The Federal Emergency Management Agency (FEMA) Dam Safety and Association of State Dam Safety Officials (ASDSO) have websites with extensive information, some of it specifically written for dam owners including, ASDSO's "Resources for Dam Owners and Operators-Being a responsible Dam Owner". FEMA and the United States Department of Agriculture's Forest Service (USDA) have published a reference under their Technical Manuals, Flyers, and Tools entitled, "Pocket Safety Guide for Dams and Impoundments" that shows the various dam components and causes, consequences, and recommended actions to common dam problems. NYSDEC also has information on dam removal resources to restore free-flowing rivers.
Information For Dam Owners
The owner of a dam is responsible for maintaining and operating the dam in a safe condition at all times so it does not constitute a hazard to life, health, or property. Because a dam impounds water or has the potential to impound water, the owner of the dam is responsible for taking appropriate measures, including periodic inspection, maintenance, monitoring, and needed repairs. It is recommended that dam owners inspect their dam(s) at least once every three months and after significant storm events such as earthquakes or floods.
For more information, refer to An Owners Guidance Manual for the Operation and Maintenance of Dams in New York State (PDF).
It is helpful to plan an inspection in advance to assure that you will observe every part of the dam. Recommended sequence for a visual inspection:
- Crest - Walk across the crest from abutment to abutment.
- Upstream/Downstream Slope - Walk across the slope in an up and down or zigzag pattern from abutment to abutment.
- Embankment-Abutment Contacts - Walk the entire length of the embankment-abutment contacts (groin).
- Outlet Conduit - Observe all accessible features of the outlet conduit.
- Spillway - Visually observe the entire length of the spillway or spillways, and all other visible features.
- Downstream Channel - Travel the route of the stream below the dam to maintain familiarity with locations of residences and property that can be affected by dam failure. Owners should be aware of new downstream development(s) and how these development(s) may impact the hazard class of their dam. Go far enough downstream to cover the area that could be affected by a dam failure.
- Downstream Toe - Walk the entire length of the downstream toe.
- Reservoir Slopes - Scout the reservoir perimeter in an effort to develop an overall familiarity with its conditions.
A more detailed inspection should also be planned on a periodic basis as described in the Owner's Guidance Manual.
Some of the conditions a dam owner may discover and should look for during the inspection:
- settlement
- turbid discharge
- structural cracking
- foundation movement
- erosion
- sinkholes
- vandalism
- animal burrows
- boils
- depressions
- voids
- debris in gates and spillways
- wave erosion
- excessive vegetation
- seeps
- soil displacement on slopes (sloughing)
This list should not be construed as an all-encompassing, last word on the problems that may be encountered.
It is important for the dam owner/operator to keep records throughout the life of the dam. Accurate records can help in the evaluation of the performance and condition of the structure over time. Regulations require that dam owners establish a permanent file to retain inspection records, including records of actions taken to correct conditions found during inspections throughout the life of the dam. Keeping good records:
- Inspection Checklist - Record a compiled list of observations made during your inspections. A checklist should be carried by the dam inspector as he/she traverses the entire structure. Samples of checklists can be found in the Owner's Guidance.
- Field Sketch - Add a field sketch of observed conditions to your inspection notes. The field sketch is intended to supplement the information recorded on the inspection checklists; however, it should never be used as a substitute for clear and concise inspection checklists. A copy of the plan view of the dam, usually included as part of the construction drawings for the dam, can be a good base for drawing a field sketch.
- Photographs - Over time, photographs provide a pictorial history of the evolving characteristics of a dam. The dam owner/operator often finds them to be great money savers because they can illustrate that some observed conditions (seepage, foundation movement, etc.) have existed for many years and may have reached a state of equilibrium. With this knowledge, quick and economical remedial actions can be developed and implemented, or avoided altogether. Photographs should be dated on the back and labeled with brief descriptions of the locations shown in the pictures, and focus on specific features of the dam as well as general views of the dam.
- Monitoring Data - It may become necessary to make measurements of various items during the course of a dam inspection. This may include measurements of seepage rates, spillway discharge rates, settlement, and for some dam owners, readings from instruments such as piezometers, crack gauges, and survey monuments. The reservoir level, referenced in a consistent manner, should be recorded at each inspection. It is important that this data also be compiled in a systematic manner and placed in the permanent file.
- Accompany Your Engineer During Periodic Inspections - Owners can learn many things from experienced inspectors, such as:
- What to look for.
- How to photograph.
- Certain features of a dam.
- What records to keep.
- How to read different types of instrumentation.
Inspection of Concrete Structures at Dams
Structural Inspections
Concrete surfaces should be visually examined for spalling and deterioration due to weathering, unusual or extreme stresses, alkali or other chemical attack, erosion, cavitation, vandalism, and other destructive forces. Structural problems may be indicated by cracking, exposure of reinforcing bars, large areas of broken-out concrete, misalignment at joints, undermining and settlement in the structure. Rust stains that are noted on the concrete may indicate that internal corrosion and deterioration of reinforcement steel is occurring. Spillway floor slabs and upstream slope protection slabs should be checked for erosion of underlying base material otherwise known as undermining. Concrete walls and tower structures should be examined to determine if settlement and misalignment of construction joints has occurred.
What to Look For During Inspection of Concrete Structures
- Concrete structures can exhibit many different types of cracking. Deep, wide cracking is due to stresses which are primarily caused by shrinkage and structural loads. Minor or hairline surface cracking is caused by weathering and the quality of the concrete that was applied. The results of this minor cracking can be the eventual loss of concrete, which exposes reinforcing steel and accelerates deterioration. Generally, minor surface cracking does not affect the structural integrity and performance of the concrete structure in the short term.
- Cracks through concrete surfaces exposed to flowing water may lead to the erosion of embankment or foundation soils from around and/or under the concrete structure.
In this case, the cracks are not the result of a problem but are the detrimental condition which leads to erosion. Proper underdrainage for open channel spillways with structural concrete floors is necessary to control this leakage. Flows from underdrain outlets and pressure relief holes should also be observed and measured. Cloudy flows may indicate that soil erosion is occurring beneath or adjacent to the concrete structure. This could be detrimental to the foundation support. - Concrete surfaces adjacent to contraction joints and subject to flowing water are of special concern especially in chute slabs.
The adjacent slabs must be flush or the downstream one slightly lower, to prevent erosion of the concrete and to prevent water from being directed into the joint during high velocity flow. All weep holes should be checked for the accumulation of silt and granular deposits at their outlets. These deposits may obstruct flow or indicate loss of support material behind the concrete surfaces. Tapping the concrete surface with a hammer or some other device will help locate voids if they are present as well as give an indication of the condition and soundness of the concrete. Weep holes in the concrete are used to allow free drainage and relieve excessive hydrostatic pressures from building up behind the structure. Excessive hydrostatic pressures behind the concrete could cause it to heave or crack which increases the potential for accelerated deterioration and undermining. Periodic monitoring of weep hole drains should be performed and documented on a regular and routine basis to ensure that they are functioning as designed. - Structural cracking of concrete is usually identified by long, single or multiple diagonal cracks with accompanying displacements and misalignment.
Cracks extending across concrete slabs which line open channel spillways or provide upstream slope wave protection can indicate a loss of foundation support resulting from settlement, undermining, or erosion of foundation soils. Erosion of foundation soils is the result of inadequate underdrainage and/or cutoff walls. Items to consider when evaluating a suspected structural crack are the concrete thickness, the size and location of the reinforcing steel, the type of foundation, and the drainage provision for the structure. Because many features are hidden beneath the concrete, an as-built drawing may be critical for a proper evaluation. - Inspection of intake structures, trashracks, upstream conduits, and stilling basin concrete surfaces that are below the water surface are not readily accessible during a regularly scheduled inspection.
Typically, stilling basins and downstream toes require the most regular monitoring and major maintenance. Stilling basins are holding ponds for rock and debris, which can cause extensive damage to the concrete surfaces during the dissipation of flowing water. The downstream toe of the dam and the stilling basin are also susceptible to undermining. Undermining of the downstream toe could affect the structural integrity and performance of the dam. Therefore, special inspections of these features should be performed at least once every five years by dewatering the structure. Investigation of these features using experienced divers may be needed.
Guidance For Developing An Emergency Action Plan (PDF) can be used to find more information.
Burrowing Animals: Rodent Control
Rodents such as the woodchuck, muskrat, and beaver are attracted to dams and reservoirs and can be quite dangerous to the structural integrity and proper performance of the embankment and spillway. It is essential that these animals and their activities be controlled to ensure proper functioning of a dam.
Beavers will instinctively try to block spillways and intake structures. Research shows that beavers react to the sound of flowing water. Such actions can raise the water level in a reservoir, which could cause upstream flooding, reduce the spillway discharge capacity (potentially causing a dam overtopping which can lead to failure of the dam), or produce sudden high outflows from the dam should the beaver structure suddenly fail. Upstream beaver dams can generate large quantities of floating debris that can clog a dam's intake and outlet structures. Beaver activity downstream can raise the tailwater elevation, which in turn can reduce the discharge from the dam or erode the downstream toe of the dam. Beavers have also been known to burrow into the upstream face of embankment dams, below the water line. Traffic can then collapse the burrow and create a partial breach at the water line.
Beaver Control
Beavers will try to plug spillways with their cuttings. Dam owners should routinely remove the beaver cuttings from spillways and intakes. Trapping beavers near dams should be encouraged, and may be done by a licensed trapper during the appropriate season, typically during the fall and winter. During the closed trapping season, permits to control destructive beavers may be issued by DEC's regional wildlife personnel. Contact regional wildlife personnel for further assistance.
Woodchucks burrow into the downstream face of a dam. When burrowing into an embankment, woodchucks stay above the phreatic surface (upper surface of seepage or saturation) to stay dry. The burrow is rarely a single tunnel. Their burrows are usually a network of tunnels and chambers with multiple entrances. It is usually forked, with several side passages or rooms from 1 to 12 feet long.
Occupied woodchuck burrows are easily recognized in the spring due to the woodchuck's habit of keeping them "cleaned out." Fresh dirt is generally found at the mouth of active burrows. Half-round mounds, paths leading from the den to nearby fields, and clawed or girdled trees and shrubs also help identify inhabited burrows and dens.
Woodchuck Control
Woodchucks will be discouraged from inhabiting an embankment if the vegetation cover is kept mowed and is properly maintained. Control methods should be implemented during early spring when active burrows are easy to find, young woodchucks have not scattered, and there is less likelihood of damage to other wildlife. In later summer, fall, and winter, game animals will scurry into woodchuck burrows for brief protection and may even take up permanent abode during the period of woodchuck hibernation. Woodchucks may be controlled by trapping or hunting any time of the year with a valid trapping or hunting license.
Muskrats burrow into a dam's upstream face. Muskrats make their homes by burrowing into the banks of lakes and streams or by building "houses" of bushes and other plants. Their burrows begin from 6 to 18 inches below the water surface and penetrate the embankment on an upward slant. At distances up to 15 feet from the entrance, a dry chamber is hollowed out above the water level. Once a muskrat den is occupied, a rise in the water level will cause the muskrat to dig farther and higher to excavate a new dry chamber. Damage (and the potential for problems) is compounded where woodchucks or other burrowing animals construct their dens in the embankment opposite muskrat dens.
Muskrat Control
Barriers to prevent burrowing offer the most practical protection to earthen structures. A properly constructed riprap and sand/gravel filter layer will discourage burrowing. The filter and riprap should extend at least 3 feet below the water line. As the muskrat attempts to construct a burrow, the sand and gravel of the filter layer caves in and thus discourages den building. Heavy wire fencing laid flat against the slope and extending above and below the water line can also be effective. Eliminating or reducing aquatic vegetation along the shoreline will discourage muskrat habitation. Where muskrats have inhabited the area, trapping is usually the most practical method of removing them from a pond, and should be encouraged. Muskrats injuring private property may be taken by the landowner, his or her family or employee, or their written designee at any time in any manner. Contact regional wildlife personnel for further assistance.
Eliminating A Burrow
One method of backfilling a burrow in an embankment is mud-packing. This simple, inexpensive method can be accomplished by placing one or two lengths of metal stove or vent pipe in a vertical position over the entrance of the den. Making sure that the pipe connection to the den does not leak, the mud-pack mixture is then poured into the pipe until the burrow and pipe are filled with the earth-water mixture. The pipe is removed and dry earth is tamped into the entrance. The mud-pack is made by adding water to a 90 percent earth and 10 percent cement mixture until a slurry or thin cement consistency is attained. All entrances should be plugged with well-compacted earth, and vegetation re-established. Dens should be eliminated without delay because damage from just one hole can lead to failure of a dam or levee.
Please note that a permit may be required to remove or disturb an animal. You should contact DEC's regional wildlife offices to inquire about controlling muskrat or beaver.
For more information, please refer to the following resources:
- The Association of State Dam Safety Officials includes the Federal Emergency Management Agency 2005 publication, Impacts of Animals on Earthen Dams - FEMA 473, under the Technical Manuals, Flyers, and Tools section of their webpage.
- Best practices for nuisance wildlife control operators.
Problems with Trees and Brush Near Dams
Trees and brush may be aesthetically pleasing and provide other benefits; however, the growth of woody vegetation on and near dams, including the downstream toe area, can lead to serious problems. Sudden uprooting of trees by strong winds can result in the movement of a relatively large amount of embankment material and create large voids in the embankment. This in turn can lower the crest of the dam, reduce the effective width of the dam, lead to instability of the embankment, and facilitate seepage. Falling trees can also cause structural damage to concrete, steel, stone, or timber structures.
The root systems of trees can be a potential hazard by allowing seepage pathways to develop through a dam. Trees eventually die and their roots decay and rot. The root cavity leaves a void within the dam through which water can enter and flow. This can ultimately lead to failure of the dam by piping (internal erosion). In general, a tree's root system may extend to the edge of the tree canopy or tree drip line.
Brush and woody vegetation prevent the proper visual inspection of the dam surfaces. The observation of sinkholes, slides, animal burrows, seeps, and other irregularities can be obscured by trees and brush. Woody vegetation can also cause excessive shade which in turn can hinder the growth of sturdy, dense grass coverage. These affected areas are more prone to surface erosion.
Excessive vegetation can provide habitat for burrowing animals, which can create problems on a dam.
Erosion Control
Grass cover is a very effective and inexpensive means to prevent the erosion of embankment surfaces. The stems and root systems of grasses tend to trap fine particles of soil, thus inhibiting the migration of these particles. A good grass cover provides an excellent means against erosion due to runoff caused by rains, and may protect the embankment during limited overtopping. Extensive testing and well documented incidents have consistently shown that a good grass cover is highly effective in preventing erosion at dams.
Maintenance
Grass cover should be routinely cut to provide a surface that can be easily inspected. In general, the grass on a dam should be cut at least twice a year. Trees and brush should never be allowed to grow on or very near to a dam including the downstream toe area! Many older dams have very large trees growing on or near them. Trees at or less than 4 inches in trunk diameter should be cut flush with the ground as part of the dam maintenance program and monitored during routine inspections for any changed conditions around the stump. For very small dams, trees less than 4" diameter may compromise the dam when removed. Removal of brush, and trees and roots larger than a 4 inch trunk diameter should be done under the direction of a qualified professional engineer knowledgeable about dam safety and maintenance, and may require a permit from the Department. When in doubt, consult with a licensed professional engineer.
For more information, please refer to Technical Manual for Dam Owners: Impacts of Plants on Earthen Dams, FEMA 534, published by Federal Emergency Management Agency, dated September 2005.
Seepage Through Earthen Dams
Wet areas downstream from dams are not usually natural springs, but seepage through or under the dam. Even if natural springs exist, they should be treated with suspicion and carefully observed. Flows from ground-water springs in existence prior to the reservoir would probably increase due to the pressure caused by the pool of water behind the dam.
All dams have some seepage as the impounded water seeks paths of least resistance through the dam and its foundation. Seepage becomes a concern if it is carrying material with it, and should be controlled to prevent erosion of the embankment, or foundation, or damage to concrete structures.
Seepage can emerge anywhere on the downstream face, beyond the toe, or on the downstream abutments at elevations below normal pool. Seepage may vary in appearance from a "soft" wet area to a flowing "spring." It may show up first as an area where the vegetation is lush and darker green. Cattails, reeds, mosses, and other marsh vegetation often become established in a seepage area. Another indication of seepage is the presence of rust-colored iron bacteria. Due to their nature, the bacteria are found more often where water is discharging from the ground than in surface water. Seepage can make inspection and maintenance difficult. It can also saturate and weaken portions of the embankment and foundation, making the embankment susceptible to earth slides.
If the seepage forces are large enough, soil will be eroded from the foundation and be deposited in the shape of a cone around the outlet. If these "boils" appear, professional advice should be sought immediately. Seepage flow which is muddy and carrying sediment (soil particles) is evidence of "piping," and is a serious condition that if left untreated can cause failure of the dam. Piping can most often occur along a spillway or other conduit through the embankment, and these areas should be closely inspected. Sinkholes may develop on the surface of the embankment as internal erosion takes place. A whirlpool in the lake surface may follow and then likely a rapid and complete failure of the dam. Emergency procedures, including downstream evacuation, should be implemented immediately if any of these conditions are noted.
Seepage can also develop behind or beneath concrete structures such as chute spillways or headwalls. If the concrete structure does not have a means such as weep holes or relief drains to relieve the water pressure, the concrete structure may heave, rotate, or crack. The effects of the freezing and thawing can amplify these problems. It should be noted that the water pressure behind or beneath structures may also be due to infiltration of surface water or spillway discharge, but should still be addressed.
A continuous or sudden drop in the normal lake level is another indication that seepage is occurring. In this case, one or more locations of flowing water are usually noted downstream from the dam. This condition, in itself, may not be a serious dam safety problem, but will require frequent and close monitoring and professional assistance.
The need for seepage control will depend on the quantity, content, and location of the seepage. Reducing the quantity of seepage that occurs after construction is difficult and expensive. It is not usually attempted unless the seepage has lowered the pool level or is endangering the dam or appurtenant structures. Typical methods used to control the quantity of seepage are grouting or installation of an upstream blanket. Of these methods, grouting is probably the least effective and is most applicable to leakage zones in bedrock, abutments, and foundations. These methods must be designed and constructed under the supervision of a professional engineer experienced with dams.
Controlling the content of the seepage or preventing seepage flow from removing soil particles is extremely important. Modern design practice incorporates this control into the dam design through the use of cutoffs, internal filters, and adequate drainage provisions. Control at points of seepage exit can be accomplished after construction by installation of toe drains, relief wells, or inverted filters.
Weep holes and relief drains can be installed to relieve water pressure or drain seepage from behind or beneath concrete structures. These systems must be designed to prevent migration of soil particles but still allow the seepage to drain freely. The owner must retain a professional engineer to design toe drains, relief wells, inverted filters, weep holes, or relief holes, and regular monitoring of these features is critical.
Regular monitoring is essential to detect seepage and prevent dam failure. Knowledge of the dam's history is important to determine whether the seepage condition is in a steady or changing state. It is important to keep written records of points of seepage exit, quantity and content of flow, size of wet area, and type of vegetation for later comparison. Photographs provide invaluable records of seepage.
All records should be kept with the Inspection and Maintenance Plan for the dam. Every inspector should always look for increases in flow and evidence of flow carrying soil particles, which would indicate that a more serious problem is developing. Instrumentation can also be used to monitor seepage. V-notch weirs can be used to measure flow rates easily and inexpensively, and piezometers may be used to determine the saturation level (phreatic surface) within the embankment.
Regular surveillance and maintenance of the internal embankment and foundation drainage outlets is also required. The rate and content of flow from each pipe outlet for toe drains, relief wells, weep holes, and relief drains should be monitored and documented regularly. Normal maintenance consists of removing all obstructions from the pipe to allow for free drainage of water from the pipe. Typical obstructions include debris, gravel, sediment, mineral deposits, calcification of concrete, and rodent nests. Water should not be permitted to submerge the pipe outlets for extended periods of time. This will inhibit inspection and maintenance of the drains and may cause them to clog. Rodent guards are readily available and should be installed where needed.
For more information, refer to National Dam Safety Program Research Needs Workshop: Outlet Works, FEMA 539, Federal Emergency Management Agency, August 2006.
Low Level Drains
A low level outlet conduit or drain is required for emptying or lowering the water level in a reservoir, lake or pond in case of emergency and for inspection and maintenance of the dam.
Typical Types of Low Level Drains
- valve located in the spillway riser
- a conduit through the dam with a valve at either the upstream or downstream end of the conduit (upstream control is greatly preferable)
- a siphon system (often used to retrofit existing dams)
- a gate, valve, or stoplogs located in a drain control structure
- stop logs across a spillway
- any combination of the above
Uses of Drains
The following situations make up the primary uses of low level drains:
- Should serious problems ever occur to threaten the immediate safety of the dam, drains may be used to lower the lake level to reduce the likelihood of dam failure. Also see Emergency Action Plan (EAP) (PDF) guidance documents for more information on dam emergency planning. Examples of such emergencies include:
- clogging of the spillway which may lead to high lake levels and eventual dam overtopping development of deep slides or cracks in the dam
- severe seepage through the dam which may lead to a piping failure of the dam
- partial or total collapse of the spillway system
- landslide around reservoir rim
- slope protection repair
- spillway repairs
- repair and/or installation of docks and other structures along the shoreline
- dredging the lake application of aquatic herbicide
Some dam owners prefer to lower the lake level during the winter months to:
- reduce ice damage to structures along the shoreline.
- provide additional flood storage for upcoming spring rains perform several repair items during this winter drawdown period.
- periodic fluctuations in the lake level also discourage muskrat and beaver habitation along the shoreline. Muskrat burrows in earthen dams can lead to costly repairs. Beaver dams and debris can plug spillways.
Common Drain Maintenance Problems
Some common problems often associated with the maintenance and operation of low level drains:
- deteriorated and bent control stems and stem guides;
- deteriorated and separated conduit joints;
- leaky and rusted control valves and sluice gates;
- deteriorated ladders and platforms in control structures;
- deteriorated control structures;
- clogging of the drain conduit inlet with sediment and debris;
- inaccessibility of the control mechanism to operate the drain;
- seepage along the drain conduit;
- erosion and undermining of the conduit discharge area;
- vandalism; and
- development of instability of earthen sections resulting in slides along the upstream slope of the dam and the shoreline caused by lowering the lake level too quickly.
Operation and Maintenance Recommendations
- All gates, valves, stems and other mechanisms should be lubricated according to the manufacturer's specifications. If you do not have a copy of the specifications and the manufacturing company cannot be determined, then a local valve distributor or your engineer may be able to provide assistance.
- The low level drain should be operated at least twice a year to prevent the inlet from clogging with sediment and debris, and to keep all movable parts working easily. Most manufacturers recommend that gates and valves be operated at least four times per year. Frequent operation will help to ensure that the drain will be operable when it is needed. You should contact DEC Environmental Permits in your region of the state to confirm if there are environmental issues with a significant release of water or sediment. All valves and gates should be fully opened and closed at least twice to help prevent sediment buildup and to obtain a proper seal. If the gate gets stuck in a partially opened position, gradually work the gate in each direction until it becomes fully operational. Do not apply excessive torque as this could bend or break the control stem, or damage the valve or gate seat. With the drain fully open, inspect the outlet area for flow amounts, leaks, erosion and anything unusual.
- All visible portions of the low level drain system should be inspected at least annually, preferably during the periodic operation of the drain. Look for and make note of any cracks, rusted and deteriorated parts, leaks, bent control stems, separated conduit joints or unusual observations.
- Low level drain control valves and gates should always be placed upstream of the centerline of the dam. This allows the drain conduit to remain depressurized except during use, therefore reducing the likelihood of seepage through the conduit joints causing internal erosion of the surrounding earth fill.
- For accessibility ease, the drain control platform should be located on shore or be provided with a bridge or other structure. This becomes very important during emergency situations if high pool levels exist.
- Vandalism can be a problem at any dam. If a lake drain is operated by a crank, wheel or other similar mechanism, locking with a chain or other device, or removal and off-site storage of the operator, may be beneficial. Fences or other such installations may also help to ward off vandals.
- Rapid draw down of the impoundment can result in a build-up of hydrostatic pressures in the upstream slope of the dam which can lead to slope failure. Lowering the water level slowly allows these pressures to dissipate.
Refer to Guidelines for Design of Dams (PDF) for low level drain capacity and design requirements.
Dam Removal
Dams are used for a variety of purposes, including recreation, public water supply, navigation, hydropower production, and as flood control structures. There are more than 5,400 active dams in New York State’s inventory of dams and DEC research suggests there are many additional small, undocumented dams.
The majority of the dams in New York were built in the early and mid-20th century to fill vital roles in New York’s economic and societal growth. However, changing conditions are decreasing the need for some dams, particularly those no longer utilized for their intended purpose or unable to be maintained in a safe manner in compliance with New York State’s dam safety regulations.
When a dam becomes obsolete, is in disrepair, or when its operation and maintenance costs outweigh its benefits, dam removal can be a wise decision. Benefits of dam removal can come in the form of diminished risk, costs, and liabilities for the dam owner, improvements to river ecology, and an elimination of the potential for downstream flooding due to a dam failure.
The removal of a dam may necessitate the services of a professional engineer who is licensed in New York State and has experience in dam safety. This webpage summarizes some of the considerations and steps involved in proposing to remove a dam. They are presented in no specific order and may be impacted by a variety of factors including but not limited to the size of the impounded lake or pond, the height of the dam, its location.
Potential Benefits of Dam Removals
- Restored natural hydrology and free-flowing riverine conditions – Allows a stream to return to a more natural hydrology, which can restore river functions and provide natural river habitats instead of the artificial pond/lake habitat created by the dam.
- Improved water quality – Temperature and oxygen levels are two water quality parameters, among others, that may be improved. Harmful algal blooms that may occur in an impounded waterbody may be diminished or avoided once the dam is removed.
- Restored aquatic connectivity of habitat – Reducing or removing a dam may lessen or eliminate an obstacle or impediment to upstream and downstream movement of aquatic species into natural habitats for spawning, feeding, and other important life cycle and survival needs. These potential benefits may apply to both resident fishes and aquatic life and those fishes (e.g., herring or American eel) that migrate long distances from an estuary.
- Reduced flood risks – Downstream risks of flooding from an unplanned dam failure may be permanently eliminated.
- Reduced public nuisance and public safety liabilities – Even a properly maintained dam may pose a safety hazard, especially for recreational users likes anglers and boaters. Removal eliminates the risks related to the dam structure.
- Eliminated operation, maintenance, and repair costs – Removal of a dam allows the dam owner(s) to avoid the costs of operating and maintaining the dam in a safe condition, including ensuring compliance with New York’s dam safety regulations requiring ongoing inspection and maintenance, engineering reports, repair, and emergency planning.
Potential Drawbacks of Dam Removals
- Potential for positive and negative implications to upstream and downstream invasive species – Dam removal can allow the upstream migration of certain invasive species into new habitats that were previously inaccessible. At the same time, the removal may help limit invasives upstream by eliminating or reducing the habitat they occupied and used upstream in the impoundment.
- Short-term impacts – There may be short-term impacts to the stream from the construction activities that are associated with the removal of a dam. Removing the dam acts to ‘reset’ the stream, and the short-term impacts may be outweighed by the longer-term ecological benefits to the stream.
- Social or cultural attachment to the dam and impounded water uses – Shifts in cultural or recreational uses may change or shift by removing a dam.
- Potential changes in stream hydrology – As with dam repairs, dam removal may necessitate the services of a professional engineer licensed in New York with experience in dam safety to evaluate anticipated changes in downstream flows and hydraulics.
Environmental Approvals and Considerations
The presence of a dam usually results in numerous environmental impacts over time. These may include accumulation of sediment behind the dam, disruption of fish migration, an increase in water temperatures, changes in stream hydraulics and flow patterns, and others. The removal of a dam may reverse these impacts, but careful planning is needed to control any negative or unintended environmental impacts that may be associated with the removal.
Whether repairing, changing, or removing a dam, each dam and each project is unique and may require permits from multiple programs within DEC and may involve a public notice and comment period to obtain public comment from individual or group stakeholders.
Scheduling a meeting with DEC Environmental Permits staff in the appropriate regional office before applying for permits (called a “pre-application conference”) is often a good way to obtain assistance and important information that will help the dam owner plan their project and make the environmental review process go more smoothly. For a list of Regional Permit Administrators and Environmental Permits offices, see the DEC website. As dam removal can tend to require the services of a professional engineer licensed in New York with experience in dam safety, it can be helpful to include the engineer in the pre-application meeting.
Permit Application Procedures
The general application procedures for a Protection of Waters permit are found on the DEC website.
In addition to the general procedures, a permit application for the removal of a dam may need to include the supplemental information listed below. Much of this information can be provided in the engineering report that accompanies an application for a Protection of Waters Permit for Work on a Dam.
- A sediment management plan estimates the quantity and quality of the sediment accumulated behind the dam and outlines a plan for how it will be managed during and after dam removal. This may include removal of all or some of the material. It may also include stabilization of some material that would remain above the flow of the water course after removal.
- A water management plan describes whether and how the impoundment will be drained during the dam removal project and any flowing water entering the work site will be managed. Methods used to drain the impoundment and manage flowing water, if found to be necessary, must also avoid or minimize the discharge of muddy (turbid) water downstream of the site. In this regard, where the dam to be removed does not have an operational low-level drain or where the drain is buried in sediment, a siphon or coffer dam or other means of drawing water from the upper portions of the impoundment to a location downstream of the work site may need to be used.
- Plans and specifications identifying the scope and design of the removal (full removal, partial removal). If a partial removal, then it may be necessary to include a dam safety hazard classification assessment, a technical review of the expected incremental rise in the surface water elevation of the remaining impoundment during a 100-year flood, and a bank stabilization design. In all, or nearly all cases, such plans need to be prepared by a professional engineer licensed in New York State with experience in dam safety.
- The methods of removal need to be described, including equipment to be used, the project schedule, the sequence of removal actions, how materials will be handled, and how the site will be accessed. In addition to the water management plan noted above, this would include other necessary measures to reduce or eliminate pollution during removal (e.g., erosion controls). For sites where access may be difficult, any temporary measures must be described, which may include temporary access roads, temporary culverts, or stream/wetland crossings.
- For projects within certain flood zones on a federal Flood Insurance Rate Map, an analysis of potential changes to upstream and downstream flood elevations and floodplain extents may be required.
- The methods that will be used to restore and stabilize the site following removal must be described. This would include the removal of any stockpiled materials, temporary water diversion, or site access measures. For areas that require planting, a landscaping plan that uses native plant material would be required. In some cases, in-stream or stream bank measures may also be needed to restore channel flows and ensure bank stabilization upstream of the structure’s former location.
- If state-regulated wetlands are present above the dam, an estimate of their size and evaluation of how the removal will change the location or extent of the wetlands would be needed. Information on the general locations of state-regulated freshwater wetland is available on the DEC’s online Environmental Resource Mapper.
- If available information indicates that there may be state-listed threatened or endangered species within or near the site, further evaluation of the potential impacts of the project on those species may be required. This information will usually be obtained in consultation with the appropriate DEC regional office during the review of the project. However, general information on the potential presence of state-listed species can also be obtained through DEC’s online Environmental Resource Mapper.
- Other information that may be identified in consultation with DEC staff during a pre-application conference.
DOW, Bureau of Flood Protection and Dam Safety
625 Broadway
Albany, NY 12233