3 ms·

> You can use the “100 year flood” values for your locale to get an upper bound on rainfall I wonder, does climate change also affect those calculations, with

by xg15 2y ago

> You can use the “100 year flood” values for your locale to get an upper bound on rainfall I wonder, does climate change also affect those calculations, with the need to adjust those bounds upwards? I remember the (technical) term "100 year flood" getting lots of ridicule in the last years because we already got multiple "100 year floods" in the range of a decade.

jnwatson 2y ago
Faster than climate change is development. The addition of impermeable surfaces like parking lots and buildings upstream causes more water to get to you. Development happens faster than the flood maps get redrawn.
- susiecambria 2y ago
  I wonder how well USA state agencies are doing in their efforts to map sea level rise, flooding, etc. and if they take into account development. I know a lot of work is being done in Virginia to address sea level rise, flooding in poor areas, etc. But because I live in a rural part of the state, I can't assess for development since it's not ubiquitous.
dv_dt 2y ago
Short answer yes, but as another comment suggested, development changes adding hard surfaces in new places in the neighborhood also rapidly change those calcs.
magicalhippo 2y ago
If it was me I'd chop a zero off those N-year estimates. But that got me thinking, how often do they adjust these estimates? Was a "100 year flood" in 1950 less than in 2000, say? Obviously it'll be different for different areas, especially between countries, but still.
mindslight 2y ago
FWIW "100 year flood" refers to the flooding that is expected to occur in that specific area on average once every hundred years, not an event that will happen only once over the whole country once per hundred years. So regardless of climate change it makes a lot of sense to be seeing many hundred year floods since mass media has us caring about many areas in parallel, not just the specific area we live.
- xg15 2y ago
  Ah, I wasn't aware of that definition. That makes a lot more sense, actually.
- tda 2y ago
  Also, as people have a life expectancy in the order of 100 years (75 perhaps?), there is a pretty good chance you will experience a 1:100 year event at some point in your life. Also a lot of infrastructure is built for 50-100 year life (or is still used after 100 years, even though it was designed for less). The 1:100 year safety level should thus be considered a bare minimum for high impact events. In the Netherlands water infrastructure is designed to withstand the 1:10.000 year storm for catastrophic events, and e.g. 1:1.000 year storm for high impact events. But even with higher safety standards, it is key to design not only for the current climate, but for the future climate over the design life of whatever you are building. Extrapolating 1:10.000 year events from 30 years of observations might seem sketchy already, but with all the uncertainties in climate change predictions is a bit of a stretch.
  - thfuran 2y ago
    There's a pretty good chance you'll experience any given 1:100 year event. You're basically guaranteed to experience quite a lot of 1:100 year events, since there are so many options.
- zejn 2y ago
  This is a misleading definition as it makes people believe that if there is a 100-year flood this year, that equal or worse flooding will not happen next year. Long term observed water levels are fed into a model with normal distribution, and a 100-year flood is then defined as 99-th percentile of the model. This means that there's a 1% chance of a 100-year flooding each year. Global warming and climate change have an effect that there's a lot more water vapour in air. Same air mass can deliver more rainfall on a given territory, pushing observed water levels into higher extremes, meaning what used to be 100-year flood is now 25 (4% yearly chance), 10 (10% yearly chance) or even 5-year (20% yearly chance) flood.
LegitShady 2y ago
the 100 year flood is basically a mathematical concept - its about the long term average recurrence interval - the probability that the given event will be equaled or exceeded in a given year. The "100 year flood" has a 1% probability to happen in a given year, based on historical data and a normal probability distribution (which is what we see in nature).
- zejn 2y ago
  First time I've see someone describe this correctly.
  - LegitShady 2y ago
    I have to admit I have a degree in structural engineering and bashed my head trying to understand various bits hydrology for some time some years ago, and still occasionally dabble in it when I'm required to (although I leave it up to more competent hydrology experts 99% of the time).

by xg15 2y ago

jnwatson 2y ago

Faster than climate change is development. The addition of impermeable surfaces like parking lots and buildings upstream causes more water to get to you. Development happens faster than the flood maps get redrawn.

susiecambria 2y ago
I wonder how well USA state agencies are doing in their efforts to map sea level rise, flooding, etc. and if they take into account development. I know a lot of work is being done in Virginia to address sea level rise, flooding in poor areas, etc. But because I live in a rural part of the state, I can't assess for development since it's not ubiquitous.

dv_dt 2y ago

Short answer yes, but as another comment suggested, development changes adding hard surfaces in new places in the neighborhood also rapidly change those calcs.

magicalhippo 2y ago

If it was me I'd chop a zero off those N-year estimates. But that got me thinking, how often do they adjust these estimates? Was a "100 year flood" in 1950 less than in 2000, say? Obviously it'll be different for different areas, especially between countries, but still.

mindslight 2y ago

FWIW "100 year flood" refers to the flooding that is expected to occur in that specific area on average once every hundred years, not an event that will happen only once over the whole country once per hundred years. So regardless of climate change it makes a lot of sense to be seeing many hundred year floods since mass media has us caring about many areas in parallel, not just the specific area we live.

xg15 2y ago
Ah, I wasn't aware of that definition. That makes a lot more sense, actually.
tda 2y ago
Also, as people have a life expectancy in the order of 100 years (75 perhaps?), there is a pretty good chance you will experience a 1:100 year event at some point in your life. Also a lot of infrastructure is built for 50-100 year life (or is still used after 100 years, even though it was designed for less). The 1:100 year safety level should thus be considered a bare minimum for high impact events. In the Netherlands water infrastructure is designed to withstand the 1:10.000 year storm for catastrophic events, and e.g. 1:1.000 year storm for high impact events. But even with higher safety standards, it is key to design not only for the current climate, but for the future climate over the design life of whatever you are building. Extrapolating 1:10.000 year events from 30 years of observations might seem sketchy already, but with all the uncertainties in climate change predictions is a bit of a stretch.
- thfuran 2y ago
  There's a pretty good chance you'll experience any given 1:100 year event. You're basically guaranteed to experience quite a lot of 1:100 year events, since there are so many options.
zejn 2y ago
This is a misleading definition as it makes people believe that if there is a 100-year flood this year, that equal or worse flooding will not happen next year. Long term observed water levels are fed into a model with normal distribution, and a 100-year flood is then defined as 99-th percentile of the model. This means that there's a 1% chance of a 100-year flooding each year. Global warming and climate change have an effect that there's a lot more water vapour in air. Same air mass can deliver more rainfall on a given territory, pushing observed water levels into higher extremes, meaning what used to be 100-year flood is now 25 (4% yearly chance), 10 (10% yearly chance) or even 5-year (20% yearly chance) flood.

LegitShady 2y ago

the 100 year flood is basically a mathematical concept - its about the long term average recurrence interval - the probability that the given event will be equaled or exceeded in a given year. The "100 year flood" has a 1% probability to happen in a given year, based on historical data and a normal probability distribution (which is what we see in nature).

zejn 2y ago
First time I've see someone describe this correctly.
- LegitShady 2y ago
  I have to admit I have a degree in structural engineering and bashed my head trying to understand various bits hydrology for some time some years ago, and still occasionally dabble in it when I'm required to (although I leave it up to more competent hydrology experts 99% of the time).