I have two questions for Outlet Rating Curve.

1. I am reviewing a model which contains, SICB tabular head rating curve. How can I check and confirm those numbers?

For other types, I saw that MTO Drainage Management Manual has Design curves, but not for SICB. How should I proceed?

2. When we have multiple type of catchbasin, let say SICB, Herringbone and fishbone type in the same subcatchment. If there is only one outlet link that connects manhole with major system node, how can I assign different catchbasins types along with their numbers?

could you please answer my above question(s)? I am in time crunch, and with your guidance I am hoping to complete rating curve table here.

Hi Hiral

We model the road gutter channels using SWMM transects and use FHWA HEC-22 Chapter 4 (formerly HEC-12) methodology for on-grade catch basin grates or curb opening inlets (applied to SWMM outlet entities). You will need to take a leap of faith when relating the splash-over velocity for your grate vs. the FHWA standard types. We typically use the FHWA P-50x100 (type 3) to represent the tiny urban grates that we use in my part of the world. Some jurisdictions (Ontario Inland Waters, City of Calgary) have done flow testing and can provide sag or on-grade rating curves.

See more discussion of methodology here:

https://www.openswmm.org/Topic/9028/outlet-instability-for-dual-drainage-modeling https://www.openswmm.org/Topic/19499/flow-by-in-swmm5

We end up developing the rating curves on an unique case basis (for longitudinal grade, and cross-fall if in non-typical superelevation areas) and for the number of grates in the group. It gets ugly if you mix several grate types within a group, but sometimes this cannot be avoided due to presence of curb vs no curb, etc.)

If you are only dealing with sag rating curves, then the inlet capacity is the sum of the rating curves as flow vs. elevation. However, more logical to add separate outlet entities each with its own rating curve and let SWMM work out the inlet capacity of each with hydraulic grade line elevation than to develop a combined rating curve outside of SWMM.

Thank You Kirby for sharing this information and insights related to Rating Curves. :)

Is it safe to assume the Side Inlet Catchbasin as weir only, to simplify the quantification of flow for various heads ?

Hi Hiral

For on-grade inlets, I'd stick with HEC-22 Chapter 4 as a reputable standard method, and includes consideration of longitudinal slope, roadway crossfall and impact of local depression near inlet, pavement Manning roughness, gutter flow. The on-grade curb opening calculation is based on simplified side weir method but has features than are more appropriate for the roadway context, you can compare the two calcs using a spreadsheet.

I'm not familiar with the term 'side inlet catchbasin' but I think that it refers to a curb-inlet only without a surface grate; please correct me if I'm wrong. (Sometimes on-grade inlets are known as 'fly-by', since the flow 'flys by' the inlet as it travels downstream). Note that for on-grate curb & gutter inlets, flow is assumed to only enter via the surface grate.

If the side inlet catchbasin is in a sag location (without velocity of flow over grate), I'd make up a rating curve for your inlet grate in sag conditions based on first principles (whether a curb inlet only, or a surface grate and curb inlet combination). Both the surface grate and curb inlet act as a weir at low head, then as an orifice when inundated. Surface grate weir can be assumed to be rectangular weir without end contractions. Weir 'Width' is perimeter of the grate excluding the curb face (if against a curb) at low head, then a bunch of orifices based on area of all slots under high head. Curb inlet can be assumed to be a series of rectangular weirs with end contractions (one for each opening), then orifice with total area of all openings under high head . The rating curves for the weir and orifice equations wont; meet or may cross, so you'll need to add an interpolated transition zone to connect the two curves. Total the two weir-orifice curves to create your sag inlet rating curve.

You can also make assumptions about the fraction of inlet clogging. Some jurisdictions mandate 50% clogging (e.g. if you need 1 inlet under 'clean' conditions, then install 2), but most jurisdictions are silent on this. We usually assume 30% derating to account for some clogging either for on-grade or sag inlets, and deal with this by simply decreasing the rating curve flow by 30% for all heads (rather than decreasing the weir perimeter or orifice area). However, we also do 'garage bag' analysis on each sag inlet, where the impact on flooding and overflow at roadway high points near the inlet is investigated if the inlet capacity was reduced to zero (as if piece of wood, poly or a garbage bag obstructed the inlet).

Also note that these inlet calculations are not typically done for local streets, but become a necessity to estimate and control gutter flow or puddle encroachment into higher speed travel lanes arterial roadways and freeways, and that these higher speed - higher volume roadways don't usually collect the same volume of litter as local or collector roadways, and often receive a higher frequency of cleaning than lower volume/speed roadways.

Thank you so much Kirby, this was really helpful. You were right, Side Inlet Catch basin are Curb inlets, and in may case, they do not have slots on the surface, as they are proposed to be bicycle friendly. I would follow HEC-22 chapter 4, completely. Thanks again for your timely and informative reply.