Posted by: Michael Caswell
« on: December 13, 2018, 07:39:06 am »
What exactly constitutes a dam? Why should the canal's raised embankments be called a dam?
Some are trying to class the Great Embankment as a levee, but according to experts Dr D H Gray and Dr D Rosgen a levee is not subject to hydro-static pressure, whereas the Great Embankment is. It has a phreatic line and is subject to seepage and has numerous continual leaks. So, it's not a levee! But what is it? some say it's 'special' and needs it's own category or classification, because it's unique. The truth is, they don't want it called a DAM because there are strict rules made by FEMA, ASDSO, NYDEC and a host of others, who all denounce large vegetation on dams.
The Great Embankment is build exactly like an earthen embankment dam (EED).
It has Rip-rap to protect it's inboard side from wave action.
It has negligible lateral current flow.
It has spillways, toe drains, constant hydro-static pressure, a phreatic line.
It's construction is exactly the same as an EED.
It is constantly under load from a large volume of water, unlike a levee, which is under load only for short duration, seldom long enough saturating soil to create a phreatic line.
This should answer these questions.
Quote from Dr Donald H Gray (Professor University of Ohio)
"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. "
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.
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. They have a Phreatic Line as they are under constant water pressure.
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.