A closer look at the CVFPP (2): Consequence modeling brings out uncertainty in risk estimates

A closer look at consequence estimation.

Learning more about how the Dutch assess flood risk and evaluate costs and benefits over the past eight months has  taught me lot about estimating loss-of-life and other consequences. Doing so from outside of the United States has allowed me to look more critically at how we perform them and how we might improve our own methods.

Part 2 of looking more closely at the CVFPP concerns Appendix 8G- Life Risk Analysis and Appendix 8F- Flood Damage Analysis. A first-round review suggests that the risk analysis methods used in the CVFPP may have underestimated the true risk facing the Central Valley (including life loss and other damages).


Section 3.3 of the CVFPP Life Risk Analysis explores various approaches to estimating life safety (including the Dutch approach), and in Section 3.4, the CVFPP proposes to use its’ own new method. A new approach should be peer-reviewed, however, especially one that my serve are the basis for critical decision-making and the investment of billions of dollars of state and federal funds.

On pages 3-27 and 3-28, DWR acknowledges limits to the study, but the following points highlight additional uncertainties in the analysis and could have effects on the real loss of life and damages experienced during a flood.

  • Inputs to the DWR Life Risk method are based on 2000 Census data and don’t account for new development behind levees since 2000, leading to an underestimation of the potential loss of life during a flood (and those that would require emergency services).  DWR’s model requires input data on a person-per-structure-level of detail and the 2010 Census data is not available at this level yet. However, conversations with US Army Corps personnel indicate that it is possible to account for the growth and still maintain the desired fine-grained data (FEMA has a method to account for it in HAZ-US as well) despite DWRs’ reluctance to do so.
  • The evacuation efficiency used is questionable for the following reasons
  • The warning times used for various impact areas may be optimistic as compared to other studies.[1]
  • One report cites,

“Furthermore, Sacramento residents have little or no experience with evacuations, and thus it is difficult to predict the fraction of at-risk populations who would choose to evacuate with much certainty.” [2]

  • Though DWR justifies this by assuming no levee breach, and therefore a 21 hour warning time and flooding only from the Sacramento River as opposed to the American(page 3-18), the point is that another scenario with 0 hours of warning time (as in the Comprehensive study) could produce a significantly different results.  This analysis therefore fails to capture uncertainty.
    • The model assumes a 100% willingness to evacuate but a recent study[3] showed 17% of residents of one Central Valley neighborhood would not evacuate if advised.
    •  Media reports during floods often mention individuals try to “wait it out” and have to be rescued in the end.  Consider the National Public Radio Broadcast from the 2011 floods:

“A thousand residents of Poplar Bluff, Missouri, were told to evacuate yesterday, when floodwaters started topping the levees there. Butler County Sheriff Mark Dobbs says one of the levees breached today, but despite repeated warnings, some residents did not evacuate and had to be rescued by boat.”  (NPR 2011)[4]

  • The model assumes a single mortality rate for an entire impact area despite the reality that mortality rates for areas adjacent levee breaches are higher than the rates in the rest of the floodplain. This is due to the fast moving and fast rising rate of water which can sweep people off their feet and cause buildings to collapse. [5]
  • Discrepancies between CVFPP and other Life Loss Analyses: 1) CVFPP Life Risk Analysis for Natomas Basin, 2) a 2005 “Urban Flood Scenario”[2] by SAFECA/David Ford Consultants, and a 3) recent assessment by Jonkman et al. in 20121 suggest  uncertainty in the Life Risk Assessment Method and the potential for DWR to have overlook critical factors. For example, the DWR Natomas impact area (SAC 36) assessment estimates a 2.5 Annualized Life Risk (which would be about 250 fatalities in a 100 year flood), while the Jonkman study suggests a significantly higher range around 500-1000 fatalities in a 100-year flood.

The Flood Damage Analysis does not consider nor communicate “societal risk.”  It  does not consider nor discuss the full range of consequences seen in large flood events like Katrina,[6] or used in risk assessment methods by other countries like the Netherlands[7]including:

    • ASCE (2007): Mass Migration from New Orleans

      Massive job loss

    • Migration of hundreds of thousands of people to other parts of the country and subsequent loss of culture
    • Short and long terms health effects due to contaminated waters
    • Analysis does not provide adequate discussion on availability of emergency shelters
    • Analysis does not appear to consider time/costs for pumping dry and rehabilitating areas/levees such as was done in the ARkStorm Report by the USGS[8].
    • It is important to note that though emergency response can be an effective tool, more Katrina fatalities occurred during evacuation than due to flood exposure.[9]

In summary, the Flood Damage and Life Risk analyses do not capture nor communicate a full societal Risk.  Annualized deaths and damages do not convey the same message as large numbers and losses from a single event.

Why is this all important?  It is important to accurately communicate the threat and potential consequences of a large flood to decision makers (CVFPB, land use planners, local governments) and to the public at large so they can make informed decisions about how best to prepare, prevent, and reduce risk. While uncertainties will always exist (which is the reason the field of risk management developed), efforts could be made to reduce uncertainty and consequently reduce risk by having more accurate information displayed using more comprehensive analyses with which to make decisions. Additionally, if DWR intends to use these analyses/methods to choose between alternatives in the future, limitations of the methods, and not accounting for uncertainties could bias certain risk reduction measures over others.

[1] Jonkman, SN. Hiel, L., Bea, R., Foster, H., Tsioulou, A., Arroyo, P., Stallard, T., and Lyndsie Harris (2012) Integrated Risk Assessment for the Natomas Basin (CA):Analysis of Loss of Life and Emergency Management for Floods.  Natural Hazards Review doi:http://dx.doi.org/10.1061/(ASCE)NH.1527-6996.0000079

[2] David Ford Consultants (2005). Urban Flood Scenario: Sacramento Area Levee Breach, Department of Water Resources http://www.ford-consulting.com/portfolio.php

[3] Ludy, J. & Kondolf, GM (2012). Flood risk perception in lands “protected” by 100-year levees. Natural Hazards. 61:(2), 829-842 DOI: 10.1007/s11069-011-0072-6

[4] Schaper, David (April 26, 2011). Severe Weather Wreaks Havoc in Midwest, South. National Public Radio  http://www.npr.org/2011/04/26/135745211/severe-weather-wreaks-havoc-in-midwest-south

[5] Rijkswaterstaat (prepared by HKV). (2004). Standard Method Damage and Casualties Caused by Flooding. DWW-2005-009.

[6] American Society of Civil Engineers (2007). The New Orleans Hurricane Protection System: What went wrong and why? http://www.asce.org/uploadedfiles/publications/asce_news/2009/04_april/erpreport.pdf

[7] Vrijling, H 2001. Probabilistic design of water defenses in the Netherlands. Reliability Engineering and System Safety 74 :337-344

[8] U.S. Geological Survey/MultiHazards Demonstration Project.  An ARkSTorm Scenario. http://pubs.usgs.gov/of/2010/1312/of2010-1312_text.pdf  

[9]Boyd, Ezra. 2011.  Fatalities Due to Hurricane Katrina’s Impacts in Louisiana.  Dissertation, University of New Orleans http://etd.lsu.edu/docs/available/etd-06092011-084046/

Posted in Flood Risk Management, Flood Risk Perception, Loss of Life | Tagged , , , , | 1 Comment

A closer look at the CVFPP (1): does increasing “protection” increase risk?

Why it benefits to look closer:

Over the past couple of months, I’ve spent considerable time reviewing the Central Valley Flood Protection Plan and some of its technical appendices.  The first Draft of the CVFPP is indeed a remarkable piece of work, especially given time and budget constraints, and DWR should be applauded. It is the first attempt of the State of California to address flood management on such a large, system wide scale… and further, it is the first of any U.S. effort to do the same since the Mississippi River Commission created the MR &T in the late 1920s.

Upon first read, the plan and its underlying analyses appear fairly straightforward and the State Systemwide Investment Approach appears to reduce risk with a portfolio that mostly focuses on levees, but considers limited bypass expansion as well. However, looking deeper into the details (what, where, and what metrics/data were used), however, revealed some concerns with regard to public safety.

Part I.

Increasing “protection” to urban levels may actually increase risk.

For some context of why the finding is important, I’ve pasted below a letter I submitted  last week with UC-Berkeley Professor Matt Kondolf to the Central Valley Flood Protection Board.

April 20, 2012

Dear Mr. Edgar and members of the Central Valley Flood Protection Board,

Thank you for the opportunity to submit comments on the Draft of the Central Valley Flood Protection Plan. We applaud the progressive step by the state of California and DWR to begin to address California’s flood risk in a comprehensive and systematic way. While there are many positive features of the plan, our brief comments today draw attention to a serious concern of ours:

Increasing flood protection in urban areas to a 200-year standard without additional measures or limits to growth for urban areas will likely induce urbanization and increase flood risk to life, property, and to the State of California.

Meanwhile, the CVFPP Life Risk Analysis incorrectly indicates the opposite—that the SSIA will reduce risk.

Where new dense development would otherwise be prohibited, Figures 3-1 (below) and 3-2 of the Plan show vast areas of land (in green) that will be ‘protected’ by 200-year urban levees and subsequently open for dense development under the State Systemwide Investment Approach. This means there will be thousands more inhabitants in places like Yuba City and Marysville on lands that are still vulnerable to floods exceeding a 200-year event.


The risk of being flooded by floods larger than the ‘design flood’ is known as “residual risk.”  The likelihood of being flooded even if ‘protected’ by 200-year levees is still remarkably high: Over a 30-year period (the typical length of a mortgage), there is a 14% chance of being flooded. Over 100 years, a 39% chance. These are the probabilities of being flooded only from larger floods overtopping a 200-year levee. The risks are actually much higher because the levees could fail from shaking in earthquakes or other failure mechanisms.

It is commonly observed that,

“There are two types of levees: those that have failed and those that will fail” -(William Hammond Hall)

Levees only reduce the probability of a flood in a given year. Thus, it is important to anticipate and plan for the eventual failure of the levees. The flooding that results from levee failure is characterized by a sudden rushing wall of water, provides little warning, and conveys sufficient force to pull buildings from foundations, sweep people off their feet, and damage critical infrastructure.

While Hurricane Katrina raised public awareness of flood risks, the images from the flood may make Californians underestimate the impact of flooding here. In New Orleans, we saw images of men calmly wading through flooded streets, pushing rafts, which they could do because water temperatures were about 85 deg F. When the levees fail in the Sacramento Valley, water temperatures will be around 50 deg F. Exposure to such cold water will bring swift hypothermia and death.

Our review of the CVFPP Attachment 8G- Life Risk Analysis

indicates that DWR may have drawn misleading, convoluted conclusions that indicate the State Systemwide Investment Approach (the Plan)  reduces risk when infact, this strategy actually increases risk. Here is how:

 Risk= Probability of flooding X Consequences of flooding

The Life Risk Assessment uses data from the 2000 Census despite the significant build out that has occurred (or will occur) in flood-prone communities of Central Valley.  Urban Levee in figures 3-1 (above) and 3-2 of the CVFPP, however, are likely to induce growth.

The data presented therefore underestimates the risk by underestimating:

  1.  the number of people exposed under any of the CVFPP approaches,
  2.  the number of people who may not survive a flood,
  3.  the number of people who would demand emergency services and shelter during a flood,    and
  4.  property and infrastructure damages that would occur.

As best we can tell, the analysis and expected 49.8% benefits from reducing life-loss (figure 2-1 in Life Risk Analysis) are misconstrued because they consider only that the probability of flooding has decreased (by increasing “protection level” to 200 years) without considering that the consequences of flooding have increased.

The population has grown since 2000, and likely with a 200-year levee, the population will continue to grow. Rerunning the calculations/model with more accurate exposure numbers could, therefore, result in fewer life-loss reduction benefits.

For a specific example: In Table 4-1 of the Life Risk Analysis, does Yuba City’s (SAC25) Life Risk Reduction from 8.2 to 2.4 include the future growth behind levees in the region? Or does it simply reduce the probability of flooding/failure, in the risk equation while not accounting for the increase in population (exposure and consequence)? If the latter is the case, it is not only inaccurate, but unethical to conclude that the SSIA is reducing the risk to loss of life.

Why it matters: communities are exposed to involuntary risk.  Increasing risk behind levees is of even greater concern because ‘protected’ communities are not aware of the risk. In 2009, we surveyed residents in an affluent, levee-protected neighborhood in Stockton to assess residents’ awareness of flood risk. We found out that residents did not understand their flood risk, had been informed that they were “not in a floodplain” – despite the fact that their houses were built below sea level (Ludy & Kondolf 2012). This study is significant because it shows that even an affluent, highly educated population with professional jobs did not understand that they were still vulnerable to severe and likely fatal flooding, and the residents were consequently unprepared for floods.

Given that people can only take measures to reduce their risk if they are aware of that risk, exposing them to this risk involuntarily means they cannot make the decision to avoid or reduce their risk. This is unethical and increases the state’s liability and will further strain state resources like those needed for emergency response and recovery.

Increased protection will only reduce risk with additional measures that limit growth and consequences.  We recognize the extreme challenge the state has in allowing communities to grow while protecting people and property. We therefore acknowledge the States’ interest in increasing the levels of protection in urban areas.

Increasing protection in urban areas, however, only generates the positive risk reduction benefits reported above where existing dense urban areas achieve a greater level of protection—or, where an existing urban area upgrades from a 100-year level of protection to a 200-year level of protection.

Increasing protection levels actually increases risk where it induces urbanization and increases the valuable life and property exposed in areas that would otherwise remain undeveloped. That levees induce urban development where it was formerly discouraged by nuisance flooding is well documented. The effect is accepted by scientists and policy analysts, going back to the pioneering work of Gilbert White (1945), and as illustrated in figure 1 below.

We therefore support increased flood protection only with additional measures that limit growth and reduce risk such as:

  • Conservation easements to prevent further urbanization of undeveloped floodplains
  • Mandatory flood insurance with risk-based premiums to reduce financial liability for damages
  • Building codes with flood resistant materials to minimize damage when a levee overtops/fails and vertical evacuations to allow people to escape fast-rising water

Sincerely Yours,

G. Mathias Kondolf, Professor and Chair, Department of Landscape Architecture and Environmental Planning,  University of California, Berkeley

Jessica Ludy, Fulbright Fellow and Lecturer, Delft University of Technology, the Netherlands 

Figure 1 (Ludy 2009). The perverse incentive of levees. Levee construction eliminates the frequent floods that reminded people the floodplain floods. The perception of protection against flooding induces new development in the floodplain, so that when the levee inevitably fails or is overtopped, there are far greater damages than would have been the case without the levee and its induced urbanization.


ASFPM (Association of State Floodplain Managers). 2005. Hurricane Katrina & Rita: Using Mitigation to Rebuild a Safer Gulf Coast.

Ludy, J. and G.M. Kondolf. 2012. Flood risk perception in lands ‘protected’ by 100-year levees. NaturalHazards 61(2):829-842. DOI: 10.1007/s11069-011-0072-6

White, G.F., 1945. Human Adjustment to Floods. Department of Geography Research Paper no. 29.Chicago: The University of Chicago.

Posted in Flood Risk Management, Flood Risk Perception, Loss of Life, Uncategorized | Tagged , , , , , , | 1 Comment

Lowdown on the high water…and room for rivers explained

So high the water was risin’ our men sinkin’ down
Man, the water was risin’ at places all around,
boy, they’s all around
It was fifty men and children come to sink and drown

—High Water Everywhere (Part 2), Charlie Patton

A great many blues musicians sang about high water and an old professor of mine once told me there are three things you can do with it:

1- store it    2-convey it to the sea     3- let it flood

Olifants River in Flood. Source: Flickr. Author “Albrigi”

Simple enough.  For years, we’ve mostly managed high waters through options 1 and 2, ignoring 3, with concrete and earthen dams and levees to keep the water away. Championed by engineers like Humphreys on the Mississippi in the mid 1850s, advanced engineering and technologies have “tamed the river,” reducing the frequency of smaller floods (as well as our memories of them) and allowed for more economically productive uses of floodplains. As a result, communities have thrived for centuries on low-lands thanks to deepened channels which allowed for navigation and trade, flat fertile soil for agriculture, residential and commercial development, and beautiful waterways as a scenic place to recreate.

FISK Map 1944, Historical Courses of the Mississippi River.

Over time, this way of management restricted the ability of a river to maintain it’s preference–option 3 above: to spread out on to a floodplain every once in a while when waters are greater than the channel capacity.

The Delta Blues remind us that such is the desired course of rivers where floodwaters use the floodplain– irrespective of  human settlement on it.  Consider  also the numerous historical courses of the Mississippi River.

Mark Twain also understood this well.

“The Mississippi River will always have its own way; no engineering skill can persuade it to do otherwise…”

As did William Hammond Hall,

“It should be fully understood then, that floods will occasionally come which must be allowed to spread.”

Giving room in the past

Humans know that we have repurposed floodplains and placed valuable assets in a rivers’ path.  We are no stranger to the fact that rivers need room in order to protect our cities, farms, industry, and way of life. We remember the Great Floods of 1927 and the multiple breaches on the Missouri and Mississippi systems during 2011.

1927 Mississippi Flood, Hamburg Louisiana. Source: American Environmental Photographs Collection, Department of Special Collections, University of Chicago Library/File

As early as 1918 in the Sacramento Valley,  and in 1928 on the Mississippi River (after 700,000 were displaced), Americans sought to make space for the river. Evacuating of 250,000 Netherlanders in the Rhine basin in 1995 inspired the Dutch to create a national program to let water back into a landscape they had spent centuries keeping dry. Most recently in the San Joaquin Valley after 1997, farmers, engineers, hydrologists, and politicians acknowledged that traditional storage and conveyance alternatives were not alone enough and started discussing a plan. (See: Yolo Bypass, Mississippi River & Trib, Room for the River, and Lower San Joaquin Bypass referenced in 1998 Preface to Kelley’s Battling the Inland Sea.)

Giving room today

Making room for rivers was just the subject of a recent Scientific American article, as well as one in the Economist. Expanding floodways made a grand entrance to the first draft of California’s Central Valley Flood Protection Plan (table 3-2) as a viable way to meet multiple objective, and  to a lesser extent, “Room for Rivers” is also considered in the 5th draft of the Delta Plan. Lastly, in 2008, California Legislation from SB 5 (California Water Code Sec 9163 (c) ) required the state consider the feasibility of a bypass on the Lower San Joaquin.

While the concept and buzz word of “Room for Rivers” might be understood by some, what follows is a brief explanation for those who aren’t quite so hydraulically inclined (great blog by the way), and some detail for those who want to know the practicalities.

  1. What does “room for rivers” mean?
  2. How do you give a river room? (and where is it appropriate to do so?)
  3. What are the benefits? 
  4. Example Projects 
  5. Challenges?
  6. Areas of further research

The explanations and project examples below demonstrate that giving room to rivers actually incorporates all three options above–storing high water, conveying high water, and allowing high water to overflow the banks from time to time.

Presented factually, this post does not advocate for or argue against certain projects– the attempt is to provide information and opportunity for a discussion. There are some additional resources provided at the bottom and please comment if you have questions.

  Continue reading

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Public Involvement in a Better Plan: Letter to the Central Valley Flood Protection Board

For better or worse, in the United States and in Calfornia especially, we have a strong element of public involvement in most planning processes.  Better because it forces transparency, allows voices to be heard, and as Americans we really can have some influence in shaping our future.  Worse because… all of those voices have different perspectives and priorities (often conflicting)– and as a result, the process can take ages.

No where has this been more prevalent over the last 30 years than in California, amidst the struggle to balance competing needs with constrained budgets in the Central Valley, the Delta, and with respect to any topic related to water.  Todays’ post briefly discusses the most recent plan in the headlines and how one group has united behind common goals and interests to improve a policy that will eventually affect multiple parties including all California taxpayers.

The Central Valley Flood Protection Plan.

California has embarked upon large scale system-wide flood risk reduction projects and policies with California Department of Water Resources‘s recently released Draft of the Central Valley Flood Protection Plan. The Central Valley Flood Protection Board must adopt (or reject) the Central Valley Flood Protection Plan (CVFPP) by June of this year  and the public has again an opportunity to play a large role. The Flood Protection Board will both receive recommendations on the plan and will make suggestions to the Department of Water Resources to amend the plan.


The original flood management system in the Central Valley was built over a hundred years ago, much of it created property by property, when at that time it protected mostly agriculture. (For a good history, read Battling the Inland Sea, Kelley 1998). Over the years however, much of the infrastructure has been degraded, levees have failed, and consequences of failure have changed. Today, the Central Valley’s flood management system protects dense urban areas including those of political importance like the capitol city of Sacramento–also critical infrastructure, unique ecosystems, and a valuable fresh water supply.  The state of California recognized the need to improve the system in 2008 and passed legislation (SB 5) to create a system-wide plan (the CVFPP) to address deficiencies in the Central Valley Flood Management system where failure could mean billions in damages for all Californian taxpayers.

To learn more about the plan and process, click here.

The overarching goal of the CVFPP is to “Improve Flood Risk Management.”

Supporting goals are to improve operations and maintenance, promote ecosystem function, improve institutional support, promote multi-benefit projects (ecosystem, water supply, etc).  The billion (5 billion in fact) dollar question lies in how?

How to manage competing interests?

Public Safety and Ecosystem Function (and agriculture and water supply) are not necessarily in conflict, despite that it has appeared that way in the past. A robust flood management approach can reduce flood risk and enhance ecosystems by incorporating both technical and non-technical measures.  Examples of successful projects can be seen in California where the Yolo Bypass has protected Sacramento from flooding since 1933, and in the Netherlands where the burgeoning Ruimte voor de Rivier programme promises to reduce water surface elevations and improve “spatial quality” on a system wide basis.  Other areas in Europe are following suit (Adaptive Land use for Flood Alleviation project).

Weir gates at Hondsbroeksche Pleij flood bypass, January 2012: the first completed Room or River project in the NL . Image credit: Almar Joling

The Department of Water Resources has made a good start in this direction as the first draft of the Plan is not limited to a levees-only approach, and it tries to appease the sum rather than the parts. But I suspect that as public comments begin to arrive to the Flood Board, we will likely see requests  to protect individual interests–requests that miss out on the sense of collective good– something that is really needed if California wants to reduce risk on a system-wide basis as opposed to piecemeal.

I came across this  letter (below)  and I’ve pasted it as an example of how 16 conservation local and national conservation  groups collaborated to provide the Flood Protection Board with clear, implementable recommendations for the Plan to reduce risk and achieve the plan’s stated goals. For full text of the letter, click here:  Conservation Community Letter on Flood Plan 2-15-12  The Board will have a public hearing on February 24 in Sacramento.

The conservation groups identify six areas for improving the plan that integrate instead of separate multiple interests:

  1. Maximize the use of cost-effective and multi-benefit flood management tools such as flood bypasses, setback levees, and transitory storage on floodplains.Fremont Weir (image credit: Katie Jagt)
  2. Specify an overarching strategy with measurable objectives for incorporating ecosystem function.
  3. Clearly state how the flood plan will be integrated with related state and federal restoration efforts within the state flood control planning area.
  1. Develop a more explicit climate change adaptation strategy to minimize projected impacts on flood risk, ecosystems, and water supply reliability.
  2. Explicitly integrate and balance flood management and water supply objectives.
  3. Provide specific guidance to enable local planning.

Signed by: American RiversCalifornia Trout,  National Wildlife Federation California Waterfowl Association,  River Partners,  Environmental Defense FundNatural Resources Defense Council,  Sacramento River Preservation Trust , TheNatureConservancy FriendsoftheRiver ,  PlanningandConservationLeague,  The Bay Institute,  Trout Unlimited,  Tuolumne River Trust PRBO ConservationScience DefendersofWildlife 

What do you think?

(See below for no-explanation-needed-image of “Why floodplains are good for fish”). This  is a fun photo and one that tends to resurface time and again)

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