Saturday, December 13, 2008

Matilija Dam - slurry design

At the December 4th Design Oversight Group (DOG) meeting, the Corps of Engineers presented further information on the details of the proposed slurry operations.

Standard slurry operations discharge the spoils into a temporary pool formed by sand berms. Dredged sand deposits in the pool, and slurry water flows out. (This can be seen every year when the Ventura and Channel Islands harbors are dredged to bypass sand down the coast.) In this case, the 'turbid' (muddy) water left over flows into the ocean where silt is slowly dissipated.

Because the Matilija Dam slurry operation will be transporting sediments much finer than beach sand, a more complex method of depositing the sediments is being proposed. The "Thickening Method" is similar to that used in mining operations.

This method uses large industrial equipment called a flocculant thickener processor. This processor adds a chemical to the water/sediment mixture, separating out the water so that a sediment 'paste' is discharged. This paste may be pumped through a pipe or trucked to a site and may form up to a 4% slope. (It may be processed for a thicker paste if a steeper slope is required.)

In the Matilija Dam slurry operations, it is desirable to recycle the slurry water, pumping it back up to the reservoir for reuse. Because the sediments are so fine, the engineers propose to use the 'thickening method' which will speed water separation and drying time at the slurry discharge point and optimize water recycling. They propose the use of a 46 ft diameter deep cone paste thickener processor, located at single central location, to process the slurried sediment. Once up and running, it will be able to provide a continuous production rate according to the amount of water in the slurry mixture. (Note that a google search turned up other more compact processes:

It is unclear how the slurried sediment will compress after drying at the disposal areas. Discussion during the December 4th meeting suggested that the material may reduce from 70lb/sq.ft to 150 lb/sq.ft, perhaps resulting in as much as a 50% reduction in disposal height. How this will affect revegetation/restoration is not clear, so adaptive management of revegetation efforts needs to be considered in the planning process.