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# Channel Drainage Problem

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## Channel Drainage Problem

(OP)
I am trying to figure a problem out.

If I have a precast concrete channel system of 5 lineal metres with x height and x width capped off at one end, how do I find out what the flow rate out of that channel is from the other end when the inflow is keeping the water level at maximum capacity inside the channel at all times.

And then I need to calculate this again for 10 & 15 lineal metres, I do know that the flow rate diminishes with every additional metre of length but am unsure how to calculate this.

Any help would be greatly appreciated.

Cheers

### RE: Channel Drainage Problem

flow rate can be determined with weir/orifice equation as appropriate for the outlet. flow depth / friction loss can be determined using mannings equation.

### RE: Channel Drainage Problem

If the "capped" storage volume is prefilled with water, then the outflow will be very close to the inflow. Although there will be some peak attenuation due to channel routing effects, the effect will be minimal for that length. This could also be modeled as a pond with weir overflow (as suggested by cvg) but again, the peak reduction will be minimal for the arrangement you have described.

You said "I do know that the flow rate diminishes with every additional metre of length"

How do you know this? What is the exact mechanism of the flow reduction that you are expecting?

Note that the storage volume below your outlet weir will have no effect on the outflow hydrograph (assuming the channel is prefilled up to the weir.)

Peter Smart

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