Gutter Spread in Hydraflow Express Tool

[mmckeonJJMML]

There seems to be an issue when analyzing several inlet scenarios with Civil 3D's Hydraflow Express tool. It seems that even when there is 100% capture, the gutter spread does not change as a result of the inlet grate size. 

Is there a way to model inlets (in both sag and on slope conditions) where the gutter spread is impacted by the inlet grate size?

[Kevin.Spear]

Make sure to use either a grate or combination inlet so that you can enter the OPEN AREA of the grate. This affects the inlet capacity.

But the real answer depends on your catchment flow. Sag Inlets operate as a weir initially, open area is not a consideration. Once a sag inlet is submerged, it acts like an orifice. This is where the open area impacts the calculation. Inlets on grade do not consider the open area of the grate. 

Look in the User's Guide (F1) under Inlets>Computational Methods to see which variables you may be overlooking.

[fcernst · Accepted solution]

Grate inlets intercept little more than the flow
conveyed by the gutter width occupied by the grate. (HEC-22)

[mmckeonJJMML]

I suppose I should adjust my question to indicate that the primary confusion is that by changing the dimensions of the grate (so that the weir width is increased) the spread does not appear to be affected. 

Logically, the flow spread would be impacted, however there seems to be an issue in having the program model it accurately. I'm concerned that there is a setting selected that should not be. 

[fcernst]

No.. it's the fact that grates have much less effect than one would think at first.. until the HEC-22 literature is thoroughly reviewed..

[Kevin.Spear]

Express only considers flow and gutter geometry for spread. It does not consider the inlet at all. I thought you were looking for captured flow and then using a custom channel to calc spread.

[mmckeonJJMML]

Thank you Kevin. That answers my question. 

Based on your answer it seems as though it would not be possible to adjust the model to account for the inlet grate - correct? Instead, it appears the best way to analyze the system would be to use the flow depth given from the (gutter spread) to analyze the inlet as a weir.  Would you agree?

[fcernst]

Express only considers flow and gutter geometry for spread. It does not consider the inlet at all....

What??

[Kevin.Spear · Accepted solution]

First, the spread calc itself does not consider inlet dimensions, only flow and gutter cross section.

However, the flow bypassed is what's left over from what's captured by the inlet. That calc is based on its efficiency; what I would consider the grate open area. In express, the open area is just the grate length times width; presuming bras, etc. are deducted from the actual physical dimensions. As @fcernst mentioned, this aspect has little to do with the width of the inlet as much as the length does.

In other words, you're not crazy. 🙂

[Charlie.Ogden · Accepted solution]

@mmckeonJJMML , for a grate on grade the "gutter spread" is measured upstream of the inlet - so that is why the size of the inlet doesn't matter. As Fred showed, it decreases when it passes the inlet - and that will be affected by the inlet size. 

However, for a grate in sag the gutter spread will be affected by the grate dimensions, and Express shows that (if it's not, please show us the parameters you are using). This is because for a grate in sag there's no bypass so the whole flow enters the grate - what is calculated is the depth over the grate for that flow to pass through - then the spread is calculated from the depth and the gutter/road geometry.

[KirkWM]

@Charlie.Ogden is right, there are several different calculations going on in express, it initially calculates spread based on flow and gutter geometry, then it calcs the inlet intercept capacity (which is different depending on if you are using grate only, combination, curb opening), then it will recalculate the spread (using flow and gutter geometry) after the inlet with the reduced (or sometimes even eliminated) by-pass flow. I have a whole AU class about using part parameters, like grate width and length and curb opening height, from the part catalog, to be linked to the Analyze Gravity Network (AGN) tool within C3D if you want to incorporate this into a robust workflow but unfortunately, you have to do a few more steps to save/preserve spread data as AGN does not save this info back to the parts (https://www.autodesk.com/autodesk-university/class/Analyze-Gravity-Networks-Beyond-Basics-2020).

[jasonvia]

What about modeling in AGN? 

We have been contemplating the Inlet Depth versus the height of flow from the gutter line. 
Jurisdictions limts of design use the 5" rule for a standard curb and 1/2 travel lane for the spreadl limits. 
Are we correct to assume the Inlet Depth is in a depression of X inches and therefore greater than the flow based on the gutter flow immediate upstream of the Inlet ? 

To add, 
In AGN using a combination inlet is more restrictive in output (higher values) then using the Generic Drainage with pipe connections Parts list structures which raised all sorts of questions, and we ran multiple value scenarios and we just dont believe the output at all. With an open area of curb inlet/hoods the intercept should be grater than a standard grate alone on grade or in a sag.  
We can repeat instances where the standalone grate calculates a lower value (higher intercept/inflow) than a combination inlet.