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

[Jeeman]

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?????

 

[cvg]

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.

 

[wolfhnd]

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.

 

[cvg]

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)

 

[BRIS]

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

 

[wolfhnd]

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.

 

[cvg]

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.