maximum water depth / diameter in open channel flow

[Geotech910]

We are checking a roof drainage pipe (30m length- 10" dia) that water depth will be about 75% of the pipe diameter. some codes require 67% max ratio. what is the potential risk of allowing water depth to exceed the code requirement given that the pipe length is only 30m (from the downspout to the sump)

 

[bimr]

The maximum flow rate does not occur under full bore conditions. The maximum discharge occurs when the water depth in circular pipes reaches 93.8% of the pipe diameter.

If you allow the water to back up, you may have a leak on the roof.

 

[cvg]

don't forget about leaves and debris. most roof drains get plenty of that and it really cuts down on the flow capacity of your pipes.

 

[BigInch]

Careful. The real danger is ponding on the roof and collapse of the structure.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[Xalii]

At 67% depth you are at 80% of the max flow.
At 75% depth you are at 91% of the max flow... (max flow is at 82% depth)
Then passing from 67% to 75% you divide by 2 the security margin...
With the forecast of stronger storm in coming years, I let you make the calculation of damage versus installing larger pipes.