Capacity of a Flat Trench

[Jeamon]

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

 

[BigInch]

Water at a higher hydraulic profile can flow to a lesser hydraulic profile, so I'd approximate that effect by reducing the upstream cross-section to a slightly lesser depth (invert elev) than what you actually have there. Calculate the resulting "virtual" slope. Then assume that the channel across the flat area only flows at the reduced depth and has that virtual slope over the entire length of the flat section.

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-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[stanlsimon]

I have always interpreted "slope" to mean average slope. If there exists a short section of flat trench, it has never been much of a problem, perhaps because the trench was draining a variable flow. In my case (big industrial washers)the waste water flow was contained in the storage capacity of the trench, not sure if your flow is pulsating or constant.

 

[LHA]

Big Inch has got it conceptually correct. But I'll go further:

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

 

[Jeamon]

Thanks for the replies. When doing the exercise described, I assume I should be using the critical depth of the channel as the minimum depth in the slope equation?

Josh

 

[LHA]

Not necessarily. You have find a point in the system where you know the actual operating water surface. At the disinfecting stage, it is probably the weirs out; at the bar screens, maybe an EQ tank in? Then calulate losses for each unique section until you get a point on the HGL into or out of the section in question. Step up or down across that section, there is the HGL.

Engineering is the practice of the art of science - Steve

 

[Jeamon]

Thanks for the reply. One other question I have relating to open trench designs...how do you usually account for minor head losses in concrete trenches? I have a concrete trench that will be turning 180deg. and I am not sure how to account for minor losses in this trench as I am sure the typical closed conduit friction factors don't apply. Any advice?

Thanks

Josh