Design Storm
Design Storm
(OP)
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?
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?





RE: Design Storm
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?
RE: Design Storm
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
RE: Design Storm
So does anyone else have any other advice for us and our fight against the township's interpretation of their ordinance?
Thanks.
RE: Design Storm
RE: Design Storm
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.
RE: Design Storm
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.
RE: Design Storm