Capacity of a Flat Trench
Capacity of a Flat Trench
(OP)
Hi,
I am working on a WWTP and we are using rectangular concrete trenches. There are numerous locations where the concrete trench has to be flat for sections (i.e. across UV lighting, around screens, etc.). When calculating the capacity of the trench I always use mannings formula but mannings obviously results in a capacity of zero for the flat sections. How do you calculate the capacity of these sections?
Thanks
Josh
I am working on a WWTP and we are using rectangular concrete trenches. There are numerous locations where the concrete trench has to be flat for sections (i.e. across UV lighting, around screens, etc.). When calculating the capacity of the trench I always use mannings formula but mannings obviously results in a capacity of zero for the flat sections. How do you calculate the capacity of these sections?
Thanks
Josh





RE: Capacity of a Flat Trench
Alternatively, the channel really does not have to be flat. You can always raise the depth at entry. Simply reduce the depth (raise invert elev) of the channel at the upstream entry to the flat area. That would give you an actual slope you could use in Manning's eq.
BigInch
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RE: Capacity of a Flat Trench
RE: Capacity of a Flat Trench
The slope in Manning's is actually the energy gradient of the fluid, not the geometric slope of the conduit. In uniform flow, it approximates the geo slope so closely that it is valid to use the two interchangeably. But it is always more valid to use the delta z in the HGL for the rise, than the delta z over the channel.
For this problem, you know flow, it is a given. You will use all the other parameters in Manning's to get delta z, or the losses over over the section. Capacity is dictated by allowable depth in channel for an open section (including freeboard), or by allowable pressure/surcharge in a closed section.
Engineering is the practice of the art of science - Steve
RE: Capacity of a Flat Trench
Josh
RE: Capacity of a Flat Trench
Engineering is the practice of the art of science - Steve
RE: Capacity of a Flat Trench
Thanks
Josh