CSU, Long Beach -- Geography 558 -- Hazards and Risk Management

Floods

Flooding has been one of the most studied of natural hazards for a long time, and it was the target of the first social science studies of hazards (Gilbert White's work in the 1930s). Many of the themes in hazards research were first developed in connection with floods, such as:

Magnitude and frequency relationships
Human exacerbation of vulnerability to large-scale disasters through mitigation against smaller-scale, more frequently recurrent hazards

Definition: physically, flooding occurs when water reaches and tops some height above a given benchmark point, posing a threat to life or property caused by rising or spilling water

Types:

Riverine: involves flooding from a customary channel
- Slow: produced by rainfall or snowmelt in the watershed
- Flash floods: sudden results of intense thunderstorms
- Riverine floods are the most common and produce the most damage worldwide
- This lecture will concentrate on riverine floods, but I'll summarize some other kinds of floods first.
Estuarine: involves unusual rise of water in a coastal bay due to a combination of two or more factors
- Storm surges as water is "bunched up" by storms' (especially tropical cyclones') winds and even by a slight bulge in the water surface due to the storm's extremely low pressure
- High daily or monthly astronomical tides
- Storm surge combining with astronomical tides results in storm tides funnelling up an estuary
- Riverine flooding entering a bay
- This is the kind of flood that the Thames Barrier was designed to mitigate for London
Coastal: severe sea storms or tsunamis
- Commonly hurricanes or other massive storms at sea create salt-water flooding on the coast. This is often exacerbated by riverine flooding produced by the torrential rainfall such storms produce onshore.
- Tsunami, or seismic shock waves (once called "tidal waves," but they are not generated by tides)
  - These are caused by large earthquakes with strong vertical components to their motion (normal, reverse, or thrust faults), which causes vertical displacement of sea water. As a whole column of water drops (from seafloor dropping or from seafloor rising and pushing up the sea, which then drops), this sets off very fast moving waves (they can move 700 km/hr). These fast-moving waves are forced to slow down as they shoal, especially if the shallowing of sea water is gradual (as on a broad and shallow continental shelf offshore). This slowing causes an increase in their amplitude when they strike the shore (we saw this with earthquake waves: as waves slow, they continue to convey their energy by increasing amplitude).
    - This is what happened in the Boxing Day tsunami of 2004 after the 9.1 Sumatra earthquake in the Sumatra Subduction Zone
    - Here is an animation of the event and how the tsunami was generated and changed shape as it approached shore (needs the QuickTime viewer): http://www.tectonics.caltech.edu/movies/outreach/sumatra/subduction.mov.qt
    - An informative site about the Sumatra quake and tsunami at Caltech: http://www.tectonics.caltech.edu/outreach/highlights/sumatra/what.html
    - The Great Tōhoku Earthquake of 2011 was another tsunamigenic earthquake, a 9.0 Mw earthquake on a destructive plate boundary (the Pacific Ocean plate is pushing under the North American plate [yes, you read that right] in northern Japan). The New York Times had a good summary of the tectonic setting of the quake and nice graphics of the plates involved and an isochron map of the ensuing tsunami at http://www.nytimes.com/interactive/2011/03/11/world/asia/maps-of- earthquake-and-tsunami-damage-in-japan.html.
- Tsunami can also be caused by large submarine landslides, again, a situation that can cause marked vertical displacement of a column of water.
  - Such tsunami offer virtually no warning, as happened in Papua New Guinea in 1998. A submarine landslide raised 15 m waves that inundated about a 20 km stretch of coast and killed 2,200 people.
  - California is at risk to such events, because we have some major submarine canyons, such as Monterey Canyon, La Jolla Canyon, and Scripps Canyon.
  - I have lecture notes on tsunami, which you're welcome to browse if you're curious about this hazard.
Miscellaneous catastrophic events causing flooding:
- Dam failures caused by:
  - Earthquake (the lower Van Norman Reservoir dam nearly failed due to the Sylmar earthquake in 1971, causing evacuation of a large segment of the San Fernando Valley until the water behind the dam was released and the facility closed). This was a near-miss actually, since the Lower Van Norman was only half full when the quake struck and knocked over the upper 30 feet of the dam, leaving it only 6 feet taller than the water it impounded! Here is a commemorative article about it: http://latimesblogs.latimes.com/lanow/2012/02/sylmar-earthquake-anniversary-dam-almost-collapse.html.
  - Extreme rainfall/snowmelt (a horrific series of dam failures in the Huang He or Yellow River watershed in China took place in 1975, when two extraordinarily strong rainstorms, back to back, caused the sequential collapse of 62 dams, starting with the huge Shimantan and Banquiao dams, killing some 85,000 - 170,000 people (depending on how you define flood-related deaths) and destroying about 1 million hectares of farmland). Here is a link to material on this staggering dam-related flood catastrophe: http://www.internationalrivers.org/resources/the-forgotten-legacy-of-the-banqiao-dam-collapse-7821.
  - Mechanical and design flaws (e.g., the 1928 collapse of the St. Francis Dam in San Francisquito Canyon, a tributary of the Santa Clarita River in Los Angeles County, the day after William Mulholland visited it and pooh-pooed concerns about cracking. The resulting 25 m wall of water killed more than 500 people that night -- this was a classic "disaster by management"). Here is a riveting photojournalistic essay on the disaster: http://framework.latimes.com/2013/03/12/st-francis-dam- collapse/.
  - Landslides into the reservoirs behind a dam (sometimes because of quakes -- example Italy in 1963, when a large quake caused a landslide into the Vaiont Reservoir, causing 100 m tall waves in the reservoir, which overtopped the dam (which, somehow, remained intact) and the flood downstream killed 2,500 people). The dam is still standing, used at a much reduced capacity for hydroelectric power. Here is a description of the engineering and geological processes involved: http://blogs.agu.org/landslideblog/2008/12/11/the-vaiont- vajont-landslide-of-1963/.
- Volcanic eruptions melting huge amounts of snow and ice, as happened most recently in Iceland, when the volcano that disrupted air travel in Europe also created flash floods from the melting of the Eyjafjallajökull glacier, forcing hundreds of people to evacuate. Wikipedia has a nice summary: http://en.wikipedia.org/wiki/2010_eruptions_of_Eyjafjallaj%C3%B6kull.

Physical dynamics of riverine flooding

These affect the floodplain, a strip of relatively smooth land bordering a stream, built of sediment carried by the stream (alluvium), and overflowed regularly in times of high water

Water spillage in excess of channel capacity creates both flooding and the landforms associated with flooding: the floodplain and the levées
- Levées are built up areas that accumulate along the riverbanks
- Water's capacity to carry its suspended load and move its bed load is a function of its velocity (C = V^3-4) -- the exponential nature of this function means that small accelerations and decelerations translate into very large changes in carrying capacity
- Once the river tops its banks during a flood, however, the water flow across the floodplain slows sharply, causing the water to deposit its load of sediment there at the banks and, to a lesser extent, out on the floodplain
- The result is a buildup in sediments on the banks, which is called the levée
- Alluvial streams are unstable through time, in addition to their usually annual overtopping of their banks, which eventually forces them to change their courses
  - Because of natural levée buildup due to overbank topping, progressively more sediment gets trapped in the channel bed
  - This means that the level of the river starts to rise above the surrounding floodplain
  - This is an energetically unstable situation
  - A particularly high water episode will, therefore, cause the stream to abandon such a channel and settle in another, lower part of the floodplain.
  - Rivers change course constantly through this mechanism, if left to their own devices
Another source of locational instability in river systems is that such alluvial streams tend to meander, rather than flow in a straight line.
- The meanders cause lateral cutting by the fastest flow in the stream (the "thalweg") being slammed into the outside bends.
- Eventually, the outside bends cut through to one another, leading to another kind of stream change of course and the construction of oxbow lakes, swamps, and bayous.
- Here is a link to one of the plates in the famous Harold Fisk maps of the Mississippi River's meanders and course changes through the last thousand years or so: http://www.adammandelman.net/wp-content/uploads/2012/03/Fisk-plate- 22-1944.jpg
Another kind of locational shift occurs when the entire system of meanders tends to drift downstream in geological time: meander drift
- This is because the entire system is acted on by gravity as a system
- Outside bends facing downstream get a little more cutting activity than outside bends facing upstream, so the meander itself moves downstream
The takeaway message, then, is that rivers are very unstable through time, and these instabilities pose flood hazards. Given the mechanisms involved, such technological solutions as dams and levées must be understood as essentially temporary solutions ... and solutions with an inevitable cap on their effectiveness (more on this in the social dynamics section)

Riverine flooding occurs in the context of the drainage basin or catchment or watershed (different names for roughly the same concept)

Water in watersheds flows overland, underground, and via channels
Watersheds, then, contain channels, slopes, aquifers, and springs, bounded by drainage divides, which express and contain these flows

A drainage divide is any raised elevation (mountain ridge, hill crest, even the gentlest "interfluve" or raised area between streams) that separates the flow of rain or snowmelt by gravity into different watersheds
Watersheds can be of any size (e.g., Boulder Creek versus the Mississippi River)
They are a nested concept (Boulder Creek is part of the St. Vrain Creek drainage, which is part of the Platte River drainage, which is part of the Missouri River drainage, which is part of the Mississippi River drainage basin)
Fluvial geomorphologists and hydrologists often express this nested hierarchy in terms of "stream order," where a first order stream might be the small trickle or brook seeping out from a spring, a second order stream collects the waters of two or more first order streams or tributaries, a third order stream collects the waters of at least two second order streams, and so on down to the mouth of the whole stream system pouring out into an ocean, lake, or, sometimes, a playa or interior basin (like the Tulare Basin in the southern San Joaquin, where the Kern River ends and never makes it to the San Francisco Bay).

Where a river debouches (pours) into a sea or lake, it typically builds up a delta because of the drop in velocity (and carrying capacity) at that point. As the delta builds up, the mouth of the stream becomes unstable and shifts about, often dividing into distributaries.
- Deltas themselves can shift around through time as seen in this map of the Mississippi Delta's various lobes of activity at various times over the last 4,600 years: http://sustainindiana.org/wp- content/uploads/2015/02/mississippi-delta1.jpg.
- At the present, it is "trying" to move back to the Atchafalaya Bay, being strenuously opposed by the U.S. Army Corps of Engineers! For more information: http://earthobservatory.nasa.gov/IOTD/view.php?id=6887.
When a river (especially in the West, with its abrupt topography) pours out of a steep canyon onto a flatter valley floor, you get the same reduction in velocity and the same buildup of a mound of deposited alluvium, which is typically fan or shell shaped. Like a delta, you may see the stream divide up into braided patterns. This feature is called an alluvial fan.
- Here's an image of one from Death Valley: http://pages.uoregon.edu/millerm/DVfan.jpeg.
- Alluvial fans present particular flood hazards: The distributive flow across them is unstable and unpredictable, they are often found at the mouth of canyons that may be prone to flash flooding (particularly if their catchment areas have been denuded by fire), and, because they are so gently sloping, they may be tempting places to build.
- For more information on the hazards of alluvial fans and California's response to them: http://aftf.csusb.edu/documents/FINDINGS_Final_Oct2010_10-29- 10_web.pdf.

Runoff is the amount of discharge from a watershed, channeled through the highest order stream at the base of the drainage basin. Many factors affect runoff:

Climate or long-term weather patterns:
- Common storm types
- Prevailing mixes of precipitation types
- Frequency of storms
- Duration and intensity of rainfall patterns
- Distribution of precipitation amounts and types over the basin
- Yearly predictability (drier climates less predictable)
- Global environmental change and associated changes in regional and local climate patterns
Weather or short-term patterns
- All the factors above in their short-term (hourly, daily, weekly, seasonal) particularity
- Antecedent precipitation and saturation: Has the soil been saturated by a series of previous storms?
Topography: elevation, orientation, steepness
- Mountains cause orographic uplift of air masses that hit their sides
  - This causes expansion ("adiabatic") cooling of the air mass as it rises up into regions with lower and lower air pressure. As the air expands, its heat energy is spread out over a larger volume, making the air cooler as the heat density declines.
  - At some point, the air is so cool that it is unable to support its load of water vapor, which then begins condensing or freezing and falling out as precipitation
  - Higher up you go, the higher the precipitation levels because of the increasing coolness of the rising air
  - But higher up more precipitation is snow (which normally reduces flood hazard by releasing water slowly in the spring/summer melt, instead of all at once after a storm event)
- Slope aspect is another factor that can affect precipitation or water retention during a storm or over the course of the year
  - Orientation with respect to the sun:
    - Ubac slopes (the ones facing the poles, like the North Pole here in the Northern Hemisphere) are shadier and cooler, so snow melts more slowly, and this can reduce flood hazard
    - Adret slopes are the ones facing the sun, and they are warmer and drier but they also can allow for too-sudden melting of snow and ice, which can increase flood hazard
  - Orientation with respect to the winds
    - Windward slopes are wetter because of orographic uplift against their sides
    - Leeward slopes are drier (think of the contrast between the windward Big Sur coastline and the immediately interior leeward slopes of the Central Coast Ranges), and this affects the runoff in a watershed.
- Direction of storm movement in relation to stream flow can be very important:
  - When a storm is moving downstream, the waters deposited upstream arrive at the same time with the deluge downstream!
  - If a storm moves upstream, this evens out the runoff experienced at the base of the watershed: First the deluge as the storm passes overhead and then the waters from upslope arrive after the initial deluge at the mouth has moved out of the system.
- Steepness of slopes: Steeper slopes feed runoff into channels faster (less chance for infiltration of surface water into the soil and groundwater system)
Geologic structure:
- Underlying rock structures determine the drainage pattern and orientation of a landscape with respect to the prevailing winds and storm paths
- They also affect infiltration (porosity), as geological structure and stratigraphy can be made up of rock materials with very different properties
Vegetation:
- Dense vegetative cover increases the infiltration of surface water in to the soil and groundwater systems, by creating micro-dams of litter and roots, which slows the surface flow down.
- This slows down the movement and concentration of water into channels as water is diverted into subsurface flow, to emerge much later from springs
- By stretching out water receipt through time, vegetation cover reduces flood hazard
- Deforestation and desertification worsen flood hazard, then, and surface denuding by fire is one of the reasons that flood hazard increases after one of our firestorms
Soil texture:
- Soil texture or the mix of soil grain sizes within the soil affects infiltration capacity or the ability of soil falling on a surface to soak down below the surface.
- Coarse-grained soils have higher infiltration rates: That's why, if you have sandy or gravelly soil, you really have to water a lot in the summer
- Fines (clays, especially, and silts) have lower infiltration capacity
  - Water is bound hygroscopically to particles (through sorption), forming thin films of tightly held water, which plants cannot access
  - Surrounding this film may be capillary water, which plants can draw and which can move in response to evaporation (wicking), and then gravitational water, which drains rapidly out of pore spaces in soils and rocks in response to gravity and becomes part of the water table and groundwater system.
  - But, since the spaces between fine particles are so small, the retention of hydroscopic water and capillary water means the soil saturates readily, too: The spaces for gravitational water to drain down through the soil are so small. Retention of water can make the soil prone to sliding and erosion through pore pressure (spaces stuffed with water expand, weakening the cohesion of soil by reducing frictional resistance), and the additional weight of the water in all those spaces adds to the gravitational loading within the slope.
  - Fines are also very susceptible to compaction. The surface can actually be sealed by the tiny pressures of raindrop impacts, and a sealed surface becomes subject to detachment and sliding as infiltration is reduced or stopped.
  - In all, clay and fine silt dominated soils have low infiltration capacity and are subject to surface failure during a heavy storm as they saturate and seal.
- But fines support more plant cover, and that can offset some of their problems with low infiltration and accelerated erosion.
Human activities really affect the performance of a watershed during precipitation:
- Built environments are usually impervious, which increases flood hazard (more surface water moving much faster because there's no infiltration)
- Overgrazing (soil compaction by hooves reduces infiltration and vegetation is removed by grazing)
- Changing fire régimes can result in increased runoff

Concept of discharge

Discharge is the volume of water passing a given point per unit of time (cumecs, cubic meters per second, or cfs, cubic feet per second). Another way of looking at it is the area of the stream cross-section (depth x width) times its velocity
Peak discharge (Q) is the maximum discharge permitted by the cross-section and the drainage basin characteristics
- Peak discharge can be expressed in terms of the "Rational Equation":
  - Q = CIA
Base flow: This is the component of discharge given by springs, soil moisture, and other sources that are at least partly independent of given storms. It does fluctuate over the course of the year, but not too dramatically and, so, it's used as a kind of baseline for measuring a flood event.
Flood hydrograph:
Flash floods are particularly extreme but very localized flood events.
- They are pretty common hazards in arid and mountainous environments, which may be subject to intense summer thundershowers
- The intensity of precipitation quickly overwhelms the infiltration capacity even of gravelly soils where the vegetation is sparse, so there is almost instantaneous overland and channelized flow
- There is, thus, little lag between the peak rainfall intensity and the peak discharge in a desert channel, and the rising limb is extremely steep: These are like walls of water coming at you!
- Because of the extreme rise in discharge, which raises velocity of flow (greater volume = less frictional resistance), these floods have tremendous erosional capacity, often loaded with mud, sand, gravel, rocks, and even boulders (or hapless campers and their vehicles): The vast bulk of desert fluvial erosion takes place in these events. As weird as it seems, water is the most influential gradational force acting in these arid environments because of the nature of flash flooding.
- Flash flooding can take place in more humid environments, too, whenever there are stationary or slowly moving clusters of thunderstorms that shoot enormous amounts of moist air straight up -- tornadoes can induce flash flooding, too, given that they are particularly rogue thunderclouds.
Urbanization and other human alterations of a watershed can create a hydrograph that resembles that of a flash flood. The rising limb is steeper, the lag between peak precipitation and peak discharge is shorter, and the peak discharge is likelier to go past bankfull stage:

Recurrence interval is an important hazards concept and it was developed early to visualize flood hazard.

Very tricky concept
Think about the "100 year flood":
- Does this mean we can expect (just) one every century?
- No, it's just the magnitude of a flood that has a 1 percent probability of happening in any given year (independently of any other years) and, thus, a probable average recurrence interval of 100 years.
Equation for recurrence Interval: I = (n + 1)/r
- I = recurrence Interval
- n = number of events in a series for which you have data
- r = rank of an event
Plot complication: We often don't have 100 years of data
- Extrapolation is used:
  - Graph your data series
  - Fit a curve to describe it best
  - Extend the curve to the 100 year level
- Problem: It is really not statistically valid to extrapolate beyond your data (but it's the best we have, a "beggars can't be choosers" situation)
The probability (p) of a flood of a given recurrence interval (I) being equaled or topped in the next whatever years (n) is given by:
- p = 1/I
- probability is 1 divided by recurrence Interval

Magnitude and frequency relationship is expressed by the concept of recurrence intervals and probabilities

The lower the magnitude level, the more frequently it will be matched or topped
The greater the magnitude level, the less frequently it will be equaled or exceeded

Cascades of Secondary Effects: Flooding can create a variety of secondary effects that can deepen a disaster.

People (and vehicles carrying them) can be dragged off and drown or be struck unconscious by debris in the water or be pinned underwater.
Flooded homes can then be invaded by mildew and mold, which are themselves very potent hazards to health (and the cost of repair can be staggering).
Topsoil can be dragged off (or buried by debris), damaging agricultural land.
Crops will be destroyed, and this can create severe economic trauma for farmers and, in the poorest areas, can result in famine.
Livestock will die from drowning, hypothermia, or starvation, which can be a very substantial economic (and emotional) loss for farm families.
Flood waters can be contaminated by any sort of chemicals, as storage tanks are pushed off their moorings, float off, and smash into objects that can rupture them, including containers of other chemicals (with mystery chemical soups the result).
Electrical and other lifelines may fail, sometimes causing electrocution hazards or, ironically, large fires.
Drinking water may become scarce as potable water sources become compromised by floodwaters or the secondary effects of flooding.
Depending on local conditions, floodwaters can trigger epidemics of such water-borne diseases as cholera, typhoid deber, and hepatitis A and such vector-borne diseases as malaria, dengue, yellow fever, and West Nile Fever.

FIRMs (Flood Insurance Rate Maps)

Developed by FEMA for our National Flood Insurance Program to delineate flood hazard zones for insurance purposes.
100 year flood is used as a baseline, which represents a compromise between greater levels of protection (precautionary principle, avoiding a Type II error) and the rapidly escalating costs for protection much beyond that level (de minimis principle, avoiding a Type I error)
Designations:
- A (and its various subtypes) mean a high risk zone, one with a 1% chance of flooding each year or a 26% chance of flooding during a 30 year mortgage
- V (and its subtypes) mean the same thing, but in coastal contexts
- B means a moderate flood zone, estimated to lie above the 100 year (1% annual) flood zone but within reach of the 500 year (0.2%) flood. It can also mean areas that can experience shallow (< 1 ft.) flooding or areas theoretically protected from the 100 year flood by levées
- C means minimal flood hazard areas, usually above the reach of the 500 year flood, though there may be localized shallow ponding of water
- X is another designation for areas that are like zones B and C -- out of reach of the 500 year flood or protected by levées from the 100 year flood
- D are the unstudied zones
FEMA's Map Service Center can generate a "FIRMette" for yourself by entering addresses at http://msc.fema.gov/.

Social dynamics

Why do people live in floodplains?

Farming
- Alluvium and the soil fertility it has created for agriculture
- Access to irrigation water
Fishing
Transportation
- Cheapness of water shipping
- Flatness and ease of land transportation (trucking, rail)
Flat surface, which is easy and cheap to build on, too
Power source
Scenic ("home with a view")

Environmental perception issues

Many individuals are unaware (or unreceptive to learning) of their risk
- There are, after all, good reasons to be on the floodplain
- To admit hazard creates cognitive dissonance, that uncomfortable sensation created when you try to hold two mutually incompatible thoughts in your head at the same time ("it's risky here!" versus "I need to stay here because of my farm, my job, my family, my gorgeous home, whatever")
- A common response to reduce cognitive dissonance is denial
- Sometimes legitimating denial is anti-government sentiment ("who do they think they are? telling me what I can do with my own land?")
Accurate risk perception is increased by recent experience with floods.
It is also improved by detailed scenario information coming consistently from a variety of trusted sources
Some people whose denial has been broken will actively seek information, and that transition to information-seeking behavior becomes an opportunity for scientists, planners, and emergency personnel to share information about the hazard, how to mitigate against it, how to prepare for it, or start working on getting away from it

Mitigations and adjustments to flood hazard

Individual or household level adjustments and mitigations include avoidance, structural mitigations, and non-structural mitigations.
- Avoiding high hazard locations
  - Would-be home-buyers really need to expend a little effort on due diligence and the Internet makes this fairly easy to do, and it should be done before viewing homes and before "falling in love" with a particular home
  - Moving out of them is an option for the individual household that may be cost-effective in the long run (and life-conserving!), but that presumes a willing, if ignorant or careless, buyer -- or a FEMA flood buyout program when you need it --
    https://https://www.fema.gov/node/454190
- Structural mitigation options:
- Individual non-structural mitigations and preparations are possible, too:
  - Working out an emergency plan and, ideally, coördinating with neighbors and other community members for sandbagging, emergency housing, communication, and the like
  - Insurance
- Fatalistically accepting vulnerability to loss is an option for adjustment, too, one daily practiced by all kinds of people, sometimes under the influence of fatalistic or predeterministic religious views, sometimes for lack of resources to do anything about it: Hope for the best while resigned to the worst.
Societal level mitigations:
- Structural mitigations:
- Structural mitigation against small recurrent hazards in this way may increase overall social vulnerability to larger, rarer events by creating a false sense of security, which leads to more people and assets at risk on the floodplain, which creates vulnerability for them and for the rest of society, too:
- Societal level non-structural mitigations
  - Land use zoning as a means of encouraging individuals, households, and businesses to avoid high risk zones (can generate political resistance as a "taking," however)
  - Buyout programs (very cost effective, according to FEMA, see link above)
  - Mandatory insurance programs (discussed above)
  - Warning systems of varying intensity, scaling up in urgency and specificity
    - Here's a rather nice one from East Anglia in England, which illustrates that progression:

Socioeconomic vulnerability patterns vary, as they do with so many hazards, having to do with occupations and differential access to resources:

Farmers, farmworkers, and farms at risk
- Even in the developed West, farmers and farmworkers are often highly vulnerable simply because of the dependency of farms on water-accessible locations
- In the poorer countries that are aspiring to develop economically, farmers are often the poorest people in society with the fewest resources to learn about impending danger, do much about it, or recover from the damages.
  - Crops and animals are destroyed
  - Soil may be washed away or polluted
  - Agricultural supplies and equipment are destroyed
  - Seed supplies may be destroyed and preclude planting
  - Deaths in the family may make it very hard to muster the labor needed to resume farming; grief alone can make resuming work very difficult
  - Floods can set off a destructive cycle of debt, from which many households can never free themselves
In the richer countries, amenity migrants may be at heightened risk (though perhaps not vulnerable to the extent of their risk through various government and insurance resources and their own wealth) -- this is a dynamic similar to California's wildfire-urban interface (WUI) situation and the increasing settlement of the "hurricane coast."
The economy as a whole is susceptible to shocks from floods, including agricultural and industrial disruptions and resulting declines in transportation. It should be noted that, while floods are horrendous problems for most people in floodplains and can set them back financially, even for the rest of their lives, some people will benefit by being able to sell goods and services necessary for others to rebuild their lives. Sometimes, this entails gouging; but many times it is quite innocent and necessary. If you work in construction or a big-box home supply store, for example, you will simply be very busy after a disaster. People in first response work may even be able to earn overtime during a flood disaster. So, overall economic effects will be negative, but there will be offsetting economic gains in particular sectors, places, and times.

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