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100 yr Storm (How often Does it Really Occur?)
8

100 yr Storm (How often Does it Really Occur?)

100 yr Storm (How often Does it Really Occur?)

(OP)
I have heard several times the probability of a 100 yr event is the 1/100 for any given year.  

I have also heard that statistically if you look at a period say 30 yrs then the odds of a 100 yr event occurring in that 30 yrs is much greater thant he relative frequancy of the union of events... i.e. one way to look at is that the odds of a 100 yr event over a 30 yr period is 1/100+1/100+1/100 etc for 30 times or 30/100 or 3/10... this makes some sense to me, but I have heard several times that the probability is acutually much higher than 3/10 that a 100 yr storm would occur in over a 30 yr period and that the statement the people always make that "... a 100 yr storm occurs on average once per 100 yrs is generally false."

Anyone good enough with statistics to explain in simple terms which is true?????  

RE: 100 yr Storm (How often Does it Really Occur?)

This general topic (but not your specific question) has been discussed in the Storm/Flood forum.  It's been a long time but I'm pretty sure that it's easier to work with the probability of it NOT happening in any one year, and you can mulitly this to get probabilty for other periods.  So probability of not having a 100 year event in a 30 year period is (1-0.01)^30 = 0.74, so the probablity of it occurring is (1 - 0.74=0.26) or 26%.  Same approach for 100 years gives 63%.  So in any 100 year period the chance of a 100 year storm is 63%.  But I believe it's still true that for a very long period of record, the 100 year storm does occur on the average once every 100 years, which is actually part of the definition of a 100 year storm.

Hope this helps, and that the old college brain cells haven't failed me.

Carl

RE: 100 yr Storm (How often Does it Really Occur?)

The 100-year storm, flood, etc. can occur ANY TIME! So look out! The 100-year storm can happen 365 days a year - though unlikely.

With that in mind, though, the proverbial 100-year event is "unusual" enough to cause substantial (note: not "significant") damage to life and property, hence, use it for your design loads, winds, floods, etc. and thus storm events/flood events that don't produce the same stages, etc. as the "100-year" event will easily be sustained by whatever project you're working on.

How's that for obfuscation?

RE: 100 yr Storm (How often Does it Really Occur?)

I think this is a code subject...if warming has some effect, the code needs to recognize. Also if it is wanted to specify different calculations for different target useful lives of the buildings the code needs to elect the closed form giving the forces...for I have seen different fromulations being in use for windstorms and earthquakes, so the nature of the phenomenon seems be influencing what statistical prediction tools we need to use. In short, a 2 pages clarification on that wouldn't do any harm in future codes and texts.

RE: 100 yr Storm (How often Does it Really Occur?)

Just a point to remember - the 100 year event is one which is EXCEEDED once in 100 years on average.

RE: 100 yr Storm (How often Does it Really Occur?)

The 100-year event has nothing to do with 100 years.  The definition is that the 100-year event has a 1% probability of occurrance during any given year.  

And no it does not average once every 100-years.

The Houston area has had 3 in the past 10 years, and over
10 in the last 100 years.

RE: 100 yr Storm (How often Does it Really Occur?)

4
From Fema website http://www.fema.gov/mit/tsd/fq_term.htm#frequt3

What is a "100-year flood"?
A 100-year flood is a flood that has a 1-percent chance of being equaled or exceeded in any given year. A base flood may also be referred to as a 100-year storm and the area inundated during the base flood is sometimes called the 100-year floodplain.


What does the "100-year flood" mean?
The term "100-year flood" is misleading. It is not the flood that will occur once every 100 years. Rather, it is the flood elevation that has a 1- percent chance of being equaled or exceeded each year. Thus, the 100-year flood could occur more than once in a relatively short period of time. The 100-year flood, which is the standard used by most Federal and state agencies, is used by the National Flood Insurance Program (NFIP) as the standard for floodplain management and to determine the need for flood insurance. A structure located within a special flood hazard area shown on an NFIP map has a 26 percent chance of suffering flood damage during the term of a 30-year mortgage.



Other information can be found at http://www.esri.com/hazards/
Online Harzard Maps http://www.esri.com/hazards/makemap.html

RE: 100 yr Storm (How often Does it Really Occur?)

So CarlB has it right...even in the final comment. I would be concerned on any definition of some level of flood assumed to be one a 100 years' flood that occurs somewhere 5 times in 20 years. This would indicate the need -or at least the convenience- to correct the stated level of the 100 year flood, for recent data would be indicating that such level would be locally and presently corresponding actually to one 4 years' flood.

RE: 100 yr Storm (How often Does it Really Occur?)

Don't forget that there are also 500 year events and PMFs (Probable Maximum Floods). Rich 2001 has it right.  100 year rainfalls don't always produce 100 year floods.  A lot depends on the prior rainfall, storage capacity in upstream reservoirs, the direction of the rain pattern in relation to the shape of the basin, speed of the event,etc.   Conversely, a less than 100 year rainfall event can produce a 100 year flood if conditions are working against you. I remember the hurricane generated front that stalled out over Albany, Texas in 1979? after a prolonged drought.  They cloud seeded into the thunderbumber for 6 hours.  I seems to recall that it produced 28 inches in 24 hours and remains the record rainfall in this area.

RE: 100 yr Storm (How often Does it Really Occur?)

If a 100-year storm occurs 5 times in 20 years the definition would need to be revised? No. Climatology is based on much longer cycles that 20 years. If you flipped a coin 20 times and it came out heads 12 times you wouldn’t revise the probability of getting heads on a coin toss to 60%.

RE: 100 yr Storm (How often Does it Really Occur?)

The occurrence of a "100 year storm" 5 times in 20 years may indeed indicate the need to review the local hydrology.  In a perfect world we would have ample data to accurately predict the 100 year storm, however, most regions in the US have less than 100 years of data.  The accuracy of rain gauge measurements taken 100 years ago could be questionable.  The number and geographic location of rain gauges may also be insufficient.  It is also very likely that continual climate change and global warming also is changing our hydrology.  Therefore, it is entirely likely that what we call a "100 year storm", may be statistically correct, however, the statistics are weak due to the lack of data.  This points out the need for continued data collection, review and frequent update of isohyetal maps and depth - duration data so that we can more accurately define the 100 year storm.  We also need to be careful not to confuse the storm with the runoff.  As stated previously, a 100 year flood may be caused by a storm which is less than a 100 year event.


 Chuck
 cgopperton@stantec.com
 http://www.stantec.com/
 

RE: 100 yr Storm (How often Does it Really Occur?)

As can be deduced from my previous post, I agree with cvg in the need of revision. Very probably what happens is that the evaluation got not so finer as to discover that locally there is a pot of higher frequency. So the geographical contour lines for the flood level adscribable to the zone needs be modified, there the probability experience is proving is higher, and it is better to come to terms with reality.

For example, in this same forum was stated it was not necessary to calculate temporary structures against earthquake (forms specifically), since the probability of the structure taking an earthquake blow during construction is too low. Now in Taiwan tha fall of construction cranes and variegate construction material has caused some deaths and a number of wounded people.

So life safety standard has failed during construction, at least the prevention of fall of objects and cranes, this last one very amenable to computation of the holding  devices.

RE: 100 yr Storm (How often Does it Really Occur?)

(OP)
Thank-you all, this has been a most interesting discourse.

RE: 100 yr Storm (How often Does it Really Occur?)

The question is really one of margin of safety.  100yr storm design is arbitrary.  What you need to consider is what damage to property and life you are will to endure to use a cheaper structure.  Since we are dealing with public money most of the time for large structure the public should but never will tell us what is an acceptable risk.  Most designs have a margin of safety so you need to take that in to consideration too.  Since all drainage design is based on observed rainfall it is pointless to consider what might happen we can only know what has happened.  Structural design is the same way, you test a beam and when it breaks you assume all the other similar beams will break at the same load.  That's just a practical way of going about you work.  If you had not noticed most of the formulas we use don't make any sense because they are full of empirical data.  Why should 100yr flood tables be any different.

  I don't think the climate is changing so fast that we cannot deal with it.  I wonder if anyone has consider that the probably of an ice age returning is in fact more likely than runaway global warming based on historic data.  Do you know what triggered the last ice age?  I would have to say that predicting the weather is not very reliable and I would guess that predicting climatic change is not any better.  The conservative thing to do is adjust as needed and keep the fear level under control.  How many automobiles does it take to release the same amount of co2 as one volcano?  In the 16th century it snowed in London in the summer.  What are we going to do if Kansas has snow in June and the wheat crop is ruined, give up bread .
  I don't think the climate is changing so fast that we cannot deal with it.  I wonder if anyone has consider that the probability of an ice age returning is in fact more likely than runaway global warming based on historic data.  Do you know what triggered the last ice age?  I would have to say that predicting the weather is not very reliable and I would guess that predicting climatic change is not any better.  The conservative thing to do is adjust as needed and keep the fear level under control.  How many automobiles does it take to release the same amount of co2 as one volcano?  In the 16th century it snowed in London in the summer.  What are we going to do if Kansas has snow in June and the wheat crop is ruined, give up bread .  This is a {do you want to live forever?} kind of discussion.

  Does anyone here really think that a 20% change in climate is going to out strip the safety factors already in place?

RE: 100 yr Storm (How often Does it Really Occur?)

Jeeman,

Your estimation of the probability of a 100 year event happening in 30 years is incorrect. 1% + 1% ... = 30% will not work.  If you use this same theory for 100 years, you will get a 100% chance that the event will occure in the 100 years (which isn't true).  The probability P of an event having a given return period T occurring at least once in N successive years is given as P=1-(1-(1/T))^N.  So, for a 30 year period, the probablity that a 100 year event will occure at least once is 26%.  For a period of 100 years, the probability that a 100 year event will occure is 63%.

Brian

RE: 100 yr Storm (How often Does it Really Occur?)

jugglerbri,
Your calculations are not correct, because your definition of a 100-year storm is incorrect. A 100-year storm does not mean that the storm has a probability of occurring every 100-years.  A 100-year storm means that the event has a
1% chance of occurring in ANY GIVEN YEAR.

RE: 100 yr Storm (How often Does it Really Occur?)

Bayou,

Yes, you are correct that the definition of 100-yr event means that it will have a 1% chance of occuring in any year, however, that number is not additive, i.e., there is not a 2% chance that a 100-yr event will occure within two years nor a 50% chance the event will happen within 50 years.  The formula I gave gives the probability of an event with a given recurance interval occuring within a givin time period.

Brian

RE: 100 yr Storm (How often Does it Really Occur?)

One thing that often difficult to wrap one's brain around is that these statistics are only for extremely large distributions.

Just for giggles, the following table is the probability of only x occurrences in 100 and 500 years, so the probability of getting none in 100 yrs is 36.6%, which means that the probability of getting 1 or MORE is 73.4%.  The probability of getting none in 500 yrs drops to 0.66%.  Note that as the overall period increases, the 1% of the period is the 50% breakpoint in the probability distribution function.

0    36.6032    0.6570
1    36.9730    3.3184
2    18.4865    8.3631
3    6.0999    14.0230
4    1.4942    17.5995
5    0.2898    17.6351
6    0.0463    14.6959
7    0.0063    10.4759
8    0.0007    6.5210
9    0.0001    3.6008
10    0.0000    1.7859

TTFN

RE: 100 yr Storm (How often Does it Really Occur?)

Ok. How about 5 yr storm. Most of the municipality design their storm sewer to handle 5 yr storm. What is the probabality of getting 5 yr storm in any given year?

RE: 100 yr Storm (How often Does it Really Occur?)

Probability and return period are inversely related.  The formula is:

p = 1/T  where p is the probability and T is return period

therefore probability of a 5 year flood in any given year is 1/5 = 0.20

RE: 100 yr Storm (How often Does it Really Occur?)

The so called 1 in 100 year event is really an event with a probability of an event of that magnitude or greater of 0.01 in any year.

A probability of this type is called a poisson distribution.  You can look up the details in any undergraduate statistics textbook.  In this type of distribution the probability of the event occurring in any time frame is independent of it occurring in the next time frame.

Some examples of this type of distribution are the arrival of calls in a telephone exchange, breakdown of a piece of equipment, defect along a wire or rope, manufacturing defects and of course storms and floods.

If a flood occurs one year the possibility that the 1 :100 flood occurs next year is still 0.01. If a flood has not occurred for many years the probability is still 0.01.

The formula is

P(n events in time t)= e exponential (-lambda*t)*(lambda*t)exponential (n) /n!

Where n =0,1,2,3 etc
Lambda=expected number of events per unit time In the storm case 0.01 (for a 30 year period the expected number of floods is 30*0.01=0.3 events)

The probability of zero events in 30 years is:
P(0)=e exponential (-0.01*30) * (0.01*30) exponential (0)/0! = 0.741

The probability of at least one 1:100 event in 30 years is

1-P(0)=1-0.741=0.259

The probability of exactly one event is

P(1)=e exponential (-0.01*30) * (0.01*30) exponential 1 / 1!=0.222

The probability of two events in 30 years is therefore

P(2)=e exponential (-0.01*30)*(0.01*30) exponential 2 / 2!= 0.033

As you can see the probability of additional events drops off quickly as n increases.

The probability of more than two events would be

1-P(0)-P(1)-P(2)= 1-0.741-0.222-0.033=0.004

Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion
www.kitsonengineering.com

RE: 100 yr Storm (How often Does it Really Occur?)

Interesting discussion.

But, is the 1% probability actually backed out from the binomial statistics, or was it a simple thing of looking over 1000 yrs and finding only 10 occurrences?

TTFN

RE: 100 yr Storm (How often Does it Really Occur?)

IRstuff

That is the real problem in generating the 1:100 (or other probability) distribution.

Once the value of the event is known calculating the probabilities is actually a simple matter as I  showed above. Its really introductory level statistics.

The real statistical work is in calculating the value of the storm (or in aerospace usage the MTBF which is the same statistical distribution).

You seldom have 100 years of data. You often only have 30  to 50 years of reliable data. Old data is unreliable because the method of taking the samples may be suspect, sample locations may have changed, only extreme data may have been recorder etc. The new hard drive that I bought with a predicted MTBF of 100,000 hours was not run for over 11  years to predict this number.

What often happens is that the 30 years of valid data is extrapolated to predict a rare occurrence.  (or many drives are run for a couple of months) As in all extrapolations accuracy degrades the further out from the data that you have collected.

If the forecasters are scientifically honest instead of stating that the 1:100 storm is x mm/hr they would state that they are say 95% confident that  that the 1:100 storm is between x and y mm /hr. They could also predict that a 99% confidence interval was some larger range.

More data points would decrease the range and higher confidence intervals would increase the range. This would allow users of the data to determine their own risk assessment.

Statistics of this nature are quite complex and beyond my statistical ability to do or explain.  Complex computer programs are often used.   One that will do this analysis that I have used in the past is SPSS (Statistical Package for the Social Sciences.)  Don’t let the name fool you, it’s a powerful and complete statistical package.

If you search for rainfall IDF (intensity, duration and frequency)  curves on line you will find some samples of the output from these programs.

Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion
www.kitsonengineering.com

RE: 100 yr Storm (How often Does it Really Occur?)

Thanks for the info.

At least, with the hard drive, there are a couple of factors that allow one to do a better job of predicting the MTBF:

>  There are actually thousands of harddrives manufactured, so the failure database could be quite substantial.

>  The individual components of the hardrives can be individually tested for their MTBF and the system MTBF could be calculated from them.  That's the basis of MIL-HDBK-217 and other prediction approaches.

>  Actual accelerated and highly accelerated life tests can be performed.  The activation energies of failures can be determined and time-temp acceleration factors are calculable.  This could be done with a few hundred harddrives.

TTFN

RE: 100 yr Storm (How often Does it Really Occur?)

I didn’t realize, but am not surprised by these assists in predicting MTBF.

My main point is that the data is extrapolated from a shorter time frame and a finite number of tests to predict  the actual failure frequency in real life and that these predictive methods involve complicated statistical techniques.

While there may be thousands of drives manufactured, the MTBF is usually published before they are made or subjected to any real world use. The high numbers will simply serve to reduce the confidence interval in predicting the actual MTBF.

Tests like the time-temp acceleration factor are in themselves based on a statistical correlation between the increase in failures and the increase in temperatures.

Calculating assembly failures from component failures is also a statistical technique.  A simplified example is if I have two components and both have to fail for the assembly to fail and the probability of failure is 50% for each then the probability of assembly failure is 25%.

Even if a extremely large number of drives was tested or you had the complete storm data for some location, it would be a statistical exercise to predict failure rates.

The predicted storm frequency or drive failure rates  could also be totally nullified by some change in the underlying conditions like climate change that had an effect on rainfall or a new motherboard that had some effect on drive reliability.

Any time you use a statistic you should ask a couple of questions. (First you have to realize that the number you are using is a statistical based number.)

What is the sample size? Is the 1:100 year storm based on 30 or 200 years of data?

How reliable is the data? Are all data points relating to the same measurement? Did they use a different type of rain gauge 150 years ago?

Has some underlying factor changed that makes the data worthless as a predictor of future events? I.e. global warming

What is the confidence interval at some statistical level of assurance? Is the range x to y and you are 95% sure that the true value lies in this range?

How important is the potential variance in the analysis to what you are doing? I can live with a wide range of storm rainfalls without any significant impact. Often a storm sewer designed for the 5 year rainfall will also accommodate the 10 or 20 year rainfall because the 5 year number forces me to use a pipe size that will carry larger flows.  In this case I really don’t care about the confidence interval. If the pipe size is just big enough for the 5 year flow then I might be concerned if the actual flow exceeds the predicted 5 year flows.
 
If you are doing any regression analysis how strong is the correlation between the independent and the dependant variable? In a weak correlation the effect of a third variable may nullify all conclusions.

Is there some logical reason for a correlation to exist? Someone once did a correlation analysis between the length of Vanna White’s hemlines on Wheel of Fortune (a US game show for those outside North America)  and the next day’s performance in the stock market. The cause and effect here is hard to see so this could simply be a statistical anomaly.

Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion
www.kitsonengineering.com

RE: 100 yr Storm (How often Does it Really Occur?)

laugh, anything that can go wrong will go wrong. maybe be use 150% of 100 year.

RE: 100 yr Storm (How often Does it Really Occur?)

I'm an x-aeronautical engineer who wandered into this thread and finds it very interesting. The misunderstanding of basic terms (or misnaming of basic terms) was also widespread. The use of probability of a failure was introduced in systems analysis. Consideration of failure in Structure and powerplant was handled differently. However the FAA has been changing these and moving toward a probability based approach there as well. There is a ton of material available from the FAA. The discussion above regarding changing/updating probability values have a correlation in that there is movement to increase the stringency for ultra-large-capacity passenger airplanes such as the Airbus 380 because the consequences of a "catastrophic" failure (even being of low probability) are so severe. The "benchmark" level was "ten to minus 9th". The fundamental philosophy for higher probability failures was (1) they must be shown to not be catastrophic, (2) the airplane has to be shown to be able to "continue safe flight", and (3)could exceed its "design loads" during the safe flight phase. Good discussion!!

RE: 100 yr Storm (How often Does it Really Occur?)

they are likely using a risk assessment approach.  Probability of failure X consequence of failure = risk.  For example:

100 year flood
probability of equaling or exceeding = .01
consequence = $200,000 damage
risk = .01 x 200,000 = $2,000

for the airliner
probablility of failure = .000000001
consequence = $100,000,000 plus all passengers lost
risk = "acceptable"?

RE: 100 yr Storm (How often Does it Really Occur?)

This is the strangest thread we have ever had .

RE: 100 yr Storm (How often Does it Really Occur?)

thus if the divorce rate is 30% and the average man gets married at aged 30 and lives to 84 there is a 99% probability that he will either die bankrupt or will be drowned in a 100 year flood.

RE: 100 yr Storm (How often Does it Really Occur?)

We just as well keep it going lol.  The whole problems is that we have enough statistical data to determine what the probability of a certain flood is but we don't have much data for determining the cost associated with that flood is.

Even worse we don't have hardly any data on what the cost of over designing at one location has on the ability to maintain the entire infrastructure of society is.

RE: 100 yr Storm (How often Does it Really Occur?)

How about this -

If you design a 100 year drainage solution, but you use conservative assumptions, provide plenty of freeboard, and use safety factors.  Do you have a 100 year system, or maybe it has 200 year capacity?

Or - you design 2 year storm drains, plus you design 100-year, 2-hour storm retention for all development.  You put catch basins every 660 feet, no matter what and also at each intersection.  What is your resulting storm drainage system capable of handling?

RE: 100 yr Storm (How often Does it Really Occur?)

Using over conservative design is a major cause of failure of drainage schemes. If you design for a 100 year storm and then provide freeboard you will then find that your downstream structures are washed out during a flood event. The correct procedure is to check the safety of downstream structures at flows resulting from upstream channels running at bank full discharge not the design (100 year) discharge. The 100 year flood is the flood that has probability of EXCEEDANCE not a probability of occurrence. Its probability of occurrence is zero.

Brian

RE: 100 yr Storm (How often Does it Really Occur?)

Interesting!

a) Downstream structures would only be washed out if they were underdesigned for the design discharge.  

A lawyer may make a case to a judge and jury that since Engineer A increased the flow in the river, Engineer B's downstream bridge was washed out.  However, Engineer B could be held liable for not providing sufficient freeboard in his design to pass the design flood.  Of course (reality check), Engineer A did not cause it to rain, so Engineer B is at fault!

b)As stated previously, the 100-year flood is the flood that has 0.01 probability of being equalled OR exceeded in any given year.  If this was zero, I would be out of work.

RE: 100 yr Storm (How often Does it Really Occur?)

I recently designed a drainage structure at a slide repair.  The existing structure was an 8x6 RCB.  I used a 6x4 pipe arch to extend it under the new slope.  Using Pondpack I determined the extent of storage required upstream and purchased enough property to allow for the detention.  

The reason I decided to purchase a detention area instead of extending the box was that the down stream structures for a mile had been corrupted by development.  There were three boxes downstream and developers had filled in between them with 60 inch RCP's to carry the water.

The reason they were getting away with this was because over the years the box at my location had been modified by debri closing off part of the openning.  The reduced capacity of the box was flooding the area up stream.

My design was for a hundred year storm which is perhaps not enough safety margin but the downstream system is sure to fail before mine.  In the event that a larger storm happens the road may be washed out but the damage to property due to the other modifications to the stream that I was not responsible for will be worse.

I considered it a social responsibility to look at more than just my organizations interest when making this decission.

I could have played it safe and left the people who modified the system down stream hold the bag. In the interest of society, which is the goal of all civil engineers to serve, I think the $30,000 it cost me to buy the detention storage was the right decission.

I'm lucky to have the luxury of acting this way because as a goverment employee I have the option to look at what is best for everyone and not just a client.

RE: 100 yr Storm (How often Does it Really Occur?)

I think that is an interesting example of how much the greater good encompasses.  

While there is the greater good of protecting people and property from the anomalous events that turn out to be not so anomalous, one could rightly ask whether it might be better that people not live in those areas that are prone to flooding.

I curious how you balance the immediate relief vs permanent relief, i.e., if the people were not there at all, there would be no need to do this in the first place and there would be no issue of whether adequate safety margin has been applied to the 100 yr event

TTFN

RE: 100 yr Storm (How often Does it Really Occur?)

The property that would be damaged in this case was developed at about the same time the road was built.  It is an odd set of events.  The fill has mostly been done by heavy equipment companies for storage.

Your right the current federal practice of buying and not repairing property is a wise one for the long term.

RE: 100 yr Storm (How often Does it Really Occur?)

Put another way, the property owners downstream weren't really left holding a bag...  They made a decision to put in the 60 inch pipes.  Obviously, these have less than the 100 year capacity and less than the existing 8 x 6 box.  They saved a lot of money by doing this.  They took a measured risk that by doing this, they wouldn't experience a flooding loss, or cause anyone else harm.  However, they could have (and should have) known that eventually, the slide would be removed, or the culvert extended, or the government would improve the entire watercourse to minimum standards, or possibly a large flood would come along and wash the slide out and the channel is back up to it's original capacity.  

Apparently, all of this was done without the oversight of a public agency.  The very reason we have public agencies is to serve the general public, which is not always well served by individuals.

RE: 100 yr Storm (How often Does it Really Occur?)

CVG - not all engineering is dictated by legal interpretations of the US litigation system. I have spent the last 30 years working on development projects in developing countries where engineering is dictated solely by economics and not by risk of legal actions.

The point is that the 100 year event is the flood that has a 26% probability of being exceeded over a 30 year life. The flood flow you calculate has practically zero probability of occurrence it has a probability of exceedance. If you design for this flood and provide freeboard then your channel will carry a higher flood flow than you have designed for. If you have designed the downstream structures for the theoretical 1 in 100 year flood and they do not have a similar excess capacity to match the design capacity of then they have a 26% probability of failure in the 30 year life. If for example the freeboard provided increases the upstream channel capacity sufficient to carry the 150 year flood then the probability of the channel flowing at bank full in its 30 year life is 18%. If you have not designed the downstream structures for this flow then you will have a very difficult task in proving that you were not negligent when the structures fail.

Some years ago I investigated a drainage project in Nigeria where downstream structures failed due to upstream drains flowing at bank full and passing up to 1.5 times the design 1 in 25 year flood. The downstream drop structures and stilling basins were designed precisely for the 1 in 25 year flood flow not for the channel capacity. The damage cost was some $20 million. The Contractor's designer was held responsible and the contractor reconstructed at his own cost.

Brian

RE: 100 yr Storm (How often Does it Really Occur?)

Just to weigh in.  The 100 year storm is taken as rainfall in 24 hours - generally.  In our area it is 7.2 inches.  We get storm cells that dump the EQUIVALENT of a 100 year storm in an hour or two.  These cause flash floods and damage; however, I have not seen 7.2 inches in 24 hours.  

RE: 100 yr Storm (How often Does it Really Occur?)

Bris

I understand and do agree with the point that it is best to design everything in the watercourse to the same standard.  Of course, someone has to set that standard.  

This is also a problem in the US.  We have recently prepared the first watercourse flood control masterplans for this region.  This was in reaction to the uncontrolled development and flood control improvements that were going on in the county on the major watercourses.  Developers try to maximize the use of their land, which means building as close to the river as possible.  They would do this by constructiong bank lining to prevent the river from migrating towards the development.  However, this just increases the risk for the owner across the river or downstream.  Prudent design might be a non-structural approach, requiring a set back from the river to allow for overbank flow or for lateral migration.  Or a consistent approach to types of lining, river profile, scour and sediment transport etc.  Improvements should all be designed using the same hydrology model and be consistent with the masterplan.

However, assuming that there is to be a design standard in place and some oversight, safety factors (freeboard is just one) are essential in engineering.  For instance, there is quite a bit of uncertainty in both hydrology and hydraulics.  We try to account for these uncertainties by providing a safety factor in the design.

RE: 100 yr Storm (How often Does it Really Occur?)

In my example you may have noted that my detention basin was design for an event which would assure the failure of the downstream system regardless of what I did.  The rule is don't make it any worse than it already is.  If my system could only hold a 50 year flood that would have been acceptable if the down stream system failed at that point too.  That is assuming that I had and I had made the situation atleast marginally better than the no build situation regardless of the flood event.

RE: 100 yr Storm (How often Does it Really Occur?)

This just keeps getting better! - don't get mad, I'm just playing devils advocate ;)

"over conservative design causes downstream flooding"

Rain causes the flooding, not designs.  
Property owners who do not protect themselves properly suffer flood damage.


Example:
Joe buys property in the country.
He decides that the best place to build his house is right next to the gurgling stream.  He builds it, but then realizes he is very close to the creek and during the first heavy rain, the water is lapping his front porch.  So, he decides to re-align the channel slightly and widen it a bit so that the next time it rains, the house won't get wet.  His engineer designs the channel for 100 year flows and puts in plenty of freeboard.  Joe is happy, because he knows that he only has a 26% chance of getting flooded before his mortgage is paid off!

The dominant discharge for the stream is about 25 years.  
Anything over that typically flows in the overbank floodplain.

Now assume that George lives downstream and built a driveway across the creek (before Joe moved in) and only put in a 25-year culvert.

A 100-year flood comes along and George's culvert and driveway are washed out.
 
George claims that it's Joe's fault, because he overdesigned the channel on his property! Theoretically, the flood water would not have reached his culvert had Joe not re-aligned the channel and made it bigger.  Joe should have matched the flow in George's 25 year culvert, even though the damage clearly was a result of a large flood and a small pipe.  

According to the theory, Joe should have simply lived with the 25 year design standard that was set previously by George and pay for his own flood damage every 10 years or so when he get flooded.  After all, George was there first.

Reality is that when the flood comes along, Joe will be high and dry in his new house.  George will lose his driveway and culvert and a good part of his front yard due to the erosion caused by the overbank flow.  But he expected
that, because it was only a 25 year design, and he took a risk. (He only had a 10 year mortgage, after all, and he saved a wad of cash on the 25 year culvert versus the 100 year bridge that his engineer originally recommended)

RE: 100 yr Storm (How often Does it Really Occur?)

CVG - I have no problem with your arguments with respect to individual riparian owners on non-regulated water courses. In UK you would not be allowed to improve the flow capacity of a stream or river without assessing the downstream impact of the increased flow. These days the  Environment Agency would most likely require a flood attenuation solution. (lessons learned from past mistakes).
 
My comments refer to main drainage and river schemes. One problem I am consistently coming across is in the design of drainage from irrigation projects. To minimise drainage cost the design allows 24 to 48 hours inundation of the drained area. Thus the design storm depth is the rainfall depth over the inundation time plus the time of concentration. The channels are designed to remove the water over 24 to 48 hours. The engineer then adds freeboard to his drain and is surprised when inundation times are reduced to only a few hours and all his bridge crossings and drop structures fail under a two year event.


Another example I have investigated recently is a major flood defence project. The project comprises several miles of earth flood embankment designed for a 1 in 50 year flood. Adjacent to urban areas a concrete flood wall is used in place of the earth embankment. The Engineer has provided freeboard is 0.5m to the earth embankment and 0.3 m to the concrete wall sections. The result is that floods, in excess of the design flood, now flood the urban areas instead of the rural areas. It may be rainfall that is causing the flood event but it is the engineer’s failure to recognise that a 50 year flood has a 0.02 probability of exceedance rather than 0.02 probability of occurrence which is causing the flooding.

The point I was making is that when you have calculated the 100 year flood to three decimal places and add freeboard you have designed the channel for a larger flood than what you have calculated. My argument is that you should then design the downstream structures for the same flood (the bank full event) that the channel is able to carry. The safety factors on structures should not be less than those on channels.

Brian

RE: 100 yr Storm (How often Does it Really Occur?)

My legal staff always recommends no change in downstream flows or discarge patterns.  Sometimes it can't be done but it is the same rule we have worked under for 30 years.  Most cities now require discarges from developments to be moderated to normal levels by detention basins.  In the above example the structure I had been working with was older than the downstream development which would have allowed me to restore it to it's original level but they still advised to not exceed the currrent level of discarge.  George was there first that is the key issue I believe but I'm not a lawyer.  

RE: 100 yr Storm (How often Does it Really Occur?)

Wolfhnd hit the nail on the head!  no change in flow OR discharge patterns.  However, if you do anything - you will change it.  For example time of drainage will change the impact of the flow downstream, even if peak flows are not increased.  And how far downstream must you check this?

I agree with both of you - downstream impacts should absolutely be considered.  However, this is clearly rarely done by most developers and not always checked by the agencies I have worked with.  Typically, peak flows are checked for the outfall on the subject property with pre-existing conditions and then with post-development conditions.  No increase in peak discharge is allowed.  Good enough for government work.  No investigation of what the effect is on the river, it is assumed that it always got into the river, and rarely an analysis further downstream to determine what existing infrastructure might be under designed.  Therefor, the reason for undertaking watercourse masterplans.  Study the watercourse, not the watershed, and establish a baseline condition and standards for future development or upgrades.

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