Tim Randle, Sediment

Tim Randle, Bureau of Reclamation

2010 Reservior sedimentation: 24 million cubic yards, consisting of sand, gravel and clay.

Scientists are getting a good idea of what the river was made up of before the reserviors were created.

These are the largest dams ever removed in US history. Lake Aldwell has the largest sediment volume associated with a dam removal.

Objective of sediment management was to figure out how much of the sediment would get out of the reserviors. Various schemes were explored to figure out how to get rid of it, but scientists figured to let the river take care of it first, but do it as controlled as possible.

Sediment impact mitigation included a surface water intake constructed and water treatment plant upgraded and constructed.

For this, we had an adapative management program so we know what to monitor and the effects that we see. Wanted to make sure the water treatment plants and flood control levies would accomodate.

The web cams are cruical to this project. We want real time information. The sediment management team stretches from California to here in Port Angeles. Cams are looking at both reserviors and the deltas as well as the dams.

Most of the monitoring is focused on the reserviors but now the river monitoring is becoming more important as the dams are now closer to being removed.

They haven’t had to slow the rate or temporarily halt dam removal during this process, so that’s good.

At Lake Mills, they thought only a small portion of the delta would erode, but needed action to erode the middle of the delta. Actions included moving a log jam, clearing an alder forest and installing a pilot channel. Once that was done, the reservior was lowered and the river widened out.

In February 2011, started to see dramatic changes to the delta in Mills, with the delta “advancing” toward the Glines Canyon dam.

in August 2011, there is very little water in the former Mills delta.

In Aldwell, there were 20-40 feet of thickness of sediment. After removal, there has been significant erosion of sediment.

Sept 2011 – river started to wind its way through the former reservior, but the storms caused the river to widen. In July 2012, there is sediment covering the pre-dam surface of the reservoir but not a lot – maybe a meter. Floods may peel away some but not much.

In Aldwell, the river isn’t following the alignment of the old river channel, but when it comes to the gooseneck part of the lake, it falls into the old channel.

Findings: sediment erosion is keeping pace with the dam removal. Lowering the reservoir at controlled rates is working well.

Shrinking Reserviors – no more Lake Aldwell, and Mills still looks like a lake, but it’s lowering. He thinks by next November, there will not be a lake anymore.

Aldwell: Elwha dam removed, half the delta is eroded, no pilot channel excavated.

Mills: Glines Canyon halfway gone, 40 percent of delta has eroded and redeposited, remaining delta thickness is 60 feet.

Loss of Lake Mills is expected this fall and suspected sediment concentration and turbidity will increase with continued removal of Glines Canyon dam.

 

 

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