Design and risk

[SMIAH]

I have a project to rebuild a dam located near the summit of a mountain, at the outlet of a lake. The discharge of the lake is made only by the dam as the water is directed into a stream to the bottom of the mountain while the river flows into a river.

The design of the dam is governed by standards that set the design return period to 1000 years. On this stream, are located several culverts, bridges and small weirs. These private works are subject to standards of the county that set the design return period at 10 years.
Some stakeholders have told me that they believe that a design return period of 1000 years should be used for the structures located downstream of the dam (i.e. they would like to override the standards).
This request appears really questionable to me.
An owner located downstream would be penalized by the presence of a work of greater importance upstream…
Has someone ever been through a similar situation?

 

[Ryb01]

Worked on a few projects with similar scope. Desiging structures downstream of the dam for a 1000 year event is excessive. However, I'm not sure I would proceed with only sizing structures for the the 10-year event either. For roadway crossings, typically a 25-100year event seems more reasonable depending on the location, size of raod, traffice requirements, etc.

If in the event that the dam failed, hopefully never, I'd suspect that the structures downstream, even if designed for the 1000-year event, would not be there after.

"An owner located downstream would be penalized by the presence of a work of greater importance upstream..." could you exapnd on this further?


Hope this helps. This is an interesting challenge.

 

[coloeng]

I believe you need to design a facility based on the level of risk associated with its failure. For example the failure of a dam carries much greater risk to the general public than the failure of a culvert under the road downstream of the dam. That is why a dam is designed with a much greater return period than a culvert is.

In Colorado dams are designed for the probable maximum event (Noah's flood). While some argue that this is extreme, others argue that if you know the event is possible and you don't design for it, if there is a subsequent failure of the dam because of the more extreme event, there should be liability for not designing for a known potential event.

Are these stakeholders asking for the County to change their design standards and force the private parties to replace their facilities based on the new standards, or are they asking the owner of the dam to redesign and replace these private facilities?

 

[SMIAH]

Interesting comments, as usual.

Actually the return period for the design of culvert is 25-year. This is a mistake.

Are these stakeholders asking for the County to change their design standards and force the private parties to replace their facilities based on the new standards, or are they asking the owner of the dam to redesign and replace these private facilities?

They are asking the owner of the dam to conduct a technical study in order to evaluate if the culverts located downstream have the capacity to pass the flow.

An owner of a dam located upstream of another dam, would have to evaluate the effects of the presence of his dam on the downstream one. While this is not the case in the opposite situation. Which is for me nonsense.

For my general culture: So in Colorado you don't have a way to classify the risk associated with the dambreak? Even a small dam has to be able to withstand a PMF?

 

[jgailla]

It is absolutely ridiculous to have downstream culverts pass a 1000 year event.
coloeng has it spot on.
So what if a road washes out under a reasonable (10 year) sotrm.

The possible economic damage if a dam breaks is significantly higher than the damage from a culvert sized for a smaller storm and overtopping or destroying the road.
For example, state roads have different criteria for design depending on the importance of the roadway, because more traffic equals more economic loss in case of a failure.

All engineering is based on economic risk. Even in the case of human life there is an economic component. The highway fatality rate could be dropped to almost nothing if the speed limit was 25 miles per hour. Most people have no conception of risk.

 

[coloeng]

After reviewing the Colorado Rules and Regulations for Dam Safety and Dam Construction, there are actually four classifications of dams in Colorado:
1) High Hazard- Loss of human life is expected in the event of failure.
2) Significant Hazard- No loss of life expected but significant damage expected in the event of failure.
3) Low Hazard- No loss of life or significant damage expected in the event of failure.
4) No Public Hazard- No loos of life and damage only to owner's property expected.
Dams are then further classified by size- Large, Small and Minor.

The inflow design flood requirements range from 0.9 x PMP for a large high hazard dam to 50 yr return period for a minor low hazard dam. The inflow design requirement is based on the perceived risk associated with the failure of the dam.

We have a similar issue with having a dam upstream of another. The downstream dam is considered a low hazard dam. Although if there were no dam downstream ours would be considered a low hazard dam, because of the dam downstream it is considered a significant hazard dam. The reason for this is that if it failed it would probably cause failure of the dam downstream.

 

[cvg]

owners downstream should never be "penalized by the presence of a work of greater importance upstream...". The dam owner is fully responsible for any changed conditions created by his dam. If building the dam increases the risk to downstream properties, than the dam owner is the responsible party to mitigate that risk. Of course some of the stakeholders may also own culverts downstream of the dam, if so then they have a right to ask that risk is not increased. This may require an inundation study and perhaps a full risk assessment for normal operation as well as for dam break.

 

[SMIAH]

In the case I’m referring to, the works on the dam aims to increase its discharge capacity so that it can allow the passage of a possible 1000-year flood. This increased capacity results in a lesser increase on the water level of the lake upstream.

The owner must study the flooded areas downstream after the dam failure and based on the residences or affected infrastructure, the risk associated with the dam is classified. According to this, measures must be taken, among others, regularly inspect the dam (the frequency of inspections is proportional to the classification). Also, the owner must communicate risk to people concerned about the consequences of failure (e.g. map with flood response time).

However, it is not usually required to ensure that downstream facilities are properly sized. And that's the whole point.

I think there is a distinction to be made between the case associated to the construction of a new dam and the one of a dam reconstruction.

 

[cvg]

depends on the scope of the reconstruction. if you are altering the discharge rate from the dam, and that rate increases than you are imposing conditions on downstream properties that could not be foreseen. If those downstream culverts were properly sized and then after the dam reconstruction, they are being overtopped (for the 10-year storm) then the dam owner has a responsability to mitigate that.

 

[SMIAH]

Very true.

 

[beej67]

I have to imagine this (sticky) issue varies quite a bit by region, but in Georgia the engineer of the upstream dam would be on the hook for a dam breach analysis for sure, assuming it was a dam big enough to fall into the Safe Dams Act. I've even seen a dam breach analysis required for a dry detention pond by some of the more stringent municipalities around here.

Something I'm unclear on, SMIAH .. these "stakeholders" who want the downstream private owners to upsize their culverts even though code doesn't require it .. what "stake" do they hold exactly? Is it the review agency itself? Concerned citizens? Perhaps the downstream owners themselves fishing for a settlement based on a perceived burden due to the new dam upstream?

Like many things in the business side of engineering, this issue smells like half science and half relationships to me. It's my experience that the best thing to do sometimes is stop with the science for a second, and try to get a handle on what each stakeholder's motivation is.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[SMIAH]

Indeed, a dam breach study had to be done.
These stakeholders are mostly concerned citizens.
It makes sense to think that they want the dam owner to pay for a study of their design works.