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DIR Return to: Just the Dam Facts! - Embankment Reference Material
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#Post#: 166--------------------------------------------------
Dr Gray says it all!
DIR By: Michael Caswell
Date: November 28, 2018, 4:48 pm
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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.
HTML https://dhgray.com/sketches-and-essays/woody-vegetarian-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
HTML https://dhgray.com/sketches-and-essays/woody-vegetarian-on-earthen-levees/
See the full article here.
HTML https://wordpress.com/post/eriecanalfacts.wordpress.com/370
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