Curb Inlet on Grade with local depression computes capture efficiency incorrectly

[BenTWhite]

Take the following curb inlet:

Property	Value
Long Slope	1%
Sx	2%
Sw	0.0
Curb Length	3 ft
Local Depression Width	2 ft
Local Depression Height	2 inches
Flow	1 Cfs

SWMM calculates a capture efficiency of around 60%, but if you run the same inlet in the FHWA Hydraulic Toolbox, you get a capture efficiency of around 48%

SWMM is calculating the upstream spread taking into account the local depression, whereas the spread computation should ignore the local depression, and be based on the incoming uniform gutter.

Here is a quote from HEC-22

Remember that for locally depressed inlets, the quantity of flow reaching the inlet would be dependent on the upstream gutter section geometry and not the depressed section geometry. (HEC-22, 4-41)

UI Issue

There is another related UI issue that I will bring up here as well, the local depression width and local depression height on the street view get messed up on save and reopen of the SWMM project and u need to re-enter them.

[LRossman]

I get an efficiency of 51.7% from the FHWA Toolbox (v.4.4) using a Manning's roughness of 0.016 compared to SWMM 2.4's 61.3%. (Release 2.4 included a bug fix for the equivalent cross slope for a depressed inlet that is used to compute capture efficiency.) Without the local depression both programs give the same result (26.8%).

I ran both programs on the two examples in the HEC-22 manual that include curb inlets with a local depression. The results were as follows:

Example	Solution Variable	HEC-22	SWMM	Toolbox
4-9a	Captured Flow (cfs)	1.55	1.54	1.47
4-9b	Inlet Length (ft)	12.5	11.5	13.9

The manual calculations appearing in HEC-22 only carry two decimal points which might explain some of the differences in the table. But it still appears that the Toolbox may have some issues of its own when implementing the HEC-22 procedures for depressed inlets. I can't find any source code for it to check its calculations against SWMM's.

I did verify the UI bug that was reported so that needs to be fixed.

[MattAndersonPE]

Lew - Best to email concerns on FHWA Toolbox page to the contact on this page: https://www.fhwa.dot.gov/engineering/hydraulics/software/toolbox404.cfm It's changed over the years - but *Ryan Lizewski *is listed today. ***@***.*** They are usually really good if you document the question to validate the issue or answer a question about the process it does. This is why Julien found the cross-slope issue. Matt Anderson Guage Engineering

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On Fri, Feb 2, 2024 at 11:52 AM Lew Rossman ***@***.***> wrote: I get an efficiency of 51.7% from the FHWA Toolbox (v.4.4) using a Manning's roughness of 0.016 compared to SWMM 2.4's 61.3%. (Release 2.4 included a bug fix for the equivalent cross slope for a depressed inlet that is used to compute capture efficiency.) Without the local depression both programs give the same result (26.8%). I ran both programs on the two examples in the HEC-22 manual that include curb inlets with a local depression. The results were as follows: Example Solution Variable HEC-22 SWMM Toolbox 4-9a Captured Flow (cfs) 1.55 1.54 1.47 4-9b Inlet Length (ft) 12.5 11.5 13.9 The manual calculations appearing in HEC-22 only carry two decimal points which might explain some of the differences in the table. But it still appears that the Toolbox may have some issues of its own when implementing the HEC-22 procedures for depressed inlets. I can't find any source code for it to check its calculations against SWMM's. I did verify the UI bug that was reported so that needs to be fixed. — Reply to this email directly, view it on GitHub <#21>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ACDUFI3JS3LFMMHSSP4QRG3YRUKV7AVCNFSM6AAAAABCVTJE7GVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTSMRUGI3DGNRUGU> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>