Vegetation, erosion and dams

From http://www.dcr.virginia.gov/sw/dsveget.htm

Introduction

With proper care, earthfill embankment dams have proven to be very effective for many years. Most embankment dams are composed of non-organic material and do not deteriorate appreciably with time. An embankment may even continue to undergo additional consolidation and strengthen with age, once the critical points of initial settlement and initial reservoir fill have passed. Nevertheless, the continuing safety of any embankment dam depends on the integrity of its earthen fill to withstand pressure from the volume of water in the reservoir.

The biggest enemy of any earthfill dam is erosion, either external (water overflow creating ruts or rills on the surface of the fill) or internal (sometimes called "piping"). External, or surface, erosion is generally obvious if one takes the time and effort to look over the dam carefully. Internal erosion is not readily visible and may not be detected until it is too late for corrective action. This can result in an emergency situation and even lead to a catastrophic failure.

Need to control vegetation

A dense cover of low-growing grassy vegetation is recommended because it will provide protection from surface erosion, but its root structure does not penetrate the embankment so deeply as to create a potential path for internal erosion.

The type of grass and its fertilization should be appropriate for local conditions. The proper vegetation should be established and maintained over the entire embankment, outlet, plunge pool and spillway area. Coverage should extend at least 25 feet beyond the abutment contacts and toe of the fill. Regular mowing throughout the year is essential so that the surface can be readily traversed by foot. Potential dam safety problems, such as misalignment, cracks, animal burrows, surface erosion, seepage, sloughing, etc. can be spotted early enough to take corrective action. Therefore, mowing should precede each dam inspection.

The trouble with trees

though woody vegetation such as trees and brush may protect against surface erosion, such growth can cause other, serious problems. These problems develop over years and may go undetected until it's too late. In addition, trees or brush can hide an embankment surface, making inspection difficult.

When trees die, it causes the roots to decay, leaving a cavity within the dam. Water leaking through such a cavity can produce a piping failure. In addition, a tree can be blown over during a severe storm, leaving a large hole in the dam in place of the uprooted root ball.

Because tree root problems occur over a long time, even decades, they often go undetected. Many times, dam failure is described as a sudden event when in fact conditions leading to the failure went undetected or ignored for years.

Control of trees on dams

Trees should never be allowed to grow on the embankment fill, at the outlet, plunge pool or spillway area, or within 25 feet beyond the abutment contacts and toe of fill. Any tree on a dam should be removed, its roots grubbed out and dense grass cover established in its place. Deviation from this standard must be based on a critical assessment by a professional engineer who specializes in dams.

Follow the general guidance below:

Condition	Action needed
Existing dam with trees	Cut and remove all trees. Grub out all roots larger than 1 inch in diameter. Grade to adjacent contour and establish cover as described below.
New dam or existing dam without trees	Establish and maintain dense cover of grass. Maintain a height of 4 to 8 inches with regular mowing to discourage growth of woody vegetation and to facilitate visual inspection for seepage, sloughs or other signs of stress. New dams should have no trees on them.

Removal of trees from existing embankments

A question dam owners often ask is, "Why can't I just cut down the trees at the surface, and then keep the vegetation properly controlled in the future?" This may be acceptable for Class III and Class IV dams if and only if accompanied by a commitment to very carefully monitor the dam and to be prepared for immediate emergency action. Eventually, however, it will be necessary to deal with the decaying roots. In other words, a decision to just "cut and watch" simply postpones the dealing with the underlying problem and may result in an emergency situation or even failure of the dam.

Removal of trees and roots on hazard-potential Class I and Class II dams must be done under the direction of a professional engineer. After cutting and removing all trees and brush, all roots should be grubbed out to assure that no roots larger than 1 inch in diameter remain. Generally, the reservoir needs to be lowered prior to grubbing the roots. The rate of decrease in the reservoir level should not exceed 6 inches per day unless otherwise directed by a professional engineer. Holes resulting from the grubbing operation should be backfilled with well-compacted soil. Upstream slopes should be backfilled with impervious soil, while more pervious soil may be used on the downstream slope. The backfill should then be graded to blend with the surrounding contour, and appropriate grasses should be established on all disturbed areas.

Trees on dams are a serious safety hazard. There is no single "cookbook" solution on the proper way to remove them . . . each case is unique. The advice of professional engineer needs to be sought and followed.

Contact DCR's Dam Safety Program staff at (804) 371-6095
or email dam@dcr.virginia.gov.

Damage Caused to Earth Embankment Dams by Earthquakes in the United States

Scot H. Dahms
4/23/04
ES 767, Global Tectonics, Emporia State University, Emporia, Kansas

The theory of plate tectonics explains that the Earth's crust is broken into many plates. The plates are in continuous contact with each other and move as a whole. They move slowly over the Earth's surface and are propelled by heat generated inside of the Earth. Plates meet along fault zones. Earthquakes are caused by the sudden release of energy from the movement of the Earth's crust mostly along fault zones. Areas along the fault zones may become stuck or locked in place storing a tremendous amount of energy. When the area moves, the energy is released in the form of seismic vibrations causing ground shaking. The sudden release of energy can cause enormous amounts of damage to structures including earth embankment dams. Damage to a dam can cause the dam to fail releasing large amounts of water into a river valley. The result can be catastrophic including loss of life and property.

Most of the early dams were built with no consideration of earthquakes. For instance, dams built by the U.S. Army Corps of Engineers in the Louisville District before 1950 were not designed for earthquake forces because designers did not consider earthquakes probable threats (USACE 1990). As more information of earthquakes was collected, the need to built dams that could withstand earthquakes was recognized. Earth embankment dams may be damaged by earthquakes in several ways including dam movement, liquefaction of fill in a dam, water waves caused by an earthquake over topping a dam, and direct damage caused by a dam being located on a fault.

The dam or parts of the dam may move vertically, horizontally or a combination of both resulting in vertical cracks or settlement of the dam. The West Yellowstone Earthquake of 1959 damaged the Hegben Dam in Montana. The Hegben Dam is an earth embankment dam with a concrete core and the earthquake had a magnitude of 7.5. The earth embankment settled more then a meter and the concrete core cracked in four places. The reservoir floor was tilted but the dam embankment did not fail (Bonilla 1991).

In 2001, the Howard Hansen Dam located northwest of Enumclaw was damaged by the Washington Earthquake. The earthquake had a magnitude of 6.8. The damage consisted of cracks in the embankment and movement of instrument housing buildings on the dam (USGS 2001).
Seismic studies of the areas considered for dam construction increase the knowledge of potential problems that may be caused by earthquakes. For dams that are in place or planned to be built, modifications can be made to the structure to increase the structural stability including crack stopping zones, grouting, adding berms, and flattening of slopes (Babbitt and Verigin 1996).



Liquefaction

Liquefaction of fill in the dam may occur. Liquefaction is the large drop in stiffness and strength of soil due to seismic movements (Byrne and Seid-Karbasi 2003).


As a result, part of a dam may slump and slide off the structure. During the Santa Barbara Earthquake of 1925, the Sheffield Dam in California failed due to liquefaction apparently in the area just below the dam embankment containing silty sand. Little damage and no loss of life resulted. The earthquake had a magnitude of 6.3 and the dam was constructed in 1917 (Bonilla 1991).

Above image is a cross section of the Lower San Fernando Dam before and after the earthquake. Image taken from USGS (1995) with written permission for educational purposes.



The Lower San Fernando Dam located northwest of Los Angeles, California was damaged by the San Fernando Earthquake of 1971. The earthquake had a magnitude of 6.7. Liquefaction in the upstream side led to a major slide involving the core, the crest, and the upstream slope along nearly half the length of the dam.


The dam did not fail because a cracked rim on the downstream side of the dam was higher then the water level in the lake. No loss of life resulted but 80,000 people were evacuated downstream of the dam. The dam was constructed between 1912 and 1915 with hydraulic fill methods and was enlarged in 1930. The older part of the dam included a clay core with outer zones of silty sand (Bonilla 1991). Hydraulic fill methods involve the mixing of fill soil with water, transporting the fill to the dam site by pipelines, depositing the fill and water on the embankment, and allowing the water to drain away. The fill is loose and is susceptible to liquefaction (VTU 2000). Because of the damage to the Lower San Fernando Dam, the California Department of Water Resources, Division of Safety of Dams analyzed over 100 dams. Of these, 60 dams have been physically modified, 19 have permanent storage restrictions, 36 have preliminary restrictions pending mitigation of deficiencies, and 4 have been removed (Babbitt and Verigin 1996).

Photograph of the Los Angeles Dam after the Northridge Earthquake of 1994. Notice cracks in the surface. Photograph used with written permission from USGS (1995) for educational purposes.



Los Angeles Dam in California was built 3,000 feet upstream of the Lower San Fernando Dam in 1976. The information from the damage to the Lower San Fernando Dam was used to design the Los Angeles Dam. In 1994, the Los Angeles Dam was damaged by the Northridge Earthquake. It had a magnitude of 6.6 that is comparable to the San Fernando Earthquake of 1971. The Los Angeles Dam only suffered minor deformation and superficial cracking (USGS 1995).


The recognization of materials susceptible to liquefaction and slumping would allow their removal before the construction of a dam would occur. During the design analysis of Patoka Dam in Indiana, loose fine sand in the foundation was found to be susceptible to liquefaction during shaking from a moderate earthquake. The revised design included removal of the valley fill and placement of the entire embankment on bedrock. The dam was built between 1972 and 1978 (USACE 1990).

Over Topping of Dam by Water

Water waves caused by an earthquake may over top a dam. Water going over the top of a dam would result in erosion to the surface and could lead to failure of a dam. Water waves over topped the Hegben Dam in 1959 (Bonilla 1991).

All dams should be constructed with ample freeboard to withstand waves caused by earthquakes (USACE 1990). For dams that are already constructed, freeboard could be increased by adding to the embankment or by lowering the spillway (Babbitt and Verigin 1996).

Fault Locations

The dam may be located on a fault and have direct damage caused by the movement of the plates. The Great San Francisco Earthquake of 1906 caused damage to the San Andreas Dam. The dam only incurred minor damage but the tunnel spillway that ran through the San Andreas Fault was ruined. The fault offset approximately 2 m in the locality of the dam during the earthquake. The dam was built from 1868 to 1870, before the fault was recognized (Bonilla 1991).

Increased knowledge of where faults are located now affect where and how dams are built. Building of dams across faults should be avoided. If the construction across a fault cannot be avoided, special measures need to be taken to ensure the structural stability of the dam. The Coyote Dam southeast of San Francisco was constructed across the Calaveras Fault in 1936. The dam was designed to accommodate 6.1 m of horizontal and 1 m of vertical fault displacement. The Palmdale Dam was originally built across the San Andreas Fault in 1891. The dam was completely reconstructed in 1969 because of the possibility of faulting. The new dam was designed to accommodate 6.1 m of horizontal and 1 m of vertical fault displacement. The Cedar Springs Dam in California was built over active faults. Because of the faults, the original design of the dam was modified including a reduction in height, shifting of the axis, and changes in construction material zonation (Bonilla 1991).

As more is learned about earthquakes and earth embankment dams over time, damage to earth embankment structures is being reduced. Older dams constructed on poor foundations, with poor fill, or with poor construction techniques are more likely to be damaged or fail. Dams constructed on firm foundations, with appropriate fill, and using current construction techniques are less likely to be damaged or fail. Knowledge from past failures, increased technology, increased knowledge of earthquakes, and improved construction techniques are the primary reasons for better structural stability in earth embankment dams.

References.
Babbitt, D. H. and Verigin, S. W., 1996. General Approach to Seismic Stability Analysis of Earth Embankment Dams. California Department of Water Resources, Division of Safety of Dams.

Bonilla, M. G., 1991. Faulting and seismic activity, in Kiersch, G. A., ed., The heritage of engineering geology; The first hundred years. Boulder, Colorado, Geological Society of America, Centennial Special Volume 3.

Byrne, M. P. and Seid-Karbasi, M., 2003. Seismic stability of impoundments. 17th Annual Symposium, Vancouver Geotechnical Society.

U.S. Geological Survey, National Strong-Motion Program, 2001. Howard Hansen Dam. World Wide Web homepage URL: http://nsmp.wr.usgs.gov/data_sets/20010228_1/20010228_hhd_pics.html.

U.S. Geological Survey, 1995. Fact Sheet-096-95, The Los Angeles Dam Story. World Wide Web homepage URL: http://quake.wr.usgs.gov/prepare/factsheets/LADamStory/

U.S. Army Corps of Engineers, Louisville District, Dam Safety Section. 1990. Earthquake Fact Sheet. Louisville, Kentucky.
Virginia Tech University, 2000. Geotechnical Engineering Photo Album. World Wide Web homepage URL:
http://cgpr.ce.vt.edu/photo_album_for_geotech/.