SSA Curb & Gutter Inlet Calculator

[cpusey]

We are trying to transition out calculations for gutter spread to be done in Civil 3D so we don't have to keep old computers around to run HY-22 that NCDOT basis its hydraulic calculations on. The Curb and Gutter calculator matches up with HY-22 for spread calculations for an inlet on grade but spread numbers are almost doubled for "inlet on sag" solutions as compared to HY-22. Can someone explain why there is such a difference? The two programs match perfectly for inlet on grade and NCDOT has been using HY-22 as a standard for years. That does not mean its right but it would be a big deal if it were not. Spread for the same inlet in a sag is computed in both software's below. 

Thanks for the help. 

[fcernst]

I have verified your HY-22 results with other software, however it won't let me use the 1' gutter width that SSA is allowing because the grate width at 2' exceeds that.

Try using 2' gutter width in SSA to see what happens.

[cpusey]

It decreased the spread somewhat of course but did not get a solution similar to HY-22.

[cid:ABD23EA3-68CD-43A7-A49E-051C1AD96E6A]

Chris Pusey, PLS | Professional Land Surveyor
office: 910-426-6777 | ext 103 | cell: 910-489-6777 | fax: 910-426-5777

409 Chicago Drive, Suite 112, Fayetteville, NC 28306
On time, every time. | www.4Dsitesolutions.com

[cpusey]

I ran it with a 2' gutter and got a smaller spread but it did not line up with HY-22. If you verified the results from HY-22 I must have something input wrong or the calculator is wrong. I have had someone else verify my inputs and we think they are the same with different results. I am beginning to think that the SSA calculator might be broken. 

[fcernst]

Ok. Yes, I don't use SSA. Never been comfortable with the results and bugs

[cpusey]

What are you using to compute the spread? I am setting up an old computer in my office now to run HY-22 but we are looking for something we can run on Windows 10. Thought SSA was the answer.

[cid:ABD23EA3-68CD-43A7-A49E-051C1AD96E6A]

Chris Pusey, PLS | Professional Land Surveyor
office: 910-426-6777 | ext 103 | cell: 910-489-6777 | fax: 910-426-5777

409 Chicago Drive, Suite 112, Fayetteville, NC 28306
On time, every time. | www.4Dsitesolutions.com

[Matt.Anderson · Accepted solution]

HY-22 was last published in 2002.

The FHWA updated the tool as the Hydraulic Toolbox as it contains the updates how it calculates spread due to better guidance provided.  It reports 4.302 feet (2-foot gutter)

For example -

• Gutter depression and local depression are now separate items that change how the results will compute. (Not in this case)
• The physical gutter parameter can't be less than the width of the grate.  (This effects how to compute spread as key difference)

Note that both HY-22 and Hydraulic Toolbox compute the same depth at curb and at the center of grate.

The rest of the computations is how do those depths translate to spread.  

In HY-22, the depth-ave is at the center of the grate, which is on the edge of gutter, so HY-22 should resolve to pavement spread of 4.3 feet + gutter depth or a total spread of 5.3 feet.    It doesn't - it reports 2.2 feet which should not make sense with the depth of flow.  The 1.2 feet of spread on the pavement is a depth of flow at the gutter edge of 0.024 feet, or 0.064 feet at the curb. HY-22 is wrong. 

As the system is in weir flow, your SSA calculations are incorrect (slope is 0.042 vs 0.04), and you've increased the width of the gutter.  Since this sag inlet performs all in weir flow, your should keep your gutter at 1 feet, but change the inlet shape to 1 foot by 5 foot (keeping the total weir flow length) as the combination inlet ignores the curb-opening back.  

The right answer should be closer to 4.302 feet (Hydraulic Toolbox) or 4.29 feet (SSA)

[fcernst]

In HY-22, the depth-ave is at the center of the grate, which is on the edge of gutter..

No, it's at the center of grate.

This grate capacity calculation involves weir/orifice control analysis using an avg depth across the entire grate. You can not take that avg depth out to the edge of grate to compute spread.

FHWA Toolbox also incorrectly computes spread for sag curb inlets, because they incorrectly use the weir flow projected depth, and take that out to the gutter to compute spread.

Test this sag curb inlet:

10 cfs

10 ft length

Sx = 0.02

Sw = 0.083

Local Depression = 3"

[cpusey]

Thank you for the insight. We are processing the info we have gotten on this thread. Very helpful.

[cid:ABD23EA3-68CD-43A7-A49E-051C1AD96E6A]

Chris Pusey, PLS | Professional Land Surveyor
office: 910-426-6777 | ext 103 | cell: 910-489-6777 | fax: 910-426-5777

409 Chicago Drive, Suite 112, Fayetteville, NC 28306
On time, every time. | www.4Dsitesolutions.com

[Matt.Anderson]

Fred -

You are correct - depth_average is at the center of the grate.  But I must protest taking me out of context -  the center of the grate is 1 foot from the edge of the curb as the grate is 2 feet wide and the gutter is the gutter is 1 foot wide.

Geometrically - something doesn't add up and hence HY-22 doesn't compute correctly.

I think you misinterpret HEC22 for sag inlets as you contradict 4.4.3.2 by saying the smaller orifice condition depth is controlling in this situation. 

[fcernst]

No, I never said this was in orifice control... this scenario is under weir control.

[fcernst]

My last example was simply lowering the grate 1" below the gutter at the flowline.

This example use the definition of local depression as "a" to compute the grate cross slope.

What do you get for spread if you use 2' for gutter width in HY22?

[Matt.Anderson]

Fred -

Ah, now I see the issue.  

You resolve spread by recomputing the geometry of the grate as if the grate slope controlled the weir flow.

However, per FHWA, the depth is computed at the edge of the depression (it's weir flow) and hence any gutter flare to accommodate the wider inlet and local depression is meaningless to the inlet during weir flow. 

The spread that the Toolbox uses is the simple geometry of Sw, Sx, and Gutter Width.

[fcernst]

However, per FHWA, the depth is computed at the edge of the depression (it's weir flow)..

No, the FHWA doesn't say that, and the depth used is the average depth over the grate and the effective weir length (see the figure next to my calculations).

I am computing depths and water surface elevations to calculate spread. I am also getting the same d_ave and d_curb as his HY22 example.

The spread that the Toolbox uses is the simple geometry of Sw, Sx, and Gutter Width...

Interestingly, I'm finding the Toolbox is also unresponsive to changes in the local depression value, so it's not behaving properly.

[Matt.Anderson]

Ie, when I doubted and sought clarification of the Hydraulic Toolbox, I emailed FHWA.   The response I received was what I indicated.

The local depression has no bearing on spread in sag because the measurement is before the depression where the spread is the greatest.  

[fcernst]

The local depression has no bearing on spread in sag because the measurement is before the depression where the spread is the greatest...

Well, that's not correct. Changes in the local depression affect the resultant depth and water surface elevation.

[fcernst]

I see a problem with sag curb inlet calculations in the FHWA Toolbox, Hydraflow Express, Hydraflow Storm Sewers and Civil 3D Analyze Gravity Networks. They all compute 17.07 for spread in this example.

Flowmaster and Stormwater Studio both compute the correct longer 23.3’ spread for the sag curb inlet, and I checked this (see below). Where Aquaveo is going wrong is saying that the 0.47’ depth is “upstream of local depression” in their output, then taking that short "depth" dimension, and placing it in the gutter to compute spread. You can’t do that. It helps if you set a datum at the curb lip and compute the water surface elevation (WSEL), then you’re dealing with absolutes.

[Matt.Anderson]

Fred -

Funny - look at the location of A-A in the plan view.  It shows the upstream of the local depression.

The one element that you are missing is the manual calculations are you are ignoring the compound gutter/road section.  The slope of the gutter depression vs. the inlet depression.  a is the inlet depression in the solution you calculated and you've lumped the gutter depression into a.

[fcernst]

No, i'm just using "d" and the gutter pan lip as a datum to compute the WSEL , then computing spread. 

Talking to other software developers they say they see a lot of confusion as to what "d" is. So, it helps to see better if you use the datum technique I show here and compute a WSEL.

[Matt.Anderson]

The difference is the gutter geometry. 

100 less 0.02*2 ft = 99.96    [2 foot gutter ignoring the compound gutters slope]

100 less 2 ft *.083 = 99.83  [2 foot gutter at compound shape]

At depth of 0.47 - the resulting WSEL becomes different and hence spread is different.

Does it make sense that a compound gutter section upstream of an inlet carries the same spread as one with uniform road Sx?