Testimony in question – Vegetation on Dams.

In the recent court case when an injunction was filed against the New York Power Authority to stop vegetation removal from the canal Earthen Embankment Dams between Brighton, Pittsford and Fairport,  Mr Donald H Gray (Professor Emeritus)  was called on to provide testimony.

In the petitioners reply Index 2018-945  it clearly states on page 4

Just so this court is not misled into thinking that the presence of trees on canal
embankments is not universally scorned, petitioners submit with their reply papers the affidavit of a highly-qualified and distinguished geotechnical engineer who opines that trees growing on the canal embankments enhance structural stability and public safety, and the removal trees will reduce embankment stability and integrity. See affidavit of Donald H. Gray, P.E., sworn to February 21, 2018 (“Dr. Gray Aff.”

I searched the Internet to find all documents written by Dr Gray, and in every case, his articles were written explicitly about NATURAL WOODED SLOPES being clear-cut.

This was obviously a completely different scenario than a levee or an embankment dam, so I wrote to Dr Gray and asked him if he had any documents referring to  levees or embankment dams.

Here is his reply.

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.

“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.

Prepared by Donald H. Gray

July 2009

LITERATURE REVIEW
——————-
EFFECT OF WOODY VEGETATION REMOVAL ON THE HYDROLOGY AND STABILITY OF SLOPES

One way to settle the argument about the effect of woody vegetation on the hydrology and stability of slopes is to cut down all the trees and see what happens. The effects of wide spread vegetation removal on the stability of natural slopes have been studied extensively as a result of a timber harvesting practice known as clear-cutting. It is useful and instructive to examine the consequences of such a practice vis a vis a policy promulgated and now being carried out by the US Army Corps of Engineers to remove all woody vegetation over 2 inches in diameter growing on or near the toe of earthen embankments or levees.

Widespread removal (clear-cutting) of trees on natural slopes and streambanks generally leads to an increase in slope failures. This is the overwhelming consensus of papers published in the scientific and technical literature as documented in the attached literature review. In some cases there may be a short- term benefit from tree removal resulting from a decrease in shear forces transmitted to a slope from wind. The adverse effect of wind depends, however, on such factors as the size and height of the canopy, stand density, and wind direction. The presence of tall, rigid trees growing near the water line of levees may also promote scour erosion around the base of the tree. These offending trees can be identified, selectively removed and/or pruned as part of an alternative vegetation management policy.

In the long run, cutting of trees on slopes leads to a gradual decrease in mass stability as a result of the decay of roots which previously acted as tensile reinforcements in the slope. Root decay can also lead to the formation of pipes in a slopes which promote internal or seepage erosion. The removal of tree canopy results in the loss of interception and evapo-transpiration which tends to promote wetter and less secure slopes. Canopy removal also results in less attenuation in the delivery rate of rainfall to the ground surface.

Considerably fewer studies have been carried out on the effects of woody vegetation removal on the mass stability of artificial embankments and levees. However, many of the results and findings from clear-cutting studies of natural slopes apply…. and are a cause for alarm in the case of a blanket, unexamined

removal policy on levees. Such a policy is tantamount in effect to an uncontrolled, national experiment. At the very least the position, size, age, etc of every tree that is removed on or near a levee should be entered into a GPS data base so that possible cause and effect relationships can be studied and the wisdom of such a policy properly evaluated at a later date.

One of the main arguments advanced by proponents of tree removal on levees is improved access, visibility, and flood fighting. It would be a great irony if all these goals were realized only to better witness, gain access to, and attempt to fight floods on a levee that is disintegrating and failing because of woody vegetation removal.

https://damfailures.org/lessons-learned/uncontrolled-vegetation-on-and-around-dams-can-hinder-inspection-and-lead-to-serious-structural-damage-significant-maintenance-costs-and-possible-failure/

Another report from Dr David Rosgen, hydrologist.

https://ecna.createaforum.com/general-discussion/important-announcement-coming-soon/msg178/#msg178

 

 

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