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Consequence Advice for Small Dam Owners

A layman's guide to dam safety consequence analysis, what to ask for, and what to expect.

Preface

Dams are an essential resource for modern life, supporting agriculture, ensuring food security, water supply for drinking, industry and recreation. Like any vital infrastructure, dams can carry risk in the event of a failure.

To manage risk, governments, regulators and water managers require analysis of dams to determine risk associated, and whether additional work, inspection or emergency planning is warranted.

Analysis of dams, and dam safety is a complex topic - and it can be a shock to discover your asset carries risk and financial liability, made harder if you dont understand the lingo and the process. If you are a small asset owner without prior industry experience, who needs to do, or get work done on a small dam then this guide is for you.

In this guide we step through risk and methods for analysing the consequence of a dam failure, what is 'appropriate' by industry terms, and what you should be asking your dams consultant to reduce your cost, and your potential liability through appropriate analysis. Specifically we discuss:

  1. How and why do dams fail?

  2. What is 'Risk', how do we calculate this, and what does it mean?

  3. What are the key steps in analysing dam break and consequence?

  4. Typical 'levels' of flood and consequence assessment - what is appropriate for me?

  5. How do we estimate risk to people - and why does this matter?

  6. How does this all combine - and how will it affect me?

  7. I need work done - what questions should I be asking?

Part 1: How and why do dams fail?

Most small dams or farm dams are what we call embankment dams, made out of compacted earth and/or rock. While there are many types of dams, this guide focusses solely on earth and rock embankment dams.

Failures for embankment dams tend to come back to a single key point, and that is that the flow of water can erode earth and rockfill. Outside of specialized risk analysis there are three general types of failure:

  1. 'Piping' or 'Internal Erosion' Failure: During construction, or as dams age, defects can occur within a dam that allows a concentrated leak under, or through a bank. If sufficient flow passes, it can begin eroding a dam from the inside, leading to a collapse and breach.

  2. Overtopping Failure: Overtopping failure is where water flows over the top of a dam, be that due to a storm of pump malfunction - and the flowing water 'cuts' into the embankment, leading to a slot - allowing the usually stored water to escape, accelerating the rate of erosion.

  3. Slope Stability Failure: This occurs when part of a dam slides or collapses due to steep slopes, poor construction materials, weak foundations, or an earthquake. This movement can allow water to escape, causing further erosion and potentially leading to a breach.

Initiation of erosion in an earth embankment overtopping trial at Lake Røssvatnet, Norway. From the paper "Two-Dimensional Model for Embankment Dam Breach Formation and Flood Wave Generation", D. Froehlich, 2004. Reproduced with permission.

Part 2: What is risk - how do we calculate it?

Risk is the chance of an event leading to something bad, like losing money or getting hurt. It’s about uncertainty and the possibility of negative outcomes in any situation.

There are two key parts that define risk as follows:

  1. Probability: What's the Likelihood that an event will happen. I.e. if you bet on a horse at the races, what are the odds you will lose, is it a 50/50 win chance, or a high stakes bet with a slim win chance but high payoff?

    In dams we call this probability an 'Annual Exceedance Probability' which can be thought of as what's the chance in a given year that an event will occur, or a dam fail. I.e. a 1/2 means there is a 50% chance each year, 1/100 means there is a 1% chance each year.

  2. Consequence: Consequence is, if the event occurs, what does it actually do? I.e. using the horse racing analogy, the consequence of losing the bet, is you lose your money.

    In dams we measure consequence based on losses to you and your business, losses to society through financial damages and loss of service, or loss of life for severe floods. Typically consequences for small dam owners are governed by loss of life which we refer to as "Potential Loss of Life".

With these two factors we can calculate risk as follows:

Risk = Probability x Consequence.

For dams this is:

Risk = What is the probability in any given year a dam will fail x What is the consequence of that dam failing.

The fist fundamental task is the assessment of consequences in the event that your dam fails – this underlies a range of subsequent actions depending on the result of the assessment. Estimating the probability of failure is a complex process requiring engineering judgement and industry experience - so for this guide, we focus on estimation of consequence which is typically the starting point for small owners

Houses in Corinda flooded during 2022 Brisbane flood, 01 by State Library of Queensland, licensed under Creative Commons Attribution 4.0 International License (CC BY 4.0)

Part 3: What are the key steps in analysing dam break and consequence?

When estimating the consequence of a dam break, the typical steps include:

  1. S1: Consider how the dam might fail: With the near-infinite variability in dam arrangements and materials, we start by identifying "Credible Failure Modes"—plausible ways a dam could realistically fail. For example, a dam on a stream may overtop during heavy rainfall, but this risk might not apply to a dam built on flat ground, which relies on pumps and has a large outflow channel.

  2. S2: Estimate the dam breach shape: This step involves estimating what the breach might look like. We use empirical (case history data) and the type of failure (S1) to estimate this. Most dams practitioners would use a piece of work by David Froelich (referenced as Froelich, 2008 or Froelich 2016/17).

    As an owner it is VERY IMPORTANT - to check how a breach shape has been estimated.

  3. S3: Estimate the breach outflow: Often estimated using the same method as the breach shape - using case history data to estimate a peak outflow (Froelich). For larger dams, or dams close to high population density sometimes we would model the development of a breach with time, to get a better result. This step can be complex, and requires industry knowledge to adopt appropriate tools.

    Owners should seek confirmation that breach estimation follows accepted practice and practitioners should be able to back up why a particular method has been chosen.

  4. S4: Estimate the flooding: Knowing how much water will come out, and how fast, we estimate the zone that will be flooded. There are a few methods ranging in complexity - which are discussed in the Part 4.

  5. S5: Estimate the location and number of people who might be at risk: Knowing where the flood will go, how high and how fast - we estimate who might be at risk, simply based on location. We refer to this as 'Population at Risk' or PAR for short This is discussed further in Part 5.

  6. S6: Estimate the Loss of Life: Knowing who might be at risk, and what the flood hazard looks like, we estimate a 'Potential Loss of Life' - or PLL for short. This is discussed in Part 5.

Simplified Dam Consequence Analysis Flowchart, Dam Safety Consulting, 2025.

Part 4: Typical 'levels' of flood and consequence assessment - what is appropriate for me?

In estimating Consequence, the extent of flooding and location of houses, roads or people are key. Australian national guidelines outline several methods stepping up in complexity (and cost):

  1. Initial Level: An Initial Level assessment is a screening level, rule of thumb type assessment that might be able to identify a consequence category that is obvious based on existing knowledge. Practitioners assume a flood wave 1/3 to 1/2 the height of the dam, and assume it stays constant moving downstream. Anyone who is in the flood area are considered to be at risk (see Part 5), and this is used to assign a consequence. This assessment should be conservative and may raise gaps than need to be assessed.

    The initial Level Assessment is only appropriate to screen out Very Low and Low consequence dams from needing further work. This should never be used to justify major investigation or upgrades.

  2. Intermediate Level: An intermediate level assessment is a calculated, but still conservative method requiring a more quantified approach. This is often a good balance for small dams - but generally Does Not Consider Attenuation or pre-existing flood conditions.

    • Attenuation is how water spreads out and slows down. For example pouring water in a bathtub, it spreads and drains slowly, reducing the flow but extending the time.

    • Pre-Existing Flood Conditions are the flood conditions that may exist BEFORE a dam fails. I.e. if the dam failed because of heavy rain, rivers may already be swollen, and the dam break becomes super-imposed on those already elevated levels. This may require hydrological studies (not covered here).

    As with the Initial Level, the Intermediate Level typically relies on the total Population at Risk (PAR) (see Part 5). However, an experienced practitioner may expand on this method to meet Comprehensive Level requirements without the need for detailed 2D modelling.

  3. Comprehensive Level: A Comprehensive Level assessment is the most accurate method. It involves a detailed review of people, property damage, and flood behavior. This assessment typically includes advanced calculations or comprehensive hydraulic modelling to estimate the flood zone, depth, and velocity. This level of detail allows for the estimation of not only Population at Risk (PAR) but also Potential Loss of Life (PLL).

    While the most precise, the Comprehensive Level assessment is also generally the most expensive.

An experienced practitioner should be able to recommend the most appropriate starting point based on a brief review. If there is uncertainty, they should provide a cost estimate that includes multiple stages in case the Initial or Intermediate methods prove insufficient.

Typical Comprehensive 2D Flood Inundation Modelling Resulting from Large Dam Breach, Provided by Albert Shen of HARC. Reproduced with Permission.

Example Flood Extent Following 'Intermediate Level' Empirical Routing of Peak Flow Using Manning's Equations. Dam Safety Consulting 2025.

Part 5: How do we estimate risk to people - and why does this matter?

Once we have our flood estimates, we can estimate who might be at risk - and whether there is a potential for a loss of life resulting from a dam break.

While only one factor, this is often the driving factor behind the assignment of a 'consequence category' for small dams - which is to say - it dictates how much effort should be assigned too dam safety.

To estimate the potential loss of life we would typically:

  1. Estimate the location and number of people who might be at risk: Knowing where the flood will go, how high and how fast - we estimate who might be at risk, simply based on location. This might include cars on roads (who we call 'itinerants'), or people in houses or building ('Permanent Population'). We call this 'Population at Risk' - or PAR for short.

    This should be based on satellite imagery, or driveby data - owners should ask HOW and WHERE population at risk have been identified. This should be backed up by data (I.e. traffic volumes, or house population) - if in doubt, more data collection may save you effort in the long run by allowing a less conservative analysis.

  2. Estimate the Loss of Life: Knowing who might be at risk, and what the flood hazard looks like, we estimate a 'Potential Loss of Life' - or PLL for short.

    There are several 'typical' accepted methods for assessing PLL - these include:

    • Graham Method: A method developed in 1999, this method collated old failure and fatality data - and provides an empirical approach based on 3 'flood severity' bands. While this was superseded by the RCEM method, it is quick and easy to apply, generally gets similar results and is still considered valid for small dams where determining the depth and velocity of a flood (2D modelling) might not be warranted.

    • RCEM Method: Released in 2014, the RCEM method builds on the Graham method and ties potential fatalities back to the depth, and speed of flood waters. While considered more accurate, this also requires a bit more data on your potential flood.

    • LifeSim: The newest method, lifesim is a software package that predicts actions taken by population. This is generally undertaken by specialists.

    Owners should understand which method will be used/has been used when arranging and receiving consequence reports.

Island Bend Dam, Snowy Hydro, Photo by Dam Safety Consulting, 2025.

Part 6: How does this all combine - and how will it affect me?

Once the consequences of a dam failure have been estimated - your dam will be assigned a 'consequence category' in-line with national guidelines.

The consequence category can be thought of as a rating on how bad a failure might be, both for you, and for the local community. This will combine an assessment of environmental, provision of services and financial damages with potential life loss (usually the driver for most small dams).

Once you receive a consequence category assessment, this will set the baseline expectations for how you manage the safety of your dam, and may require you to undertake additional work or analysis on your dam.

Regulations differ around Australia, so your responsibilities will likely be explained to you by either the regulator in your area, or by your dam safety professional.

Small Concrete Hydropower Wier, Part of the Hydro Tasmania Fisher Power Scheme, Mackenzie Plateau, L Johnson, 2024.

Part 7: I need work done - what questions should I be asking?

In this guide we have covered off, what, how and why we undertake consequence assessments for dams, as well as some of the typical methods used. If you are getting work done, it is in your interest to be an informed owner and understand what you are being offered.

When getting a quote for work, or checking work received there are several key questions you should be asking including:

1) What level of consequence assessment in accordance with ANCOLD 2012 Guidelines on Consequence Categories for Dams can I expect?

Typical Methods: Initial Level, Intermediate Level, Comprehensive Level.

Additional Notes: Initial level should only be used to screen out Low and Very Low consequence dams from needing further work.

2) What consequence assessments will be considered?

Typical Methods: Sunny Day, Flood or BOTH.

Additional Notes: Your practitioner should be able to clearly outline the reasons for selection. Typically, an on-stream storage will require BOTH.

3) How will the breach shape, and peak discharge be estimated?

Typical Methods: Rule of Thumb, Empirically (i.e. Froehlich 2017) or Modelled.

Additional Notes: Rule of thumb should be used to screen Low/Very Low ONLY.

4) How will the flood extents and/or depths velocities of the flood be estimated?

Typical Methods: Rule of Thumb, Empirically (I.e. use of manning's paired with peak outflow) or modelled.

Additional Notes: Rule of thumb should be used to screen Low/Very Low ONLY.

5) How will Permanent Potential Loss of Life (PLL) be assessed?

Typical Methods: Grahame (1999), RCEM (2014), LifeSim Modelling.

Additional Notes: Other methods may be available - however should be well justified by a consultant and backed up by industry literature.

6) How will Itinerant Population at Risk be assessed?

Typical Methods: Grahame (1999), Campbell (2013), Woodman (2016), LifeSim

Additional Notes: Itinerant population estimates should be based on traffic estimation, or counts - or if estimated, the reasons should be clearly defined.

Remember, your consultant should be able to clearly define how and why methods have been chosen - and initial level analysis should only be used as a screening for Low/Very Low consequence dams.

Rapidan Dam failure, 5 by U.S. Army Corps of Engineers, licensed under Creative Commons Attribution 4.0 International License (CC BY 4.0)

Closing

We hope this guide has been helpful, if you have any questions - or suggestions on how we can improve please don't hesitate to reach out via our Contact Page.

Dam Safety Consulting would like to thank Angus Swindon, of Tasset Consulting for his review and valuable input into this page.

Additional Resources

If you would like to know more about consequence assessment, or estimation there are some excellent guides on offer from regulators around Australia.

Users are encouraged to see:

Dam Safety Consulting would like to thank Angus Swindon, of Tasset Consulting for his review and valuable input into this page.

Experts in Dam Safety, Consequence Analysis, Modelling and Risk.

Contact

Info@Damsafe.com.au

+61 404 725 171

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