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LID area and impervious runoff calculations

Hi users,

Just a query regarding LID's in SWMM. Let's say that impervious sub-catchmant A has a total area of 100 m2. 

Let's say that I deploy a LID into sub-catchment A (a rain barrel) and set its surface area as 10 m2 (thus it occupies 10% of sub-catchment in A).

In doing this, does SWMM therefore calculate the runoff from sub-catchment A using an 'adjusted' area of 90 m2?




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  • Yes, it adjusts the non lid area,  i am attaching an example model with two subs to illustrate this.  The loading to the lid is the runoff from the 90 square meters + the rainfall over the LID.  You can most easily see this by making the lid cover all 100 square meters.


    • Hi there Robert,

      Thanks very much for your quick reply. OK - just another query on this. Back to my example. Let's say my 100 m2 sub-catchment (before LID) is 100% impervious. I deploy the 10 m2 LID. I select that 50% of the impervious area drains to the LID. SWMM will calculate that runoff from 45 m2 flows into the LID? 

      • The total load to the lid will be the runoff from the 45 square meters + the rainfall. 


        • Thanks a lot Robert.

          Hey - I've got another query if you don't mind. 

          I've just stumbled across a strange SWMM error - for all my links, I cannot seem to view any output. I get an error "no objects to display" even though I know there is data there. Any advice? Cheers.

          • Hello Matt,

            Under Options/Report what do you have for link reporting? do you have all links checked?


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