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Trust Is Knowledge

It is estimated that since 1950, over 20,000 dams have been built in Australia It is also estimated that over 50% have either failed, or perform below expectations Building a farm dam is not a case of digging a hole and waiting for it to fill with water. Many farm dams fail because they were not properly designed, planned or built Farm dams should be planned and built with the same care you would apply to building a house. This is a comprehensive  guide to good dam design, and what you should look for in a dam designer

The purpose of dams

It is important for the dam designer to understand the priority purpose for the dam being built. Dams have many purposes:

• Water storage for irrigation & stock
• Human & domestic consumption
• Flood control
• Recreation
• Decoration

No matter what their purpose is, a key characteristic of good dam design is longevity. Some operating dams are over one thousand years old and the great majority of dams built in the nineteenth century are still operating fully today.  

Balancing considerations

Good dam design is all about finding the right balance between maximising water storage capacity, sound engineering principles and cost of construction. Key points for good dam design are:

• There are limits on dam sizes
• Site topography and soils are important
• Usage requirements will also determine dam dimensions
• Where you build your dam is critical
• When to build a farm dam is important to its success

Selecting a dam designer

The design of dams is a specialised and complex task for the following reasons:

• All dams are different.
• The consequences of dam failure may be disastrous.
• The force of water pressure is enormous.

State of the art dam design is founded on a sound, proven technical basis (criteria, standards, guidelines, methods) and trained, experienced, conscientious, and competent designers. Dam designers should have completed an engineering technologist program, or have the equivalent training based on experience. Dam designers must also have experience in:

• the investigation, design, construction and inspection of dam projects in order to exercise appropriate judgement in determining and assessing design objectives and requirements
• classifying foundation and embankment materials
• classifying hazard potential
• evaluating pertinent project details
• creating a long-term management plan for the dam

The correct sequence of successful dam design

Good dam design is an iterative process of comparing and selecting from alternative plans until the most acceptable plan is identified.

Areas for investigation are:

1. Preliminary identification of needs and opportunities
2. Preliminary decisions on possible alternative plans for providing for the needs and opportunities
3. Preliminary estimate of prospective differences among the alternatives, expressed in physical or nonmonetary terms
4. Translation of descriptions of the differences among the alternatives into rough estimates of the benefits and costs in monetary terms
5. Evaluation of nonmonetary effects of the plan, such as expected environmental effects
6. Analysis and comparison of the rough monetary and nonmonetary estimates, and selection of those alternatives justifying further study
7. Selection of a single plan to determine the optimum project size with consideration to available budget

Water and good dam design

Water has always been an important factor for farming & agriculture. This fact has been reinforced by the recent run of dry years. With low rainfall, high evaporation rates, permeable soils and flat landscapes – there is a lack of abundant runoff and surface water resources. To further compound and complicate this situation – vegetation clearance for agriculture has altered the hydrology and salt movement in the landscape. The result is that many farm dams struggle to capture enough water to sustain existing stock and domestic requirements. This is why good dam design is essential to future water security – because water is now the limiting factor for potential expansion of livestock enterprises. Good cost effective dam design involves capturing the most water with the least excavation possible (also known as ‘storage ratios’) Highly desirable sites may have a storage to excavation ratio of 10:1 or higher We aim to provide professional technical information and advice to support landholders, farmers, earthmoving contractors and natural resource management (NRM) workers in key areas of dam siting, design, construction and management. This design advice ensures stable, safe and effective dam structures – particularly in areas concerning dam construction where the failure of built structures may endanger life, property or important environmental assets.

Critical dam design factors

On-ground conditions are important for various design aspects of farm dam establishment and management. The following critical factors are essential to consider when designing a dam:

• the suitability of on-site materials  
• the levels of technology required
• the catchment management needed to establish a useful water supply  
• the economic viability of the dam – cost vs need
• rainfall
• evaporation
• dominant soils
• geology
• soil sodicity (indicating dispersion)
• acid sulphate soils
• saline seepage
• proportions of clay in soils
• local flooding histories

Placement of a dam

Factors to consider when seeking a site for a dam include:

• type of dam
• size and storage ratio required
• catchment or source of water
• soils for foundation and bank materials
• proximity of dam should be as close as possible to where water will be used
• spillway or by-pass site
• protection of sensitive environments

Types of farm dams

The siting, type and purpose of a farm dam will all be inter-related. Different factors will play a greater role depending on the specific situation. Stock dams must be deep enough to allow for losses through evaporation, so a site with deep suitable clay soils is best. It is often beneficial, where possible, to take advantage of natural topography to maximise the water storage, compared to excavation volume (storage ratio). Farm dams can be broadly divided into 2 categories – on-stream or off-stream On-stream dams have dam walls placed on, or constructed across a watercourse or drainage path for the purpose of holding back and storing the natural flow of that watercourse or the surface water runoff flowing along that drainage path.  On-stream dams have the potential for large water storage from minimal earthworks – however they must be designed for destructive environmental flows – and large on-stream dams need extra care with design and construction. Off-stream dams are used to store water that is diverted from a watercourse or surface water run-off, by a wall or other structure. Off-stream dams will normally capture a limited volume of surface water from the catchment above the dam.

Optimal catchment design

A good dam site needs sufficient catchment, of a suitable quality. Decisions on catchment placement will be affected by:

• local flood history
• quantity and quality of small to large runoff events
• catchment erosion and sedimentation issues
• potential pollutants or contaminants from agricultural land use (e.g. fertilisers, pesticides, disturbed soils)
• degradation issues (salinity, acidity)
• potential for enhancing catchment runoff
• do upslope soils run water well?

Catchment of water will be reduced by permeable soils, areas of minimum or no till cropping and large tracts of native vegetation. It is normal that soil moisture will need to build up – before catchments will wet up and run water – following drought or extended dry periods.

Enhancing water catchment with good swale design

The use of water diversion drains (swales) can extend the catchment area by a factor of 10, and sometimes enable dams to be placed high in the catchment (avoiding potential saline seepage).

Spillway design

Dams, particularly gully dams in large catchments, have significant risks of failure during large floods. Large flows can cause spillway erosion and failure, and overtopping and failure of banks. Good dam design should consider the availability of sufficient space, or opportunistic natural structures, to provide an adequate spillway. The site must be topographically suitable to enable the spillway to be constructed as an integral part of the dam, and spillway flows should be returned to the normal drainage route before they leave a property. Spillways need to be designed to carry peak flood flows following heavy rainfall, typically based on a 1 in 100 chance flooding event and be resistant against erosion. A good spillway will carry peak flows, working with adequate freeboard to protect the dam wall from ‘overtopping’ which can cause serious failure and wash out of the dam wall.

Flood history

Knowledge of historical flooding heights, severity and frequency will help with siting and design, particularly spillway design. Siting dams out of flood prone areas (with a sacrificial diversion bank) may avoid potential flood-induced damage to the main dam wall.

Good dam design and suitable dam construction material

Dams are often built using unsuitable soil types, and good dam design ensures this never happens The volume of available suitable clay is first estimated at the proposed site. If there is insufficient clay at the preferred construction site, we test for sources of good clay as close as possible to it. Construction costs will increase dramatically with distance to the clay source.

Testing the suitability of clay soils

Achieving a ‘watertight’ seal in dams requires soil porosity (ie. soil pores / air voids) to be minimised. Watertight dams need high density/ low porosity soils – this requires the right mix of fines (clay) content. Fine materials (clay particles) will help block up soil pores, and this is aided by low to moderate dispersion and compaction. Compaction is required at the correct moisture content such that clay clods are crushed and air voids squeezed out. Only clays can hold water – but some clays require special treatment and others should not be used:

• slaking soils are prone to erosion
• highly dispersive soils are prone to ‘tunneling’ or tunnel erosion

Soil moisture

Correct soil moisture is critical for construction purposes. The clay soil will layer and compact more easily, and reduce leakage and settlement after the bank has been built. The dam embankment should be constructed from soil that is sufficiently moist to be pliable without crumbling, but not so wet as to excessively stick to, or flow away from machinery.

Dam design and compaction allowances

Compaction is achieved while the dam is being built, and it is essential to allow 5 % extra height for settlement. For a bulldozer built dam with no compaction other than the pressure of the bulldozer tracks, 10 % extra height in the embankment should be allowed for settlement.

Dam embankment design

There are three major types of construction techniques for dam walls:  

• Homogeneous dams – are constructed from one material (usually 20-30% clay)  
• Zoned dams – use a clay core surrounded by a more pervious shell. These are more stable, and have reduced construction costs with steeper slopes.  
• Diaphragm dams – are used when suitable clay is limited. A compacted clay layer acts us an impervious blanket over more pervious sand or gravel.

Good embankment design ensures that no tree branches, roots, organic or other perishable materials are placed in the embankment. The principles of good embankment design are:

• Excavation to commence at the downstream toe.  
• Each series of excavations should extend across the full width of the dam before the next layer is commenced.  
• As excavation proceeds the machine will be working progressively upstream
• Where the soil from the excavation is of varying types of material, the embankment should consist of two zones. The more impervious material should be placed in the upstream zone and the pervious material in the downstream zone
• Material to form the bank should be spilt from the blade on the uphill journey so that it is deposited in a layer not more than 20cm thick.
• No material should be tipped over the downstream batter.  
• Each layer must be thoroughly consolidated by rolling or traversing with suitable compaction machines before the next layer is placed.
• The distribution and gradation of the materials throughout the impermeable zone of the embankment should ensure that this zone is free from layers of material differing materially in texture or gradation from the surrounding material.  
• If the natural moisture of the soil is less than this limit, water should be added in borrow areas until the moisture content is raised to the limit required
• The embankments must make allowance for final settlement

After the embankment has been completed, sufficient top soil previously stripped from the area of the construction should be uniformly spread over the embankment so that a grass cover can be established

Other important features of dam design

Batter slopes

Slopes in contact with water should be no steeper than 3 horizontal:1 vertical. Slopes steeper than this are prone to eroding and difficult to compact with machinery. Downstream batter slopes on dam walls should be flat enough, so that the seepage line emerges as low as possible.

Protection from drying

Shrinkage cracks in dam walls are a concern when heavy clays are subjected to wetting and drying cycles. Covering the crest of the dam with 150-225 mm of sandy soil will provide a mulch to help prevent the clay drying out.

Protection for erodible (dispersive / slaking) soils

Dispersive and slaking soils are prone to erosion under the impact of rainfall, water inflows or wave action. Measures to control this may include:  

• Applying a layer of topsoil over the dam wall and batter slopes.  
• Establishing drought-proof grasses (not trees!) on dam banks and diversion drains.  
• Protecting batters and the embankment from wave action with tyres or additional width of crest
• Install plastic or a rock or concrete chute to prevent erosion on dam inflow slopes. Unprotected steep inflow slopes are prone to gullying erosion which can move up the hillslope, and mobilise sediment into dams.

Controlling sediment inflows

Sediment traps are an important part of good dam design They encourage uniform, slow flows so that water-borne sediments slow and settle out before reaching the dam

Plastic lined dams

Lined dams are an option where impervious materials are not found close to the site. Polyethylene liners can be used and are one of a number of dam-lining options Lined dams may be combined with options to enhance catchment runoff (e.g. plastic sheeted catchments) Once installed, the plastic lining will maximise runoff rates and create a reliable, good quality water supply for stock and domestic use. The plastic sheeting is rolled out, welded together and buried at the edges (in trenches) to keep it in position. The low point in the catchment area acts as a collection point, allowing time for water to flow into the dam, and also as a sieve to remove leaves and other debris. Good dam design allows landholders to make their own assessments on the cost-effectiveness of different lining options.

Building farm dams during droughts

Water scarcity during drought periods often causes an upsurge of interest in farm dam building, but construction under such dry conditions is fraught with problems. It is better to defer building dams until conditions are favourable because dams built in dry periods are more prone to failure.

Avoiding potential headaches with good dam design

Professional dam design can protect dams against some of the main causes of failure such as:

• Erosion
• Rilling and piping
• Seepage
• Leakage
• Cracking
• Deformation
• Spillway failure
• Erosion
• Animal damage
• Remedial works to fix these problems may have limited success, be time consuming and very expensive.

Who should design your dam?

It is worthwhile to choose a dam designer who has experience and a good project record. Very few general earthmovers have the experience, knowledge or the ability to build a dam. Try to find someone who specialises in dam construction. If you choose an experienced dam designer, you can reduce the chance of problems occurring during and after the building of your dam. The important message here is to get it right during the dam design phase. It will save you a lot of money and headaches Talk to us about professional dam design  

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