The World Bank recently published the 177-page Urban Flood Risk Handbook: Assessing and Identifying Interventions by Scott Ferguson, Mathijs van Ledden, Steven Rubinyi, Ana Campos, and Tess Doeffinger.
The book is primarily targeted to flood-project managers and city officials. However, the informative illustrations and easy-to-understand writing make it a useful primer for anyone who wants to become more knowledgeable about flooding.
The World Bank bills the handbook as a “roadmap for conducting an urban flood risk assessment in any city in the world.”
What will the average person get from it? The ability to understand flood professionals, the issues they face, and how they evaluate problems/solutions.
It also made me realize that Harris County’s equity framework, which the county uses to allocate flood-mitigation funds, fails to incorporate many factors recommended by the Department of Homeland Security and others.
Outline of Handbook
The book contains five chapters.
Chapters cover the following sub-topics.
Intro and Chapter 1: Defining Risk
Much of the book has a checklist quality to it. For instance, what should professionals quantify when defining flood risk?
- Hazards and their probabilities
- Exposure – an inventory of elements and activities affected by hazards, for instance:
- The population and its assets such as homes and belongings, private businesses, and industrial assets
- Infrastructure such as roads, drinking water, sanitation, drainage, and flood protection infrastructure
- Public infrastructure like health care and school facilities
- Environmental and cultural assets
- Economic activities.
- Vulnerability – the degree to which people, property and infrastructure can be adversely affected
Here, for instance, I was surprised to learn that Harris County’s equity framework considered some, but not all of the factors that make areas vulnerable. For instance, it doesn’t really incorporate infrastructure!
Chapter Two: Hazard Assessment
The chapter on Hazard Assessment begins with a list of “boundary conditions” typically used within a flood model.
Five conditions typically include: (1) rainfall, (2) infiltration, (3) flow, (4) water levels and waves, and (5) pumps and flow control structures.
But just measuring rainfall can be difficult depending on the number and types of gages in use. “Most meteorological services only provide daily values (mostly from manual gauges), which provide part of the overall rainfall characteristics. However, the distribution and intensity within a single day are essential,” say the authors.
Automated gages that can measure short (sub-daily) periods are critical in informing regulations that affect the design of infrastructure. Short, high-intensity bursts are a significant factor in urban flooding. However, many areas have gages measured manually once per day. (The Flood Control District’s are all automated.)
As I scanned the text of this chapter, flashbacks from Harvey kept recurring. When I read this sentence, I thought of the Lake Conroe discharge designed to prevent the flooding of Lake Conroe homes. “Discharge rates from control gates normally relate to both upstream and downstream water levels…” [Emphasis added.] Did the dam operators consider downstream water levels at the time? The case is still pending in the courts.
In chapter 2, I also found the most illuminating discussion of differences between 1D, 2D and 3D modeling that I have ever read.
Ditto for a description of Manning’s Coefficient, which is used to estimate the impact of friction on the speed of water, which in turn affects the rate of accumulation and flood height.
Trees and buildings create friction for floodwater that slows it down. But concrete and clear-cutting speed it up.
Such information is used to quantify the potential consequences of flooding to homes, businesses, and people across an area.
Chapter 3: Risk Assessment
Chapter 3 focuses on Risk Assessment and lists types of infrastructure that the Department of Homeland Security defines as “critical sectors.”
Again, Harris County’s Equity Framework (used to distribute flood-mitigation funds) considers none of these.
Chapter 4: Interventions
Chapter 4 discusses types of interventions (solutions to flooding). They fall into three main categories.
The chapter then addresses the limitations, applicability, costs, benefits, and environmental/social impacts of each.
It’s too complicated to summarize here. But it’s fascinating to see how flood professionals evaluate the range of options.
The last chapter addresses the business side of flood mitigation, i.e., bidding jobs. I’ll skip that here and close by saying this. Even after studying flooding for six years, I felt lights turning on almost every time I turned a page in this book.
I had a high level understanding of most of the concepts. But this gave me a much fuller understanding. I found myself constantly thinking, “Aha, so that’s how that fits in.”
Kudos to the World Bank and the authors. They have provided a valuable addition to the literature.
Posted by Bob Rehak on 9/24/23
2217 Days since Hurricane Harvey
Used with permission of World Bank