pumping led to false claim of rapid sea level rise. (unpublished work by the author)

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Figure 2, image courtesy Google el al., general overview of region.

The malagabay article is more about Velez-Malaga, Almayate.

The whole coast has ground water usage problems, particularly from rapid urbanisation. A lot of this stalled when Spain got itself into serious fiscal trouble.

For an overview of the whole coast including the part under discussion. in Spanish, 241 page PDF “HYDROGEOLOGICAL ITINERARY FOR MEDITERRANEAN COAST ANDALUZ” here (2.8MB)

Below is a specific paper dealing with the Velez basin, in English…

In addition, the construction in the 1990s of a dam upstream has resulted in a decrease in aquifer recharge and in a significant advance of salt water intrusion during the 1991-1995 period. The 1996 to 1998 abnormally humid period has led to an effective flushing of the deltaic aquifer.
At present, the main objective of hydrological policy is to use the reservoir to supply water for both irrigation and human consumption, and to stop pumping from the aquifer to prevent new episodes of marine intrusion.

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Figure 3, From Groundwater Quality Monitoring in a Coastal Mediterranean Aquifer
Affected by Agricultural Contamination and Seawater Intrusion-
Extrusion Processes (Vélez River, Andalusia, Spain)
J. L. García-Aróstegui 1 , M.C. Hidalgo 2 , J. Benavente 3

CONCLUSIONS
In the Mediterranean southern Spain, the first half of the 1990s decade has
been characterized by a severe drought. Consequently, the groundwater extraction from the Vélez river aquifer has experienced a very important increase in order to satisfy the water demand for both irrigation and urban supply. Furthermore, the aquifer recharge has diminished during this dry period due to the starting of functioning of a reservoir system upstream, which has made practically nil the infiltration of stream water. This situation has favoured the salinization of most of the deltaic sector of the aquifer by seawater intrusion.
Apart of the above mentioned process, the groundwater quality deterioration during this period is also related to the intensive agricultural practices, with application of very high doses of fertilizers.
The abnormally humid period registered during 1996-1998 has considerably enhanced the aquifer recharge and, in addition, the pumping rates have been reduced.
http://www.olemiss.edu/sciencenet/saltnet/swica2/RioVelez.pdf

No borehole depths or extraction data came to light. (Spanish language makes search difficult)

As far as the specific observation about seashore stand is concerned I have several observations.

The tidal range is tiny but there is a surprisingly large annual range, of the order of +-40mm to +-60mm (plus and minus). Assuming typical shore slope for sand on a benign coast, this is likely to be 2 or 3 degrees. This points to an annual tideline variation of over 2 metres.

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Figure 4, Medicane, Mediterranean severe storm from 1996, Image Wikipedia, associated article here

Google Earth provided illuminating historic images. Rather obviously humans migrate onto the natural beach but nature speaks, can’t stay there.

There are signs of coastal scour, longshore drift. There are no breakwaters.

On detail looking this remarkable change was found near a river mouth, presumably the result of storms, although human activity is possible.

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Figure 4, coastal change at 36.726425° -4.106616° during 2007 .. 2009

The above poses a problem. I do not want to upset either Google or Microsoft so in general I try to avoid conflicts. In this case the technical pressure is strong, images are complementary.

Bing have spectacular close detail of the above pointing to a significant story. Taking a guess, this is productive farmland on the Velez delta. A sea defense dyke is being built.

For those prepared to look use Googel Earth timeline I recommend a look close to the above at 36.728424° -4.101427° where significant change interacting with humans can be seen.

When scour occurs there is likely to also be deposition, the ever changing sea.

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Figure 5, approximate annual sea level variation.

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Figure 6, Malaga sea level without annual showing little general variation but with surprisingly large short spikes.

Conclusion

I have no particular idea why a large range variation has been noticed. There are probably several factors at work.

Post by Tim

Comments
  1. tallbloke says:

    Really interesting. Thanks for taking a closer look at this Tim. I wonder how much of the annual range is due to thermal expansion. Beaches come and go, the tourism industry spends large sums carting truckfulls of sand around. Storms, longer term current and tidal changes, rainfall patterns. As you day, lots of factors to untangle.

  2. tchannon says:

    Reckon the outflow from the now damned Velez will be much lower than say 1000 years ago, less storm silt out to sea. Coastal erosion is no longer countered. Looking at the high res aerial photography points to lots of human activity. Bet a good chat with the older residents, any fishermen, as well as local municipal engineers would tell a lot. Looks quite possible it is partially cosmetic surgery Rog.

    Coastal erosion is a fascinating subject, full of learning the hard way. Tends to have winners and losers. It only takes say a marina or whatever built at an unfortunate point to start change elsewhere.

    The few times I have visited Med. seashores, I usually keep well away, I’ve been surprised at the extent of the sand, didn’t make sense in relation to the tide. Maybe this is about episodic storms, the general position of the sea is one thing, storms then sweep a much larger area.

  3. ren says:

    It will be dangerous.

  4. ren says:

    Sorry, Tallbloke will be wrong.

  5. ren says:

    Current situation. In Scotland may be snow.
    http://pl.sat24.com/pl/eu/km