Providing interception for roof

[bcsatloswray]

Sorry for the long-winded post.
I am not a drainage engineer, but have somehow ended up with this project in Cardiff UK where I need to provide a drainage scheme for the new structure and the sticking point is the interception criteria. Basically, the first 5mm of rainfall isn't allowed to drain out of the site and must either infiltrate to the ground, be captured in rainwater harvesting units (only if there is a daily useage requirement) or be captured by vegetation and evapotranspirated. I cannot infiltrate water as the site is contaminated, I cannot use rainwater harvesting due to the local council's restrictions, and using the SuDS manual I would need around 2.9 square metres of vegetative space per 1 sq.m of roof, since the roof is 545 sq.m, the vegetative space required becomes so large it effectively acts as lined permeable paving draining an impermeable area which the SuDS manual states does not comply with the interception criteria. I know a green roof is one solution, but I would like that to be a last resort due to cost.

My thinking is that I could use a storage tank connected to the downpipes which would fill up for the initial rainfall, then as is continues to rain, the siphon effect empties the tank ready to store the initial rainfall from the next storm. Like a pythagorean cup. Once again I am not a drainage engineer so please let me know if this makes any practical sense. Thanks! Attached cross section attempting to clarify.

 

[cvg]

something like this?

 

[CarlB]

If I understand your concept, the initially captured water is released to the downstream drainage system once the volume is exceeded. So this is a form of detention, but I don't believe meets the intent of "capturing", which is to not allow the runoff to proceed downstream.

 

[swazimatt]

isn't your option the same concept as using an orifice controlled detention tank - you are not retaining the first 5mm on site, you are just detaining it until it fills and empties. If your proposal is allowed i would think a detention tank with orifice would be better as the flow is lower to avoid increasing peak runoff from the site

What is the reason for holding the first 5mm? is it to catch the first flush (ie treatment) or discharge capacity issues. if it is treatment the stormceptor cvg posted would manage this, Spel also make something similar. Some systems are designed to catch the first flush and then as flows increase they are diverted around the treatment device