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Dam Owner’s Guide 

To Plant Impact On 

Earthen Dams

FEMA L-263 / September 2005
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Introduction


The Trouble With Trees 

One would not have to look further than 

down one’s own street to appreciate the benefits of 

trees. From shade to habitat, urban wilderness to visual 

interest, communities nationwide are enhanced by the 

enduring presence of trees.While trees enhance most 

corners of a community, it is important to understand 

their limitations, especially relating to earthen 

embankment dams. Located on dams, trees and other 

woody vegetation that would otherwise bring charm 

to a city park could interfere with adequate inspection 

or safe operation of dams, or even cause dam failure. 

Private landowners and state and federal agencies 

use earthen embankment dams as reservoirs to store 

water for farm and city water supplies, hydropower, 

navigation, and recreation. Earthen embankments 

are also used as wastewater lagoons. Composed of 

compacted earthen fill and often near water sources, 

embankment dams are naturally conducive to 

vegetation growth. As such, a troublesome condition 

can arise if problem vegetation, such as trees, brush, 

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Tree roots distress an emergency spillway sidewall. 

ivy, and other deep-rooted plants, become established. 

Out of 48 states, 29 document problem vegetation as 

the cause of dam failures or unsafe dam operations 

in their states.The U.S.Army Corps of Engineers, 

the National Park Service, and the Natural Resources 

Conservation Service document several similar cases 

at the federal level. State dam safety officials and other 

experts agree that trees and woody plants on dams are 

a major concern, one which can cause substantial and 

costly damage if not properly dealt with. 

You, the dam owner, are the first line of defense 

towards the appropriate maintenance and safe 

operation of dams nationwide.Towards these goals, 

this brochure was developed to inform federal, 

state, and private dam owners on the dangers and 

identification of problem vegetation on earthen 

embankment dams.This brochure describes how 

problem vegetation can adversely affect dams and 

offers dam owners a quick quiz to determine whether 

their dam may be at risk for problems related to 

inappropriate vegetation. 

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Some states estimate 

that trees are present 

on 90–95% of their 

regulated earthen 

dams and on a 

substantial number 

of their smaller, 

unregulated dams. 

A disappearing act: inappropriate vegetation overgrows an 

embankment dam, now abscured. 

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The Roots 

Of The Problem 

Trees and brush rely on an extensive root system to 

provide oxygen, nutrients, and moisture as they grow. 

In the beginning, the tap root stabilizes the young 

plant. Once secure, the tree or brush develops a root 

ball and a lateral root system that extends from the 

now less important tap root. In general, the root ball 

is located below the trunk, and the lateral root system 

extends outward from the trunk to the “drip line” of 

the plant’s canopy.The large root ball and extensive 

roots of trees and brush easily penetrate earthen dams; 

while the roots stabilize the plant, they destabilize the 

dam.Though low-growing ground cover plants such 

as kudzu and crown vetch do not establish deep root 

systems, they provide a dense cover that can obscure 

the dam surface. 

Many dam owners plant woody vegetation on their 

dams to provide pleasing landscape effects, and others 

opt not to remove existing trees and vegetation to 

avoid related expenses or environmental permitting 

issues. Moreover, many dam owners believe false 

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The Truth Is... 

Tree roots do not stabilize soil masses, 

but instead loosen embankment soils 

and create seepage paths. 

Dense, low-growing groundcover or tall 

grasses are not desirable embankment 

plants, and can actually obscure 

serious dam problems. 

Even one tree on any size dam is cause 

for concern. 

benefits of woody vegetation on dams or simply do 

not see it as a threat to the safe operation of their dam.

The truth is, problem vegetation can cause a cascade 

of adverse effects that undermine the integrity of 

earthen dams. Specifically, erosive conditions, adverse 

hydraulic effects, and obscured dam inspections can 

result from problem vegetation on earthen dams, and 

can lead to maintenance difficulties and potentially 

dam failure. 

Lateral Trunk 

Root System 

Root Ball 

Tap Root 

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Adverse Effects 

Of Problem 

Vegetation 

Erosion Impacts: Constructed of local, non-organic 

material, earthen dams can function effectively for 

many years as long as erosion, the largest threat to an 

earthen dam, is controlled.Though there can be several 

causes of earthen dam erosion, the most common 

and widespread is problem vegetation growth.Trees 

and woody vegetation can make a dam susceptible to 

erosion in several ways: 

 The decaying roots of dying vegetation create a 

seepage path in the embankment for stored water 

or wastewater.This path can lead to internal erosion 

(piping) of the embankment. 

 Woody vegetation shades the embankment and 

reduces dense grass coverage, which can make 

affected areas more prone to erosion. 

 Woody vegetation loosens compacted soils of the 

embankment. 

 Roots can penetrate existing cracks and joints in 

the foundation rock and embankment, potentially 

leading to internal erosion and seepage. 

Hydraulic Impacts: Like any other type of dam, 

earthen dams are designed to manage a certain 

amount of water or wastewater. Proper reservoir 

management requires that all the hydraulic structures 

work to their design capacity.Trees and woody 

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vegetation can impair the hydraulic efficiency of the 

dam in several ways: 

 Uprooted or overturned trees and large brush 

reduce the freeboard of the reservoir. 

 Vegetation in the emergency spillway reduces the 

spillway capacity. 

 Uprooted trees displace a large amount of soil, 

lowering the dam crest and reducing the effective 

width of the dam. 

 Falling trees and large vegetation can damage dam 

facilities, such as the emergency spillway and outlet 

structures. 

Dam Inspection Impacts: Operational inspections 

are conducted regularly to ensure proper function 

and integrity of the dam crest, slope, outlets, channel, 

spillway, and toe.The inspection relies heavily on 

observation to determine seepage, cracks, slumps, 

and similar hazards. As such, problem vegetation can 

decrease the effectiveness of an inspection in many 

ways: 

 Dense ground cover obscures animal burrows, 

which can undermine dam integrity. 

 Dense ground cover obscures dam defects. 

 Heavy vegetation limits access to critical inspection 

areas. 

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How Is Your Dam 

Doing? 

Even though earthen dams are common across the 

U.S., a national policy regarding woody vegetation 

management on earthen dams is not in place. 

Moreover, over half of the states do not have a specific 

policy for managing problem vegetation on dams; 

those states with policies typically rely on case-bycase evaluations before recommending vegetation 

remediation approaches. Combine this with an 

intermittent state inspection schedule, and clearly 

the dam owners are the first line of defense in 

protecting their dams.The quick quiz on the 

following pages will assist dam owners in 

determining whether their dam is at risk for 

problems relating to inappropriate vegetation. 

1. Does your dam look like this? 

Embankment 

Abutment 

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Dam owners should 

check their dam 

frequently—vegetation 

that is acceptable now 

can grow into or support 

problem vegetation over 

time. 

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Emergency 

Spillway 

Abutment 

Toe 
2. Does your dam support the 

following vegetation? 

Trees and shrubs. 

Vines such as kudzu 

or ivy. 

Arid-region plants such 

as cacti, sagebrush, 

desert broom, salt 

cedar, mesquite,cypress, 

cottonwood, paloverde, 

or palm trees. 

Dense, perennial plants 

such as crown vetch. 

Tall vegetation. 

3. Do the following buffer areas 

contain problem vegetation?  A 25- to 50-foot buffer beyond the embankment, 

abutment, and toe. 

 A 15-foot buffer around buried pipe systems. 

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If you answered “yes” to any of the these 

questions, then you should contact your State Dam 

Safety Official to determine whether inspection and 

mitigation is required. Based on this inspection, 

the official will develop the most appropriate 

remediation approach based on the extent and 

type of vegetation and the condition of the dam. 

Often, problem vegetation is too extensively 

established, or trees and brush are simply too large for 

removal by the dam owner. For example, removal of 

large trees sometimes also requires removal of their 

stumps and roots, which can leave large voids in the 

dam. In these and similar cases, the input of a qualified 

professional engineer is necessary to remediate 

vegetation while preserving dam integrity. Conversely, 

some state policies allow dam owners to remove 

smaller trees and other vegetation on their own, but 

some amount of dam repair, such as filling with 

compacted soils, is usually necessary. Coordination 

with the State Dam Safety contact is advised before any 

vegetation removal begins. 

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Establishing And 

Maintaining 

Desirable 

Vegetation 

In addition to regular inspections, proper vegetation 

maintenance is key to reducing the risks associated 

with problem plants. Planting and mowing (at least 

twice yearly) of regionally appropriate species of lowgrowing grass, such as Bermuda or fescue, prevents 

problem vegetation from moving in. A recent survey 

indicates that many dam owners consider the costs of 

regular mowing too high. However, if one considers 

the costs of woody vegetation remediation, which is a 

potential outcome of avoiding grass cover and annual 

mowing, the economics of grass establishment and 

maintenance becomes more desirable. For example, 

though variable based on dam size, site conditions, 

and regional costs, a rational estimate for annual 

mowing maintenance equals about $100 per acre, 

compared to about $2,500 per acre for tree removal, 

clearing, and grubbing. 

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Conclusion 

Maintenance of appropriate earthen dam vegetation 

reduces several risk factors that contribute to dam 

failure such as erosion, hydraulic inefficiency, and 

inadequate inspections. Moreover, maintaining 

desirable vegetation will prevent problem vegetation 

from becoming established and is more affordable 

than remediation, which may be necessary if problem 

vegetation is allowed to take root.This brochure is 

intended to help dam owners nationwide identify and 

mitigate problem vegetation before adverse effects 

occur. 

For information on obtaining a print, CD, or online 

version of this publication or the technical manual go 

to www.fema.gov/fima/damsafe. 

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