Modeling a sloped bottom, level top infiltration trench

[sajensen]

thread789-247172

This is a follow-up question on my posting regarding using Rectangular "pipe" for modeling gravel-filled sloped trenches with embedded perforated pipes.

I am trying to model stage-storage volume for the subdrainage pipes and gravel filled trenches on a synthetic turf field with the following parameters:

1. The surface of the field is flat in the main section with a small area where the surface slopes at 1%
2. The subdrainge pipe/trenches are sloped at 0.5% in the flat area and 1% under the sloped area
3. The subdrainage pipe/trenches at the two field areas are separate but connect to a common "collector" pipe

Using rectangular "pipe" for the trench and embedding the perf. pipe within works fine where the slope of the pipe/trench matches the slope of the finish grade since the "trenches" have a constant, uniform depth below the field subgrade.

My question is, on the "flat" section of the field how do I model a pipe trench where the vertical depth of the trench on the upstream end (trench is 12" wide x 10" deep) is different than the depth on the downstream end (trench is 12" wide x 28" deep)?

Do I model the trench in two parts with the bottom sloped section as 12" x 10" with the embedded 4" perf pipe and then model the top portion as a 12" wide x length of trench x variable depth "pond" that routes to the "trench" below?

What I've been doing is modeling the trenches as "rectangular" and using the depth of the downstream end as the "depth" of the trench. But, with the trench bottom on a slope the "top" of the trench effectively ends up ABOVE the surface of the field towards the upstream end. (I've been just discarding any stage-storage volumes above the subgrade elevation of the field which is the elevation we use to determine storage).

I am hoping there is a simpler, more accurate way within HydroCAD to model a variable depth, "non-rectangular", gravel-filled trench with embedded perf. pipe on a slope? The ability to input a variable-depth option would allow modeling trenches that have a sloped bottom/level top AND those trenches where the bottom slopes differently than the top.

Thanks for any ideas/suggestions!

Shawn

 

[psmart]

To model the variable trench height you'll need to use custom storage. This means you'll have to calculate the trench surface area at multiple elevations.

However, unless the water surface elevation actually rises above the top of the trench at the low end, there will never been any water in this wedge-shaped volume, and you wouldn't need to include it in the model. As a precaution, set the pond's flood elevation to the top of the low end of the trench, so you'll get a warning message if you ever reach this elevation.


Peter Smart
HydroCAD Software

http://www.hydrocad.net