HW/D Maximum Values 1

[civilman72]

I recently reviewed a local municipalities (near Denver) drainage standards and was a little surprised to see the following statement as it applies to inlet control for culverts under roads:

"The maximum allowable headwater for the 100-year design flows will be 1.5 time the culvert diameter. Also, headwater depth may be limited by the street overtopping..."

Does anyone know the reasoning behind limiting the HW/D to such a "small" amount? I don't design small culverts with huge amounts of head, just to "shove" the storm conveyance through, but also limiting the HW/D to 1.5 seems way too conservative, particularly for a 100-year storm event.

Any opinions would be appreciated.

 

[RWF7437]

Just guessing, but here are several reasons that might apply:

1. You wouldn't want a very small culvert under a high fill. Such a culvert might easily clog and would be difficult, and expensive to clear or replace.
2. If such a culvert failed structurally, the same difficulties would result.
3. The roadway embankment acts as a dam. A very high "dam" might be subject to catastrophic failure during a severe rainfall event.
4. Exit velocities for a small culvert under high head might be erosive and endanger downstream properties.

Have you asked the Agency why they have this requirement ?

good luck

 

[civilman72]

RWF,

Thanks for the response, and I agree with all the statements you made. However, I was not interested in installing a very small pipe on a very large fill slope, but rather use (what I consider to be) a rational HW/D value of say 3. IMO, this would seem appropriate for a major storm event.

For instance, I have a road that is approximately 8' above existing ground. I place a 24" culvert under the road and am only allowed to have one-more foot above the pipe? In order to limit the HW/D you would have to install more pipes, or bigger pipes, but re-stating over again, this seems way too conservative especially for a storm that happens once every 100 years.

Second thing that makes no sense to me is that by their standards you are allowed to overtop the road (12" at edge of shoulder), but somehow you can't exceed a 1.5 HW/D? Allowing the overtopping of the road during a major storm event makes sense to me; it basically limits the amount of drainage infrastructure you need to install to convey only your minor storms. But by limiting the HW/D during a major storm appears to create exactly the oppposite affect - creating way too much infrastructure for the most common storm events.

I'm still waiting for a reply from the "guy" at the jurisdiction... all the others I have spoken with so far are clueless.

 

[RWF7437]

A so called 100 year storm has nothing to do with 100 years. The probability of such a storm being equaled or exceeded in 100years is 64%, NOT 1%.

Hope you get an answer from the people who make up these rules.

good luck

 

[civilman72]

I should clarify that I understand that there is not a 1% chance of a storm being equaled or exceeded every 100 years. Rather, in very simple terms, there is a 1% chance EACH YEAR that a "100-year" event will occur.

But I gotta ask, where did you get the 64%? I've recently read that the assumed rainfall events may have been underestimated by NOAA, particularly for the East Coast. This is considering that they have had numerous events in the last 10 years that have exceeded their pre-determined "100-year" storm event rainfall.

Thanks for the response...

 

[RWF7437]

http://web.umr.edu/~rogersda/umrcourses/ge301/What Is a 100 Year Flood.htm

 

[RWF7437]

A possibly better way of viewing these events, and speaking about them, is to refer to them by their annual probability of occurrence. The so called 100 Year storm is, by definition, the storm which has a 1 percent probability of occurring in any one year. If we want to know how frequently such a storm might occur over a longer period of time, that probability can be calculated by:

Px = 1 - ( 1-1/N)x

Where: Px is the probability of occurrence in x number of years

1/N= the Probability of Occurrence in any one year

For example, if we want to calculate the probability of occurrence of the 100 year flood over 100 years the calculations would be:

Px = 1 - ( 1-1/100)100

Px = 1 - ( 1- .01) 100

Px = 1 - ( 0.99)100

Px = 1- 0.366

Px = 0.634

In other words, there is a 63 percent probability that the 1 Percent storm will occur one or more times over the next 100 years.

This kind of calculation can be done for any selected range of frequencies and time periods. This has been done in Figure 7-3.

Figure 7-3 Probability of the N Year Event Occurring in x Years

 

[cvg]

Having several events in 10 years which exceed the previous 100-year storm estimate is possible. But it does lead one to question whether the original 100-year estimates for precipitation were valid...

It is very likely that given the amount and quality of data for both precipitation and stream runoff varies from fair to non-existant in most of the US, that estimates of 100-year rainfall are an estimated guess and in some cases, only a WAG. Denver is one area where extreme weather conditions is not only possible, but the norm and perhaps the UDFCD feels that large safety factors are necessary for culverts. Although, I agree that the policy seems a bit extreme.

I looked at an old version of the manual (1969) and it has design charts with HW for round culverts up to 3 times the diameter of the pipe. for arches it is up to 2.5 times the height of the arch. (Section 4.2.1) So apparently this changed policy (or perhaps wording error in the manual?) was introduced in a more recent version of the manual.

 

[RWF7437]

cvg,

If you want to see the probable error of estimate for some states visit the link and click on one of the states that is shaded blue.
Sorry CivilMan2, don't mean to hijack your post.

http://hdsc.nws.noaa.gov/hdsc/pfds/index.html

 

[cvg]

what I see for my area is that with 90% confidence, I can only get within plus/minus 20% for the 100-year, 1-hour storm precip depth. Quite a variance! Unfortunately, data for Colorado is a bit more sparse.

 

[civilman72]

Hey, thanks for all the interesting points of view. And don't worry about hijacking the post; it's led to some other interesting discussions that are also relevant to the original post.

For instance, I guess one of the easiest ways municipalities can deal with the uncertainties that are apparent within hydrology calcs (i.e. rainfall data), is that they decide to overcompensate by making excessively conservative standards like the one stated at the beginning of this thread. Somewhat aggravating, but also somewhat understandable.

 

[MattMurawski]

Some jurisdictions limit HW/D in part because high HW/D values can be associated with sediment transport problems.