SWM an storm sewer design

[JrEngCa]

Hi everyone this is my first tread. This is a really cool forum! Here is my question:

In my city we restrict the flow from a site to pre-developed conditions for a 1:2 year storm based on SWM requirements. Since the system is designed to surcharge, what kind of capacity should the storm sewers have? On private property I usually design for 1:2 year developed flows. Some engineers design for more than that even when releasing at1:2 year pre-developed flows. Wouldn't sizing the sewer based on 1:2 year pre-developed flows be enough since we are restricing the flow to this level? The flow control at the outlet is going to cause surcharging so does it really matter if the sewer pipes themselves only provide a level of service equal to what the outlet is giving?

I hope soeone can help me. I don't like over designing especially for industrial sites that does not really require a high level of service.

Thanks,
Pete

 

[blueoak]

I worked in northern California and development sites had to maintain runoff flows for 2 yr events to predevelopment conditions. To some degree this is a water quality requirement where CalEPA pretty much only requires treatment up to the 2 yr event because this is about 90% of the total runuoff over a period of time. This allowed stormwater basins to be built now and not include huge areas due to future development. It was also a stream health since the 1.5 year event (sorta) moves the most material and by limiting the small storms you can pretty much slow downcutting in urban creeks and outfalls. I really think you are probably dealing with the second choice since CA municipalities are required to investigate and protect erosion in all of the new NPDES permits and the sorta of chicken little method is the control of the 2 year event. It is really much more complicated than this, but ... you get the picture.

Saying all of this, you weren't allowed to use overland flow for events less than a 10yr event. Therefore you needed an overflow to any detention or flow control device so that anything less than a 10 year was underground (at least on the public side of the project).