Adjusting subcatchment parmeters with a LID
We are having an ongoing in-house debate as to adjusting subcatchment parameters when an LID is placed in an EXISTING subcatchment.
My specific example entails converting a portion of the PERVIOUS area of a subcatchment to a LID (e.g. bioretention area). My question is: if the LID is added to the pervious portion of the subcatchment do I adjust the imperviousness of the subcatchment?
I'd say yes. The manual indicates to adjust %s when adding a LID. If one had a 1.2 ha site w 50% impervious and then put 0.2 ha into a LID on the pervious area that would leave 1.0 ha, 0.6 ha imperv and 0.4 ha pervious and impervious area would have to be adjusted to 60%.
I think what we have been doing agrees with this. We've been recalculating the imperviousness on the basis of the reduced pervious area but keeping the same total subcatchment area. Would you agree with this?
I'd have to reread the manual but I thought the catchment size got reduced according to the size of the LID.
Well, I'm certainly confused over this - I'm clearly not reading the manual properly. It would seem that it depends on how SWMM is handling the catchment areas internally, that is does it recalculate the overall catchment areas internally or does one have to recalculate it as a data input. I've been told both answers from different individuals. Does someone know for sure please?
I think the subcatchment parameters only apply to the non-lid area portion. So if you add in LIDs, you would have to adjust your subcatchments parameter accordingly. It's explained in Chapter 5 (5.3.14) in User's Guide to SWMM5.
Thanks Jennifer. I've looked at, and am looking at 5.3.14. I don't see anything about changing the catchment size, only the catchment parameters (imperviousness, width). The sketch in 5.3.14 appears to show the same catchment size.
The SWMM5 engine internally adjusts the impervious and pervious area. Here is an example of how it does this calculation:
The Subcatchment area is 1 hectare, the prevent impervious is ½ hectare divided into ¼ hectare sections with and without depression storage. The pervious area is ½ hectare. The LID Bio-Retention area is ¼ hectare or 25 percent of the Subcatchment. The SWMM5 engine divides the Subcatchment into nonLID and LID sections and the impervious area and pervious areas are automatically reduced internally. The internal pervious area is now 0.375 hectares, the nonLID area is now 0.75 hectares and the two impervious area are 0.1875 hectares each for an overall total of 1 hectare. The width and slope are not adjusted by the engine.
Nice explanation Bob, but you lost me. I understand that SWMM adjusts area automatically, but will need a diagram for my less astute mind to follow what you described. Any chance one could be provided?
Sorry that this question has become so confusing. When the LID feature was added to SWMM it was debated how the area occupied by the LID should be accounted for. Should it be a separate area not included in the area specified for the subcatchment it sits in, or should its area be part of its parent subcatchment. We decided to go with the latter.
So when a user specifies the Area property for a subcatchment it refers to the total area occupied by both the LID surfaces and the non-LID surfaces. The Percent Impervious property refers to the amount of impervious non-LID area divided by the total non-LID area. The Slope and Width properties should be set with respect to the non-LID portion of the subcatchment.
For LID units that occupy their entire subcatchment, the Area value is the area of the LID unit and the other subcatchment parameters (Percent Imperviousness, Slope, Width, Depression Storage, etc.) are ignored.
Also, I’m not sure what “Chapter 5 (5.3.14) in User's Guide to SWMM5” refers to. The official User’s Manual for SWMM, “Storm Water Management Model User's Manual Version 5.1, EPA/600/R-14/413b, September 2015” is available from the EPA SWMM web site and covers this material in Chapter 3, Section 3.3.14.
Sorry! My bad, I was referring to the 13th edition of the SWMM guide produced by CHI by accident.
I think the adjustment is automatic based on the option in Lid editor " % occupied by lid". In each simulation the auto adjustment will be taken care off by swmm through the indirect loop in loop operation in the source code. Thanks.
The User's Guide to SWMM5 - 13th Ed. was a hardcopy reference manual published by CHI between 2005 and 2010 as a temporary measure while waiting for EPA to release the official SWMM reference manuals. It was a blend of SWMM5 documentation, relevant SWMM4 theory and original content. It is now obsolete (EPA has released superior reference material) but still available in PDF form here:
We'll see if we can mark it more obviously as obsolete or remove it altogether.
Just to be clear, there is no internal adjustment made to the Percent Impervious value of a subcatchment with LID units. It’s up to the user to make the proper adjustment. Take for example a 10 acre subcatchment which has 50% imperviousness before any LID units are added (i.e., 5 acres). Then suppose 20% of the original pervious area (1 acre) and 40% of the original impervious area (2 acres) are replaced by LID units. The amount of non-LID area is now 7 acres of which 5 – 2 = 3 acres is impervious. The user (not the program) has to re-set the subcatchment’s Percent Impervious to 3 / 7 = 43%. The percent of the subcatchment occupied by LID is 3 / 10 = 30%.
The percent impervious adjustment can’t be done internally by SWMM because it has no way of telling how much pervious v. impervious land was replaced by LID. The user has to bring this information to the particular LID setup they are trying to model. The effect that LID usage has on the resulting non-LID area Percent Impervious will depend greatly on the type of LID employed and various site configuration constraints.
I may have over thought this thread (I have almost for sure over thought this thread!) but...it seems to me that you have 7 acres of non-LID area of which 5 - 1 = 4 acres is impervious with a revised % impervious of 4/7 = 57% with the percent of the subcatchment occupied by LID as 3/10 = 30%?
OK, we both agree on 7 acres of non-LID area. But I said that 40% (not 20%) of the original 5 IMPERVIOUS acres was replaced by LID leaving 3 impervious acres remaining which results in 3/7 = 43% non-LID impervious. Yes?