Design Storm

[SiPaul]

Most of the municipalities in our area require storm drains to be designed for a 10 year storm. The municipality that I am in technically requires a 5 year storm for some streets and alleys.

I would like to change the requirement to a minimum of 10 year storm for all conduits, inlets, etc.

What is the best argument for changing the design storm? Given that different municipalities use different storm frequencies how does one logically determine a 10 year storm?

Any thoughts?

 

[TVE]

Hmm.. tough one...

Your best arguement would probably be flooding damage resulting from only sizing the sewers for a 5 Year storm.

Next would be that either the upper tier or other higher level authority requires the higher design storm.

Third best is probably something along the lines of "Does anyone know why we have a lower standard than everyone around us?".

The biggest arguement against the change would most likely be "And where are we going to find the money to pay for these larger size sewers?".

I guess the first real question is "Is there a need to change the standard?". If you aren't having problems, why do you want to increase the standard and its associated constructino costs?

 

[cvg]

before you just up and change the standard, you had better have
a)backup from the city manager
b)discuss / reach concensus with the home builders association (or other such organization)
c)valid technical reasons for the change

this will not be a popular change - expect a fight from the development community

 

[landerso30]

Here is an interesting dilema we are having with a site in PA. The township stormwater ordinance states that the storm sewer pipes shall be designed to convey the 25 year storm, and that the EGL & HGL shall be contained within the system. The township engineer, who happens to be a another private firm, is interpreting this to mean that the HGL & EGL for the 25-year storm shall be contained within the actual pipe. As long as I have been designing pipes, I have always interpreted the "system" to include not only the pipe, but also the manholes and inlets; as long as they arent surcharged. Obviously our pipes are alot larger in order to contain the HGL & EGL within the actual pipe. We are thinking of trying to get some evidence and ammo to try and fight this. We have established a few reasons for doing so. First, the storm system will be over designed, and its a huge development, and secondly, it will be a huge cost to our client. Also in order to meet the demands of the township engineer, exsiting offsite pipes will have to be upsized because in existing conditions the HGL/EGL arent within the pipe, just cause we are tying into them.
So does anyone else have any other advice for us and our fight against the township's interpretation of their ordinance?

Thanks.

 

[SiPaul]

It is nice to see that these "old" issues get some new life with new perspectives. I have generally come to the conclusion of others who have replied to this thread that the main indicator should be the flooding. I am in charge of storm drains for a large (older) city and many of the municipal drains are arguably undersized. However, even those that may only pass a 2 year storm as determined by computations do not experience substantial flooding problems. This may be due to an overly conservative model or our modeling techniques may simply be overly conservative as a rule. It is hard to justify a Capital expenditure for a problem that only exists on paper. Generally these "undersized drains" are the large ones that drain at least a square mile. However, even the smaller ones don't have big problems. The problem areas are almost all in sumps. I think that to "fight" a municipality a complaint history could be the best indicator of adequacy of a drain. Albeit, one would have to have a 10 year storm in the history.

 

[cvg]

I would agree with your interpretation of "system". It is obvious that the drainage system is comprised of pipes, manholes, catch basins, inlets, etc. To state that it is only the pipe is somewhat arbitrary.

Unless you expect debris in your stormwater or possible blockage, the water surface will not likely reach the energy grade line. In addition, if you have some freeboard in your inlet structures, manholes etc. than you still won't experience flooding. And temporary pressure flow in a storm drain (under an extreme event such as a 25-year flood) is not likely to be harmful to the system. So, in effect the system will be able to convey the design flow.

As a policy matter, if the township desires to maintain EGL within the pipes, it should be described in the ordinance as below the soffet of the pipe.

 

[LeoSC]

I believe that it makes more sense to design the overall system taking into account the concept of minor and major systems: the minor system conveys the more frequent storms, say up to a 5-year storm, and the major system is designed so that the 100-year storm can be conveyed safely. This concept is used successfully in Ontario.

Furthermore, when one is forced to examine the major system, the resulting design is safer because areas where large storms could cause flooding are eliminated.

Aside from the issue of the very high cost, there is another aspect to consider when one designs a storm sewer system for the 10-year or the 25-year storms: one has to make sure that the runoff actually makes it into the pipe, i.e. the inlets have to be fairly substantial. This leads to issues of safety, aesthetics, etc., and more cost.

 

[cemorway]

The only caution i would like to add is consideration to downstream systems. If they have been designed to handle a postdevelopment 5 year event for the upsteam drainage system, you may create adverse conditions downsteam. I agree, 10 yr is pretty common for sizing storm sewers.