Information For Dam Owners

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.

Dam Inspections by Owners

Periodic Maintenance Inspections

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).

Inspection

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.

What to Look For During Inspection

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.

Keeping Records

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

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.

Detection

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.

Control

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.

Monitoring

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

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.