Author Topic: Dr Gray says it all!  (Read 56 times)

Michael Caswell

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Dr Gray says it all!
« on: November 28, 2018, 05:48:40 pm »
Dr Gray explains the difference between earthen embankment dams and levees.

Earth dams and levees are both constructed embankments.
They differ mainly in their purpose and type of hydraulic loading.   Earth dams are constructed across a stream or river to create an impoundment.  The impoundment provides water storage, flood control, and recreational opportunities.  An earthen levee is constructed parallel to a river or stream to prevent flooding of adjacent low-lying areas during periods of high water.  An earthen levee is subjected mainly to tangential-hydraulic forces (scour) whereas a dam is subjected to dynamic hydraulic forces (wave action).
I don’t know of any earth dams where woody vegetation was purposely planted or allowed to grow on a face of the dam.  Such is not the case with earthen levees where vegetation can often be found growing on both the landward and water sides.
 An earthen dam is designed and built to prevent overtopping and minimize through-flow (seepage).   Overtopping of an earth dam is prevented by building the dam high enough.   By comparison earthen levees have been built mainly to withstand breaching that can occur during overtopping, through-flow and lateral scour.  The presence of vegetation on a levee helps prevent erosion that can occur during overtopping and lateral scour.
  I’ve attached a revised version of the article sent to you earlier that examines the risks and benefits of woody vegetation on earthen levees.

According to Dr Gray,  The Erie Canal Embankments are not Levees, but Earthen Embankment dams. The body of water, once called Oxbow Lake, behind the dam is a reservoir, not subject to ‘scouring’ as there is no lateral flow, and it is constructed across the flow of the stream flowing at The Oxbow.

I believe Dr Gray was misled into thinking these were wooded slopes or levees, as he had not actually visited the site.

He said
“I don’t know of any earth dams where woody vegetation was purposely planted or allowed to grow on a face of the dam.“

So, according to Dr. Gray, he states that our canal embankments are indeed dams, for the following reasons.

1. They are constructed across streams.

2. They have a constant level of water, maintained by spillways.

3. There is no lateral flow of water so no ‘scouring’ of the banks is in effect.

4. They are subject to Hydraulic Dynamic forces – pressure and waves.

5. They are not designed to over-top, hence the spillways.

6. Trees are not allowed on dams (See D H Gray, USACE, FEMA, ASDSO)

6. Phreatic line problems can cause seepage, boils etc.

Levees on the other hand constitute –

1. Embankments are build parallel to the water flow of a river or streams.

2. They are only operational in time of flood. (Holding back water)

4. They are designed to withstand over-topping.

5. Trees help withstand scouring caused by rapid flow of water in flood situations.

6. No spillways build into a levee.

7. Phreatic line problems virtually non existent.

He also sent me the following document about Levees.

Woody Vegetation on Earthen Levees 2

See the full article here.
« Last Edit: November 28, 2018, 05:55:04 pm by Michael Caswell »

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