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Monday, July 8, 2013

Building an Earth Dam

Rye Patch Dam, Nevada

What is the purpose of the dam?

"The purpose of a dam is to impound (store) water, wastewater or liquid borne materials for any of several reasons, e.g. flood control, human water supply, irrigation, livestock water supply, energy generation, containment of mine tailings, recreation or pollution control. Many dams fulfill a combination of the above functions.

Manmade dams may be classified according to the type of construction material used, the methods used in construction, the slope or cross-section of the dam, the way the dam resists the forces of the water pressure behind it, the means used for controlling seepage and, occasionally, according to the purpose of the dam.

The materials used for construction of dams include earth, rock, tailings from mining or milling, concrete, masonry, steel, timber, miscellaneous materials (such as plastic or rubber) and any combination of these materials."

"In many tropical, subtropical and Mediterranean climates, dry season agriculture and the pre-rainy season establishment of food and cash crops cannot be under- taken without large quantities of water. To rely upon streamflow at a time when temperatures and evaporation are often at a peak can be unrealistic and risky. It may become essential for a dam to be constructed on a river or stream to allow for off-season storage of vital water supplies. Although primarily for irrigation, such structures can be used, either separately or combined, for fish farming1, stock and domestic water purposes, drainage sumps, groundwater recharge, flood ameliora- tion and conservation storage."


Types of Embankment Dams

"Embankment dams are the most common type of dam in use today. They have the general shape shown here.

Materials used for embankment dams include natural soil or rock, or waste materials obtained from mining or milling operations. An embankment dam is termed an “earthfill” or “rockfill” dam depending on whether it is comprised of compacted earth or mostly compacted or dumped rock.

The ability of an embankment dam to resist the reservoir water pressure is primarily a result of the mass weight, type and strength of the materials from which the dam is made."

Also good mock videos of dams collapsing. 

Chencha earth dam, Ethiopia


Site Planning

What is the start date and date of completion?

"Site selection is a critical component to the success or failure of an earth fill dam. Consider the following points: 

The dam must have the potential to fill with runoff (most years) or store sufficient water between runoff events that fill the reservoir. It is essential that the dam and reservoir have sufficient depth and volume to last through extended periods of drought.

Topographical features such as slope, width and height of dam, as well as reservoir capacity will influence construction costs. A topographical survey of the proposed dam site will be required to estimate costs, prepare necessary information for licensing and provide construction details for the contractor.

Soil conditions must be suitable for both compaction and the prevention of seepage losses through the dam. It is highly recommended that some pre-construction soil testing be done at the proposed site or sites. This testing can be accomplished by digging five or six test holes or test pits where the dam and reservoir are to be located. Soils should be checked to depths three feet below that of any proposed excavation for the dam or reservoir.

An assessment of the hazard potential downstream should a dam failure occur.

A good location for a spillway that will effectively handle runoff and minimize erosion.

Watershed activities that can affect the water quality or quantity of runoff."


Dam Planning

Dam Capacity

"The water storage capacity of a dam and reservoir can be estimated as follows: 

Dam capacity =
[Reservoir Length x Reservoir Width (at the dam) x
Depth of the Water (maximum)]

[400 feet x 100 feet x 12 feet] / 3 = 160,000 cubic feet 

160,000 cubic feet x 6.25 gallons per cubic foot = 
1 million gallons 

This calculation provides a close estimate on steep sided V-shaped water channels, but the accuracy of the estimate is poor on flatter, U-shaped water channels. 

Note: The reservoir must store enough water between runoff events for a secure supply and have good storage characteristics to prevent excessive evaporation losses."

Dam Design

"A typical design of a small earth-fill dam is shown below. For stability, the upstream slope must be a minimum of 3:1. Erosion protection is required to protect the dam from wave action. This protection can be achieved with a combination of smaller and larger rocks (or other suitable material) and, with smaller projects, a floating log boom. 

The downstream slope requires a minimum 2:1 slope, seeded with native grasses to prevent surface erosion. 

The top or crest of the dam should be a minimum of 10 feet wide (preferably 15 feet) to accommodate road traffic and minimize the potential for erosion. 

The crest elevation should be a minimum of three feet above the Full Supply Level (FSL) of the reservoir. 

The dam should be fenced to prevent livestock traffic, as this traffic can be a major cause of slope and crest degradation."


Site Clearing

"The area to be covered by the embankment should be pegged out prior to commencement of any works. The embankment and the area to be excavated should be cleared and grubbed.

Topsoil should be heaped in areas outside of the area to be covered by the embankment and all trees, scrub and roots removed. Topsoil should be placed in layers not exceeding 200 mm and planted with grass if it is to be left for a considerable time (more than 6 months). This will conserve the integrity of the topsoil.

All saturated material in the embankment area must be pushed well clear of the site and must not be used in the embankment.


Dam Construction

Diversion Tunnels:
"To build a Dam the engineers must first de-water the part of the river valley in which they wish to place the dam. This is usually achieved by diverting the river through a tunnel. The tunnel is built through one side of the valley around the planned construction area. A series of holes is drilled in the rock. Explosives are placed in the drill holes, blasting takes place and broken rock is then removed. This procedure is repeated many times until the tunnel is completed."

"Work on diverting the river starts in summer when river levels are low. Earth-moving equipment is used to build a small dam (called a cofferdam) upstream of the main construction area. This acts as a barrier to the river and causes it to flow through the diversion tunnel. Another cofferdam is built downstream of the main damsite to prevent water flowing back into the construction area."

Site Construction

The area covered by the base of the dam must be stripped of all vegetation and organic soil. The organic soil can be stockpiled and used on the downstream slope of the fill. All slopes steeper than 1.5:1 on sides of draw should be flattened to minimum of 2:1. 

Key trench
A key trench (cutoff trench) is excavated below the base of the fill upstream of the centerline of the fill. The key trench is incorporated in the design for two reasons: to anchor the dam to the base material and to prevent piping (seepage under the fill). 

The key trench should be a minimum of three feet deep for a dam the height of 10 to 12 feet. It should extend the full length of the dam and reach one third to one half of the way up the side slope of the draw."

Dam Construction
"Fill construction
The dam must be constructed from an impervious (clay or clay-based) material. A simple field test to determine the suitability of the material for compaction requires adding a small amount of moisture to a handful of material and then mixing to the consistency of putty. 

Next, try rolling the material between the palms of your hands. The material has good compaction characteristics if it can be rolled to the diameter of a pencil, approximately six inches long, then bent into a loop without separating the material. Several attempts may be required to obtain the proper moisture level to do the test. 

Construction material taken from the surrounding hillsides or an excavation in the reservoir area must be placed close to horizontal in the fill in six inch layers and compacted. If the material is dry, moisture will have to be added, and suitable compaction equipment such as a sheepsfoot packer used to obtain the proper compaction. 

A simple test to evaluate proper compaction is to place the edge of the heel of a hard-soled boot on the fill and push down hard with all your weight. If only a mark is left, compaction is satisfactory. If the heel sinks in, compaction is poor. No rocks over six inches in diameter should be placed in the fill. 

Start construction by filling the key trench with well-compacted material, and continue adding six inch layers until the maximum height is obtained. The top of the dam at the center of the draw should be built 10 per cent higher than the design to allow for settlement of the fill. 

A riparian pipe should be placed through the bottom of the fill during construction, along with a frost-free valve (curb stop) set well back in the fill to ensure frost protection. This pipe and valve system will allow water to be released downstream to a stock-watering trough, or to other water storage facilities during times of water shortages. "

"Embankment Dam: Forces
Water pushes against the embankment dam, but the heavy weight of the dam pushes down into the ground and prevents the structure from falling over."


Building the spillway or stream return

"The purpose of the spillway is to pass flood flows without overtopping the dam wall. Particular attention must be paid to providing adequate width and depth (or freeboard) of the spillway as per the specifications given in the dam permit. The following guidelines apply to spillways:

1. The absolute minimum width of a spill way is three metres.

2. Minimum spillway dimensions are given on the permit. Alteration of the spillway by the installation of culvert pipes should be treated with great caution. Adequate allowance for the passage of the specified flood flows must be maintained by increasing the amount of freeboard or increased spillway width if culvert pipes, or any other obstruction, is used or required in the spillway. Specific engineering advice must be sought before changing, modifying or obstructing a spillway in any manner.

3. The spillway should be cut in solid material (preferably rock) that will resist erosion. The stream return should be channelled back to the original watercourse and stabilised with a suitable size riprap consisting of rock or other materials such as Reno Mattresses or Gabions that will resist erosion and subsequent deposition of soil materials downstream.

4. In no circumstances should a spillway be blocked by either logs becoming wedged in the spillway or the spillway being purposely filled in to increase the capacity of the dam. The spillway of a dam is purposely designed to pass a very extreme flood. Reducing the size of the spillway increases the risk of the embankment overtopping and ultimately failing because the size of the spillway is limited to only passing smaller flood events than what the dam was originally designed for and the freeboard of the dam is reduced.

5. It is the dam owner’s responsibility to maintain the dam in a safe condition at all times including the maintenance of an adequate spillway able to pass the specified flood flows."


Finishing the Dam


Filling of the Dam

"When the dam is completed the diversion tunnel is closed and the lake begins to fill. The closure of the diversion tunnel has two phases. During low flow a large re-usable steel gate is lowered across the entrance. The diversion tunnel is then permanently blocked off by the construction of a concrete plug."



"Once the dam has been constructed, regular maintenance and inspections are required to ensure it remains in a good operating condition.

Dams over 10 metres in height and dams with hazard categories of ‘Significant’ and higher require the following regular inspections to be carried out:

Weekly or more frequent inspection to be carried out by the owner.

Biennial or intermediate inspections and surveillance reports carried out by a suitably qualified person (as defined by the Water Management (Safety of Dams ) 2003 Regulations).

Comprehensive surveillance reports carried out every 5 years by a suitably qualified person such as a Class A competent Engineer.

The maintenance and inspection requirements detailed above do not always apply to Lower hazard category dams. In such cases the dam owner should read their dam permit to verify what maintenance and inspection requirements must be undertaken.

It is good practice for the dam owner to inspect their dam on a regular basis to ensure that the dam is operating in a safe manner. Such inspections should include the following as well as any other matters the dam owner thinks necessary to inspect:

Inspection of the spillway to ensure it is not blocked by logs or trees growing in the spillway or deliberately blocked to increase the capacity of the dam (section 3.16 provides information on spillways);

Inspection to ensure that trees have not become established on or near a dam embankment. Tree roots can cause the embankment to crack leading to dam stability problems (section 3.15 provides information on vegetation). The highest plant growth that should be allowed on a dam embankment is pasture grass to protect against erosion.

Seepage from the dam should be monitored on a regular basis. Seepage is generally normal in all dams and should not be a concern unless it increases over a time or the water becomes turbid (dirty). An increase in turbid water is an indication that the embankment may be eroding internally which may lead to piping failure. If concerned, a suitably qualified and experienced person should be consulted.

It is suggested that the dam owner use a logbook to record observations from any dam maintenance and inspections visits for future reference."


Water quality and drought proofing

"There are a number of ways to improve the water quality in a dam. At the planning stage, avoid sites where watershed activities can allow poor quality or contaminated runoff to enter the reservoir of the dam. Examples of these sites include heavily cultivated fields or areas where a heavy concentration of livestock manure exists. 

The design of the reservoir can also help improve water quality. The deeper the excavated reservoir, the better the water quality and the more drought proof it will be from evaporation losses.

Stripping the topsoil from the flooded area of the reservoir will reduce the amount of nutrients available for plant and algae growth. The more plant and algae growth generated, the more rot and decay are generated and cause the water quality to deteriorate. Regular treatments to help control plant growth will help maintain water quality."