How Improbable is a One-In 1,000 Year Flood Event?

The Governor of the State of South Carolina points out to reporters in every press conference that the floods are a one-in-1000 year event. In other words, the devastation from these floods is not our fault, this is an extremely improbable event and there is no way we could have been prepared for it.

The cost of the flood is estimated at USD 1 billion and rising.

Suppose someone offered you a lottery ticket with a USD 1 billion payout and told you that only 1,000 tickets were being sold, how much would you be willing to pay for that ticket?

Is there a problem with the way we present our water data to the public and decision-makers?

People are easily fooled by statistics and they are also desensitized to reporting of threats from the environment.  Recent research  Newell et al. (2015) concludes that the assumption that “summaries of the nature and scale of disasters will lead people to reduce their exposure to risk” is false. In other words, disaster reporting actually increases the appeal of a disaster prone region. This paradox may partly be explained by a conclusion that after a disaster the region will be safe for the next 1,000 years.

What is our responsibility for this absurd conclusion?

For starters, what is the provenance of the one-in 1,000 frequency estimate?  How was the uncertainty of that estimate communicated to end-users and decision-makers?  What are the assumptions implicit in this frequency analysis and what is the validity of those assumptions in the current climate and landscape? What is the role of cascading dam failures in the severity of flooding? National geographic proposes 4 hidden causes of dam failures that are relevant to this case. In other words, what are the preventive investments that could have dramatically reduced the cost of payout in this lottery?

People are not very good at understanding concepts like probability, uncertainty, and risk.

What can we do differently to help them understand, as they re-build in the floodplain, that it is nearly certain they will be devastated again, we are only uncertain about exactly how soon?

If we want to refine the estimate of how soon, we should invest more in relevant and reliable water data. Please comment below.

Photo credit: Photo by Ryan Johnson  |  Flooding in North Charleston  |  The aftermath of the flooding in North Charleston, South Carolina caused by over 15 inches of rainfall resulting from Hurricane Joaquin.  |  Taken on October 4, 2015  |  License


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9 responses to “How Improbable is a One-In 1,000 Year Flood Event?”

  1. We can start by using the morphological definition of floodplain and stop defining flood risk areas by the moving goal line of probability based techniques. Nature has etched flood limits in the terrain and if we respected those limits we would have far fewer economic disasters. By defining probability based limits we allow development right to the boundary without being aware that it’s an artificial line. Municipalities also need to make developers aware that a 1% AEP flood line (incorrectly referred to as a “once in 100 year flood”) is not a risk assessment. The service life of infrastructure must be accounted for when assessing risk: for example a structure with a 100 year service life has a 63% probability of being impacted by a 1% flood event. What do people do in their daily lives that includes a risk of losing everything they own that comes even close to 63%? Nothing, that would be stupid, and yet floodplains are filled with houses.

    We have some work to do.

    • Hi Marcel,
      How would you respond to the criticism that the morphological definition etched in the landscape is a record of what has happened as opposed to what could happen? The floods that created those marks were the result of a perfect storm of meteorological conditions where those conditions are perhaps more, or perhaps less, probable in the current and future climate than they were in the past climate.Those marks were created from events at a time when the upstream landscape may have been mostly forest and when the river may have been running freely. Flood works and stream bank hardening reduce risk until such a time that they don’t. When they do fail they tend to fail spectacularly. Those spectacular failures can totally change the course of the floodplain. I am not disagreeing with you, I am hoping you will expand on what seems like a good idea.

    • Johan Linderberg October 23, 2015 at 1:40 am

      Hello Marcel

      I am very glad that there are somebody else that put this in writing. When city planners and developers use floodplains for new housing they are asking for trouble. They are inadvertenly causing more problems and pushing expenses on to the public or new houseowners.

      In my hometown we had a suburb housing area built in an area formerly a marchy/bog area with a small creek/ditch dewatering it. It was owned by the municipality and was sold to a developer. In the seventees we abstracted a lot of groundwater inside the citylimits causing a drawdown in a large area.

      During the start of the new millenium we have reduced the pumping rate to less than 25% of the former maximum. In 2005 the area was flooded by a heavy rain event. 7 houses where floode. It was a 50 year event! Then 9 months later the same area was hit by a 100 year event, and then again a 50 year event!

      As a joint company taking care of drinking water and sewage we proposed buying up these properties and remowing theses houses to prevent new flooding.

      When politicians and the public demands some actions against flooding and property damage there is not always a technical solution of larger pipes, levees or other systems. The only true and reliable solutions would often be to remove these houses prone to flooding.

      In the old days people where smarter, they developed cities on hills above the water line. In modern times people rely on a technical solution but there will always come an event causing flooding.

      We tend to forget why we see more flooding events today around the world than 100 yeras ago. If you look at the total urbanized areas today there are far greater areas covered with pavement and housing developments. Most greater cities grew from some small settlement close to sea or rivers (for easy acces to water/transportion) to these very large metropolises and every square meter of flat land turned into some kind of development. Regardles or oblivious to the fact that these flat areas where created by flooding and deposition of sediments by the river.
      If you make a city in a very narrow valley next to a small stream this will be prone to excess water flow every spring or fall, mudslides, rockfalls and other “unpredictable” events.
      Why do we not realize these facts, but only sit back and argue whose fault it is of this flooding event.

      Climate change, insufficient piping systems or…..?

      Maybe it is the uncomfortable fact that much of these problems are caused by bad planning and not seeing the obvious of no housing in these floodplains.

      There are some rules that we all have to abide by. Water runs downhill, hardrock areas, claysurfaces are impervius to infiltration and the more impervius surfaces you have within the catchment area the more water will flow faster and greater to the lowest part of the catchment (the gravitational law).

      I hope city planners and politicians are reading this. Wake up and be smarter about water managment.

    • When we talk about “flood Plain”, we have recognized that at different times of the year, some large volumes of water will be discharged into our rivers causing over topping of the normal river banks and water overflowing into these flood plains. Hence the advice that only activities of temporary nature should be allowed in these areas. This is made to result in minimal suffering, loss of lives and destruction to property in the event of flooding. However, driven by the ever increasing desire to “develop” any open space, we continue to see permanent structures coming up and settlements in these plains. This results in massive suffering of the settlers, loss of lives and destruction to property.

      It does not matter whether we know exactly when the flooding will occur. the altitude of short term gain must be dealt with firmly is we are to deal with flood disaster risks effectively.

      This calls for effective water management in our urban areas, proactive political thinking that short term gains will not be beneficial in the long run and effective regulatory institutions to ensure flood plains are cleared of “permanent development”

      Flooding itself is not a problem. We need the water. It is the related disaster risks which we must deal with effectivelt

  2. I would respond that allowing rivers and streams to operate using the morphological definition that Marcel described is infinitely (ok, maybe that’s a statistical exaggeration) better than what we have historically done, so this makes it a logical starting point. Obviously there is a great deal of uncertainty about what will happen in the future, but we can be reasonably certain about what has happened in the past. Why would we ignore this history and build in proven floodprone areas? The tendency to straighten, and defend (e.g. levees, walls, etc.) removes the inherent hydraulic resilience of the system through the reduction of storage, and the time of concentration. So not only would conserving the morphological floodplain reduce risk in the short term, but would provide greater resilience to the uncertainties associated with future development, climate change, etc.

    • Hi David,
      I am totally convinced that we should have adopted this approach 100 years ago. As well as averting untold amounts of risk we would have accumulated a wealth of natural capital created by high-functioning river systems. Unfortunately, it isn’t 100 years ago anymore. Flooding in Europe a year ago destroyed properties that were hundreds of years old. This year droughts are exposing previously concealed features never seen before. The past is no longer a reliable predictor of the future. The thing that we need to be paying attention to is no longer the extremes themselves but the first and second derivatives of the extremes. In other words, it is not just what our risk has been but also how quickly our risk factors are changing that needs to inform decision-making. There is little real uncertainty in the observation that risk factors have accumulated since floodplain developments first began. One aspect of the problem is that we have no good way of communicating that risk is a complex function of factors and that we currently experiencing a collision of land-use and climate change factors that are bound to result in great misfortune. How can we communicate that risk is increasing with a confidence that can compete with conventional wisdom that is anchored in a history full of manageable, rather than catastrophic, events?

  3. “History is a vast early warning system” Norman Cousins

    No one can predict the future, but we can make educated guesses based on what we know about the past. Reading the morphology of a river will go a long way to predict what it can do, and identifying floodplain is one of the easiest things to do.

    How we get messed up is using statistical methods employed by amateur hydrologists (typically water resource engineers) as the basis for predicting flood potential. The statistical distributions used for flood frequency analysis were designed for INTERPOLATION of physical phenomena unrelated to river flows, but most importantly the method is analytical, not predictive. It can only analyze data it is provided, which is typically not from the site being examined, and rarely includes actual flood events, so in effect the ‘model’ must predict a flood from non-flood data. Statistical models completely ignore the fact that flooding, for the most part, is cause by rain. This decoupling of the cause-effect cycle can, and usually does, grossly under-predict what a river can do. If the cause, ie large intensity widely distributed rainfall event covering the entire upper basin, is considered, then the existing floodplain looks quite reasonable.

    Modern humans have a very limited view of our environment. If we don’t have first-hand experience of a flood, then it looks extreme when it does happen even though the footprint of that event covers many previous footprints etched in the terrain. There’s a recent movement called ‘room for the river’ which gives the river the room it needs to operate. Even though it rarely needs its floodplain (‘rarely’ in human time scale) that floodplain is the river’s home. If we build our homes in the river’s home we will lose; water always wins. Always.

  4. I think Johan is hinting at the root of the problem so I’ll take a stab at shining some light on it:

    People in cities don’t just build houses in floodplains, they must be permitted to do so by local planning authorities: the Planning Department. Planning Departments don’t have in-house water resources engineers to delineate floodplains, rather they rely upon outside experts to do that for them, generally, consultants (I am one of those, BTW). Consultants don’t just conjure up criteria from thin air, they use regional or national standards for floodplain delineation techniques which are all based on probabilities derived from existing stream flow data. Those standards were created some time in the last century with the philosophy that a 1% flood is rare enough that it’s an acceptable risk.

    As I mentioned above, a 1% AEP flood presents a 63% risk to infrastructure designed for a 100 year service life, so which ever governmental authority established the 1% criteria is then (unknowingly) passing on that risk, through consultants, to local Planning Departments and finally to property owners. None of those groups, especially the property owner, has the knowledge to appropriately assess the risk, although the fact that they cannot get flood insurance should be a red flag. They happily live in the floodplain blissfully unaware of the risk and comforted by the (mistaken) knowledge that the “100 year flood” won’t happen in their lifetime.

    When the inevitable flood impacts occur, blame is placed on the flood being ‘extreme’ (“it was a 1:1000 year flood of biblical proportions, no one could ever have expected it” or “climate change”) and the recovery cost is borne by several layers of government or by the property owner. After that the consultants go back and use the one new data point to recalculate the 1% AEP flood, reissue the flood maps, the municipalities then buy back properties in the new flood area and the cycle starts anew.

    The problem started with us, the water resources community and the responsibility to correct the problem (as highlighted by the article we are commenting on) rests with us. WE created the incredibly misleading “100 year flood” concept, so we have to fix it. While it is true (statistically anyway) that a particular magnitude flow has a 1% probability of being equaled or exceeded in any given year, and while it is also true that 1% is also 1/100, it is an illogical and misleading leap to say that it the same as occurring once in 100 years. It MAY have happened once in the past 100 years (if you are so lucky to have 100 years of data) but no one can predict the future. THAT is the problem and it is so engrained in the system that it permeates through to land use planning being applied in deep seated ignorance.

    From my perspective, I would like to see the entire floodplain (morphological definition) identified as flood hazard area with a ban on development. Consider this: if a natural floodplain is inundated is it an economic disaster? No. If a floodplain with 3000 homes is inundated, is it an economic disaster? Yes. The solution is pretty obvious to anyone that knows anything about how floodplains work, and that is us, the water resources professionals. Let’s fix this.

  5. I am late to this discussion but have to agree with the general sense or tone that the Morphological definition of the floodplain should be a starting point.
    The future of our climate is for change, whether we are looking at geological change or otherwise.
    Even if we cannot agree to remove all the infrastructure that exists inside that floodplain we should at least agree to prevent all redevelopment and new development in these areas that we know will be flooded, we just don’t know when.
    The morphological floodplain is an ideal place for parkland, golf courses (not the clubhouse) and other non permanent infrastructure, possibly campgrounds if you have sufficient early warning systems that trailers can be moved out.
    One part of the permanent infrastructure that is very difficult to remove but is very subject to the floods is our water treatment facilities and sewage treatment facilities all on rivers. Both are typically located where the incoming load is most accessible. Next to the source for the water treatment facility and at the bottom of the hills for the wastewater.
    Early in the discussion it was asked “would anyone invest their entire life in something that had a 63% chance of failing?” and the presumed answer was not likely. The actual answer, based on the location of physical infrastructure across Canada, is of course they would if the presumed reward was high enough and they were convinced that someone was around to cover their losses. In Canada’s case until recently losses were covered by government because insurance was not considered to be available but now insurance is available and risks will be identified (and costed). The new question is will that make a difference?
    We can only hope so and use all of the persuasive powers of our professions to direct a safer outcome for the future(s).

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