Earthen Dams

Construction, Function and Failures

CarolinaFireJournal - Bob Twomey
Bob Twomey
07/21/2017 -

Most of us in a rescue career may see some level of dam failure, especially when heavy rainfall associated with hurricanes pass through our state, or during extended periods of sustained rainfall. 


Pictured is a small picturesque private pond that could be found anywhere in the state.

In this article, I will explain how dams are constructed — or supposed to be constructed — how certain parts of an earthen dam function, and what can happen when they fail, and how they usually fail. This is a very technical article, but in order to understand dams and dangers that can be associated with their failure, the rescuer must understand some physical concepts about them, just like you must understand proper extrication methods in hybrid vehicles.

Since most concrete dams are so well constructed and are very strong, rarely will any of us ever see failure with this type of dam. However, since most farm ponds and small lakes in our communities are constructed of earth, I will focus on these structures.

There are thousands of earthen dams in North Carolina, from farm and agricultural uses to residential and recreational uses. Most of these dams were engineered and designed properly, with adequate piping systems and emergency by-pass spillways. Some were built without the benefit of proper engineering. Some are also not properly maintained as required under state dam  safety laws. Others are well maintained and are as safe today as the day construction was completed.

My non-rescue working career was with the Natural Resources Conservation Service. I served over 37 years with that agency, and as a result, designed and oversaw construction of many ponds and dams in North Carolina, nearly all of which were associated with agriculture. I was privileged to work on large flood control dams in North Carolina as well, and thus learned a great deal about dam construction.

Dam Classifications

The graph above shows various components of earthen dams.

For the rescuer, an understanding of the “hazard classification” of dams is very important. In North Carolina, all dams fall under the hazard classifications: Class A, B, or C. Class A dams are considered low hazard, class B are intermediate hazard, and class C are high hazard dams. What constitutes a low hazard dam? Basically, should this classification of dam fail, the downstream damages would not include the potential for loss of life or infrastructure. Basically, there would be stream flooding, damage to low travel volume roads, culverts, small bridges, cropland damages and so forth. The likelihood of death or injury is low, as is significant road or utilities damage. However, high hazard dam failures would be failures that have a high likelihood of downstream injury or death, substantial damages to roads, utilities and infrastructure, buildings damaged or destroyed, and severe flooding. Intermediate hazard dams fall in between, with some risk to life, property, and infrastructure.

You can see that these hazard classifications are usually associated with the height of a dam and the volume of water impounded behind it. Generally, dams with roads immediately down stream, or houses, buildings and bridges, would fall under the intermediate or high hazard classifications due to potential loss of life and structural damages. However, small lakes or ponds with high dams that have infrastructure immediately below them can be classified as high hazard if there is the same potential for loss of life or substantial damages.

There are over 5,683 dams in North Carolina. About 3000 of them are considered “regulated” by the state, meaning, they are intermediate or high hazard dams that require state inspections regularly due to their potential for substantial downstream destruction or loss of life, should they fail. The North Carolina Dam Safety Section of the North Carolina Division of Environment has jurisdiction with inspection of dams, both public and private. The purpose of the inspections are to ensure that dam owners are properly maintaining their dams, that potential problems with dams are discovered in a timely fashion so that repairs can be made BEFORE the integrity of the structure is compromised. Does this mean that the owners of small dams should not be as equally diligent with inspections and maintenance of their dams? ABSOLUTELY NOT! On the contrary, you will more likely see failures of smaller, low-hazard dams due to their design characteristics and often reduced or neglected levels of maintenance.

Basic Components of Dams

Now you know what the hazard classifications of dams are and what that means relative to dam height and the size of the impounded pool. But in order to understand the common ways earthen dams fail, you must understand the basic components of any earthen dam. Any of these important components, if not installed correctly or if compromised years after construction, can lead to failure of the structure. During this section of the article, you will need to refer to the components diagram in the article.  I will try to simplify the engineering concepts down to the basics.

Rescue personnel must understand the following statements: ALL earthen dams are saturated to some degree. Next, the only thing holding back the impounded water is the fact that the weight of the soil in the dam — and the way it is constructed — weighs more than the weight of the water from the pool pushing against it. Profound statements, considering what is downstream of many dams. Think about this a moment and let it sink in.

Two of the most important components of any earthen dam are the core and the core cut-off trench. The core, constructed of the best — sandy clay-sandy clay loam — soil on the landowner’s property, is instrumental in reducing the amount of soil saturation within the dam. It also adds structural integrity to the entire dam. The core cut-off trench “anchors” the core — and thus the dam — to the ground upon which it is built. It also prevents water seepage under the core and dam. A dam leaking a lot of water under it is not going to hold much water. It is also likely, if seepage continues, to undermine the dam, leading to eventual structural collapse and dam failure.

Next, all earthen dams must have some type of water level regulating device. In smaller ponds and lakes, this is usually accomplished by a pipe system. A vertical pipe, called the riser, maintains the water level in the pond at a constant elevation. It connects to a pipe under the dam, called the barrel, which carries water falling into the riser out to the downstream toe of the dam. This discharge water flows into a stilling basin at the end of the pipe, which absorbs the energy of the water flowing out of the barrel. The barrel pipe has anti-seep collars fastened to it to prevent water within the saturated part of the dam from flowing along the outside of the pipe, causing soil erosion within the dam itself. This could easily cause the dam to slump and fail catastrophically. All dams are required to have a drain in them, such that water may be emptied from the pool area in a controlled fashion, such as during an emergency or when repairs are needed to the dam.

Note the low section of this dam that was overtopped, eroding the dam all the way down to the bottom of the pond.

Dam failed at riser pipe and barrel at dock in pond. All piping washed away and total pond drained in minutes.

This pond’s waterline is too close to the top of the dam, affording little runoff storage, and main pipe system too small, resulting in overtopping of dam and complete failure.

All dams have a “front slope” and a “back slope.” These inclined surfaces are built upwards as the core is built upwards, in layers, and are required to be thoroughly compacted during construction. The whole dam rises as a unit during construction. All earthen dams have some type of emergency spillway, which is used to convey large volumes of water around the dam that the piping systems cannot carry. An earthen dam should NEVER be allowed to overtop. They are not designed to be overtopped, and being made of fill material, can seriously erode to the point of complete failure of the dam.

If you look at it this way, it will all make sense: the roof of your house is a watershed. Rainfall upon the roof runs off. The gutters, like a creek in a landscape watershed, function to carry off this runoff. When we get large storms through North Carolina, as during a hurricane, it sometimes is not possible to have a pipe system big enough to carry all of this watershed runoff — such as when your gutters cannot handle the volume of water coming off your roof watershed. If earthen dams lack sufficient pipe size to carry this runoff under the dam, the pool would fill and water would flow over the dam; hence the presence of an emergency bypass spillway (ESW). You will find most emergency spillways in North Carolina to be graded to a specific width and depth and most will be grassed spillways. The dimensions of the ESW are relative to the expected high rainfall in the part of the state where you live and the design engineer determines the calculations.

Now, all of this information above has value to the rescuer, because once you KNOW the components of an earthen dam, you can begin to feel more confident in assessing dangerous situations with dams when — not if, but when — we get heavy rainfalls and flooding. The next part of this article will go into detail about reading the signs and symptoms of potential or imminent dam failures. Yes, in a way, this is just like recognizing signs and symptoms of your medical patients and acting accordingly.

Causes Of Earthen Dam Failures

I mentioned above that earthen dams are, to some degree, saturated. That means mud, no matter how properly compacted the soil in the dam may be. Mud is mud, and this mud has hydrostatic pressure against it from the weight and pressure of the impounded pool. Imperfections within this dam can cause problems, because water can and will flow through a dam if allowed to do so. This phenomenon is called “piping.”

Several causes that can start the piping process are tree roots decaying inside the dam, muskrat burrows within the dam, water flowing along the outside the barrel pipe instead of through the pipe, insufficient compaction of the soil when the dam was constructed, dam construction with improper soils —as in too sandy or gravelly — decaying stumps or logs that were improperly buried in the dam when constructed, and many more reasons. The point is, that when water begins to flow through a channel within the dam, the flow carries soil particles with it. In time, this channel gets bigger and bigger, allowing for more water to flow through it, until one day, sections of the dam sink, or fall in, easily leading to catastrophic dam failure.

Pond storage volume almost completely gone, filled with sediment, brush, grass and trees. There is no storage, so heavy runoff could easily overtop the dam causing more of a mudslide should the dam fail.

Another major cause of dam failures is due to the deterioration of the metal pipes through the dam. Corrugated metal pipe, depending upon the thickness or gage of the pipe, has a life expectancy of 20 to 30 years. When the pipe rusts over time, it can allow water that was flowing through it to now flow around the outsides of the pipe. Sometimes the pipe even collapses, causing a dramatic slump in the dam, thus a weak spot, in the saturated part of the dam. Guess what is likely to happen?

The following eight photos show what happens when pipe systems fail, the dam slumps, and water flows through and over the collapsing section of dam. Some of the dams were overtopped due to a lack of runoff storage between the permanent water level and the top of the dam. Other dams were not properly designed, with pipe systems too small to carry even moderate flows, steep side slopes, insufficient storage, or adequately sized emergency spillways.

Inspecting Dams

So, as rescuers, what does all this mean? Low hazard dams/ponds that fail in rural settings may not cause excessive damages downstream or wash away houses or outbuildings, and may not claim lives. But the damages done to streams filled with sediment and debris from a sudden release of water are expensive to clean up, and streams choked with sediment and debris no longer have the water-carrying capacity they once had. Future heavy rainfalls can then cause flooding not previously seen before, and this CAN affect homes and other buildings, roads, utilities — even lives, should an area not be flood prone before the break, now become flood zones. 

If you have ever seen a dam fail, you can attest to the awesome power of moving water. The ground shudders, tons of earth moves in a matter of seconds to a few minutes, stream channels sometimes are gutted down to bedrock, and tons of soil, trees and debris are deposited for long distances downstream. A wall of moving water, mud and debris can shatter a house like a matchbox, throwing people into the raging flows with little chance of survival. Roads, culverts, utilities — all are destroyed in a matter of minutes. Normal daily life and routine is sometimes disrupted for months. Your normal access routes for fire, rescue and EMS are gone in seconds.

Most fire departments perform inspections of buildings and warehouses and other facilities within their response areas so that they know what they will be facing should fire erupt. I would suggest that the same inventory of dams and ponds be done within your districts, so that you know what is there, how high the dams are, how large the pools are, what is downstream should a particular dam fail, and then PREPLAN for how a response might be performed. Generally, dam owners won’t mind you cataloging their ponds, but seeking permission to enter upon their land and telling them why you are doing it and inviting them along with you is a good idea.

Some things that you might look for are dams with excessive water along the downstream toe of a dam — refer back up to the drawing — slumps, sloughs or “sunk in” areas on the wet or dry slopes of the dam, failing piping systems, large amounts of water flowing into the stilling basin from AROUND the barrel instead of through it, or even vortex swirls in the pool area, where there is no pipe causing the vortexes. These are all signs of problems on an earthen dam. Sometimes, after very heavy rainfall events, the back slope of the dam may actually slide, as in a mudslide. This is because the soil becomes super-saturated, and acts more like a “flowable solid” under the effects of gravity, just like a landslide. This can, if large enough, significantly weaken the structure of the dam. Look for dams, large and small, that have steep side slopes. These are much more susceptible to sliding that flatter side slopes during heavy rainfalls. Look for “stress cracks” on the top edges of a dam, as these can indicate the slope has settled and is subject to sliding.

Nice pond, but permanent waterline near top of dam, little runoff storage, and structures downstream. Normal rainfall/runoff years may function fine, but in heavy rainfall events, dam may easily be overtopped.

Emergency bypass spillways clogged with trees or debris cannot function properly when the time comes. Often, through neglect, ESW’s are not mowed or maintained. Trees and heavy brush grow in them, and this sometimes changes their relative elevation to the permanent pool and top of the dam. Thus, when they are needed to carry excess flows safely around a dam, they cannot. Likewise, trees should never be allowed to grow on an earthen dam. Grass is the preferred ground cover on dams. It allows for easy inspection of the dam after storms and during regular maintenance inspections and protects the soil from erosion hazards.

After the breach of a dam, as rescuers you can expect the possibility of all kinds of rescue to be associated in this, again depending upon the size of the pool area, dam height, speed of failure, what is in the flood inundation area below the dam, terrain and land features, and whether there is continuous rainfall after the dam fails. Knowing in advance things to look for on dams will help you be prepared for what can be expected. When large storms approach, weakened dam and dams deficient in long-term maintenance should be monitored. North Carolina Emergency Management can be of assistance to you in securing needed rescue resources when you suspect a dam could fail, and afterwards, if it does fail. Your local Soil and Water Conservation District Office and local Natural Resources Conservation Service personnel can be a good source of information as well.

I know I have thrown a lot of information at you in this article. The intent has not been to make you dam engineers, but rather to expose you to common types of failures of earthen dams, of any kind: ponds, hog lagoons, waste ponds in agriculture production, dairy waste storage ponds, etc. If you have questions, feel free to email or call me and I will help as I can.

Remember, chance favors the prepared mind. The more you know and learn, along with experience over time, the better and more effective rescuer you will be.

Bob Twomey is the founder and past chief of the North Carolina High Level Extraction Rescue Team, Inc. a volunteer helicopter search and rescue support team based in Transylvania County, North Carolina. He is the senior helicopter pilot for Wolf Tree Aviation, LLC operating out of Transylvania Community Airport. He has served in numerous officer and training positions in the Transylvania County Rescue Squad. He has been active in SAR for 41 years. He can be reached at 828-884-7174 or [email protected]

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Issue 32.4 | Fall 2018

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