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Geotimes
 Published by the American Geological Institute
Newsmagazine of the Earth Sciences

October 2000


News Notes
 Field Notes

Subsidence threatens Venice
Scientists have discovered a stone walkway beneath Venice, Italy, that dates back to the second century.  It provides new insight into historical rates of ground subsidence in a region increasingly plagued by floodwaters. Their work has serious implications for a major public works project, still in the planning stages, to employ 79 mobile floodgates where the Venetian Lagoon meets the Adriatic Sea and block rising tides from flooding the city.
Albert Ammerman and Charles McClennen, an archaeologist and geologist from Colgate University, dated the stone path and successive archaeological remains built on top of the path. They found that the ground subsidence and simultaneous rise in sea level resulted in an average rise in relative sea level of 13 centimeters each century. Over the past century, the rise in relative sea level has reached a historical peak at 23 centimeters, in part due to an ill-planned industrial complex that pumped water from beneath the city from 1930 to 1970 and accelerated ground subsidence. 

If the rate of ground subsidence follows the current quickening trend, Venetians may find themselves standing knee-deep in some pretty foul water before the century is over. The proposed flood control project will install gates that will automatically rise and stop the flow of water between the Venetian Lagoon and Adriatic Sea whenever the tide rises 100 centimeters above the 1897 average. Ammerman and McClennen calculate a potential rise in relative sea level of 30 centimeters over the next 100 years that takes into account a small safety margin (4 centimeters) and any potential rise in sea level due to global warming, in addition to the historical average. 


    Proposed floodgates that will cut off the lagoon
    from the Adriatic Sea during especially high tides
    may excacerbate pollution in the Venetian Lagoon. 
    Philip Greenspun.
If their predictions are right, and the flood gate project is completed, the tide could rise enough to close the gates 150 times per year in an especially bad year.  Because high tides are also seasonal, the exchange of water between the lagoon and sea could be cut off almost daily during winter months. The result would be noxious quantities of pollutants trapped in an already foul-smelling lagoon, unable to be flushed away by daily tides. The impact on the lagoon ecosystem would be devastating.

Ammerman and McClennen published their work as a policy forum article in the Aug. 25 Science.


 Flowing freely



This aerial view of the Elwha River delta shows the current
flow of sediment into the Pacific Ocean. National Park Service.
Removing the Elwha Dam and Glines Canyon Dam from the Elwha River in Washington state will return the river delta to its pre-dam condition and cause sediment to build up in the upper and lower reaches of the river, according to a report released on Sept. 11 by the U.S. Geological Survey. 
 The Northwestern Power and Light Company constructed the Glines Canyon Dam in 1927 without the fish passage facilities that were required by law at the time. Since then, the once famous chinook salmon run has dwindled from 380,000 to 3,000 in the 1990s. A 1992 congressional mandate requires that the two hydroelectric dams on the upper reaches of the Elwha River the Elwha Dam and Glines Canyon Dam must come down in order to restore the salmon run. When removed, the Elwha Dam and Glines Canyon Dam will be two of only a few hydroelectric dams intentionally destroyed in the country.

The USGS undertook an experiment in 1994 on the effect that the removal of water from Lake Mills, the lake formed above the Glines Canyon Dam, will have on the redistribution of sediment further down the river. 
 Researchers dropped the water level by 18 feet to expose the delta to erosion by the Elwha River. Channels were cut into the delta and the unstable banks eroded back at a rate of 80 feet per day, transporting as much as 10,000 tons of sediment per day. 



Laura Wright
 

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