Catie George wrote:Lots of questions (mostly for you to think about). I am personally not a fan of small unengineered dams, so this colours my response.  

First... What is downstream of this dam if it were to break? Even a 4' dam breaking can cause an amazing amount of damage if there is enough of a pond behind it.

Second- what are your local laws regarding dam construction, and is this a year round stream? Here a dam like this may require an environmental assessment and most likely a permit.

Are you intending for the dam to overtop or how does the water escape? What are the slopes like (of the valley, and grade of stream, and flow volumes)? What is the foundation soil type?

Why quickcrete? Would it not be easier to just build an earth embankment with a  bit of a spillway? Also, you would want to embed the concrete dam in the ground a foot or two to prevent the water just piping beneath it and eventually toppling it/rapid release of water, and likely would want 2-3 courses of bags in width... this sounds expensive. I also anticipate the bag joints and corners becoming places for frost jacking to eventually break the concrete and ruin it- earth bags with reasonably silty soil (20-30% silt and clay) and some sort of protective cover might be cheaper/less backbreaking and about as water tight.

If you do use quick crete, I would suggest making sure it is in the dry season, so water doesnt wash away the cement in the concrete before it cures (cant remember the technical term for this).

Safest (less prone to breaking due to lower pressures, less risk if break due to less of a pond) and possibly more effective/less time consuming/less material probably would be if you were to make a series of weirs rather than one 4 ft tall dam. A couple 1-2' tall structures would  definitely increase groundwater penetration without some of the risks of a taller structure. Plus, they are less disrupting to habitat and less prone to fill up with silt.

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Catie George wrote:Those gullies look like they may see a fair amount of water during big storms to me. You definitely want to figure out how you plan for the dam to empty once filled, even if it is a very rare occurance. if I recall,  over topping is the most common method of dam failure.

I once saw the result of a catastrophic failure of a beaver dam (which caused a cascade of beaver dam failures), which included moving 6' boulders and scouring out a 6' channel in the hillside, removing 2 old bridge  foundations, and washing out a road, and completely removing an island from a small river. Water is powerful stuff.

I suppose, if I was determined go make a 4' tall dam on my own property, I would probably consider building a 8'-10' wide at the crest homogeneous earth structure, with 2.5h:1V to 3H:1V slopes  with a layer of filter cloth and large gravel to 6" rock covering the upstream side, and a spillway made of a concrete channel drain (long piece of concrete with 2 raised sides) about 1 ft lower than the crest and more filter cloth and rock to protect the downstream slope below the spillway. Or a lined rock fill dam, with liner on the upstream face trenched in at top and bottom. Good liner with UV protectants should have a 20-100 year lifespan.  This is not engineering advice, I have never seen the site, no idea about seismicity, soil types, rainfall, catchment, etc,etc, but these are the dam types I have seen be both relatively easy to build and long term successful.

If you used earth bags, you could cover with gravel.

Seriously though,  I would have nightmares about dam failure and liability. I have spent far too much time swearing about previous small dam builders to be comfortable with them. IMO half assing a dam is rather like half assing electrical systems. Either you regret it, or the next person regrets it.

Oh- and if you decide to build a concrete dam, it doesnt work, and you decide to later build an earth embankment, make sure to take the concrete out before you build the earth embankment, as the concrete will likely make the earth dam more likely to fail.

The 10 most important things to be considered in design of embankment dams

Amr El-Sayed, Ph. D., CCM

Amr El-Sayed, Ph. D., CCM

Cost Control Manager at EPC Consultants, Inc.

Published Aug 2,

Introduction

 The designer engineer’s responsibility is to provide safety. The designed structures must act with integrity giving due consideration to the purpose of the project and the ultimate effects of the project on fellow human beings.

 At the same time the Engineers are responsible to the community for the cost of the structure. There is always a limit to the finance, so any cut in cost must not sacrifice safety.

 The Engineers also carries a legal responsibility, and are responsible at all times for both what they do and what they say.

 Sequence of Dam Design

 1- Specify the purpose for the dam project

• Water supply (requires a high reservoir)
• Irrigation
• Silt retention
• Transportation
• Electricity generation
• Recreation and beautification (requires a constant reservoir level)
• Flood mitigation (requires a low reservoir)

2- Architecture layout and choosing the best spot for the dam

• In the planning stage possible dam sites will have been chosen from contour maps and aerial photography, selected primarily on topography. A narrow gorge is best, hoping for minimum quantities in the dam and a valley opening upstream to provide the required storage. There maybe alternative sites along the length of a river and hence further investigation has to be done to ascertain the best possible position.
• Depends on many considerations, such as the narrowest stream width, the location of the reservoir, geological formation at the site, and what the purpose of the dam was.

 3-Site Investigation

• Most failures are due to lack of appreciation of how the particular dam site would react to the superposition of the dam and reservoir. It is therefore essential that a detailed site investigation takes place and Engineers appropriately use the results.
• A Geologist will assist the Engineer in the selection of the dam site, and a construction Engineer will study the access and possible sources of materials.

 4- Laboratory and Field Testing

• All the parameters used in the design such as soil shear strength, unit weight, maximum dry density,…….etc, should be estimated from different types of field and laboratory tests.

5- Hydrology study

• Hydrology is a science of prediction - the likelihood of recurrence of natural events. Mathematicians may try to predict events based on past history but Nature is unpredictable as to time and magnitude of occurrence.
• Based on past information the low flow characteristics of the river will control the storage required and hence the normal full supply level of the reservoir. High flow records and flood forecasting techniques provide the basis for design of the spillway, and hence the flood storage required above normal full supply level.
• The hydrology study also involves determining the storage capacity of the reservoir, the workable lake elevation for navigation or power supply, and the design of emergency spillway.

6- Loading and Factor of Safety – Static and Dynamic Loading

Both static and dynamic (like earthquake) loads acting on the body of the dam are calculated.

The minimum factors of safety for embankment dams would be:

 Upstream Slope

Immediately after completion with full construction pore pressure                    1.3-1.5

Following rapid drawdown (slip circles between high and low water levels)    1.2-1.3

Downstream Slope 

Earthquake and Reservoir Full                                                           1.2

Reservoir full - steady seepage                                                           1.5

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In an area subject to earthquakes the following factors are indicative of acceptable values:

Seismic coefficient                                     0.1            FoS             1.8

Seismic coefficient                                     0.3            FoS             1.15

 7- Foundation Design

• The foundations of a dam must be able to withstand without unacceptable deformation the loads imposed upon it by the structure, both immediately after filling the reservoir and in the long term.
• With time, deterioration by saturation and percolation of water can occur, whilst soft rocks and clays usually exhibit lower residual strengths under sustained loading than under rapid testing. It is the 10-20m of rock immediately below the dam that is of greatest importance.
• Terzaghi’s advice might well apply to foundation testing - "...because of unavoidable uncertainties involved in the fundamental assumptions of the theories and the numerical values of the soil constants, simplicity is of much greater importance than accuracy. “The Engineer must use all the available resources, concentrating on the zones of foundation that appear weak and that will be subject to stresses once loaded.”
• Construction of a dam and filling of the reservoir behind it create load stresses on the floor and sides of a valley that did not exist previously.
• The kinds and distributions of imposed stresses created by a dam on its foundation depend on the shape of the dam and the materials used in its construction.
• Dams built of masonry or concrete can be considered to behave as cohesive, rigid, monolithic structures. The stresses acting on the foundation is a function of the gross weight of the dam as distributed over the total area of the foundation on which the dam rests.
• Earth and rock fill dams exhibit gross semi-plastic behavior, and the pressure on the foundation at any point depends on the thickness of the dam above the point.
• The pressures exerted by earth and rock-fill dams resemble in some respects those exerted by the water in a reservoir, but pressure distribution is modified by the fact that the materials of construction have some inherent strength, and fail only after some threshold stress has been exceeded. Pressures exerted by water in the reservoir behind a dam are hydrostatic and increase linearly with depth.

 8- Seepage control design

• Seepage under an embankment is much more dangerous than that for a concrete dam, since embankments are usually built on soft material which is liable to be scoured out and it is also vulnerable to influx of water; whereas a concrete dam is usually built on rock which is not worn away so rapidly by the scouring action of water; and even then a defective dam will not necessarily be endangered by passage of water through it or even under it.
• Stored water behind dams, gives rise to three basic seepage problems, which can lead to difficulties and in serious cases to total failure:

1. Piping occurs when water picks up soil particles and moves them through unprotected exits, developing unseen channels or pipes through a dam or its foundation.
2. Heave or slope failures caused by seepage forces.
3. Excessive loss of water.

• Three basic methods for controlling seepage are:

1. Use of filters to prevent piping and heave
2. Seepage reduction
3. Drainage

 9- Slope Stability

• Failure of an embankment dam can result from instability of either the upstream or downstream slopes. The failure surface may lie within the embankment or may pass through the embankment and the foundation soil. The critical stages in an upstream slope are at the end of construction and during rapid drawdown. The critical stages for the downstream slope are at the end of construction and during steady seepage when the reservoir is full.
• It is common to install piezometers to measure pore water pressures and compare data with the predicted values used in design. Since pore water pressures are a dominant influence on the factor of safety of slopes, remedial action should be taken if the factor of safety, based on the measured values, is considered to be too low.
• To ensure stability a number of conditions must be investigated:

1. The slopes must be safe against surface slipping. To ensure this the slopes must be no steeper than the angle of repose
2. The dam must be safe against sliding on the foundation
3. The mass of the embankment must be safe against a circular arc failure or composite linear failure. This is likely to occur within an earth core or weak foundation

 10- River Diversion design

• Regardless of the type of dam, it is necessary to de-water the site for final geological inspection, for foundation improvement and preparation, and for the first stage of dam construction. The magnitude, method and cost of river diversion works will depend upon the cross-section of the valley, the bed material in the river, the type of dam, the expected hydrological conditions during the time required for this phase of the work, and finally upon the consequences of failure of any part of the temporary works.
•  At most sites it will be necessary to move the river whilst part of the dam is constructed; this part will incorporate either permanent or temporary openings through which the river will be diverted in the second stage. If the first diversion is not large enough the initial stages of construction will be inundated, if the second stage outlets are too small, the whole works will be flooded.
•  At some sites there is a distinct seasonal pattern of river flows and advantage can be taken of such conditions but noting that Nature is random.

Statistical study about Dam Failures in California

 Dam failures are most likely to happen for one of six reasons: 

• Overtopping, caused by water spilling over the top of a dam
• Structural failure of materials used in dam construction
• Stability failure of the foundation or other features that hold the dam in place
• Cracking caused by movements like the natural settling of a dam
• Inadequate maintenance and upkeep
• Piping—when seepage through a dam is not properly filtered and soil particles continue to progress and form sink holes in the dam. 

 There have been a total of 45 dam failures in California. Failures have occurred for a variety of reasons, the most common failure being overtopping. Other dams have failed due to specific shortcomings in the dam itself or an inadequate assessment of the surrounding geomorphologic characteristics. The first notable dam failure occurred in in Sierra County, while the most recent failure occurred in . The greatest catastrophe relating to California dam failures was William Mulholland’s infamous St. Francis Dam, which failed in . Overall, there have been a least 460 deaths from dam failures in California.

 According to the introduction and the statistics of dam failures, the 10 most important things to be considered in design of embankment dams:

1- Site investigation:

In order to design the most efficient structure, we have to know what the structure is facing with respect to soil and geological formation, and how the dam will act at the specific site.

2- Laboratory and Field Testing:

The tests done at the construction site are the base for dam design. If any tests gave bad results, the design will be based on poor information, and will be poor design.

3- Seepage control design:

Cracks in the embankment dames are inevitable (Jim Sherrard), so a good design for seepage control makes a difference between good and bad design. The seepage control design includes filter design, toe or chimney drains, and/or adding a core with low permeability. 

4-Hydrology study

Good Hydrology study will determine the U.stream water level, and thus determining the required height of the dam, and the elevation of the spillway. Poor investigation might lead to dam overtopping.

5- Loading and Factor of Safety - Dynamic Loading

For a dam to act efficiently, all the loads acting on the dam (either external or internal) should be calculated accurately. Taking into consideration all the loads will result in a good and safe design, otherwise it might lead to unsafe structure.

6- Foundation Design

The foundation is a very important element in the embankment dam. It must carry all the dam and water loads safely without failing under excessive settlement, and it is also should be designed to protect the dam against seepage and piping.

7- Slope Stability

Slope stability check is very important as many dams failed from insufficient design checks for slope. The slope check should include:

1. Just after construction condition
2. Steady seepage, and
3. Rapid drawdown

8- Specify the purpose for the dam project

The purpose of the dam determines many factors in dam design. There is no fixed design for every dam. The purpose of the dam should be taken into consideration in dam design. For example, a dam with power plant, should be designed to withstand extra dynamic effects from power machines.

 9- Architecture layout and choosing the best spot for the dam

Based on Geology formation, choosing the best place for the dam is very essential for a successful design. The location of the dam might decrease or increase construction cost. 

10- River Diversion design

Moving the river away from the dam construction area provides low groundwater level, and thus enables to operate without major dewatering problems.

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