Dam Owners 

FEMA 534 / September 2005 

Technical Manual for 

Impacts of Plants on Earthen Dams 
TABLE OF CONTENTS 

Page 

Preface i

 Acknowledgements ii

 Glossary

Chapter 1: Introduction 1-1 

2-1 

3-1 

4-1 

5-1 

6-1 

7-1 

Chapter 2: Problems with Tree and Woody 

Chapter 3: Tree Growth and Tree Root 

Chapter 4: Earthen Dam Safety Inspection 

Chapter 5: Controlling Tree and Woody 

Chapter 6: Dam Remediation Design Considerations 

Chapter 7: Economics of Proper Vegetation 

 Vegetation Growth 

 Development Requirements 

and Evaluation Methodology 

 Vegetation Growth on Earthen Dams 

Maintenance 

v 
PREFACE 

Damage to earthen dams and dam safety issues associated with tree and woody 

vegetation penetrations of earthen dams is all too often believed to be a routine 

maintenance situation by many dam owners, dam safety regulators, and engineers. 

Contrary to this belief, tree and woody vegetation penetrations of earthen dams and their 

appurtenances have been demonstrated to be causes of serious structural deterioration and 

distress that can result in failure of earthen dams. For the first time in the history of dam 

safety, a Research Needs Workshop on Plant and Animal Impacts on Earthen Dams 

(Workshop) was convened through the joint efforts of the Federal Emergency 

Management Agency (FEMA) and the Association of State Dam Safety Officials 

(ASDSO) in November 1999 to bring together technical resources of dam owners, 

engineers, state and federal regulators, wildlife managers, foresters, and members of 

academia with expertise in these areas. The Workshop highlighted the realization that 

damage to earthen dams resulting from plant and animal penetrations was indeed a 

significant dam safety issue in the United States. The purpose of this Technical Manual 

for Dam Owners, Impacts of Plants on Earthen Dams is to convey technology assembled 

through the Workshop by successful completion of four objectives. These objectives are 

as follows: 

1. Advance awareness of the characteristics and seriousness of dam safety 

problems associated with tree and woody vegetation growth impacts on earthen 

dams; 

2. Provide a higher level of understanding of dam safety issues associated with 

tree and woody vegetation growth impacts on earthen dams by reviewing current 

damage control policies; 

3. Provide state-of-practice guidance for remediation design considerations 

associated with damages associated with tree and woody vegetation growth on 

earthen dams; and 

4. Provide rationale and state-of-practice techniques and procedures for 

management of desirable and undesirable vegetation on earthen dams. 

i
ii

ACKNOWLEDGEMENTS 

The editors of this Technical Manual for Dam Owners, Impacts of Plants on Earthen 

Dams wish to acknowledge the support of the dam safety organizations and agencies and 

many dedicated individuals that made significant contributions to the contents of this 

Manual. 

Sincere appreciation is extended to past and present members of the Subcommittee on 

Dam Safety Research of the Interagency Committee on Dam Safety (ICODS), now the 

National Dam Safety Review Board Work Group on Dam Safety Research, for their 

support of the proposal to convene a Research Needs Workshop on Plant and Animal 

Impacts on Earthen Dams (Workshop), and to the members of ICODS for recommending 

funding for the Workshop through the Federal Emergency Management Agency 

(FEMA). 

Appreciation and sincere gratitude are extended to the members, and especially to the 

full-time staff, of the Association of State Dam Safety Officials (ASDSO) for their 

support and coordination of the federally-funded project that culminated with convening 

of the Research Needs Workshop on Plant and Animal Impacts on Earthen Dams at the 

University of Tennessee in Knoxville, Tennessee, on November 30-December 2, 1999. 

The editors of this Manual are especially appreciative of the continued support, patience, 

and dedication of Susan Sorrell and Sarah Mayfield of the ASDSO staff. 

The Steering Committee of the Workshop was comprised of the following individuals 

who contributed significantly and diligently to making the Workshop a truly historical 

dam safety technological event: 

Dr. B. Dan Marks, P.E. (Chairman) Charles Clevenger, MS (Deceased) 

Dr. Bruce A. Tschantz, UT Knoxville William L. Bouley, USBR 

David K. Woodward, NCSU Sarah M. Mayfield, ASDSO 

Susan A. Sorrell, ASDSO (Project Coordinator) 
iii

Participants in the Workshop brought together diverse technologies, experiences, and 

scientific developments to create a significant contribution to dam safety in the United 

States. The editors of this Manual acknowledge the valuable contributions of the 

following Workshop participants: 

Matthew A. Barner, Wright State Univ. Douglas E. McClelland, USDA Forest Service 

William L. Bouley, USBR Dr. James E. Miller, USDA-CSREES/NRE 

Charles Clevenger, MS (Deceased) Dr. Dale L. Nolte, USDA-APHIS/WS/NWRC 

Dr. Kim D. Coder, Univ. of Georgia Richard D. Owens, USDA-APHIS/WS 

Gary Drake, Reemay, Inc. Tom Renckly, Maricopa County, AZ 

Edward Fiegle, GA Dam Safety Dr. David Sisneros, USBR 

James K. Leumas, NC Dam Safety Boris Slogar, OH Dam Safety 

Dr. B. Dan Marks, Marks Enterprises Susan A. Sorrell, ASDSO 

Sarah M. Mayfield, ASDSO Dr. Bruce A Tschantz, UT Knoxville 

Dr. Marty McCann, NPDP-Stanford David K. Woodward, NCSU 

The contents of this Manual were significantly enhanced by editorial reviews and comments 

of the ASDSO Manual Overview Committee. The editors of the Manual are sincerely 

appreciative of contributions made by the following individuals: 

Timothy G. Schaal, SD Water Rights Program Daniel M. Hill, Burgess & Niple, Ltd. 

R. David Clark, MA Office of Dam Safety James K. Leumas, City of Raleigh, NC 

Lori Spragens, ASDSO Executive Director (Project Coordinator) 

 

Because of the efforts of the many individuals previously mentioned, the editors are 

confident that users of this Manual will develop a better understanding and gain a greater 

appreciation of the seriousness and magnitude of problems associated with the effects of tree 

and woody vegetation root penetrations on the safety of earthen dams and their 

appurtenances. 
iv

 
v 

GLOSSARY 

This glossary provides the definitions of some of the basic terms used in this Manual and 

is not intended to be a comprehensive glossary of terms associated with dam safety. A 

more extensive resource of dam safety terms and definitions is available through the 

many references provided at the end of each chapter of the Manual. 

Absorption - the process of being taken into a mass or body, as water being taken in by 

 plant roots. 

Abutments - the interface between the sides of a valley containing a dam and the dam 

 embankment. Right and left abutments are referenced by viewing the dam 

 while facing downstream. 

Adsorption – the adhesion of an extremely thin layer of molecules to the surface of solid 

 bodies or liquids with which they are in contact. 

Appurtenances – structures associated with dams such as spillways, gates, outlet works, 

 ramps, docks, etc. that are built to allow proper operation of dams. 

Berm – a horizontal step or bench in the embankment slope of an earthen dam. 

Biological Barrier – an herbicidal releasing system, device, or material designed to 

 exclude root growth and/or penetration of plants into a protected 

 underground zone (such as a dam embankment). 

Boil – a typically circular feature created by the upward movement of soil particles by 

seepage flowing under a pressure slightly greater than the submerged unit weight 

of the soil through which seepage is occurring. 

Breach – a break, gap, or opening in a dam that typically allows uncontrolled release 

 of impounded water. 

Capillary Rise – the rise of water in the voids of a soil mass as a result of the surface 

 tension forces of water. 

Clearing – the removal of trees and woody vegetation by cutting without removal of 

 stumps, rootballs, and root systems. 
vi 

Crest – the near horizontal top surface of an earthen dam, or the control elevation of a 

 spillway system. 

Diameter at Breast Height (dbh) – the diameter of a tree measured at about four feet 

(breast height of average person) above the ground 

surface. 

Drainage System – graded and/or protected pervious aggregates in a dam designed to 

 collect, filter, and discharge seepage through the embankment, 

 abutments, or foundation. 

Earthen Dam – a dam constructed of compacted natural soil fill materials selected to 

 minimize embankment seepage while maximizing workability and 

 performance. 

Embankment – an earthen or rockfilled structure having sloping sides constructed of 

 select compacted fill materials. 

Failure – a (dam) incident that results in the uncontrolled release of water from the 

 impoundment of a dam. 

Freeboard – the vertical distance from the normal operating water level of an 

 impoundment to the crest (top) of the dam. 

Grubbing – the removal of stumps, rootballs, and lateral root system of trees and woody 

 vegetation. A construction operation that is typically done following the 

 clearing operation. 

Herbicide – a chemical substance or mixture designed to kill or maintain undesirable 

 Plants that may include herbaceous plants, vines, brush, and trees. 

Hydraulic Height (of a Dam) – the vertical distance from the normal operating water 

 level of the impoundment to the invert of the outlet 

 works or downstream outlet channel. 

Hydro-seeding – the technique of applying grass seeds, fertilizer, agricultural lime, and 

 seedbed mulch to seeded area in a pressurized aqueous mixture. 

Lateral Root System – roots of trees and woody plants that extend laterally from the tap 

 root and/or rootball to provide lateral support and nutrient 

 uptake for the plant. 
vii 

Line of Saturation – the leading boundary of the progression of saturation of soil in an 

 embankment exposed to an increasing head (source) of water 

 (impoundment). 

Line of Wetting – the leading boundary of the progression of wetting (partial saturation) 

 of soil in an embankment exposed to an increasing head (source) of 

 water (impoundment). 

Maintenance – routine upkeep necessary for efficient inspection, and safe operation and 

 performance of dam and their appurtenances. Labor and materials are 

 required; however, maintenance should never be considered to comprise 

 dam remediation. 

Mowing – the cutting of grass, weeds, and small-diameter woody vegetation by 

 mechanical devices such as mowers, bush hogs, and other vegetation cutting 

 machinery. 

Mulching – the application of protective material such as straw, fiber matting, and 

 shredded paper to newly seeded areas. 

Operation (of a dam) – activity by a dam owner for the necessary and safe use and 

 performance of a dam, the appurtenances of a dam, and the 

 impoundment. 

Owner – any person or organization that owns, leases, controls, operates, maintains, or 

 manages a dam and/or impoundment. 

Phreatic Surface – the upper boundary (surface) of seepage (water flow) zone in an 

 embankment. 

Piping – the progressive downstream to upstream development of internal erosion of soil 

as a result of excessive seepage that is typically observed downstream as a hole, 

or boil, that discharges water containing soil particles. 

Remediation – restoration of a dam to a safe and intended design condition. 

Revegetation – restoration of desirable ground cover vegetation (i.e. grasses) to 

 disturbed areas designed to prevent embankment surface erosion. 

Rootball – the root and soil mass portion of a tree or woody plant that is located directly 

 beneath the trunk or body of the tree that provides the primary vertical 

 support for the tree or woody plant. 
viii 

Root Penetration – intrusion of plant roots into a dam embankment so as to interfere 

 with the safe hydraulic or structural operation of the dam. 

Root System – roots contained in the rootball and the lateral root system collectively 

 comprise the root system of trees and woody plans and provide both 

 lateral and vertical support for the plant as well as providing water and 

 nutrient uptake for the plant. 

Seeding – application of a seeding mixture to a prepared seedbed or disturbed area. 

Seepage – the flow of water from an impoundment through the embankment, abutments, 

 or foundation of a dam. 

Seepage Line – the uppermost boundary of a flow net, or the upper surface (boundary) of 

 water flow through an embankment (see Phreatic Surface). 

Slump – a portion of soil mass on an earthen dam that has or is moved downslope, 

 sometimes suddenly, often characterized by a head scarp and tension cracks on 

 the crest and embankment slope. 

Spillway Systems – control structures over or through which flows are discharged from 

 the impoundment. Spillway systems include Primary or Principal 

 Spillways through which normal flows and small storm water flows 

 are discharged and Auxiliary or Emergency Spillways through which 

 storm water flows (floods) are discharged. 

Stripping – the removal of topsoil, organic laden materials, and shallow root systems by 

 excavating the ground surface (surficial soil stratum) after grubbing an area. 

Structural Height (of a Dam) – the vertical distance from the crest (top) of the dam to 

 the lowest point at the toe of the downstream 

 embankment slope, or downstream toe outlet channel. 

Stump – that portion of the trunk or body of a tree or woody plant left after removal by 

 cutting during timber harvesting and/or clearing of trees and woody plants. 

Stump Diameter – the diameter of a tree or woody plant at the ground surface. 

Tap Root – the primary vertical root in the rootball that is the origin of development for 

 the rootball and lateral root system growth. 
ix 

Toe of Embankment – the point of intersection of the embankment slope of a dam with 

 the natural ground surface. 

Weeds – shallow-rooted, non-woody plants that grow sufficiently high as to hinder dam 

 safety inspections and do not provide desirable embankment slope protection 

 against surface runoff. 

Woody Vegetation – plants that develop woody trunks, rootballs, and root systems that 

 are not as large as trees but cause undesirable root penetration in 

 dams. 

Zone of Aeration – the partially saturated zone of a soil mass above the zone of 

 saturation (above the height of capillary rise of water in a soil mass). 

Zone of Saturation – the saturated zone of a soil mass above the phreatic surface defined 

 by the height of capillary rise. 
1-1 

Chapter 1 

Introduction 

At the time Joyce Kilmer dedicated his famous poem “Trees” to Mrs. Henry Mills Alden, 

he was undoubtedly inspired by the beauty of a healthy living tree, and rightly so. For those 

that do not remember, the first verse of this famous poem is as follows: “I think that I shall 

never see / A poem lovely as a tree.” Most people are inspired and impressed by the 

splendor of trees; however, dam owners, operators, inspectors, dam safety regulators, 

engineers, and consultants might find the following verse more nearly appropriate. “I think 

that I shall never see / A sight so wonderful as a tree / Removed from an earthen dam / 

Whose future safety we wish to see.” This paraphrased verse is not intended to debase the 

great works of Joyce Kilmer; but rather, is intended to draw attention to the fact that trees 

and woody vegetation growth have no place on the embankment of an earthen dam. 

Dam safety regulators and inspectors, engineers, and consultants are frequently 

confronted with grass roots resistance in the issue of removal of trees and woody 

vegetation from earthen dams. This resistance is often associated with sentimental, 

cultural, ecological, legal, and financial issues. A fundamental understanding and 

technical knowledge of potential detrimental impacts of trees and woody vegetation 

growth on the safety of earthen dams is necessary in order to address these issues. 

Purpose 

The purpose of this Manual is to provide the dam owner, operator, inspector, dam safety 

regulator, engineer, and consultant with the fundamental understanding and technical 

knowledge associated with the potential detrimental impacts of tree and woody 

vegetation growth on the safety of earthen dams. In addition to objectives related to 

raising the knowledge level of detrimental effects of trees and woody vegetation growth 

on the safety of earthen dams, the contents will provide the user of this Manual with an 
Chapter 1 Introduction 

1-2 

understanding of the methods, procedures, and benefits of maintaining a growth of 

desirable ground covering vegetation on the embankments of earthen dams. 

Scope 

The editors of this Manual have organized the contents in a sequential manner in order 

that the reader and user of this Manual can develop the desired fundamental 

understanding and gain the technical knowledge associated with the detrimental impacts 

of tree and woody vegetation growth on earthen dams. Chapter 2 deals with the problems 

associated with tree and woody vegetation growth on earthen dams. Chapter 3 presents 

some common misconceptions about tree growth and tree root development. These 

misconceptions are contrasted with factual data about tree growth and tree root 

development. 

Chapter 4 presents a recommended earthen dam inspection protocol and procedures for 

determination of potential impacts of tree and woody vegetation growth on earthen dams. 

Chapter 5 begins the presentation of proper vegetation management on earthen dams. The 

user of this Manual is presented with methods and procedures for maintaining desirable 

vegetation growth, while also controlling tree and woody vegetation growth. 

Chapter 6 presents a number of remediation design considerations associated with the 

removal of trees and woody vegetation from the embankments of earthen dams. This 

chapter also presents a recommended phased-remediation procedure for removal of 

undesirable vegetation (trees and woody vegetation) from earthen dam embankments. 

Chapter 7 is a succinct factual presentation of costs associated with either continual 

proper vegetative maintenance or long-term dam remediation construction after tree and 

woody vegetation removal. The contents of this chapter should make every dam owner 

cognizant of the need for proper operation and maintenance relative to vegetative growth 

on earthen dams. 
Chapter 1 Introduction 

1-3 

Implementation 

While this Manual may not be considered highly technical relative to the presentation of 

complex engineering calculations for the solution of potentially serious earthen dam 

safety problems, this Manual does present a combined sixty-five years of research and 

practice in dam safety engineering associated with tree and woody vegetation growth 

impacts on earthen dams. This Manual is presented in a manner to be beneficial to the 

entire dam safety community (dam owners, dam operators, dam safety inspectors, dam 

safety regulators, dam safety engineers and consultants). Dam safety engineers and 

consultants can utilize this Manual as a reference for recommendations for proper 

maintenance of desirable vegetation growth, control of undesirable vegetation growth, 

and remediation dam design associated with the removal and control of trees and woody 

vegetation growth on earthen dams. Dam safety regulators and dam safety inspectors can 

utilize this Manual as a guideline for the inspection of earthen dams relative to tree and 

vegetation growth dam safety issues and for the direction of dam owners and operators in 

the proper method and procedures for maintaining earthen dams without detrimental 

vegetative growth. Dam owners and operators can utilize this Manual to establish proper 

operation and maintenance programs to promote the growth of desirable vegetative 

growth on earthen dams and/or remove and control the undesirable tree and woody 

vegetation growth on earthen dams. 

The last verse in the famous poem Trees by Joyce Kilmer is as follows: “Poems are 

made by fools like me / But only God can make a tree.” Again, the author will 

paraphrase this last verse, not to debase the great works of Joyce Kilmer, but to make a 

distinct point. “Only God can make a tree / But not removing trees from dams / Is 

done by fools like me.” 
Chapter 1 Introduction 

1-4 

There is yet much research and study to be done relative to the growth of proper 

vegetative cover on earthen dams. However, there is no doubt that trees and woody 

vegetation have no place on the embankment slopes of an earthen dam. The authors of 

this Manual intend to continue technological development in the area of controlling tree 

and woody vegetation growth on earthen dams. The authors would appreciate 

documentation of unusual cases of tree and woody vegetation growth related to safety 

issues associated with earthen dams. Documentation of these issues can be communicated 

through ASDSO and/or directly to the authors of this Manual. 
2-1 

Chapter 2 

Problems with Tree and Woody Vegetation Growth

According to the 1998-99 National Inventory of Dams (NID) data, there are approximately 

76,700 dams of significant size1

 and hazard category in the 50 states (USCOE, 1999). Most 

of these dams are regulated by the jurisdictional states, but many are not because of specific 

exemption clauses or different size or hazard restrictions. Because some states have lower 

size definitions for their dams than used for the NID count, the actual number of stateregulated dams is much higher (about 94,000). In Tennessee over 40 percent of the 

approximately 1000 inventoried dams not subject to regulation because of statutorily named 

county exclusions or agricultural use exemptions. Most of these unregulated dams and some 

of the regulated dams in Tennessee have troublesome trees and brush growing on their faces 

and crests. Some states estimate that as many as 95 percent of their regulated dams have 

trees. Figure 1 illustrates the general magnitude and range of the tree growth on regulated 

dams in 48 states where this information is reported (ASDSO, 2000). About half of the 

state-regulated dams are estimated to have excessive tree growth. 

Figure 1. Estimated percentages of state-regulated dams having trees.

 

1

 Inclusion in the National Inventory has been defined under P.L. 99-662 and P.L. 92-367 to include dams 

that are at least 25 ft. high or 50 acre-feet of storage (excluding low hazard dams less than 6 ft. high or 15 

acre feet of storage) and dams that due to location may pose a significant threat to human life or property in 

event of failure. 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-2 

Most dam safety engineers, including state and federal officials, consultants, and other experts 

involved with dam safety, agree that when trees and woody plants are allowed to grow on 

earthen dams, they can hinder safety inspections, can interfere with safe operation, or can even 

cause dam failure. In the past, engineers and dam safety experts have not always been in 

agreement about the best way to prevent or control tree growth, remove trees, or repair safetyrelated damages caused by trees and woody vegetation. However, all dam engineers agree that 

a healthy, dense stand of low-growing grass on earthen dams is a desirable condition and 

should be encouraged. 

From November 30 - December 2, 1999, a joint ASDSO/FEMA-sponsored workshop was held 

in Knoxville, Tennessee, for the purpose of inviting a panel of experts to discuss various 

problems, policies, and practices associated with plant and animal penetrations of earthen 

dams. Much of this manual follows up the work and recommendations produced by the 

workshop participants for engineers and owners to use in managing problems associated with 

both plant and animal intrusions. This chapter will discuss the consensus of current attitudes, 

issues, and policies involving woody vegetation penetrations of earthen dams, by state and 

federal officials, researchers, and practitioners active in dam safety. 

Attitudes Toward Woody Plant Growth on Dams 

The Association of State Dam Safety Officials (ASDSO) sent out survey questionnaires to dam 

safety officials in all 50 states and to federal representatives to the Interagency Committee on 

Dam Safety (ICODS) to determine state and federal agency attitudes about the effects of trees 

and woody plant growth on dams under their jurisdiction (ASDSO, 1999). 

In this survey the state and federal agency representatives were asked (1) if they considered 

vegetative growth to be a problem on dams, (2) if they had specific policies or operating 

procedures for removing unwanted vegetation and trees on dams and if they didn’t, how did 

they handle such problems, (3) what legal, financial, environmental or other constraints did 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-3 

they have in dealing with unwanted vegetation problems, (4) to provide documented evidence 

and examples where vegetation has negatively affected the safe operation or has contributed to 

the failure of dams, (5) to provide references to current or past research regarding the effects of 

plants and trees on dam safety, and (6) to provide example cost and other information related to 

rehabilitation and remediation of dams having problem woody plant growth. This chapter 

summarizes the collective state and federal attitude, and practice toward woody plant growth 

on dams. 

 

Problems Caused by Trees and Woody Plants 

Of the 48 states that responded to the above seven questions (Alabama and Delaware did not 

reply), all state dam safety officials indicated that they consider trees and plant growth on dams 

to be a safety problem. One eastern state dam safety engineer goes so far to say that trees are 

probably the major problem that he has to deal with. He notes further that most of the trouble 

occurs because owners (and some engineers) do not recognize trees as problems and become 

complacent as trees slowly grow into serious problems. Both state and federal officials agree 

that trees have no place on dams. Federal agencies like the 

Corps of Engineers, U. S. Bureau of Reclamation, and 

TVA, which own, operate and maintain their own dams, do 

not allow trees to grow on their structures. Figure 2 shows 

a problem dam in Nebraska where tree roots have been 

reported to penetrate the chimney drain and thus affect the 

operation of 

the dam. 

 

The problem most commonly noted by state 

officials is that trees, woody vegetation, briars, 

and vines interfere with effective safety 

inspections. Figure 3 illustrates this problem 

for a dam located in Tennessee. 

Figure 2. Example dam with problematic 

trees in Nebraska. 

Figure 3. Example dam with inspectionhindering trees in Tennessee. 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-4 

Figure 4 gives a breakdown of the percentage ranges of regulated dams where the 48 reporting 

state dam safety officials shown in Figure 1 estimate that trees and brush hinder safety 

inspections in their respective states (ASDSO, 1999). While half the states report having only 

20 percent or fewer dams with significant trees and woody vegetation that hinder inspections, 

vegetation on an estimated 30,000 or nearly a third of the collective state-regulated dams, is 

reported to obstruct effective dam safety inspections. 

Figure 4. Estimated percentages of state-regulated dams where trees and 

brush are considered a deterrent to effective safety inspections.
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-5 

Other dam safety problems caused by woody vegetation growth are: 

• Uprooted trees that produce large voids and 

reduced freeboard; and/or reduced x-section for 

maintaining stability as shown in Figure 5. 

• Decaying roots that create seepage paths and 

internal erosion problems. 

• Interfering with effective dam safety monitoring, 

inspection and maintenance for seepage, 

cracking, sinkholes, slumping, settlement, 

deflection, and other signs of stress 

• Hindering desirable vegetative cover and causing embankment erosion 

• Obstructing emergency spillway capacity 

• Falling trees causing possible damage to 

spillways and outlet facilities 

• Clogging embankment underdrain systems 

• Cracking, uplifting or displacing concrete 

structures and other facilities 

• Inducing local turbulence and scouring 

around trees in emergency spillways and 

during overtopping as shown in Figure 6. 

• Providing cover for burrowing animals 

• Loosening compacted soil 

• Allowing roots to wedge into open joints and cracks in foundation rock along abutment 

groins and toe of embankment, thus increasing piping and leakage potential. 

• Root penetration of conduit joints and joints in concrete structures 

Figure 5. Serious damage by uprooted tree to 

embankment stability at a dam in Oregon.

Figure 6. Tree root induced scouring on crest and 

downstream face of Coffey dam in Kansas.
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-6 

Current Policies and Procedures 

Twenty-four of the 48 responding states noted that they had formal policies and/or operating 

procedures for addressing tree and woody plant growth issues. These policies usually include 

one, or some combination, of the following: 

• Trees are not allowed to grow on dams or near toe and abutment 

• All trees and stumps must be removed, but roots may be left 

• All trees, stumps, and roots must be removed 

• All trees must be removed, but root systems of "small" trees may be left; root systems of 

 "large" trees must be removed 

• Dams are treated on a case-by-case basis -- usually under the direction of a qualified 

 professional engineer. 

For those states that choose to distinguish between "small" and "large" trees, the definition 

basis ranges from two to eight inches in diameter; most use a size of four or six inches in 

carrying out their policies. 

Of the remaining 24 states indicating that they have no formal policies or procedures, the range 

of recommended procedures to dam owners varies widely. Some states evaluate dams on a 

case-by-case basis, while other states require owners either to maintain their dams, to remove 

vegetation for inspection, or to use other means for dealing with plant problems such as 

requiring a qualified engineer to be retained, depending on the dam hazard classification. 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-7 

In summary, states follow several schools of thought and considerations in dealing with trees 

and vegetation on existing and new dams: 

Existing Dams: 

 Distinguish between “small” trees and 

 “large” trees 

 Remove all trees, stumps, and roots from 

 dam embankment 

 Cut trees to ground level, but leave stumps 

and roots 

 Cut trees, remove stumps, but leave roots 

 Consider case-by-case basis 

 Breach, remove, or decommission dam 

 Require retention of a qualified engineer by 

owner 

 Do nothing. 

Chapter 4: Dam Remediation Design Considerations

presents recommended procedures for removal of 

trees and dealing with tree and woody vegetation 

related problems. 

Figures 6 and 7 illustrate extensive efforts necessary 

to restore a heavily wooded earthen dam to a 

desirable vegetated and maintained condition. 

New Dams: 

 Establish effective ground cover and hope for the best in continual maintenance 

 Use vegetative barriers such as bio-barriers, 

 or use silvicides/herbicides/chemical treatment. 

Figure 6. Trees cut prior to removing stumps 

and roots from dam. 

Figure 7. Completed remediation job after removing stumps, seeding, fertilizing & mulching. 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-8 

Constraints to Removing Trees and Plants 

Several state and federal dam safety officials reported constraints to removing and/or 

controlling unwanted trees and other vegetation. Constraint categories explicitly cited by state 

dam safety officials (number of states in parentheses) are given below: 

 Financial limitations by owners (13 states) 

 Environmental regulations and/or permits (10 states) 

 Legal issues (6 states) 

 Aesthetics (5 states) 

 Threatened/endangered species issues (2 states) 

 Media (1 state) 

 Sentimental reasons (several). 

States indicated that the greatest constraint to removing unwanted trees and plants and 

repairing a structure infested with roots is limited financial capability by the owner. States 

such as Kentucky try to work with the owner to minimize the financial burden without 

threatening public safety. Ohio has recently established two low-cost loan programs to assist 

qualified public and private dam owners in funding safety-related improvements to their dams, 

including repairs mandated by the state dam safety program. 

Environmental constraints range from limitation of the use of certain herbicides or chemicals 

for controlling vegetation and for treating stumps and/or roots near water bodies; to prohibition 

of, or air quality concern for, burning cleared vegetation. Unless exempted, vegetation removal 

and maintenance around dams may conflict with wetland protection regulations. In 

Washington, environmental issues can pose a major hurdle to removing trees, but ultimately, 

public safety takes precedence over environmental concerns. In Arizona, problems with timeconsuming environmental permit requirements for larger plant removal projects are sometimes 

encountered. 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-9 

Some states have limited legal power to force owners to remove trees and vegetation from 

dams. This lack of authority may cause delays and expensive and time-consuming litigation to 

obtain an order. Other states, like Maine, do not have specific laws that force owners to 

remove vegetation from their dams, and removal orders have yet to be tested. One state, South 

Carolina, notes that if the owner will not voluntarily cut or remove unwanted vegetation, the 

only course is to start legal action against the owner. Because legal help is limited, such help is 

normally requested for the "most extreme cases." This means that only a few owners can be 

forced to do something about their vegetation. In New Hampshire, legal assistance is 

sometimes necessary to perform enforcement functions. In Oregon, if there is a problem with a 

recalcitrant owner, a Proposed Order can be initiated by the Oregon Dam Safety Program to 

correct the situation if it is determined to be an immediate threat to the integrity of the 

structure. However, this process can be rather lengthy and expensive when staff time, 

materials, and attorney fees are included in the costs of preparing for a contested case hearing. 

In the end, most dam owners have the right to contest state directives to remove trees and other 

plants through administrative and legal processes and judicial appeals. 

In some states, concerns have arisen when dams are located in parks or environmentally 

sensitive areas, especially when endangered or threatened species habitat is involved, in turn 

creating legal constraints. 

Aesthetics and sentimental reasons are often used by dam owners and their neighbors to resist 

removing trees and undesirable vegetation. This is particularly true if owners have 

intentionally planted ornamental trees and shrubs on their dams to provide shade or fruit, or to 

improve looks. Some owners believe that the more woody vegetation on a structure, the better 

-- thus making it very difficult for state dam safety officials to request its removal. 

The power of the press has had major influence on tree removal programs in some cases, 

especially where the target dam is owned by a poor or downtrodden citizen or insolvent 

municipality. Heated controversy between public safety interests and private owners or 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-10 

interest groups was generated through various newspaper stories and letters to the editor in 

1990 over the removal of 500 mature cottonwood trees on two dams owned by an 85-year-old 

widowed rancher who at the time was suffering from serious illness. The news stories, which 

cast the owner as being targeted because she was vulnerable, influenced the owner's neighbors 

to encourage her to take a stand against further removal of 500 remaining trees because they 

felt that enforcement of the state dam safety act "would cause more harm than good." 

While these constraints affect the ability of many states to enforce their regulations, some states 

such as Arkansas, Georgia, Colorado, Iowa, Maryland, Montana, New Jersey, North Carolina, 

and Tennessee report no major constraints to enforcement and consider the safety of dams to be 

of primary importance. 

Federal agencies appear to have fewer constraints than states relative to mandating the upkeep 

and maintenance of jurisdictional dams. However, some federal agencies noted that they must 

make sure that they comply with the National Environmental Policy Act and the Endangered 

Species Act prior to initiating tree and plant control and management. Isolated constraints at 

the National Park Service involving funding priorities, historic preservation, and disruption of 

visitor services may override safe operation and maintenance needs at some dams. Local 

watershed districts that are often poorly funded are responsible for the operation and 

maintenance of many of the USDA/NRCS flood control dam projects. 

Vegetation-Caused Problems and Failures 

Twenty-nine states indicated documented evidence 

where vegetation on dams has either caused dam 

failure or negatively affected their safe operation. 

Sixteen states had no documented evidence and five 

states had no response. Several states provided photos 

(Figure 8) and information on tree caused failures or dam Figure 8. Exposed tree roots in overtopped 

dam. 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-11 

safety problems. The most recent documented dam failure due to tree root penetration 

occurred in May 1999 at an unnamed Air Force Academy dam near Colorado Springs. Here, 

an approximately 13-ft. high dam with a pond capacity of less than 5 acre-feet of horse stable 

waste water failed, releasing its contents and injuring a horse in a stable located about 100 

yards downstream. The failure occurred after more than 7 inches of rain had fallen in the 

previous 72 hours. The dam had several pine trees on its crest and faces, and the breach 

opening exposed an extensive, deep root system. Roots up to 4 inches in diameter were found 

in the breach area. Figure 9 shows an example of a large root exposed in the bottom of the 

channel at the breach. The dam had not overtopped, and the failure was attributed to internal 

erosion of the decomposed granite embankment material along the roots. A tree had been 

located directly over the breach. 

 Figure 9. Large pine tree root located in the channel of the breach opening of a failed 

 Air Force Academy waste lagoon pond dam (David Eyre, Senior Civil 

 Engineer, Air Force Academy, Colorado, 1999). 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-12 

At the Federal level, USDA/NRCS referred to documented cases where dam failure has been 

determined to be caused solely by trees, and noted that trees have also masked other more 

serious seepage problems, which went undetected. 

Past and Current Research 

Other than a few references to the University of Tennessee Tree Growth Report (Tschantz, 

1988), only one or two other citations for tree or woody plant-related research were identified 

by the state dam safety officials (USDA/SCS, 1981). The surveyed Federal agencies had 

relatively little to offer in the way of references to current or past research regarding the effects 

of tree and plant growth on dam safety. The Corps of Engineers referred to geotechnical and 

other related program research conducted at the Waterways Experiment Station, published as a 

technical report series, Repair- Evaluation-Maintenance-Rehabilitation (REMR). One recent 

study for the St. Paul District showed that a hole formed by a blown-down tree in the 

downstream toe area can produce a potentially dangerous increase in hydraulic seepage 

gradient and internal erosion or piping problems in dikes (Duncan, 1999). The USDA/NRCS 

referred to the 1950's research work done at the ARS Hydraulics Laboratory in Stillwater, 

Oklahoma, on Flow in Vegetative Channels, which could have application to some emergency 

spillways. 

A recent literature review, sponsored by ASDSO/FEMA and conducted for the Steering 

Committee on Plant and Animal Penetration of Earthen Dams, researched available material on 

the effects of woody plants on dam safety (Tschantz et al, 1999). All types of sources and 

searches were inventoried, including ASDSO conference and workshop proceedings, ASCE 

technical journals and articles, USCOLD, direct e-mail and telephone contacts of selected 

federal and state agency officials, universities, research laboratories and other data bases 

accessible through the National Technical Information Service (NTIS) and National 

Performance of Dams Program (NPDP). While only a few references were found on recent or 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-13 

current research of tree and plant effects on dam safety, several references on federal and state 

practices, policies, and procedures for dealing with trees and vegetation were cited in such 

topical areas as: 

 

 woody plant physiology 

 documented examples of woody plant-caused dam failures, operation, and 

maintenance problems 

 case histories related to tree-caused dam failures 

 current and past federal, international, and other research activities 

 federal, state, international, and other organizations' policies, procedures and 

practices for preventing and remediating woody plant problems, and 

 federal, state or private cost documentation for removing or controlling trees and 

woody plants. 

Costs of Removing Trees and Tree Related Remediation 

Limited cost information for removing trees and brush or for repairing damages caused by 

vegetation at dams was available from the states or federal agencies. Most state dam safety 

officials indicated either that they did not have the data or that the owner or his consultant 

would have that information. Virginia reported that, while costs can be nominal, in extensive 

tree growth situations where grubbing is required, $10,000 to $20,000 per dam is common and 

that at one dam; the tree-clearing cost was about $40,000. Missouri reported that such costs 

could range from $1,000 to $10,000 depending on how badly the dam is overgrown with trees. 

A prominent North Carolina geotechnical engineering firm stated that ten different contractors, 

working in North Carolina, South Carolina, and Georgia, reported recent bid prices ranging 

from about $1500 to $3000 per acre for cutting trees at ground level, removing stumps and root 

balls, and grubbing the area to remove perimeter roots. Contractors were advised that clearing 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-14 

and grubbing would be done on embankment slopes ranging from 1.5:1 (Horizontal to Vertical) 

to 4:1 (Horizontal to Vertical), within possible wet areas in the lower 1/3 to 1/2 of the 

downstream slopes, and on earthen dams ranging in height from 25 to 50 feet. Table 1 

compares cost experiences reported by state dam safety officials in different regions of the 

country for clearing and grubbing trees from dams. 

 Reporting 

 State 

 Number 

 of Dams 

 *Cost 

 per acre 

 Survey 

 Comments 

 Georgia More than 25 $1,000 to 

 $5,000 

Based on consultants' feedback; cost varies depending 

on dam face conditions such as slope steepness, degree 

of wetness and tree density. 

 Oklahoma 1 

1 

$900 

$1,150 

2 acres of d/s slope over 2-1/2 day period 

3-1/2 acres, current proposal estimate. 

 South Dakota Several $100 to $200/Acre 

 

Usually 10 - 20 trees per dam 

Nevada 1 $532 

Based on 3 hourly laborers working for 2 weeks on 

3.25 acres of willow & mesquite removal on d/s dam 

face (~1995) 

Michigan 

General DNR 

construction 

cost experience 

$3,500 

$6,000 

$12,000 

Light clear/grub (diam.<6") 

Medium clear/grub (diam.<12") 

Heavy clear/grub (diam<24") 

Tennessee 7 

 $1,540 (Ave.) 

(approx. range = 

$1030 to $3290) 

Total clearing, grubbing & reseeding cost for 7 dams = 

$16,705 @ ~1.5 acres per dam. Jobs included range of 

tree sizes & heavy brush. (1995-98) 

Texas 1 $5,500 Part of overall site clearing and grubbing contract for 

new dam in East Texas (1995) 

Ohio 1 $10,000 

Cost included clearing, grubbing, mulching and 

seeding. Heavily wooded; hundreds of trees removed 

from d/s slope (1999) 

Minnesota Current 

estimates from 

Minnesota 

consultant 

 Small Projects 

$1350 

$2800 

$4475 

$4225 

$6775 

$960 

Clearing brush with brush saw - no grubbing 

Clearing brush by hand - no grubbing 

Clear and grub brush, incl. stumps 

Cut & chip up to 6" trees; grub/remove stumps 

Cut & chip up to 12" trees; grub/remove stumps 

16 m-hrs @ $60/hr to clear and grub small trees 

(diam. < 6") for less than one acre projects 

*Reported costs not indexed 

Table 1. Cost Comparisons for Clearing, Grubbing and Removing Trees from Dams. 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-15 

While the range of remedial costs varies widely, depending on several factors, it appears that 

about $1,000 - $5,000/acre may be a reasonable baseline to use for rough estimating purposes, 

with the lower figure applicable to small and low-density tree growth and the larger figure 

appropriate to mature, very dense tree stands. 

 

A typical 25-foot high by 750-foot long earthen dam having 3:1 (Horizontal to Vertical) 

embankment slopes, a 15-foot crest width, and a freeboard of 10 feet above normal pool has 

approximately two acres of exposed crest and face area for potential tree growth. Total costs 

for clearing and grubbing trees for such a dam would be in the range of $2000 to $10,000 

depending upon the local site conditions. 

Several site-specific factors can influence tree removal costs. These include size and type of 

trees, growth density, total job size (number of acres), location of growth (crest and/or both 

faces), embankment slope steepness, slope condition (such as degree of wetness or surface 

texture), degree and type of required surface treatment (backfilling, use of herbicides or biobarriers, mulching, seeding, fertilizing, etc.), and regional labor and construction indices. 

The U. S. Bureau of Reclamation reported detailed cost data using three herbicidal application 

methods (aerial, cut-stump, and ground-based foliar-application) in its 1987-93 program to 

control salt cedar along waterways in seven states of the Upper Colorado Region. Application 

costs ranged from about $60/acre for aerial spraying to about $1000/acre for cut-stump and 

spray methods (Sisneros, 1994). The National Park Service indicated that it has done tree 

removal with the assistance of the U. S. Bureau of Reclamation, but cost information is not 

readily available. 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-16 

Summary 

Trees appear to be a major dam safety issue for many states. Based on recent survey responses 

from 48 states, it is estimated that about one half of the state-regulated dams have trees 

growing on them. The same reporting states estimate that an average of nearly a third of the 

dams that they regulate have sufficient trees, brush and other growth to hinder effective safety 

inspections. 

Current state and federal policies, procedures, and practices relating to tree and woody plant 

removal, control, and management for dam safety are generally fragmented and inconsistent 

among state and federal dam safety agencies. However, all state and federal agency dam 

safety officials and experts agree that trees have no place on dams and need to be managed 

and controlled on both existing and new dams for at least three important reasons: (1) trees 

and dense vegetation hinder effective dam inspections; (2) tree roots can cause serious 

structural instability or hydraulic problems, which could lead to dam failure and possible loss 

of life; and (3) trees and brush attract burrowing animals, which can in turn cause serious 

structural or hydraulic problems. 

 

The fragmentation among state and federal agencies applies only to procedures about how and 

to what extent the trees and their roots should be removed and resulting cavities remediated to 

ensure a hydraulically and structurally safe dam. Other chapters in this Manual present 

methods and practices for controlling trees and woody plants and for remediating damage 

caused by trees and other woody plants. 

While limited information is available, a sampling of state dam safety officials and other 

experts report that the cost of removing trees and brush from the face of a dam can broadly 

range from about $1,000 to $10,000 per acre, depending on several factors. Typically, the cost 

of clearing and grubbing trees from dams falls into the $1,000 - $5,000 per acre range. The 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-17 

broad range of costs is not surprising as most dam safety engineers agree that tree removal 

costs are very much site specific. Controlling vegetation annually is relatively inexpensive, but 

removing trees on and repairing damage to neglected dams may cost owners several thousand 

dollars. 

Most dam safety experts agree that research needs to be done on determining the relationship 

of plant and tree species to root penetration of artificial environments such as embankment 

dams; the interaction between root systems and the phreatic zone and surface; and development 

and understanding of various types of physical, biological, and chemical treatment and barriers 

for controlling root growth. Because many existing dams exhibit dense growths of trees and 

woody vegetation with deep-penetrating root systems, engineering methods need to be 

developed for understanding, predicting, and stabilizing the effects of these root penetrations to 

minimize internal erosion and failure. Dam safety experts agree that both technical and nontechnical pamphlets and brochures, practice manuals, web-based documents, workshops, and 

guidance materials need to be developed for educating dam owners about the problems caused 

by trees and woody vegetation. Engineers, dam safety officials and inspectors, developers, and 

contractors must be provided technical training and information relative to the control and/or 

safe removal of trees and other undesirable woody vegetation from earthen dams. 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-18 

References 

1. Association of State Dam Safety Officials (ASDSO), State Survey: Animal and Vegetative

Impacts on Dams, Part I - Vegetation on Dams (7 questions), September 1999. 

2. Association of State Dam Safety Officials (ASDSO), State Survey, Percentage of Trees on

 State-regulated Dams (2 questions), January 2000. 

3. Soil Conservation Service (SCS), U. S. Department of Agriculture, Technical Note 705 –

Operations and Maintenance Alternatives for Removing Trees from Dams, South Technical 

Center, Fort Worth, April 1, 1981, 8 pages. 

4. Tschantz, B. A. and Weaver, J. D., Tree Growth on Earthen Dams: A Survey of State Policy

 and Practice, University of Tennessee, Civil Engineering Report, November 1988, 36 pages 

 plus Appendices A and B. 

5. Tschantz, B. A., Wagner, C. R., Jetton, J. W., and Conley, D. C., Bibliography on the

Effects of Woody Vegetation on Dams, compiled for the Association of State Dam Safety 

Officials (ASDSO) Steering Committee on Plant and Animal Penetration of Earthen Dams, 

University of Tennessee, September 1999, 18 pages. 

6. Tschantz, B. A., Overview of Issues and Policies Involving Woody Plant Penetrations of

Earthfilled Dams, Presentation and Proceedings, ASDSO/FEMA Specialty Workshop on 

Plant and Animal Penetrations on Dams, November 30 - December 3, 1999, 8 pages. 

7. Duncan, J. M., Review of Corps of Engineers Design for Rehabilitation of the Perimeter

Dikes around Cross Lake, Minnesota, Report submitted to St. Paul District, Corps of 

Engineers and R. Upton, Ad Hoc Committee Chair, Cross Lake, July 14, 1999, 16 pages 

plus Appendices A through C. 

8. Sisneros, D., Upper Colorado Region Salt cedar Cost Analysis/Evaluation, U. S. Bureau of 

Reclamation, Research and Laboratory Services Division, Environmental Sciences Section, 

Denver, Co, Final Report, Memorandum No. 94-2-2, February 1994, 272 pages. 

9. U. S. Army Corps of Engineers (USCOE), in cooperation with the Federal Emergency 

Management Agency (FEMA) and Association of State Dam Safety Officials (ASDSO), 

National Inventory of Dams - 1998-99, CD-ROM NID-GIS, v. 1.0, with Information 

Booklet, September 1999. 
 Chapter 2 Problems with Tree and Woody Vegetation Growth 

2-19 

10. Marks, B. Dan, S&ME Engineering, Inc., Arden, N. C., Faxed communication on recent 

 contractor-bid clearing and grubbing costs, February 23, 2000. 

11. Association of State Dam Safety Officials (ASDSO), Report on Specialty Workshop #1:

 Plant & Animal Impacts on Earthen Dams, Knoxville, Tennessee, Nov. 30 – Dec 3, 1999, 

 June 2000. 

 
Chapter 3 

Tree Growth and Tree Root Development Requirements 

The purpose of this chapter is to provide the reader and user of this Manual with a basic 

understanding of plant physiology related to fundamental processes of tree growth and 

tree root development. It is not the intent of this chapter to delve into a detailed biological 

study of trees and woody vegetation, but to provide the reader with a fundamental 

understanding of the requirements for tree growth and tree root development while 

attempting to dispel some of the misconceptions and myths associated with tree and 

woody vegetation growth, particularly as related to tree root development. 

Common Myths and Misconceptions 

There are many misconceptions and common myths relating to trees and woody 

vegetation that have been accepted by many people without a scientific basis. Many of 

these common myths and misconceptions relative to plant physiology have originated 

from uneducated interpretations of observations associated with tree growth and tree root 

development. Some of these myths and misconceptions associated with trees and woody 

vegetation affect correct interpretation and understanding of the impact of such growth on 

the safety of earthen dams. The more common myths and misconceptions must be 

dispelled so that a new level of understanding about the impacts of trees and woody 

vegetation on earthen dams can be properly developed. Trees and woody vegetation, like 

all living things, must have oxygen, nutrients, and water (moisture) to survive. Without 

these requirements, tree roots cannot continue development and tree growth cannot 

continue. The root system of trees and woody vegetation is in simplified terms comprised 

of two major components that are the root ball, typically directly below the trunk of the 

tree, and the lateral or perimeter transport root system that typically extends beyond the 

‘drip line’ or vertical projection of the canopy of the tree. 

3-1 
Chapter 3 Tree Growth and Tree Root 

 Development Requirements 

3-2 

Tree Tap Roots are thought by many to be the primary root system for all ages and types 

of trees and woody vegetation. In fact, the taproot is the first root to develop from the 

seed or reproductive source. This central root is an extension of the stem and differs from 

the stem only in that the root contains nodes for development of additional roots. Once 

the taproot has stabilized the young plant (tree), the root ball begins to develop and the 

taproot becomes less important than other roots that grow laterally from the taproot. The 

developing root ball provides vertical support for the tree as well as providing nutrients 

and water (moisture) to the tree. Roots extending laterally from the root ball increase the 

stability of the tree while functioning to collect and store nutrients, oxygen, and water for 

the tree. While it is true that some trees have more clearly defined taproots, taproots of 

most trees do not extend significantly far below the massive root ball of healthy trees. 

However, taproots are more predominant in locations where trees grow in deep deposits 

of loose dry soils. 

Tree Root Soil Stabilization is likely the most common misconception associated with 

tree growth and tree root development. How many times has the reader heard, or perhaps 

mistakenly said, “If it were not for those trees and tree roots this slope would really 

be eroded or unstable – those tree roots are really ‘holding’ that soil slope”. Many 

otherwise knowledgeable and educated individuals believe the myth that tree roots 

actually stabilize soil masses by ‘holding’ the soil together. This misconception leads 

many people to believe that heavy tree and woody vegetation growth is actually 

beneficial for steep embankment slopes. Tree root development that is necessary to 

provide nutrients for tree growth and stabilize the tree actually loosens the soil mass. 

Laterally extending tree roots could be thought of as being nature’s original application 

of the geotechnical engineering design concept of soil nailing. Root penetration stabilizes 

the tree and loosens the soil mass within which the tree roots are developing; the 

converse is a myth and certainly not true. 
Chapter 3 Tree Growth and Tree Root 

 Development Requirements 

3-3 

Groundwater Penetration by tree root systems is another common myth and 

misconception believed by many otherwise knowledgeable individuals. Although 

Cypress, Tulip Poplar, some Willow and Water Birch tree species appear to have root 

systems that are submerged, nutrient root systems of trees cannot survive beneath the 

water table or the phreatic surface (seepage line) in an earthen dam. Trees and woody 

vegetation depend upon their transport root systems to provide the major portion of the 

oxygen demand for continual tree growth and tree root development. Most species of 

trees and woody vegetation quickly die of suffocation once the lateral transport root 

system and root ball are inundated. This phenomenon can be visually observed in many 

areas of Arkansas, Mississippi, and Louisiana where large tracts of timber have been 

artificially flooded for duck hunting. If these flooded tracts of timber are not drained 

seasonally, the timber (trees) die as a result of suffocation. Similarly, beaver activity 

causes significant losses in the timber industry every year as a result of inundation of 

harvestable timber. Tree roots do not penetrate the water table or the zone of saturation 

where oxygen demands of the tree cannot be met. If the zone of saturation or water table 

is raised above the level of tree roots for an extended period, the tree will die as a result 

of suffocation. Tree root development and tree growth cannot occur when moisture 

contents in the soil mass are greater than about forty percent. 

Soil Moisture Uptake of many species of trees far exceeds that which most individuals 

would estimate as a normal requirement of water for continual tree growth and tree root 

development. It is not uncommon for most species of healthy mature trees to absorb 200 

to 300 gallons of water per day if this amount of water is available to the lateral transport 

root system. Reduced availability of soil moisture will curtail continual tree root 

development until such time that the soil mass is replenished with sufficient moisture to 

allow resumption of tree root development. Tree root development and tree growth 

cannot occur in soil masses having moisture contents less than about twelve percent for 

extended periods. 
Chapter 3 Tree Growth and Tree Root 

 Development Requirements 

3-4 

Woody Vegetation Control Versus Dam Performance is an issue that is clearly 

misunderstood by many dam owners, operators, inspectors, dam safety regulators, 

engineers, and consultants. Tree and woody vegetation root penetration is not a beneficial 

effect on the performance of earthen dams. As indicated previously, tree root penetration 

does not stabilize a soil mass, particularly an embankment slope. Quite the contrary, tree 

root penetration loosens the soil of an embankment slope and creates a condition more 

conducive to surface water penetration and slope failure. Earthen dams are not unlike 

other engineered structures in that they must be properly maintained in order to perform 

as perceived in the original design of the structure. 

When does routine vegetation maintenance and control become a dam safety and/or dam 

performance issue? The author is of the opinion that vegetation maintenance and control 

on an earthen dam ends, and the need for an engineered earthen dam rehabilitation plan 

begins, when effects of an improper vegetation maintenance and control program create 

conditions that are detrimental to the structural integrity of the earthen dam. For 

example, an earthen dam that exhibits a dense growth of grasses and weeds that are waist 

high, but is free of significant woody vegetation growth, is an earthen dam that is in need 

of proper vegetation maintenance and control to allow proper inspection of the dam. 

However, waist-high grasses and weeds would not typically affect the structural integrity 

of the earthen dam. Conversely, an earthen dam that supports a dense growth of four to 

eight inch diameter trees that preclude proper access for inspection is a dam safety and 

performance issue. Dense growths of trees and woody vegetation not only present a 

hindrance to proper dam safety inspection, but also are detrimental to the structural 

integrity of the earthen dam. Proper removal of trees and woody vegetation from earthen 

dams is a dam safety and performance issue that must be conducted in accordance with 

properly designed dam remediation plans and specifications. 
Chapter 3 Tree Growth and Tree Root 

 Development Requirements 

3-5 

Tree Root Characteristics and Requirements 

As previously indicated, root systems of trees and woody vegetation consist of two 

primary components that are the root ball and the lateral transport root system. While all 

tree and woody vegetation roots have a primary function of providing oxygen, nutrients, 

and water to the plant, they also provide stability for the plant. The root ball that is 

typically directly below the trunk of the tree provides vertical support while the lateral 

transport roots provide lateral support for the tree. Root systems of trees and woody 

vegetation growing on dam embankment slopes will typically be asymmetrical as a result 

of the need for the tree to be stabilized in the sloping embankment soil mass. The lateral 

transport roots will typically be better developed on the uphill side of the tree than on the 

downhill side of the tree. Dr. Kim D. Coder at the University of Georgia has conducted 

extensive studies and research on tree growth and tree root development requirements 

and characteristics. He has developed data from these studies and research programs that 

relate tree trunk size to root ball diameter and lateral transport root system diameter. 

These data are presented in Table 1 below. 

Table 1: Typical Rootball and Root System Sizes for Various Tree Sizes 

 

Tree Diameter, inches Rootball Diameter, feet Root System Diameter, feet 

4 to 5 6 10 to 12 

6 to 7 8 16 to 18 

8 to 9 10 20 to22 

10 to 11 12 26 to 28 

12 to 14 14 30 to 32 

15 to 18 16 38 to 46 

19 to 23 18 48 to 58 

24 to 36 20 60 to 90 

37 to 45 22 92 to 112 
Chapter 3 Tree Growth and Tree Root 

 Development Requirements 

3-6 

During the presentation of common myths and misconceptions about tree growth and tree 

root development, requirements of trees and woody vegetation for continual growth and 

root development were discussed. Based upon research and studies conducted by Dr. Kim 

Coder, requirements for tree and woody vegetation growth and root development are 

tabulated in Table 2. 

Table 2: Root Growth Resource Requirements 

Requirement Minimum Value Maximum Value

 Soil Oxygen Content 2.5% 21.0% 

 Soil Air Voids 12.0% N/A 

 Soil Bulk Density (Clays) N/A 87 pcf 

 Soil Bulk Density (Sands) N/A 112 pcf 

 Water Content of Soil 12.0% 40.0% 

 Limiting Soil Temperatures 40oF 94oF 

 Soil pH Values 3.5 8.2 

Soil air void content is one of the most critical factors for continual tree root 

development. This factor is critical since both soil density and soil oxygen content are 

dependent upon the amount of air voids present in a soil mass. Because of the importance 

of soil air void content, Dr. Coder conducted extensive research to determine limiting air 

void contents for various soil types required for continual tree root growth (See Table 3). 

Table 3: Limiting Soil Air Voids for Root Growth in Various Soil Types/Textures 

 

Soil Type/Texture Air Voids, % 

Sand 24 

Fine Sand 21 

Sandy Loam 19 

Fine Sandy Loam 15 

 

Loam 14 

Silt Loam 17 

Clay Loam 11 

Clay 13 
Chapter 3 Tree Growth and Tree Root 

 Development Requirements 

3-7 

Utilizing weight-volume relationships for various soil types and textures, Dr. Coder was 

able to determine the limiting (maximum) dry density of soil that would allow continual 

tree root development. Results of these correlations between minimum soil air void 

content and maximum soil dry densities required for continual tree root development are 

presented in Table 4 below. 

Table 4: Limiting Soil Dry Density for Root Growth in Various Soil Types/Textures 

 

 

In an attempt to relate the research data developed by Dr. Coder to geotechnical 

engineering data developed from over 200 earthen dam projects, the author has compiled 

a comparative list of soil properties for various soils that have been found in earthen dam 

embankments. The ranges given in the data presented in Table 5 below are associated 

with soil in a loose condition and soil in a compacted state that might be required in the 

construction or remediation of an earthen dam. The user of this Manual must be aware 

that these soil parameters are typical values and should not be relied upon for design of 

new earthen dams or design of remediation plans for existing dams. 

Soil Type/Texture 

Dry 

Density, 

pcf 

Sand 112.3 

Fine Sand 109.2 

Sandy Loam 106.1 

Fine Sandy Loam 103.0 

Loam 96.7 

Silt Loam 90.5 

Clay Loam 93.6 

Clay 87.4 
Chapter 3 Tree Growth and Tree Root 

 Development Requirements 

3-8 

Table 5: Summary of Typical Soil Parameters 

Soil 

Type 

Specific 

Gravity 

Void 

Ratio 

Porosity, 

% 

Dry Density, 

pcf 

Permeability, 

cm/sec 

Sand 2.62 to 2.66 0.40 to 0.90 30 to 45 90 to 115 0.01 to 0.0001 

Silt 2.60 to 2.68 0.50 to 1.20 35 to 55 75 to 110 0.001 to 0.00001 

Clay 2.66 to 2.72 0.60 to 1.40 40 to 60 70 to 105 0.0001 to 0.0000001 

As one can see from the tabulated summary of typical soil parameters, continual tree root 

development cannot occur in soils that are well compacted. One of the best methods of 

controlling tree and woody vegetation growth on new earthen dams and existing earthen 

dams where remediation requires placement of additional embankment fill soil is to 

compact the embankment fill soils to a high degree of compaction. Increased compaction 

of embankment fill soils reduces the air void content and limits the amount of surface 

water that can infiltrate into the embankment slope. However, a good ground cover of 

grasses can be established in well-compacted soils since the depth of grass root 

penetration is minimal and the surficial soils will typically sustain the shallow grass root 

penetration. 
Chapter 3 Tree Growth and Tree Root 

 Development Requirements 

3-9 

References 

1. Association of State Dam Safety Officials (ASDSO), Report on Specialty 

Workshop #1: Plant & Animal Impacts on Earthen Dams, Knoxville, Tennessee, 

November 30 – December 2, 1999, June 2000. 

 

2. Coder, K. D., Tree Root Growth Control Series: Root Growth Requirements and 

Limitations, Univ. of Georgia, Cooperative Extension Service Forest Resources, 

Publication FOR98-9, March 1998, 8 pp. 

3. Coder, K. D., Tree Root Growth Control Series: Soil Constraints on Root 

Growth, Univ. of Georgia, Cooperative Extension Service Forest Resources, 

Publication FOR98-10, March 1998, 8 pp. 

 

4. Coder, K. D., Engineered to Fail? Tree Root Management on Dams, Abstract, 

University of Georgia, Athens, November 1999, 1 page. 

5. Dickerson, William C., Integrative Plant Anatomy, Harcourt Academic Press, 

New York, 2000 
Chapter 4 

Earthen Dam Safety Inspection and Evaluation Methodology 

The purpose of this chapter is to illustrate dam behavior during the initial years of design 

life and to present a suggested inspection and evaluation methodology. An example 

earthen dam configuration will be presented in order to illustrate earthen dam behavior 

and to develop the suggested inspection methodology. 

Example Earthen Dam Configuration 

The example earthen dam is assumed to be a high-hazard dam having a structural height 

of about 33 feet and impounding a lake area of about three acres at normal pool elevation. 

The contributing watershed of the lake is about 320 acres (0.5 square mile) with a base 

flow of about one-half (0.5) cubic feet per second (cfs). 

The configuration of the example earthen dam consists of an upstream slope of 2:1 

(horizontal to vertical), a crest width of fifteen feet, and a 3:1 (horizontal to vertical) 

downstream slope. The dam has a freeboard of four feet making the hydraulic height of 

the dam about 29 feet. The dam is founded on relatively impervious (compared to the 

embankment fill soil) material with a down gradient slope of about three percent. The 

example earthen dam section has a key trench directly below the centerline of the dam 

crest that has a bottom width of ten feet and side slopes of 1:1 (horizontal to vertical). 

The dam crest has a two-percent slope toward the impounded lake and the upstream slope 

has no protection system against tree and woody vegetation growth or wave erosion. The 

embankment of the example earthen dam is assumed to be homogeneous. Figure 1 is a 

representation of the example earthen dam configuration with the theoretical seepage line 

intercepting the downstream slope at about one-third the hydraulic height of the dam. 

Rule-of-Thumb: The phreatic surface intercepts the downstream slope of a 

homogeneous earthen dam at a vertical distance of about one-third the hydraulic 

height above the toe of the downstream slope, provided there is no internal drainage 

system in the dam embankment. 

4-1
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-2 

Based upon data provided for the example earthen dam, this dam would be listed on the 

National Inventory of Dams (NID). In addition, the example earthen dam would be 

classified as a small-size, high-hazard dam by most state dam safety regulations. 

Figure 2 illustrates the example earthen dam with an embankment subdrain system 

located within the downstream embankment slope. The subdrain or embankment drain 

system is located at about the point of interception of the seepage line with the 

downstream slope if there was no embankment toe drain system within the downstream 

slope. As a result of the presence of the embankment subdrain system, the seepage line 

through the dam embankment has been modified (lowered) from the location of the 

theoretical seepage line for a homogeneous earthen dam embankment. The seepage line 

within an earthen dam is often mistakenly considered to have a permanent location. 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-3 

However, the location of the seepage line is continually changing as a result of many 

influential factors. Fluctuations in the pool elevation, seasonal and long-term 

climatological conditions, and the growth of trees and woody vegetation in close 

proximity to the seepage line are some of the factors that influence changes in the 

location of the phreatic surface within an earthen dam embankment. 

Important moisture regimes other than the steady-state seepage line (phreatic surface) are 

often not given proper consideration in the evaluation of the performance of earthen 

dams. The zone of saturation is located immediately above the phreatic surface or 

seepage line where embankment fill soils have become saturated as a result of capillary 

rise caused by capillary forces in the soil voids. Figure 3 illustrates the presence of zones 

of saturation associated with that of a theoretical seepage line location without an 

embankment subdrain system as well as that of a modified seepage line location with an 

embankment toe drain system. The height of capillary rise (thickness of the zone of 

saturation) is directly dependent upon the effective mean diameter of soil voids within the 

earthen dam embankment. The effective mean diameter of compacted soil is dependent 

upon the effective particle size (De) of the compacted embankment fill soil. Soil within 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-4 

the zone of saturation is completely saturated; however, there is no flow or gravity 

induced movement of water unless some external force disturbs the soil. This 

phenomenon is often observable during the inspection of downstream slopes of earthen 

dams. Seepage and free flowing water can be seen on the downstream slope of an older 

dam below the point of interception of the seepage line if no embankment subdrain is 

present. Above the point of interception of the seepage line with the downstream slope, 

the soil is saturated and the Zone of Saturation can be observed for a significant distance 

above the seepage line intercept in some cases. In the Zone of Saturation, pore water may 

be observed to fill tracks made in the water-softened embankment soil. However, once 

the tracks are filled by pore water released from the disturbed soil there will be no 

continued flow or seepage from the embankment. This condition is often confused with 

the presence of embankment seepage. Installation of a subdrain location in this situation 

may lower the phreatic surface relatively quickly; however, months or even years may be 

required to drain the zone of saturation because of tensile forces or negative pore 

pressures in the embankment fill soils. 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-5 

Embankment Wetting, Saturation, and Seepage 

Prior to presentation of the behavior and performance of an earthen dam embankment 

during the initial years of the design life, one must have an understanding of relationships 

between various velocities of moisture movement and water flow through compacted 

embankment soils. First, consider the relationship between the optimum compaction 

moisture content of an embankment soil and other moisture content properties. 

Rule-of-Thumb: The optimum compaction moisture content as determined by ASTM 

D-698 (standard Proctor compaction test) is approximately two to four percent below 

the Plastic Limit (PL) of most soils and about three to five percent below the saturation 

moisture content of the same soils. 

Compacted soils will typically increase in moisture content from the compaction 

moisture content to about the PL of the soil relatively quickly after construction of an 

earthen embankment. The rate of wetting is much greater in soils compacted dry of 

optimum moisture content than in soils compacted wet of optimum moisture content. 

Although compacted soils may undergo wetting or increase in moisture content relatively 

quickly when exposed to a source of water, the rate of saturation is much slower because 

air trapped in discontinuous soil voids must be dissolved in soil pore water during the 

saturation process. Embankment wetting and saturation are not associated with seepage 

or the flow of water through a homogeneous earthen dam; however, relative velocities of 

wetting and saturation can be related to values of steady-state seepage velocity, 

permeability, or hydraulic conductivity of compacted embankment soils. 

Figure 4 is an illustration of the example earthen dam with relationships between various 

soil water flow velocities and permeabilities. First, consider the relationship between the 

vertical and horizontal permeability of a compacted homogeneous embankment soil. 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-6 

Rule-of-Thumb: The horizontal permeability of a compacted homogeneous 

embankment soils are typically about nine times to ten times (one order of magnitude) 

greater than the vertical permeability. 

Variation between the horizontal permeability and vertical permeability is the result of 

the internal structure of compacted soils. This variation does not account for poorly 

compacted lifts since the embankment is assumed to be homogeneous. Consequently, if 

laboratory permeability tests indicate that a compacted embankment soil exhibits a 

hydraulic conductivity value of about 0.000004 centimeters per second (cm/sec) then the 

horizontal permeability of this compacted embankment soil will be about 0.000036 to 

0.00004 cm/sec. Second, consider Darcy’s Law that is the basis for all theories and 

analyses associated with the flow of water through soil masses. Darcy did not account for 

soil voids relative to soil solids in derivation of his equation. As a result, the area of 

discharge is the total cross-sectional area through which flow is occurring. If one assumes 

that the hydraulic gradient producing flow through a soil mass is equal to one (unity), 

then the discharge velocity (Darcy’s flow velocity) is equal to the permeability value of 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-7 

the soil. The actual flow velocity in the voids of the soil is often identified as the seepage 

velocity and is approximately equal to the discharge velocity divided by the porosity 

value (expressed as a decimal) of the soil. Assuming that the compacted embankment soil 

in the example earthen dam has a porosity of forty (40) percent (0.40), the seepage 

velocity of the soil would be about 2.5 times greater than the discharge velocity. Third, 

consider the wetting velocity or the velocity of the line of wetting. The wetting velocity is 

the rate at which soil increases in moisture content up to about the PL when exposed to a 

free water source. The line of wetting can often be observed as it progresses through soil 

masses, particularly soils that are dry of optimum moisture content. The wetting velocity 

is the sum of the seepage velocity and the capillary velocity or the velocity of wetting 

attributable to capillary forces in the soil. 

Rule-of-Thumb: The wetting velocity or the velocity of the line of wetting through 

compacted soil is about one order of magnitude (ten times) greater than the seepage 

velocity. 

Applying this factor to the previous comparison between seepage velocity and discharge 

velocity, one finds that the wetting velocity is about 25 times greater than the discharge 

velocity. Based upon the foregoing discussion of earthen dam embankment wetting, 

saturation, and steady-state seepage velocities, consider the illustration in Figure 5. This 

figure illustrates embankment wetting, saturation, and steady-state seepage during the 

early years of the design life of an earthen dam. Assume that laboratory testing indicates 

that embankment soils of the example dam embankment have a permeability or hydraulic 

conductivity value of 0.02 foot per day. The discharge velocity would be about 0.008 foot 

per day with a hydraulic gradient of about 0.4 resulting in a horizontal discharge velocity 

of about 0.08 foot per day. The associated seepage velocity would be about 0.02 foot per 

day with a soil porosity of about 40 percent and the horizontal seepage velocity would be 

about 0.2 foot per day. The velocity of the line of wetting or the wetting velocity would 

be about 2.0 feet per day. 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-8 

Based upon the estimated normal inflow from the contributing watershed, the lake 

retained by the example dam should reach about fifty percent volume in approximately 

twenty days and reach normal pool elevation in about forty days. Solid lines in Figure 5 

illustrate the location of the line of wetting at various time intervals. The line of wetting 

should reach the downstream slope in about ninety days. Note: The compacted 

embankment soils remain partially saturated after passage of the line of wetting. 

Dashed lines in Figure 5 illustrate the line of saturation at various time intervals. The line 

of saturation moves at the seepage velocity that is about one-tenth the value of the 

wetting velocity. When the line of wetting has reached the downstream slope in about 

ninety days, the line of saturation is still at about the vertical from the intercept of the 

normal pool with the upstream slope. Based upon this rate of progression, the line of 

saturation will not reach the surface of the downstream slope and steady-state seepage 

will not be initiated for about 900 days (about 2.5 years), provided that no external 

influences affect the rate of wetting and saturation. 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-9 

The estimated maximum steady-state seepage rate for the example dam will be about 5.5 

gallons per day per foot of dam. Before leaving Figure 5, imagine that the example dam 

contains an embankment subdrain system as indicated in Figures 2 and 3. The rate of 

progression of the line of wetting and the line of saturation will both be affected by the 

presence of the subdrain system. 

Even without the presence of an embankment subdrain system, the time required for the 

line of wetting could encompass an entire growing season depending upon the time of 

year that the dam was completed. More importantly, the time that is required for the line 

of saturation to intercept the downstream slope might encompass two or three entire 

growing seasons. Tree and woody vegetation growth can become quite dense and 

relatively large within the initial two to three growing seasons if not properly controlled. 

The initiation of tree and woody vegetation growth on the downstream slope begins the 

soil moisture uptake cycle so that the line of saturation and the seepage line may never 

completely develop and intercept the downstream slope. The condition represented by 

Figure 6 might initially be considered to be beneficial to the stability of the dam 

embankment. However, one must understand that as the tree and woody vegetation 

growth continues compacted soils of the dam embankment are continually loosened by 

the penetration of major tree and woody vegetation root systems. Furthermore, trees that 

might appear healthy to an untrained inspector may be an unhealthy specimen and have a 

premature death leaving penetrating root systems to rot inside the dam embankment. 

Additionally, soil nutrients in the compacted soil embankment of an earthen dam may not 

be sufficient for development of growth beyond which the tree cannot be properly 

sustained without premature death. Regardless of the cause, trees and woody vegetation 

do die and cease to uptake soil moisture that they previously used. This change in soil 

moisture uptake will affect the zone of aeration, zone of saturation, and the location of 

the seepage line in the vicinity of the unhealthy or dead trees and woody vegetation. 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-10 

The Mid-Life Crisis of an Aging Earthen Dam 

Once an earthen dam embankment has become impregnated with numerous trees and 

woody vegetation penetrations, routine and even major maintenance activities will likely 

not be sufficient to regain the original design life of the dam. At this time in the life of an 

earthen dam, previously identifiable maintenance problems have become serious dam 

performance and dam safety issues. Restoration through an engineered dam remediation 

design and remediation construction is typically required to bring the dam to acceptable 

standards relative to dam safety requirements. 

Figure 7 illustrates some of the problems and dam safety issues that can be created by 

uncontrolled or non-maintained tree and woody vegetation growth in what has been 

termed by the author as the ‘Mid-Life Crisis’ of an earthen dam. Seepage flow may be 

emerging from rootball cavities of blowdowns (uprooted trees) because they are no 

longer using soil moisture and the seepage line has adjusted upward toward the surface of 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-11 

the slope. Removal of mature trees by woodcutters deletes the soil moisture uptake of the 

removed trees thus further modifying the location of the seepage line closer to the surface 

of the downstream slope. Rootballs and root systems of otherwise healthy trees located at 

and beyond the toe of the downstream embankment slope become inundated by the 

adjusted seepage line. Since trees cannot live through prolonged submergence of their 

major root systems, these trees will become unhealthy and die leaving decaying rootballs 

and root systems as serious penetrations in the earthen dam. Rootball cavities remaining 

from blowdowns (uprooted trees) and their relationship to the seepage line create 

conditions susceptible to potential slope failure of the downstream embankment slope. 

Restoration of the example earthen dam illustrated in Figure 7 to a safe condition cannot 

be brought about through routine maintenance activities. An engineered dam remediation 

design and remediation construction will be required to restore this dam to a safe 

condition and original design life. 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-12 

Inspection and Evaluation Methodology 

The effectiveness, economics, and constructability of dam remediation designs for 

earthen dams begin and end with proper evaluations of the characteristics and seriousness 

of deficiencies as related to dam safety issues. All tree and woody vegetation growth on 

earthen dams is undesirable and has some level of detrimental impact upon operation, 

performance, and safety of an earthen dam. However, not all tree and woody vegetation 

growth on earthen dams imposes the same level of impact on operation, performance, and 

dam safety. Dam owners, regulators, inspectors, and engineers must develop an 

understanding of the impact of tree and woody vegetation growth relative to location on 

the dam configuration. Proper evaluation of the seriousness of dam safety issues related 

to tree and woody vegetation growth on earthen dams is typically associated with the 

location of the undesirable plant growth on the dam embankment. 

A few examples of the variability of seriousness of plant penetrations are presented 

herein to begin the learning process. The presence of a twelve-inch diameter tree on the 

downstream side of the crest of an earthen dam typically does not pose the same degree 

of impact on potential dam safety as a twelve-inch diameter tree located in the lower 

portion of the downstream slope. Conversely, a twelve-inch diameter tree in the upper 

portion of the downstream slope does not typically create the same level of seriousness as 

an unhealthy twelve-inch diameter tree on the upstream slope or front crest of a dam 

having a narrow crest width. Ornamental shrubs having shallow root systems along a 

wide roadway crossing the crest of an earthen dam will not impose the same level of 

seriousness as similar shallow rooted woody vegetation growing on the lower portion of 

the downstream slope. 

The purpose of developing a well-defined inspection and evaluation methodology is to 

allow the establishment of dam remediation design priorities. Most anyone having a basic 

understanding of the seriousness of tree and woody vegetation growth to the safety of 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-13 

earthen dams can inspect an earthen dam and recommend removal of all trees, stumps, 

and root systems. However, inspectors and dam engineers must develop a definitive 

inspection and evaluation methodology in order to prioritize the seriousness of various 

locations of tree and woody vegetation growth on earthen dams. 

Many individual dam owners do not have economic resources to undertake extensive 

dam remediation projects to bring an earthen dam into safe operation and performance 

conditions if the dam exists in a severely deteriorated condition. These owners often have 

to budget dam remediation projects over a scheduled maintenance and remediation 

construction period. Dam safety regulators, inspectors, and engineers that have developed 

and utilized a well-defined dam safety inspection and evaluation methodology can 

communicate priorities to dam owners so that the needed dam remediation design 

components can be completed in a prioritized manner. All too often dam safety regulators 

and engineers overwhelm dam owners with dam deficiencies without consideration of 

prioritization of deficiencies on dam safety, performance, and operation. 

Dam Safety Inspection and Evaluation Zones 

Five dam safety inspection and evaluation zones have been identified within the 

geometric configuration of a typical earthen dam. The delineated zones, illustrated in 

Figure 8, are not numbered in any implied order of seriousness relative to the impact of 

tree and woody vegetation growth, but have simply been numbered from upstream to 

downstream. The seriousness and potential impacts of tree and woody vegetation growth 

within each inspection and evaluation zone will be discussed during the description and 

identification of the delineated dam safety inspection and evaluation zones. 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-14 

Inspection and Evaluation Zone 1 begins on the upstream slope of the earthen dam 

embankment at about four feet below normal pool elevation. Zone 1 extends laterally to 

the centerline of the crest of the dam. Tree and woody vegetation growth in Zone 1 is 

more critical relative to dam safety in the case of dams having a narrow crest width than 

those having a wide crest width. Zone 1 also includes the area subject to damage resulting 

from wave erosion and frequently recurring rapid drawdown events. 

Inspection and Evaluation Zone 2 includes the entire width of the crest of the dam. 

Zone 2 overlaps Zone 1 by one-half the crest width. Overlapping a portion of Zone 1 with 

a portion of Zone 2 was done to emphasize the critical portions of both zones. Zone 2 is 

typically considered to be one of the least critical zones relative to dam safety issues 

associated with tree and woody vegetation growth. However, careful inspection of Zone 2 

often reveals evidence of serious dam safety issues such as tension cracks, slope failure 

scarps, and erosion features that may or may not be related to tree and woody vegetation 

growth originating in other dam safety inspection and evaluation zones. 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-15 

Inspection and Evaluation Zone 3 extends from the centerline of the crest of the dam to 

a point on the downstream embankment slope that is about one-third of the structural 

height below the crest of the dam. Zone 3 overlaps Zone 2 by one-half the crest width and 

is typically considered the least critical zone relative to dam safety issues associated with 

tree and woody vegetation growth. The seepage line and zone of saturation in this portion 

of an earthen dam embankment are typically sufficiently far below the surface to allow 

excavation of tree rootballs on the downstream slope of the dam without installation of a 

drain or filter system. A portion of Zone 2 has been overlapped by Zone 3 to draw 

attention to the most critical portion of Zone 3 that is the downstream portion of the crest 

of an earthen dam. 

Inspection and Evaluation Zone 4 extends from a point on the downstream 

embankment slope that is about one-third the structural height of the embankment to the 

toe of the downstream embankment slope. Zone 4 is one of the two most critical zones 

relative to dam safety issues associated with tree and woody vegetation growth as well as 

other potential dam safety issues. This zone typically contains the interceptions of both 

the zone of saturation and the seepage line with the downstream slope. The close 

proximity of the zone of saturation and seepage line to the surface of the downstream 

embankment slope in this zone is a critical factor relative to dam safety issues associated 

with tree and woody vegetation growth. Tree and woody vegetation growth in this Zone 

4 must be of major concern to everyone associated with the safety of an earthen dam 

and must be evaluated carefully relative to prioritization of dam remediation 

requirements. 

Inspection and Evaluation Zone 5 extends from the mid-height of the downstream 

embankment slope to a distance of one-half the structural height beyond the toe of the 

downstream embankment slope. This zone typically contains the interception of the 

seepage line with the downstream embankment slope and potential boiling (soil piping) 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-16 

action beyond the toe of the downstream embankment slope. As such, this zone is critical 

relative to long-term, steady-state seepage stability considerations for an earthen dam. 

Tree and woody vegetation growth in this zone rapidly develops into serious conditions 

that directly affect the safety of an earthen dam. Zone 5 overlaps Zone 4 to draw 

attention to the more critical portions of both Zone 4 and Zone 5. As in the case of Zone 

4, Zone 5 is typically considered to be one of the two most critical zones relative to dam 

safety issues associated with tree and woody vegetation growth. Tree and woody 

vegetation growth in Zone 5 must be a concern to all involved in the safety of an earthen 

dam. Maintenance and/or engineered dam remediation must be undertaken 

immediately in the event that tree and woody vegetation growth is significant within 

Zone 5. Control of tree and woody vegetation growth well beyond the toe of the 

downstream embankment slope cannot be over-emphasized. This area of an earthen dam 

is critical to overall stability and potential dam safety issues associated with embankment 

and foundation seepage. 

The dam safety inspection and evaluation methodology set forth herein can be easily 

modified and/or extended to meet the needs of specific dam owners, dam safety 

regulators and inspectors, and engineers. This proposed methodology for dam safety 

inspections and evaluations should provide a basic plan that will allow the reader to 

customize and/or improve existing dam safety inspection and evaluation programs. 
Chapter 4 Earthen Dam Safety Inspection 

 and Evaluation Methodology 

4-17 

References 

1. Casagrande, Arthur, “Seepage Through Dams”, Contributions to Soil

Mechanics: 1925 – 1940, Boston Society of Civil Engineers, pp 295-336 

(Originally Published in the Journal of New England Water Works 

Association, Volume LI, No.2, June 1937. 

2. Cedegren, Harry R., Seepage, Drainage, and Flow Nets, John Wiley & 

Sons, New York, 1967. 

3. Marks, B. Dan, “The Behavior of Aggregate and Fabric Filters in Subdrain 

Applications, Research Report, Department of Civil Engineering, 

University of Tennessee, Knoxville, Tennessee, February 1975. 

4. Means, R. E., and Parcher, J. V., Physical Properties of Soils, Charles E. 

Merrill Publishing Company, Columbus, Ohio, 1963. 

5. Parcher, J. V., and Means, R. E., Soil Mechanics and Foundations, Charles 

E. Merrill Publishing Company, Columbus, Ohio, 1968. 

6. United States Department of Agriculture (USDA), Natural Resources 

Conservation Services (NRCS), (formerly Soil Conservation Service, 

SCS), Technical Note 705 – Operations and Maintenance Alternatives for 

Removing Trees from Dams, South Technical Center, Fort Worth, Texas, 

April 1981. 

7. United States Department of Agriculture (USDA), Natural Resources 

Conservation Services (NRCS), (formerly Soil Conservation Service, 

SCS), Technical Engineering Notes – OK-08 (Revised) RE: Control of 

Trees and Brush on Dams, Oklahoma State Office, Stillwater, Oklahoma, 

February 1990. 
Chapter 5 

Controlling Trees and Woody Vegetation on Earthen Dams 

The establishment and control of proper vegetation on an earthen dam are essential to 

maintaining a safe dam. Effective, shallow-rooted, vegetative cover is necessary to 

reduce and prevent embankment slope erosion. Trees and other undesirable deep-rooted 

vegetation should be prevented from being established for the following reasons: 

• Permit effective inspection and monitoring of embankment crest and faces 

• Allow for adequate access to dam for normal and emergency operation 

• Prevent structural damage from embankment piping and internal erosion, unstable 

slopes from toppled trees, concrete wall/slab joint cracking/displacement, and 

other problems 

• Reduce possibility of root-blocked drains 

• Prevent blockage of spillway channel 

• Discourage rodent and other animal activity by eliminating food source and 

habitat 

• Eliminate expensive tree and brush removal and remediation costs 

• Reduce impression of owner neglect 

Consequently, dam owners should observe these four important rules: 

1. Existing trees should be removed and not be allowed to mature on earthen dams, 

abutment groins, or around water conveyance structures 

2. Trees or shrubbery should never be planted on or around new or existing dams 

3. Existing trees should be watched closely until they are removed 

4. Grasses and shallow-rooted native vegetation are the most desirable surface 

covering for an earthen dam. 

Dam owners should be especially aware of dangerous or potentially hazardous tree 

conditions such as decaying or dead branches; lightening-caused splits; stripping or 

breakage; leaning, uprooted or blown-down trees; and seepage around exposed tree roots 

located along embankment slopes, especially in vulnerable downstream toe or abutment 

areas. Outward leaning trees may result from a slumping embankment condition that can 

be an indicator of slope instability. Any of these conditions warrants immediate attention 

by the owner and a qualified engineer. 

5-1 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-2 

Woody vegetation and tree growth creating undesirable root penetrations in earthen dams 

can be controlled or prevented by proper management of root growth into new dams and 

dams that have previously been cleared of trees by proper removal procedures. In this 

manual, some of the characteristics of woody vegetation and tree growth are presented 

relative to the aging of an earthen dam. Remedial dam repair design procedures and 

construction techniques are presented for proper removal of trees of various sizes in 

various areas of the geometric configuration of an earthen dam. Proper management and 

control of woody vegetation on new and previously repaired dams (tree removal projects) 

are based upon an understanding of soil conditions that limit root growth, factors that 

affect or promote root growth, and various procedures and techniques that can be used to 

stop, redirect, and/or reduce the rate of root elongation. 

The purpose of this chapter is to provide a basic understanding of requirements for 

healthy root elongation, and to provide an introduction to some techniques and 

procedures that can be utilized to manage and control undesirable woody vegetation and 

tree growth on earthen dams. Development of a basic level of tree-literacy combines 

basic understanding of soil properties and characteristics with basic understanding of 

requirements and characteristics of healthy tree root elongation and tree growth into a 

single conceptual understanding of management and control. 

 

Healthy Tree Growth Requirements 

The primary requirement for healthy tree growth is an environment for continual 

elongation of tree roots. Continual elongation of tree roots is essential to healthy tree 

growth for the following reasons: 1) respiration that requires a continual flow of oxygen 

to root tissue through soil pores; 2) soil moisture uptake that requires continual 

availability of soil pore water that can be captured by root tissue; 3) nutrition that requires 

root systems to make continuously renewed soil/root surface contact to provide needed 

elements and nutrients for healthy tree growth; and 4) support and stabilization that 

requires soil-to-root surface contact to resist externally applied loads. 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-3 

Managed tree root growth control is required to prevent or minimize dangerous impacts 

on dams. To constrain root growth, identification of soil attributes and it’s supporting 

environment that promote or limit growth is required. By understanding what soil 

conditions limit growth, various tools and techniques can be used to stop, redirect, or 

inhibit tree root growth and elongation. The following discussion on root growth 

requirements, limitations and mechanics is based on a series of publications authored and 

furnished by Dr. Kim Coder of the University of Georgia Cooperative Extension Services 

(Coder, FOR98-9, -10, -11, & -13, 1998). The reader is referred to these well-referenced 

publications for further and more detailed information. 

Trees are not much different from all living organisms, relative to biological needs. Trees 

must have (1) oxygen gained through respiration, (2) water gained through adsorption 

and absorption, and (3) nutrition gained through adsorption and absorption, and (4) a 

stable foundation to withstand external forces. General root growth resource 

requirements are summarized in Table 1. Roots utilize soil spaces for access to water and 

essential element resources, and soil mass to provide structural support. Soil minerals 

surround the water-filled and air-filled voids or pores. These pores are continually filling 

and draining with water and air, depending upon the availability of water, water uptake, 

and atmospheric air. Root growth follows pathways of interconnected soil voids. Such 

voids result because of space between soil particles, between soil structural units (i.e., 

blocks, plates, aggregated soil, etc.); along soil fracture lines, lenses, joints, and various 

interstitial interfaces; and through paths of biological origins such as decayed or shrunken 

roots, animal burrows, etc. Better means of controlling growth can be developed by 

understanding resource levels that encourage and limit root growth (Coder, FOR98-9, 

1998). 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-4 

Root Resource Requirements 

Minimal Maximum 

Oxygen in soil atmosphere 2.5% 21% 

Air pore space in soil (for root 

growth) 

12% - 

 

Soil bulk density restricting root 

growth 

- 1.4 g/cc (clay) 

(note: 1 g/cc = 62.4 pcf) 

- 1.8 g/cc (sand) 

 

Penetration strength (water content 

dependent) 

0.01 kPa 

(note: 1 kPa = 1kN/m2 = 10 mbar 

= 0.145 psi) 

3 MPa 

 

Water content in soil 12% 21% 

Root initiation (O2% in soil 

atmosphere) 

12% 21% 

Root growth (O2% in soil 

atmosphere) 

 5% 21% 

Progressive loss of element 

absorption in roots (O2% in soil 

atmosphere) 

15% 21% 

 

Temperature limits for root growth 40oF/4oC 94oF/34oC 

 

PH of soil (wet test) pH 3.5 (acidic soils) pH 8.2 (alk. soils) 

Table 1. General list of tree root growth resource requirements (After Coder, FOR98-9, 1998). 

Roots survive and proliferate where adequate water is available, temperatures are warm, 

oxygen is present and other essential resources are concentrated. They generally tend to 

be shallow, limited by available oxygen and water saturation in deeper soil. However, 

near the base of the tree, deep-growing roots can be found, but are aerated by soil fissures 

and cracks and around roots where mechanical forces exerted by wind loads on the tree 

loosen the soil. 

The ability of primary root tips to enter soil pores, open soil pores and elongate through 

pores is dependent upon the force generated by the root and the soil penetration 

resistance. As the diameter and length of an expanding root increase, its strength to resist 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-5 

structural failure and its expansive force it can generate both increase. The chance for 

structural failure increases with longer and smaller diameter roots, while short and thick 

roots generate significant force but minimize structural failure. Radial expansion of the 

root structure immediately behind the tip also helps to fracture or reduce penetration 

resistance in the soil ahead of the elongating root tip. 

Roots use the mass of the tissues behind the tip, including root hairs, lateral root 

formation, and microbial entanglements to minimize the length over which root 

elongation force (or pressure) is expressed, thus reducing structural failure potential. As 

the root elongates, only root tissue within about six root diameters behind the tip is 

involved with force generation. Root tissue further back will act as an anchor and 

support base against the soil. Root tip pressure can be enormous and can range up to 9-

15 MPa (9,000-15,000 mbars, 130-215 psi, or 18,700 – 31,000 psf)), with 1MPa or about 

15 psi being most cited. Thus a typical root tip diameter of one millimeter is capable of 

generating up to about a 0.25-pound force. While tree roots cannot produce enough 

pressure to penetrate concrete, pipes, and most plastics or metals, they do take advantage 

of cracks, holes, joints and faults already in materials and exacerbate cracks and faults by 

growing root mass within, beneath, or around materials. When water supply is short, or 

when temperatures increase, diameter of roots are sacrificed to facilitate more elongation. 

Roots can lose more than one-third of their diameter under dry conditions, leaving roots 

thinner and elongating at a slower rate. Such conditions can generate passageways and 

set up the possibility for piping and internal erosion conditions in an earthen dam. 

Additionally, the loss of root contact with the soil and potential for mechanical failure of 

the elongating root system can lead to poor tree support, thus making a tree vulnerable to 

wind forces and possible upending. Tree roots are opportunistic in the colonization and 

control of resource space. The attributes that make a root an ideal resource gatherer for 

the tree conspire to make roots soil matrix explorers and fault exploiters. To prevent, 

control or eliminate roots from the soil infrastructure, dam owners and dam design 

engineers need an understanding of environmental conditions that limit and promote root 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-6 

growth. The foregoing discussion is summarized in terms of the four main requirements 

and conditions for tree growth and tree root development as follows: 

Trees need to breathe. Oxygen is required for healthy tree growth through continual root 

elongation. In order for proper root respiration to occur, oxygen must continually move 

through soil pore spaces to the root tissue. Tree roots are not the only living things in the 

soil pore system that is competing for oxygen. As oxygen flows toward an otherwise 

healthy root system, enormous numbers of aerobic organisms can utilize portions, and 

perhaps all, of the available soil pore space oxygen before it can be utilized by root 

systems. If all of the oxygen is used before reaching the root system, changes must occur 

in the characteristics and growth rate of the root system. Trees have the ability to generate 

energy for short periods using carbohydrates in low or non-oxygen environments. 

However, this process is taxing on tree growth, and is approximately twenty times more 

inefficient than under normal oxygen availability and respiration conditions (Rendig & 

Taylor, 1989; Coder, FOR98-10,1998). Air-filled voids in soil must be of sufficient size 

and continuity to allow carbon dioxide to move away from the root system and oxygen to 

move to the root system in order to sustain healthy root elongation and tree growth. 

Water-filled voids resulting from saturated soils around roots inhibit this process at a rate 

10,000 times less than air-filled voids (Rendig & Taylor, 1989; Coder, FOR98-10, 1998). 

When oxygen drops below two to five percent of atmospheric content, root growth and 

the root’s ability to generate elongation force significantly declines (Souty & 

Stepniewshi, 1988). Table 2 summarizes air void content requirements of various soil 

texture and types that limit root elongation. The table data shows that, for most 

embankment soils, trees need at least 10-25% air-filled voids in order to promote healthy 

growth. In summary, Roots that cannot breath die, resulting in unhealthy, unstable, 

and/or dead trees. 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-7 

Soil Texture Root-limiting % pores normally filled with 

air 

Sand 24% 

Fine sand 21 

Sandy loam 19 

Fine sandy loam 15 

 

Loam 14 

Silt loam 17 

Clay loam 11 

Clay 13 

Table 2. Root growth limiting air-pore space values by soil texture (After Coder, FOR98-10, 1998) 

Trees need to drink. Second behind the need for oxygen is a tree’s requirement for 

water. Water uptake of trees occurs both by adsorption and absorption. In the same 

manner as that described for oxygen supply, tree root systems depend upon the flow of 

soil pore water to the root system to continually uptake sufficient water to sustain healthy 

root elongation and tree growth. Soil voids that are sufficiently small to prevent continual 

flow of pore water can limit the amount of water that elongating roots can use within the 

soil matrix. Often, the moisture uptake is typically lower than that required for root 

elongation and healthy tree growth. As noted in Table 1, root elongation and healthy tree 

growth cannot be sustained where average soil moisture contents are less than about 12 

percent nor greater than about 40 percent. Soils that restrict free moisture movement 

preclude healthy root elongation (penetration) and healthy tree growth. Compacted 

soils limit pore space and therefore tend to limit supplies of both oxygen and usable 

water to trees. 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-8 

Trees need nourishment. Third, roots systems must provide nutrition for healthy root 

elongation and tree growth. Root elongation is required to encounter needed minerals, 

nutrients, and companion microorganisms in the soil mass. Root elongation must be 

continuous since replenishment of nutrients in soil is a long-term process that will not 

meet the requirements of stationary root systems and trees. Elongating or growing root 

systems continually encounter soil pores of various sizes. Soil pores that are larger than 

root tips create little resistance to root elongation; however, as soil pore sizes approach 

the size of root tips and/or become smaller than root tips resistance to root elongation 

increases significantly. Soil pores that are much smaller than root tips may be deformed 

in weak or soft soils; however, these small soil voids will reject root penetration in dense 

or strong soil masses. Roots cannot ‘squeeze’ into small, rigid soil pores within soil 

masses where soil strength and density preclude soil deformation and, therefore, growth 

is inhibited. High strength, dense soil masses containing limited required nutrients for 

healthy root elongation will not sustain healthy tree growth. 

Trees need foundation support. Tree stabilization and support is provided by both 

components of the tree root system. The root plate (root ball) provides vertical support 

for the weight of the tree much the same as a shallow foundation system provides support 

for a building column. However, tree root systems must also resist laterally applied 

external loads (i.e., wind loads). Lateral root systems provide required lateral support 

capacity against horizontal forces through development of soil-to-root frictional forces 

(nature’s own application of “soil nailing”). Inadequate root elongation results in 

reduction of base and lateral support, resulting in an unstable tree that becomes unhealthy 

and/or subject to failure under laterally applied loads. Dense, compacted soil masses 

preclude proper lateral root elongation thus creating unstable, unhealthy trees that are 

subject to premature failure. 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-9 

In summary, whether in design of new dams or in maintenance of older existing dams, 

engineers and dam owners need to appreciate the forces, conditions and resources that 

control and affect the health and stability of trees so as to prevent or discourage trees 

from growing on new or re-constructed dams or to understand why/how trees respond to 

given and changing conditions on existing earthen dams. 

Tree Root Elongation Management and Control 

There are at least eight well-documented methods and tools available to control and limit 

tree (root) growth through the application of tree root elongation processes, resource 

availabilities, and soil preparation characteristics. These methods take advantage of 

depriving the tree roots of ideal resource needs for healthy growth discussed above. 

While these methods have been primarily used in urban or agricultural applications and 

settings, some methods are directly applicable to use on earthen dams and include the 

following methods described by Coder (FOR98-11): 

1. Intelligent designs and applications that include techniques and materials based upon knowledge 

of tree growth and root development requirements. Here, minimizing available soil material faults 

or interfaces and tree root spaces are the preferred means for controlling and discouraging tree 

growth with the philosophy ‘Build it correctly and they will not come!’

2. Root kill zones utilizing cultivation methods, sawing and cutting, trenching, vibratory plows, and 

chemicals to control, discourage, and remove root structure. However, these methods often result 

in damaging or killing the tree that, perhaps, should have been removed in the first place. 

3. Root exclusion zones utilizing soil structure changes, soil compaction, water/aeration, stress, 

anaerobic conditions, soil injections and slurries, soil additives, and chemicals to prevent roots 

from colonizing the soil structure areas due to applied physical or chemical changes to the soil. 

Changing the soil structure, pore space volume or drainage/aeration matrices can generate a soil 

environment that roots cannot effectively grow and sustain. A variety of physical- or chemicalbased soil altering materials (i.e., soil injected clay slurry or cement solutions) can be effective, at 

least over the short term if adequate soil volume is treated. Compacting soils appear to be a very 

good way to prevent root colonization. High density soils increase the resisting strength of these 

soils to root penetration and deprive the roots from needed oxygen and available water. Certain 

types of clay soils, freeze-thaw cycles, biological activity, and poor soil compaction can, over 

time, produce root-accessible pore space. Soil or infrastructure building material additives that 

neutralize or sterilize the available minerals and nutrients such as nitrogen gas, sulphur, sodium, 

zinc, borate, salts, or herbicides may produce serious environmental consequences, short-lived 

results, and non-targeted damage potential. Other methods or additives may be cost-prohibitive. 

See Figure 6 at end of chapter for root clearance zones.
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-10 

4. Air gap systems designed to provide temporary and permanent air spaces for root pruning and 

lack of root support by use of large cobble stone barriers and drain systems. One of the more 

effective means of controlling tree root growth is providing stone matrices that dry quickly, create 

large air gaps, have poor water-holding ability, and are impermeable to systematic root 

penetration. Gravel layers or areas having at least 3/4 inch stone size or clean, graded, mediumsized rubble (crushed brick remnants or recycled paving and other materials), provided it is not 

covered or filled in with sand, are reported to produce large enough air gaps to discourage root 

growth. 

5. Barrier systems using commercial root traps, root deflectors, containment devices, metals, screens, 

plastics, paints, and inhibitors. One of the easiest and most available materials used to control root 

growth are various types of 2D-type screens and barriers. While some barriers are not completely 

effective, many types have been shown to be effective. A list of mechanical, biological and 

chemical tree root growth control barriers, products and systems is shown in Table 3. 

Table 3. Selected list of tree root growth control barriers (after Coder, FOR98-11, 1998) 

The costs of these products will likely continue to decrease as the demand for these products 

increases in the future. Of the barriers shown in the list, three types are most commonly used: 

traps (root engaging and constricting), deflectors (walls), and inhibitors (chemical constraints). 

Combined features of the barrier, the site, and barrier installation and maintenance are critical to 

their effectiveness, but no barrier should be assumed to stop all roots under all conditions. Most 

types of mechanical and chemical barriers have limited effectiveness lives and this should be 

factored into any long-term cost analysis. The reader is directed to the Table 3 reference source 

and other related publications for details on commercially-available root barriers. 

 

a. Copper sulfate-soaked, synthetic, non-woven fabric 

b. Copper screen 

c. Cupric Carbonate (CuCO3) in latex paint 

d. Fiberglass and plastic panels 

e. Fiber-welded geosynthetic fabric/mesh 

f. Galvanized metal screen 

g. Ground-contact preserved plywood 

h. *Geomembranes and heavy rigid plastics 

i. Infrastructure aprons and footings 

j. Metal roofing sheets 

k. Multiple layers of thin plastic sheets 

l. Nylon fabric/screen 

m. Permeable woven geosynthetics 

n. Rock-impregnated tar paper/felt 

o. *Slow-release chemical barriers 

p. Thin layered bitumen & herbicide mixtures 

q. Woven and non-woven slit-film plastic sheets 

 

 *Common commercial tree growth control products available 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-11 

6. Directed growth systems to concentrate roots in desired directions, guide root growth along 

channels, allow root survival in desired areas, and create root culverts or layers. As noted earlier, 

roots are opportunistic and grow and proliferate where there are good supplies of resources. 

Understanding root elongation, colonization, and survival processes allows growth-favoring soil 

layers, corridors, and areas to be designed for directing or deflecting roots away from infrastructures 

where tree roots can be harmful. Several methods or systems are used to attract, deflect, channel or 

lead roots in a direction or area as needed. One attraction method used is called “baiting” and 

involves providing ideal essential soil condition resources in a direction away from an infrastructure. 

The net result is a much higher survival and growth rate in that part of the root system as opposed 

near infrastructures where root damage can occur. Water, growth nourishment elements, and oxygen 

should be limited and compaction should be maximum near infrastructures. 

Another method is to “shepherd” roots to desirable locations using trenches, channels, layers, 

raceways, tunnels, and other devices that are surrounded by root control obstacles, barriers, or 

resource constraints. Growth channels filled with rich, well-aerated, ecologically healthy growth 

medium will encourage root colonization and survival in areas away from sensitive infrastructure 

targets.

7. Selection of desired species of trees that require lower soil oxygen environments, have improved 

root morphology, and are more effective species for long-term solutions. This method focuses on 

choosing and planting available tree species that can survive under rather limited or harsh 

environmental conditions. Several tree species are available that are small in size, have shallow 

and less aggressive rooting, and are slower growing. Dam owners, however, should be reminded 

again that trees in general are not a good plant option and have no place on dams; instead, more 

desirable, native grasses should be planted and maintained. 

 

8. Creating avoidance zones to separate tree growth from earthen dam embankments and dam 

appurtenances where root damage may be critical thus establishing biological-free zones that 

reduce potential problems. This method simply recognizes that there are places where trees are 

acceptable and other places, namely dams, where they are not (see Figure 6).

The most practicable of these methods for use on earthen dams are those associated with 

intelligent design development, exclusion zones, kill zones, and barriers. Within this 

group of suitable methods the combination of intelligent design development and 

exclusion zones are the most effective. With an understanding of the previous meshing of 

soil properties with healthy tree root elongation, it is not difficult to develop an intelligent 

design scheme for new dams and the remedial repair of existing dams. An intelligent 

design philosophy associated with dam embankment design and construction would 

involve proper embankment soil compaction as the means of exclusion of root 

elongation. 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-12 

In summary, there are many tools, methods and options for minimizing or preventing tree 

root-caused damage to earthen dams. The most important management (and design) 

concept to understand is how tree roots are invited to be associated with interstitial 

elements and colonize soil matrices and discontinuities, and resource availability areas. 

Our responsibilities as owners and dam design engineers must lie with creating and using 

any or a combination of the numerous root growth control tools and techniques that are 

tree-literate so that trees do not have the opportunity to become a safety problem to 

embankment dams and their appurtenances in the first place. 

Exclusion by Embankment Compaction 

Design and construction practices of using optimum compaction of embankment soils 

reduce potential settlement of embankments, increases shear strength of the embankment 

soils, decreases the permeability of the embankment soils, and minimizes long-term 

changes in the physical and engineering properties of soils. When embankment soil 

compaction results in the attainment of desirable objectives from a geotechnical 

engineering behavior perspective of earthen slopes, compaction of embankment soils also 

precludes tree root growth and elongation as a result of exclusion of most of the 

requirements for healthy root elongation and tree growth. As has been previously noted, 

densely compacted soils discourage root elongation through increased resistance, lowered 

oxygen levels, and reduced available water. Traditional embankment soil compaction 

specifications require that the soil be compacted to about 95 to 98 percent of the standard 

Proctor maximum dry density as determined by ASTM D-698. Furthermore, most 

properly written soil compaction specifications generally require that compaction 

moisture contents be maintained about two percent below to three percent above 

optimum moisture content. At these degrees of compaction and at these moisture 

contents, soil oxygen content, water content, and soil pore size are not available for 

healthy root elongation and tree growth. Even if there is sufficient moisture content in the 

soil to otherwise sustain healthy root elongation, the soil pore sizes are so small that 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-13 

available pore water cannot be effectively moved to the root system. Consequently, the 

compacted dam embankment fill soil produces an exclusion system that mechanically 

impedes healthy root elongation and tree growth. Table 2 provides a summary of 

minimum air voids for various soil types required to impede root elongation for healthy 

root and tree growth. 

Maintenance Mowing and Kill Zones 

The second most effective method of controlling woody vegetation and tree growth on 

dam embankments is through the use of native grass or ground cover with maintenance 

mowing, and using kill zones where necessary around critical structures to control trees 

and other undesirable nuisance-types of vegetative growth. Maintenance mowing should 

be done at least twice per year with one mowing scheduled for spring after initiation of 

new spring growth and the second mowing scheduled for late fall immediately prior to 

the first killing frost or freeze (See Chapter 7). The spring mowing should be a very close 

cutting of all vegetation to allow maximum sunlight to penetrate to desirable grass cover 

species. The fall cutting should not be as close as the spring cutting to provide maximum 

resistance to surface runoff erosion and to provide cover for desired wildlife species 

(quail, rabbit, grouse, songbirds, etc.). 

In areas where regular maintenance mowing is not practical to control woody vegetation 

and tree growth, the selective use of herbicides might become necessary to control small 

woody vegetation and tree growth. There are many commercially available herbicides 

that are environmentally safe to use in most applications. However, one must always be 

careful in the use, or overuse of herbicides, because they are design to kill and/or impede 

(slow) plant growth. Overuse of herbicides may contaminate areas of the dam 

embankment to such an extent that desirable grass cover cannot be effectively grown. 

One must always follow manufacturers recommendations when using herbicides, or 

better yet, solicit the advice of the nearest USDA/NRCS agent prior to using herbicides to 

control woody vegetation and tree growth on earthen dams. 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-14 

Chemical Barrier Systems to Inhibit Root Growth 

Commercially available barrier systems are effective in controlling root elongation and 

growth; however, many of these barrier systems are relatively expensive and cannot be 

justified for placement over the entire earthen dam embankment. These barrier systems 

are often economical for placement on portions of earthen dams where accessibility is 

difficult after construction and/or where particularly problematic and nuisance woody 

vegetation and tree growth is likely to occur. 

One typical biocide product, called “Biobarrier©” is marketed and promoted, among 

other applications such as sidewalk and landfill cap protection, to prevent tree and plant 

roots from penetrating dams. The product consists of long-term, slow release nodules 

containing Trifluralin herbicide, that are bonded to a geotextile fabric as shown in 

Figure1.

Figure 1. Chemical biocide barrier installation showing slow-releasing biocide nodules attached in a 

woven fabric matrix and installed under a cover of soil, mulch, gravel or stone (Biobarrier©). 

This particular barrier is environmentally acceptable to EPA and indicated to be effective 

against all types of roots around pipes, hardscapes, and dams and levees. While the 

product is guaranteed for 15 years, its life is inversely proportional to environmental 

temperature conditions. 

 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-15 

For example, its effectiveness is expected to be about 40 years at 20o

C (68oF) and about 

100 years at 15o

C (60oF). For deep soil cover, it is expected to last 100 years; for near 

soil-surface weed control installations, where temperatures are higher and cycle daily, the 

projected life is expected to exceed the guaranteed 15-year life. Figures 2 and 3 show an 

application of this product on a 25-foot high and 350-foot long earthen dam to prevent 

deep penetration of deep-rooting native trees and woody vegetation such as willows, 

sagebrush, and chokecherries. 

Herbicidal Applications 

Herbicidal delivery to control undesirable vegetation depends on several considerations 

which include (a) types of plants and weeds (herbaceous, vines, trees, brush, 

phreatophytes, etc.), (b) site conditions (geology/sinkholes/karst), topography, (c) 

proximity to water bodies, (d) riparian land use, (e) sensitive environmental factors 

(Federal, state & local regulations; potential off-site wind drift over water or land), and 

(f) application factors (dosage, placement, retention time, plant growth stage, 

physiological factors, and method of application). A very important consideration is for 

the user to follow the herbicide manufacturer’s warnings and instructions. The user is 

Figure 2. Earth dam installation of chemical Figure 3. Installed chemical barrier on a 

dam in Montana (Kershner, 1992)
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-16 

also encouraged to consult with a local county extension office or agent to obtain advice 

on the best and safest herbicide to use and on what recommended application technique 

to use. While there are several herbicidal delivery methods available, the most common 

techniques are shown below in Figure 4. 

• Foliage spraying 

• Tree injection 

• Frill or girdle treatment (slash through bark then spray or paint) 

• Basal bark spraying 

• Cutting tree and poisoning stump 

• Soil treatment 

• Other 

 Figure 4. Herbicide delivery application methods 

Some of these techniques and herbicides used are illustrated in Figures 5a – 5f. The U. 

S. Department of Agriculture (SCS, now NRCS) published a useful methods, treatment 

points, and time of treatment guidelines for controlling trees and brush on dams, 

including some of the applications listed in Figure 4 (USDA, 1988). Table 4 summarizes 

the USDA recommendations. With the exception of Krenite, which is applied to the 

foliage, 2,4-D is the only approved herbicide for poisoning trees on dams. 2, 4-D is 

manufactured by several companies and is sold under several trade names. In all cases, 

the user is cautioned again to follow the manufacturer’s instructions and should consider 

the manufacturer’s label instructions to supercede recommended instructions in the 

USDA table. 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-17 

Figure 5. Applications and techniques for different herbicidal deliveries to trees and brush, with 

example commercially-available herbicides listed. 

 Figure 5a. Frill cut application. Figure 5b. Basal bark spraying. 

Figure 5c. Hypohachet application. Figure 5d. Cut-stump application.

Figure 5e. Backpack foliar application on dam. 

 Vanquish 

 Weedone 

 Weedmaster 

 Crossbow 

 Banvel 

 Roundup 

 Accord 

 Arsenol 

 Chopper 

 Pathway 

 Tordon 

 Touchdown 

 Spike 

 Garland

 Vanquish 

 Weedone 

 Chopper 

 Pathway 

 Tordon 

 Arsenol

 Touchdown 

 Crossbow 

 Access

 Lower 18”: 

 Ester derivatives 

 to penetrate bark

Crossbow 

Pathway 

Tordon 

 Weedone

Apply treatment 

within an hour 

 of cutting tree
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-18 

Method of Application Recommended Time 

• Cutting trees and poisoning stumps • Growing season 

• Injection • Anytime 

• Foliage spraying • Last two (2) months of growing season 

• Frill treatment (trees larger than 4” dbh) • Anytime 

• Basal spraying (trees smaller than 6” dbh) • Growing season 

• Prescribed burning (trees smaller than 2” 

dbh) 

• See technical specifications 

Table 4. Recommended methods and time of herbicide treatment application (USDA, 1988). 

USDA recommends that trees killed by herbicide should be removed within the year 

following treatment to prevent front slope from falling into the reservoir and plugging the 

spillway. Downed trees on the back slope should also be removed to prevent potential 

problems of seepage, erosion, burrowing animals, etc. 

The reader is referred to the USDA guideline for detailed discussion on each of the six 

treatment methods listed in the above table. These methods can be applied to establish 

tree and woody plant clearance or avoidance zones on and around dams as illustrated in 

Figure 6. 

 

Figure 5f. Tractor spraying application on dam. 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-19 

 

 

 

 

 Figure 6. Tree clearance zones for embankment dams and dikes. 

Groin 

Concrete 

structure

Drains

Structures 

Groin 

Toe 

area

Slope 

Spillway

Concrete

structure
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-20 

Desert Plants 

Deep-rooted desert plants, when left unchecked, can propagate rapidly on earthen dams 

located in arid and semi-arid regions of the U. S. Some of these deep-rooted plants 

include Desert Broom shown in Figure 7, Salt Cedar, Mesquite, Cypress, Cottonwood 

and Paloverde. All of these species require considerable effort to control and should not 

be allowed to become established anywhere on dams. Palm trees can be a problem in 

that they are shallow-rooted, but develop a large root ball that can produce large cavities 

when toppled during high winds. Upstream and downstream access roads, in place at 

many dams, should be utilized to create a buffer zone between these species and the toe 

of dams. 

Figure 7. Deep-rooting Desert Broom Plant 

The Maricopa County, Arizona, Flood Control District (MCFCD) recommends, in cases 

where deep-rooted plants are two feet in height or less, that they be controlled with a 3-

5% solution of Roundup® Pro (Renckly and Drake, 1999). If the plants are over two feet 

in height they should be hand cut to ground level. The stumps should be treated within 

the first five minutes by an almost straight mix of either Roundup Pro® or Garlon 3AGarlon 4®, depending on the temperature conditions. MCFCD recommends that when 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-21 

treating Salt Cedar near waterways that Rodeo be sprayed at a 3-5% solution with six 

ounces of Siltwet® per acre added. This is sprayed on plants two feet in height and 

under. Plants over two feet are hand cut and the stump treated with an almost straight 

solution of Rodeo® within five minutes of cutting the plant. 

Revegetation on earthen dams is recommended to minimize erosion on the embankment 

slopes and to provide natural landscaping for earthen dams. MCFCD recommends 

hydro-seeding over labor-intensive hand-seeding to revegetate dam embankments. 

Figure 8 illustrates hydro-seeding operations on a floodway dam. Seed, water, tack 

material and a wood fiber or paper mulch are mixed in a hydro-seeder and sprayed 

directed onto the slopes. The seeds are encapsulated in the mulch and tack material until 

enough moisture is present to begin the germination process. 

Figure 8. Hydro-seeding operations on a floodway dam in Maricopa County, Arizona 

(Renckly & Drake, 1999). 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-22 

MCFCD has found that it takes 2 to 3 years before “significant” vegetative cover results 

are achieved because of the arid climate and high degree of embankment compaction. 

MCFCD determines the desirable seed mix by first laying out a test acre on the dam 

embankment and a plant count is then taken of all the different plant species that are 

native to the area and placed on the test acre. This plant count is converted by the seed 

supplier into the amount of seed needed to germinate the desired amount of the species 

per acre. The amount of pure live seed (PLS) applied for individual plant species also 

varies by availability from the local seed supplier. Table 4 shows a seeding mixture 

specified for one of the District’s dams and is typical of specified hydro-seeding mixes. 

No deep-rooted species are allowed in the seed mix. 

MCFCD has found that revegetation efforts have successfully reduced erosion problems, 

but has attracted both desirable and undesirable animals. 

SEEDING MIXTURE 

 

Common name Scientific Name Pounds of Seed Per Acre 

Purple three-awn Aristida purpurea 4 

Indian Wheat Plantago insularis 3 

Needle Grama Bouteloua arstiodoides 1 

Desert Marigold Baileya multiradiata 1 

Mexican Gold Poppy Eschschotzia mexicana 1 

Creosote Larrea tridentata 8 

Brittle Bush Encelia farinosa 2.5 

Bursage Ambrosia deltoidea 2 

** Note: Apply 1500 pounds of wood fiber mulch in Hydro-seed mix, plus 150 – 200 

 pounds tack material per acre.

Table 4. Typical seed list specified for a flood control dam managed by the Maricopa County, 

Arizona, Flood Control District (Renckly & Drake, 1999) 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-23 

References: 

1. Coder, K. D., Tree Root Growth Control Series: Root Growth Requirements 

and Limitations, Univ. of Georgia, Cooperative Extension Service Forest 

Resources, Publication FOR98-9, March 1998, 8 pp. 

2. Coder, K. D., Tree Root Growth Control Series: Soil Constraints on Root 

Growth, Univ. of Georgia, Cooperative Extension Service Forest Resources, 

Publication FOR98-10, March 1998, 8 pp. 

3. Coder, K. D., Tree Root Growth Control Series: Methods for Root Control, 

Univ. of Georgia, Cooperative Extension Service Forest Resources, 

Publication FOR98-11, March 1998, 9m pp. 

4. Coder, K. D., Selected Literature: Root Control Methods, Univ. of Georgia, 

Cooperative Extension Service Forest Resources, Publication FOR98-13, 

March 1998, 4 pp. 

5. Coder, K. D., Root Growth Control: Managing Perceptions and Realities, 

Proceedings, Second International Workshop on Tree Root Development in 

Urban Soils, International Society of Arboriculture, March 5-6, 1998, edited 

by D. Neely and G. Watson, pp. 51-81. 

6. Coder, K. D., Engineered to Fail? Tree Root Management on Dams, Abstract, 

University of Georgia, Athens, November 1999, 1 page. 

7. Rendig, V. V. and H. M. Taylor, Principles of Soil-Plant Interrelationships, 

McGraw-Hill, 1989. 

8. USDA-SCS, Technical Notes (OK-8), Control of Trees and Brush on Dams, 

Stillwater, Oklahoma, April 5, 1988. 

9. Sisneros, D., USDI-USBR, Res. And Lab. Serv. Div., Upper Colorado Region 

Saltcedar Cost Analysis, Memo 94-2-2, February 1994. 

10. USDI-USBR, Water Operation and Maintenance, Bulletin No. 150, 

Guidelines for Removal of Trees and Vegetative Growth from Earth Dams, 

December 1989. 

11. Biobarrier©, Application Manual, Root Control System,BBA 

Nonwovens/Remay, Inc. Product Information, August 1999, Old Hickory, 

Tennessee. 

 
Chapter 5 Controlling Tree and Woody Vegetation 

Growth on Earthen Dams 

5-24 

12. New Hampshire DES Environmental Fact Sheet, Tree Growth on Dams, WDDB-8, 1997. 

13. Ohio Department of Natural Resources, Dam Safety: Trees and Brush, Fact 

Sheet 94-28, July 1999. 

14. Pennsylvania DEP, Fact Sheet – Vegetation/Erosion Control on Dams, 31-40 

FS,DEP1909,June1997,http://www.dep.state.pa.us/dep/deputate/watermgt/WE/

FACTS/fs1909.htm. 

15. Renckly, T., Drake, G., Plant & Animal Management Practices on Flood

 Control Dams, Maricopa Co., Arizona, November 1999. 

16. USDA-SCS (NCRS), S. Tech. Serv. Ctr., Technical Note 705, Operations &

 Maintenance Alternatives for Removing Trees from Dams, April 1, 1981. 

17. Univ. of Tenn. Agr. Extn. Serv., R. Bullock, Chemical Vegetation

 Management on Non-cropland, Bulletin PB-1538, December 1995. 

18. Univ. of Tenn. Agr. Extn . Serv., G. Rhodes & G. Breeden, 2001 Weed

 Control Manual for Tennessee, Bulletin PB-1580, December 2000. 

19. Kershner, C., Geotextiles - It’s Only Natural, Land and Water, January 1992. 

20. STS Consultants Ltd., ASDSO Working Group, Dam Safety Guidebook, 

 1985. 

 

21. Association of State Dam Safety Officials (ASDSO), Report on Specialty 

 Workshop #1: Plant & Animal Impacts on Earthen Dams, Knoxville, 

 Tennessee, November 30-December 2, 1999, June 2000. 

 
Chapter 6 

Dam Remediation Design Considerations 

Specific dam remediation design considerations, procedures, and techniques will be 

considered for each of the previously identified dam safety inspection and evaluation 

zones. Figure 1 presents these zones as a review prior to discussion of potential dam 

remediation design considerations for each zone. Dam remediation design alternatives 

presented herein should be considered examples. These remediation design examples 

should not be considered the only alternatives for use in dam remediation design to 

correct deficiencies associated with tree and woody vegetation growth on earthen dams. 

Some additional dam remediation design alternatives presented for correction of tree and 

woody vegetation growth related deficiencies also provide positive correction of other 

deficiencies and protection against other types of earthen dam deterioration. 

6-1 
Chapter 6 Dam Remediation Design Considerations 

6-2 

Inspection and Evaluation Zone 1 

Figure 2 illustrates potential problems that can occur in Zone 1 with respect to tree and 

woody vegetation growth on earthen dams. This illustration also depicts the occurrence 

of wave erosion, vehicle access, and surface runoff erosion. Potential problems illustrated 

include instability of relatively large trees on the upstream slope and dam crest, and 

alteration of the seepage line as a result of wave erosion. 

Dam remediation design techniques necessary to address potential problems illustrated in 

Figure 2 are illustrated in Figures 3 and 4. Dam remediation construction typically 

requires lowering of the normal pool elevation and/or complete drawdown of the retained 

reservoir. This is particularly true for dam remediation construction in Zone 1. The 

normal pool elevation should be lowered as far ahead of the scheduled dam remediation 

construction as practicable. 
Chapter 6 Dam Remediation Design Considerations 

6-3 
Chapter 6 Dam Remediation Design Considerations 

6-4 

Tree and woody vegetation growth in Zone 1 must be undercut to remove all stumps, 

rootballs, and root systems developed by tree penetrations as illustrated in Figure 3. The 

required depth of undercutting typically extends to near the limits of Zone 1, which is 

about four feet below normal pool elevation. In the case of earthen dams with narrow 

crest widths, the backslope of the undercut area will typically extend to near the 

centerline of the dam crest or the downstream limits of Zone 1. Subsequent to 

undercutting affected areas of Zone 1, the undercut area must be thoroughly inspected to 

confirm that all major root systems (greater than about one-half inch in diameter) have 

been removed during the undercutting operation. Following inspection and approval of 

the undercut area by the engineer, suitable backfill should be placed in the excavation and 

properly compacted to the dam remediation design limits. Backfill should consist of 

approved embankment fill material and should be compacted to a minimum of 95 percent 

of the maximum dry density of the fill soil as determined by the standard Proctor 

compaction test (ASTM D-698). In conjunction with the undercutting and backfilling, the 

dam remediation design should include a slope protection system to deter future tree and 

woody vegetation growth and reduce the potential for wave and surface runoff erosion. 

Figures 4(a) through 4(c) illustrate various configurations of rigid (concrete) upstream 

embankment slope protection systems. Figure 4(a) illustrates a concrete slab being placed 

directly on the upstream slope from about three feet below to about two feet above 

normal pool elevation. While this system is somewhat limited relative to the area of 

protection, the most critical aspect of this system is that it provides no filtration and/or 

drainage system beneath the concrete slab. Continual wave action and the buildup of 

hydrostatic pressures beneath the concrete slab will eventually result in downward 

movement of the slab. Figure 4(b) illustrates a better dam remediation design utilizing a 

concrete slab slope protection system. This slope protection system has been improved 

over the original system by covering a larger area of the upstream slope and by providing 

a filter system beneath the concrete slab protection system. The author is of the opinion 
Chapter 6 Dam Remediation Design Considerations 

6-5 

that the dam remediation protection system shown in Figure 4(c) is the most desirable 

and cost effective design for use of reinforced concrete for a protection system. The 

reinforced concrete wall provides a gentle slope to flat backfill area that can easily be 

maintained by mowing to preclude tree and woody vegetation growth. In addition, this 

dam remediation design alternative can be used to provide a wider effective dam crest 

and provides excellent protection against wave erosion. 

NOTE: Reinforced concrete wall and slab systems constructed on the upstream slope 

must always be provided with filtration/drainage systems to reduce the potential for 

development of excessive hydrostatic pressures and internal erosion and scour of soil 

from beneath the structures. The referenced figures are presented for illustrative 

purposes and should not be used for actual dam remediation design without proper 

design analyses to confirm any indicated dimensions of the drawings. 

Alternative flexible upstream slope protection system designs for use in Zone 1 are 

shown in Figures 4(d) and 4(e). The author has utilized both of these flexible slope 

protection systems effectively to reduce potential tree and woody vegetation growth on 

upstream slopes and to provide resistance to wave and surface erosion. Figure 4(d) 

illustrates a typical gabion wall system while Figure 4(e) illustrates the use of a 

Mechanically Stabilized Earth (MSE) wall system for protection of the upstream slope of 

an earthen dam. 

NOTE: Granular backfill material used in design and construction of these flexible 

wall systems must be protected against soil contamination and internal erosion of 

retained soil by an effective geotextile filter/drainage material and/or a graded 

aggregate filter. These figures are presented herein for illustrative purposes and should 

not be used for actual design without proper design analyses to confirm any indicated 

dimensions of the drawings. 
Chapter 6 Dam Remediation Design Considerations 

6-6 

Inspection and Evaluation Zones 2 and 3 

Potential problems associated with tree and woody vegetation growth on earthen dams in 

identified Zones 2 and 3 are illustrated with dam remediation design procedures in Figure 

5. Potential problems illustrated for Zone 2 include the growth of mature trees having 

stump diameters greater than twelve inches. Mature trees having stump diameters greater 

than eight inches are illustrated at various locations throughout Zone 3 and in the overlap 

area of Zones 2 and 3. 
Chapter 6 Dam Remediation Design Considerations 

6-7 

Two dam remediation design procedures are illustrated in Figure 5 for removal of trees of 

various sizes. This illustration implies that trees located in the overlap area of Zones 2 

and 3 having stump diameters less than about twelve inches could be cut flush with the 

ground and left in place for future treatment of the decayed stump and rootball system. 

However, removal of all stumps, rootballs and root systems is always the better and more 

conservative approach to removal of mature trees. Subsequent to cutting of trees having 

stump diameter less than about twelve inches in the overlap area of Zones 2 and 3, the 

surface of the stump can be treated with a protective coating similar to polyurethane that 

will prolong the decaying process. Conversely, the referenced illustration indicates that 

any trees in Zone 2 upstream of the overlap area of Zones 2 and 3 having stump 

diameters of twelve inches or greater should be treated by total removal of the tree, 

stump, rootball, and root system. The suggested dam remediation design and construction 

procedure suggested for complete removal of trees, stumps, rootballs, and root systems in 

Zones 2 and 3 consists of the following activities: 

1. Cut the tree approximately two feet above ground leaving a well-defined 

stump that can be used in the rootball removal process; 

2. Remove the stump and rootball by pulling the stump, or by using a 

track-mounted backhoe to first loosen the rootball by pulling on the stump 

and then extracting the stump and rootball all together (this is much the 

same procedure a dentist would use in extracting a tooth); 

3. Remove the remaining root system and loose soil from the rootball cavity 

by excavating the sides of the cavity to slopes no steeper than 1:1 

(horizontal to vertical) and the bottom of the cavity approximately 

horizontal; and 

4. Backfill the excavation with well-compacted soil placed in relatively thin 

lifts not greater than about eight inches in loose lift thickness. Compaction 

of backfilled soils in these tree stump and rootball excavations typically 

requires the use of manually operated compaction equipment or 

compaction equipment attached to a backhoe. 

 NOTE: All disturbed areas must be protected by seeding and mulching. 
Chapter 6 Dam Remediation Design Considerations 

6-8 

Figure 5 further illustrates that trees located in Zone 3 that have stump diameters greater 

than about eight inches should be treated by total removal. The removal procedure should 

be the same as previously described for larger trees in Zone 2. Trees having stump 

diameter of less than about eight inches could be cut flush with the ground and treated 

with a waterproofing sealant similar to polyurethane to prolong the stump and rootball 

decaying process. Again, complete removal of the stumps, rootballs, and root systems of 

all mature trees is a better and more conservative method of remediation. 

Inspection and Evaluation Zone 4 

Figure 6 illustrates potential problems associated with tree and woody vegetation growth 

in Zone 4 of an earthen dam with suggested dam remediation design and construction 

procedures. 
Chapter 6 Dam Remediation Design Considerations 

6-9 

Young immature trees having stump diameters less than about six inches can be removed 

by cutting flush with the ground and treating the stump with a wood preservative and/or 

sealant to prolong the decaying process. This procedure is based upon the fact that 

immature trees of this size typically have not developed a rootball and/or root system that 

will significantly impact the zone of saturation or the seepage line in Zone 4. 

Trees having stump diameters greater than about six inches must be treated by complete 

removal; however, the dam remediation design and construction procedure for total 

removal of trees in Zone 4 is somewhat more complicated than total removal of trees in 

previously discussed zones. Treatment of mature tree penetrations in Zone 4 involves the 

following activities: 

1. Cut the tree approximately two feet above ground level leaving a prominent 

stump for use in the rootball extraction process; 

2. Remove the stump and rootball by pulling the stump or extracting with a 

track-mounted backhoe after loosening the rootball by pulling on the stump 

from different directions; 

3. Clean the rootball cavity to remove loose soil and the remaining root system 

by excavating the rootball cavity with maximum 1:1 (horizontal to vertical) 

side slopes and a horizontal bottom; and 

4. Install a subdrain and/or filter system in the tree penetration excavation and 

backfill with compacted soil placed in maximum loose lifts of eight inches. 

 

 Note: Backfill placed in all tree removal excavations must be compacted to a 

 minimum of 95 percent of the maximum dry density as determined by 

 ASTM D-698. 

 Note: Subdrain and/or filter systems installed in tree removal excavations in 

 Zone 4 may be incorporated into major subdrain systems to be installed in the 

 overlap area of Zones 4 and 5. 
Chapter 6 Dam Remediation Design Considerations 

6-10 

Inspection and Evaluation Zone 5 

The author identified Zone 5 as one of the two most critical zones for tree and woody 

vegetation growth on an earthen dam. Figure 7 illustrates some of the problems that can 

occur with tree and woody vegetation growth in Zone 5. The major adverse feature in 

Zone 5 is typically the interception of the downstream embankment slope by the seepage 

line. The author is a strong advocate of the installation of embankment subdrain systems 

during dam remediation design and construction even though the earthen dam may have 

been provided with an embankment subdrain system during original design and 

construction. 

One must understand the impact of tree removal in Zone 5 on the seepage line and the 

quantity of seepage that will occur subsequent to dam remediation in this zone. As 

indicated by Figure 7, trees in Zone 5 having stump diameters less than about four inches 

can be cut flush with the ground and the stump treated with a waterproof sealant to 
Chapter 6 Dam Remediation Design Considerations 

6-11 

prolong stump and rootball decay. Trees having stump diameters greater than about four 

inches must be removed completely. If the embankment toe drain or subdrain system is 

installed in advance of tree removal in Zone 5, the rootball cavity can be backfilled with 

compacted soil, provided seepage does not emerge from the excavation and/or the tree is 

located beyond the toe of the embankment slope. Tree rootball cavities existing beyond 

the toe of the downstream embankment slope generally require the installation of a filter 

system and in some cases a weighted filter system as indicated in Figure 7. The weighted 

filter system may be converted to a weighted drain system by installing a drain and outlet 

pipes connected to the outlet pipe of the embankment subdrain system. 

Summary of Dam Remediation Design Considerations 

A summary of dam remediation design considerations for treatment of tree and woody 

vegetation on earthen dams is presented below. Dam remediation design procedures and 

techniques are presented for treatment of various size trees in the identified dam safety 

inspection and evaluation zones. 

Remedial Repair Zone Procedures and Techniques

Zone 1 Remove all trees, stumps, rootballs, and root 

system; clean rootball cavity; and backfill with 

properly placed and compacted soil backfill. Install 

tree and woody vegetation and wave erosion 

protection system on the upstream slope from about 

four feet below normal pool elevation to about three 

feet above normal pool elevation. 

Zone 2 Cut trees in overlap area of Zone 2 and Zone 3 

having stump diameters of twelve inches or less 

flush with the ground and treat the stump with a 

waterproof sealant to prolong stump decay. 
Chapter 6 Dam Remediation Design Considerations 

6-12 

Completely remove trees having stump diameters of 

about twelve inches and greater, and backfill 

rootball cavity with properly compacted backfill 

soil. 

Zone 3 Cut trees having stump diameters of about eight 

inches and less level with the ground and treat the 

stump with a waterproof sealant to prolong stump 

and rootball decay. 

Completely remove all trees having stump 

diameters greater than about eight inches and 

backfill the cleaned rootball cavity with compacted 

backfill soil. 

Zone 4 Cut all trees having stump diameters of six inches 

or less flush with the ground and treat the stump 

with a waterproof sealant to prolong stump and 

rootball decay. 

Remove all trees having stump diameters greater 

than about six inches, install subdrain and/or filter 

systems, and backfill with properly compacted soil 

around the filter/drain system. 

 

Zone 5 Cut all trees having stump diameters of about four 

inches and smaller flush with the ground and treat 

the stump to prolong stump and rootball decay. 

Install a major embankment toe drain or subdrain 

system to lower the phreatic surface, filter, collect, 

and discharge embankment seepage. Incorporate 

major subdrain with tree rootball and stump 

removal where possible. 

Remove all trees located beyond the toe of the 

downstream slope having stump diameters greater 

than about four inches. Install weighted filters 

and/drain systems in rootball cavities where seepage 

boiling and soil piping is likely to occur. 
Chapter 6 Dam Remediation Design Considerations 

6-13 

Tree and Woody Vegetation Growth Control Program 

Many individual dam owners and small dam owner organizations are not financially 

capable of undertaking comprehensive dam remediation projects in one major 

construction contract. Therefore, they must undertake dam remediation programs in a 

sequential manner. The following sequential dam remediation program for controlling 

tree and woody vegetation growth provides the owner, regulator, and engineer with a 

reasonable opportunity to effectively evaluate the condition of an earthen dam and to 

prioritize dam remediation relative to observed dam safety issues. 

1. First Year: Cut all tall grasses, weeds, underbrush, and trees and woody 

vegetation having stump diameters of four inches or less 

flush with the ground and treat all cut stumps with a 

waterproof preservative to prolong rootball and stump 

decay. 

 

2. Second Year: Cut all trees in Zones 1 through 4 having stump diameters 

of six inches or less flush with the ground and treat the 

stumps to prolong stump and rootball decay. Keep all zones 

mowed and/or maintained to preclude renewed growth of 

previously cut woody vegetation. Repair most severe 

animal penetrations that exhibit seepage flows and/or cause 

unstable slope conditions on Zones 1, 4, and 5. 

3. Third Year: Initiate comprehensive remedial dam repair investigations, 

analyses, and preliminary design. Remove all trees from 

Zones 1 through 3 having stump diameters less than about 

eight inches by cutting flush with the ground and treating 

the stump with a preservative to prolong stump and rootball 

decay. 

4. Fourth Year: Finalize remedial dam repair design and begin construction 

of remedial repairs for all plant and animal penetrations 

that require special remedial dam repair design 

considerations. 
Chapter 6 Dam Remediation Design Considerations 

6-14 

5. Fifth Year: Finalize remedial dam repair construction and begin an 

operation and maintenance program that will preclude the 

need for future remedial dam repair associated with plant 

and animal penetrations of earthen dams. 

NOTE: Earthen dams that exhibit severe dam safety deficiencies 

and dam safety issues that cannot be prolonged as a result 

of potential imminent dam failure are not subject to the use 

of this type of sequential dam remediation program!!! 

 
Chapter 7

Economics of Proper Vegetation Maintenance 

Regular maintenance on a dam, especially attention to trees and brush, is known to be critical to dam

safety for several reasons (Tschantz, 2000):

 Overturning or uprooting trees causing large voids and reduced freeboard; and/or reduced

cross-section for maintaining stability

 Decaying roots of dead trees causing potential seepage paths and piping problems

 Interfering with effective dam safety monitoring, inspection and maintenance for seepage,

cracking, sinkholes, slumping, settlement, deflection, and other signs of stress

 Hindering desirable vegetative cover and causing embankment erosion

 Obstructing emergency spillway capacity

 Falling trees causing possible damage to spillways and outlet facilities

 Clogging embankment underdrain systems

 Cracking, uplifting or displacing concrete structures and other facilities

 Inducing local turbulence and scouring around trees in emergency spillways and during

 overtopping

 Providing cover for burrowing animals

 Loosening compacted soil

 Allowing roots to wedge into open joints and cracks in foundation rock along abutment

groins and toe of embankment, thus increasing piping and leakage potential.

State and federal dam safety officials and other dam safety experts agree that trees have no place on

dams. Federal agencies and some states do not allow trees to grow on dams. However, it is

estimated that about a third of the nation’s 77,000 inventoried dams have sufficient woody

vegetation to hinder effective dam safety inspections (ASDSO, 2000; Tschantz, 2000). Most states

require dam owners to remove trees and undesirable vegetation, but the cost of clearing and

grubbing trees and restoring the dam embankment slopes and crest is often cost prohibitive for many

dam owners, usually running into thousands of dollars. It would seem that regular control of woody

vegetation and maintaining the surface on an earthen is relatively inexpensive, compared to

removing trees on and repairing damage from neglected dams such as shown in Figure 1.

7-1
Chapter 7 Economics of Proper Vegetation Maintenance

7-2

Figure 1. Restored Downstream Slope on Fishing Creek Dam, Maryland (1991-92)

Likewise, it is important that owners maintain desirable vegetation on their dams on a regular

schedule to avoid the expense of periodically removing undesirable heavy brush and mature trees.

 Early control is generally viewed to be the most cost-effective means of avoiding potential adverse

effects on these structures from their continued growth (USBR, 1989). The bulk of maintaining a

dam usually involves keeping the grass mowed and brush trimmed. An important question arises,

“How much is a dam owner justified in spending to maintain a dam on a regular or annual basis

to avoid having to bear the heavy cost of removing trees?” A correlative question then follows,

“How often should a dam be mowed to control undesirable woody growth?”

This chapter attempts to answer these questions, but there are many variables and site-specific

factors which need to be considered. Some assumptions also need to be made.

Tree Removal Costs

The cost of clearing and grubbing a dam depends on the size and type of trees, growth density, total

job size (i.e., number of acres of trees), location of growth (crest and/or both faces?), embankment

face steepness, slope condition (such as degree of wetness or surface texture), degree and type of

required surface treatment (backfilling, use of herbicides or bio-barriers, mulching, seeding,

fertilizing, etc.), and regional labor and construction differences.
Chapter 7 Economics of Proper Vegetation Maintenance 

7-3 

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

Georgia Michigan Minnesota Nevada Ohio Oklahoma Tennessee Texas

$Cost/acre

Minimum cost/acre

Maximum cost/acre

The reader is referred to Table 1 in Chapter 2 and Figure 2 below for unit area tree removal cost 

comparison experiences reported in a survey by eight state dam safety officials in different regions 

of the country. The survey data shows that the cost of clearing and grubbing trees and other woody 

vegetation varies widely within and among states, but generally ranges from about $1000 to $5000 

per acre, depending on site-specific conditions (Tschantz, 2000). 

Figure 2. Comparison of tree removal costs per acre of dam surface area reported by 8 states. 

These data compare favorably with the $1500 - $3000 bid price data for three Southeastern states 

discussed earlier in Chapter 2 for cutting trees, removing stumps and rootballs, and grubbing the area 

to remove roots for different dam conditions. While not included in the above Figure 2 chart data, 

Massachusetts’ dam safety personnel reported in 2000 that, based on its own in-house experience, 

some local consultants and other sources, “broad area” tree removal costs ranged from $5000 and 

$6000 per acre or from about $800 to $1000 for individual 18-24 inch trees in their region. One dam 

safety official, from Tennessee, provided detailed cost data for clearing trees from seven dams in 

that state from 1995-1999. The cost for clearing and grubbing trees and for reseeding for one typical 

dam in 1998 is described for the reader in Table 1. 
Chapter 7 Economics of Proper Vegetation Maintenance

7-4

Dam Height 22.3 ft.

Length of Dam 830 ft.

Freeboard above Normal Pool 8 ft.

Density of trees ≤ 6 inches diameter primarily on downstream

face

“Moderate”

Approximate surface area of downstream face ≅1.3 acres

Approximate dam face slopes 3H:1V

Amount of brush cutting “Moderate”

Stumps grubbed out Yes

Amount of hand work “Considerable

”

Total job cost for clearing, grubbing & reseeding $4275

Unit area job cost $3290/acre

Year job completed 1998

 Table 1. Tree clearing/grubbing and reseeding cost for a “typical” dam

 located in Fayette Co., Tennessee (Bentley, 2000)

For comparison purposes, general sitework cost information is available from various construction

cost books. General cost data for cutting and clearing out individual trees and for clearing wooded

area is shown in Table 2 from one source (BNi, 2001). Indices are normally provided for factoring

in regional cost differences. Other cost book sources provide detailed material, labor and equipment

requirements for estimating site clearing costs (Means, 2001).

Clear small size wooded area:

• Light density

• Medium density

• Heavy density

Cut trees & clear out stumps:

• 9 to 12 inches diameter

• To 24 inches diameter

• 24 inches and up

$3,607/acre

$4,900/acre

$5,880/acre

$290 per tree

$370 per tree

$490 per tree

Table 2. General tree cutting and clearing construction cost data (Bni, 2001).

Similar general tree clearing and grubbing, chipping, seeding, mulching and fertilizing data for

estimating construction costs in various regions of the country are also available from other sources

(Means, 2001; AC&E, 2002). For example, 2001 Means cost data gives tree cutting, chipping,
Chapter 7 Economics of Proper Vegetation Maintenance

7-5

clearing, and grubbing costs for trees 6-inches or less to be $2975/acre and stump removal to be

$1425/acre for a total unit cost of $4400/acre. For trees up to 12 inches the cost is $6925/acre, and

for trees up to 24 inches the cost is $15,250/acre (Means, 2001). If burning is allowed, the cut and

chip costs can be significantly reduced. Hydro or air seeding, including seed & fertilizer is

estimated to be 35¢/square yard (about $1700/acre) (Means, 2001). Mulching would add to this

cost.

Maintenance Costs

For most dams, maintenance means keeping the crest and dam embankment slopes mowed and

trimmed. The cost of mowing a dam depends on many factors, including geographical location,

accessibility, condition of slopes as discussed above, degree of public use and desired aesthetics,

type of vegetation and frequency of mowing. Cost also depends on whether the work is done

directly by private owners, subcontracted commercially, or done by in-house state or federal

maintenance crews. Table 3 summarizes these factors. The availability of slope mowers as

illustrated in

Table 3. Factors Affecting Dam Maintenance Cost

• Region of country • Embankment slope steepness

• Type of ground cover & vegetation • Mowing frequency

• Accessibility to dam • Local labor costs

• Surface condition • Type of maintenance provider

• Size of job (surface area) • Degree of public use; aesthetics

Figure 3 illustrates the use of a slope mower for easing the burden of maintenance for state and

federal agencies and for other multiple or large dam owners.

Figure 3. Example of slope mower

 (Terratrac© photo used with

 permission from AEBI North

 America, Inc.)
Chapter 7 Economics of Proper Vegetation Maintenance 

7-6 

Most public works dams usually get mowed at least twice a year, in the early fall and late spring. 

Many subdivisions, homeowner associations, and/or residential developments typically mow dams, 

located in high-visibility areas, about once a month to every six weeks. One geotechnical 

consultant, who specializes in embankment dam rehabilitation, uses a “rule of thumb” mowing 

estimate of about $100 per acre with a minimum fee of $200 to $250 per mowing job (Marks, 2000). 

 1998 bid prices for mowing general right-of-way areas along East Tennessee highways averaged 

about $32 per acre, with a range of about $28 to $38 per acre for four jobs (TDOT-Region 1, 2000). 

The U. S. Corps of Engineers, Nashville District, furnished recent annual mowing costs for three 

District dams, including some proximate recreation zones, having total mowing areas ranging from 8 

to 27 acres. The average mowing cost for these three dams was about $55/acre and ranged from 

$43.42 to $78.24/acre (Corps, 2000). 

The Tennessee Valley Authority furnished similar estimated in-house annual mowing cost data 

associated with general dam safety grounds maintenance activities for its dams. However, TVA’s 

annual cost data included labor, supervision, slope mower fuel, parts, equipment, etc. and averaged 

slightly over $600/acre for 31 saddle and main embankment dams with a cost range from about $45 

to $2000/acre (TVA, 2000). 

A dam owner is advised that, in addition to mowing cost, the total annual maintenance expenditure 

should also include the expenses of dam inspection(s), minor repairs and rehabilitation of various 

structural components, removal of obstructions from emergency and service spillways, and other 

safety or operational costs associated with maintaining a dam. 

Example maintenance cost analysis 

The following example illustrates a rational procedure for answering the two earlier questions: 1) 

how much should a dam owner spend yearly to maintain a typical earthen dam to control trees and 

woody vegetation growth while avoiding bearing the cost of removing mature trees at a later date? 
Chapter 7 Economics of Proper Vegetation Maintenance

7-7

and 2) how often should an earthen dam be mowed to maintain acceptable ground covering

vegetation? Maintenance expense in this example is for mowing only. Assumptions for this

example are as follows: 

1. Dam Description:

• Length = 900 feet

• Crest width = 15 feet

• Embankment slopes (upstream and downstream) = 3:1 (horizontal to vertical)

• Height = 35 feet

• Normal pool = 10 feet below crest

• Nearly vertical end abutments

2. Economic Analysis Assumptions:

• 30-year project analysis period

• Annual rates of return rates = 4, 6, 8, 10, and 15%

• Zero annual inflation on recurring costs

3. Maintenance Assumptions:

• Assume that 10-year old brush and trees are mature enough to significantly

hinder effective inspection. Trees of this age can reach in size from 6 to 8

inches in diameter, depending upon species, tree density and other

environmental conditions

• Mowing costs = $100 per acre (with a minimum fee of $250 per mowing)

• Trees can grow on all exposed upstream and downstream embankment slopes

and the crest of the dam

• Assume tree removal, including clearing and grubbing, costs = $2500/acre

• Seeding & mulch not included in surface restoration costs.
Chapter 7 Economics of Proper Vegetation Maintenance 

7-8 

Economic Analysis Calculations 

Charts have been prepared and attached at the end of this chapter as a tool in helping to estimate 

mowing areas (or tree stand estimates) for different dam configurations. Chart 1 can be used to 

determine dam embankment slope area in acres for four slopes ranging from 1.5:1 to 3:1 (horizontal 

to vertical) and for dam lengths of 200 and 500 feet. Linear interpolations and ratio extrapolations 

can be made for other slope configurations and dam embankment lengths, respectively. Note that 

when determining the area of an upstream embankment slope, the equivalent dam height entered into 

the chart is the vertical distance between normal pool and crest elevation. Chart 2 is used to estimate 

dam crest area for three convenient lengths; crest areas for other actual dam crest lengths can be 

calculated from direct ratios. A self-guiding Chart 3 is provided to allow for small abutment area 

reduction corrections to be estimated and applied to slope area determined from Chart 1. 

 For the assumed example dam given above, make the following computations: 

1. Use the attached charts to estimate total mowable and potential tree-covered dam area: 

(a) Downstream Embankment Slope (35 ft. high, 3:1 slope, 900 ft. length): 

A1 = 1.28 acres x 900/500 = 2.3 acres (Use Chart 1; no abutment area reduction 

correction∗) 

(b) Crest (15 ft. wide, 900 ft. length) 

A2 = 0.17 x 900/500 = 0.31 acres (Use Chart 2) 

(c) Upstream Embankment Slope (10 ft. high exposure, 3:1 slope, 900 ft. length): 

A3 = 0.37 x 900/500 = 0.67 acres (Use Chart 1) 

(d) Estimated total dam area to be restored ≈ 2.3 + 0.3 + 0.7 = 3.3 acres 

2. Estimated 10-year cycle clearing and grubbing job costs, over a 30-year analysis period, starting 

 with end of 10th year: 

Total Estimated Cost = 3.3 acres x $2500 per acre = $8250 

∗ For this example, the abutment slopes are assumed vertical or 0o, but total slope area reduction for a 30o abutment 

 would be only ≈ 0.25 acres (see Chart 3). 

 
Chapter 7 Economics of Proper Vegetation Maintenance 

7-9 

3. Find the annual break-even cost balance between mowing and recurring clearing and 

grubbing, using the sinking fund factor (SFF), assuming 4, 6, 8, 10, and 15% discount rates 

for a 30-year period. A sinking fund is an equivalent annual amount to be set aside and left 

to grow at a certain interest rate into a specified amount at the end of a predetermined time 

period. 

It is assumed in this example that mowing and clearing and grubbing costs do not change over 

the 30-year analysis period and that the dam safety inspections are not hindered for up to 10-

year tree growth. By assuming a zero inflation rate for these costs, the results of this exercise 

are not dependent on the selected period of analysis; therefore, the annual values are valid for 

a 50- or 100-year period as well as for a 30-year period. 

♦ Annualized clearing and grubbing cost = $8250 x (SFF, i, N years) 

 where the SFF = i/[(1 + i)N – 1] 

 

 and i = discount rate (expressed as fraction) 

 N = time period, in years, between tree clearing and grubbing 

♦ Mowing job cost = 3.3 acres x $100/acre = $330 

♦ Equivalent number of mowings per year = (Annualized clearing & grubbing costs)/($330 

 per mowing) 

The following Table 4 shows that the annualized clearing and grubbing costs and equivalent number 

of annual mowings varies somewhat with the discount rate. For this example, at a 6%

discount rate, this dam owner would be able to justify about two mowings per year at $330 per 

mowing to avoid having to shell out $8250 every 10 years for clearing the dam of trees and woody 

vegetation. The owner could afford to mow once or twice a year, even at a relatively high 10% 
Chapter 7 Economics of Proper Vegetation Maintenance 

7-10 

Table 4. Annualized Cost Comparison for Assumed $2500 per acre for a 10-Year Cycle 

Clearing and Grubbing Payout. 

discount rate. By mowing on a regular basis, the owner would also realize side benefits of a more 

aesthetically pleasing dam -- one that would be viewed more as a community asset than a liability, 

be accessible and inspection friendly, and be less attractive to unwanted burrowing animals. 

If $5000 per acre or $16,500 for 3.3 acres, rather than the $2500 per acre and $8,250 per job, had 

been assumed for tree clearing costs over the 10-year cycle control period, the justifiable annual 

costs for mowing would double for the same discount rates. For this higher restoration cost, the 

owner would be justified to mow 3 or 4 times per year, depending on the cost of money. The 

following Table 5 illustrates this assumption. 

Assumed discount rate, i Annualized 10-yr frequency 

clearing and grubbing cost 

Equivalent number of 

mowings per year 

4% $1374 4.2 

6% $1252 3.8 

8% $1138 3.4 

10% $1036 3.1 

15 $ 813 2.5 

Table 5. Annualized Cost Comparison for Assumed $5000 per acre for a 10-Year Cycle 

 Clearing and Grubbing Payout. 

Assumed discount rate, i Annualized 10-yr frequency 

clearing and grubbing cost 

Equivalent number of 

mowings per year 

4% $687 2.1 

6% $626 1.9 

8% $569 1.7 

10% $518 1.6 

15% $406 1.3 
Chapter 7 Economics of Proper Vegetation Maintenance 

7-11 

Equivalent cost in number of mowings/year

0

1

2

3

4

5

0 2 4 6 810 12 14 16

Discount rate (%)

5-year C&G cycle

10-year C&G cycle

For a more conservative 5-year tree growth cycle and a $2500 clearing and grubbing cost 

assumption, the annualized clearing and grubbing costs would be $1523, $1464, $1406, $1351, and 

$1224 for the same discount rates, respectively. The corresponding justifiable mowings would be 

4.6, 4.4, 4.3, 4.1, and 3.7 per year. Similarly, justifiable mowings for an assumed $5000 clearing and 

grubbing cost would double the justifiable mowings to 9.2, 8.9, 8.6, 8.2, and 7.4 per year. Figure 4 

compares 5 and 10-year annualized costs for $2500/acre clearing and grubbing payouts. 

Figure 4. Comparison of Annual Tree Clearing and Grubbing 

 Costs for 5 and 10-Year $2500/acre payouts. 
Chapter 7 Economics of Proper Vegetation Maintenance 

7-12 

Realistically, unit area costs would likely be reduced substantially for more frequent clearing and 

grubbing or bush hogging of smaller growth. Obviously, the above values will be different if the 

costs are assumed to escalate each year. For example, assuming a modest 3% annual inflation factor 

results in an increase in the clearing and grubbing cost from $8250 to $14,900 for a 30-year analysis 

period. 

Summary 

Cost data obtained from the private, state and federal sectors show that dam maintenance and tree 

removal and dam restoration costs can vary widely, depending on several factors. 

It has been demonstrated, by way of example and reasonable cost assumptions, that dam owners can 

economically justify mowing their embankments 2 to 8 times a year, depending on local factors and 

costs, to prevent trees and other woody vegetation from maturing to a point that could compromise 

dam safety and require major capital outlays. It appears extremely economically efficient for dam 

owners to control woody growth on at least an annual basis, to avoid the large cost of removing 

mature brush and trees every 5 to 10 years and to comply with state inspection requirements. 

So, how much should a dam owner spend on maintaining his dam? At least enough to keep it 

mowed and trimmed a couple times a year – probably something in the neighborhood of $500 to a 

$1000 annually for most dams, if contracted. Keeping a dam mowed a minimum of twice a year 

does not appear to be an unreasonable financial burden for most small dam owners. A dam owner 

must understand that spending a few dollars on annual vegetative maintenance and upkeep, such as 

mowing, will pay dividends over the long run for an asset (and potential liability) such as a dam. 
Chapter 7 Economics of Vegetation Maintenance

References

1. Architects, Contractors, and Engineers (AC&E), Guide to Construction Costs,

Division #2 – Sitework & Demolition, Cyber Classics, Inc., 2002.

2. Association of State Dam Safety Officials (ASDSO), State Survey: Animal and

Vegetative Impacts on Dams, Part I - Vegetation on Dams (7 questions),

September 1999

3. Association of State Dam Safety Officials (ASDSO), State Survey, Percentage

of Trees on State-regulated Dams (2 questions), January 2000.

4. Bentley, L., Memorandum: Cost of Dam Clearing on Seven Tennessee Dams,

February 2000.

5. BNi Building News, General Construction 2001 Costbook, Sections

02110.01-02110.50 (Sitework), 2001, p. 18.

6. Marks, B. D., S&ME Engineering, Inc., Arden, N. C., Faxed communication on

recent contractor-bid clearing and grubbing costs, February 23, 2000.

7. Means, R.S., Building Construction Cost Data 2001, Site Preparation Section

02230, 2001, pp. 42-43.

8. Soil Conservation Service, U. S. Department of Agriculture, South Technical

Service Center, Fort Worth, Engineering Technical Note 705, Operations and

Maintenance Alternatives for Removing Trees from Dams, April 1981, 8 pp.

9. Tschantz, B. A., Overview of Issues and Policies Involving Woody Plant

Penetrations of Earthfilled Dams, Presentation and Proceedings, ASDSO/FEMA

Specialty Workshop on Plant and Animal Penetrations on Dams, Nov. 30 - Dec.

30, 1999, 8 pp.

10. Tschantz, B. A., Current Problems, Practices and Policies on Tree and Woody

Plant Penetration of Dams, paper presented at ASDSO National Dam Safety

Conference, Providence, R. I., September 2000.

11. Tennessee Department of Transportation (TDOT), Region 1 Supervisor,

Mowing contract bid data – 1998, March 2000.

7-13
Chapter 7 Economics of Proper Vegetation Maintenance 

7-14 

12. U. S. Bureau of Reclamation, U. S. Department of the Interior, Guidelines for

Removal of Trees and Other Vegetative Growth From Earth Dams, Dikes, and 

Conveyance Features, Bulletin No. 150, Water Operation and Maintenance, 

December 1989, pp. 1-3. 

13. U. S. Corps of Engineers, Nashville District, email information 

 furnished by D. Williams, March 2000. 

14. Tennessee Valley Authority, Knoxville, TVA Maintenance Data, 

 Email attachment information furnished by J. Morse, April 2000 and August 2001.
Chapter 7 Economics of Proper Vegetation Maintenance 

7-15 

Chart 1. Dam face area for different geometries

0

20406080

100120

0.00 0.50 1.00 1.50 2.00 2.50 3.00 

Dam face area (acres)

Dam height - Hd (feet)

Z= 1.5 2 2.5 3 Z= 1.5 2 2.5 

L = 200 ft L = 500 ft

1

Z Hd

Face slope: 1 vert : Z horiz.
Chapter 7 Economics of Proper Vegetation Maintenance 

7-16 

Chart 2. Dam crest area by width and length

0

102030405060

0.00 0.10 0.20 0.30 0.40 

Crest area (acres)

Crest width (feet)

L = 100 ft L = 200 ft 
Chapter 7 Economics of Proper Vegetation Maintenance 

7-17 

Chart 3. Abutment area corrections vs. dam height for 2.5H:1V & 3H:1V side slopes, with 45o abut

 and corresponding minimum dam crest length

0

50

100150200250300350400450500

0.0 0.5 1.0 1.5 2.0 

Area correction (-acres)

(Multiply by 58% for 30o abutments)

Minimum crest length of dam (feet)

4

3

2

4321 Enter dam height on right (5

Obtain abutment area correcProject line up to min. crest Project left to obtain minimucrest length for using full are

Minimum crest lengthline for given area correction(for 2.5:1 or 3:1 slopes)

Abutment area correction l

Z

1

Abutmentangle HD

2.5:1