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1. [Model Application] Some thoughts on SWMM infiltration, LID and other parameters
The core mechanism and setting points of infiltration and LID parameters in SWMM model are as follows:
1. The influence of catchment area mode on infiltration and LID. Subarea Routing SWMM divides sub catchment areas into permeable areas (PERVIOUS) and impermeable areas (IMPSERVIOUS), which are further divided into two categories: with and without depressions.
. Subarea Routing controls the priority of water flow paths through the Percent Routed parameter: OUTLET mode: runoff from permeable and impermeable areas flows directly to the discharge outlet, while LID facilities independently process designated areas. PERVIOUS mode: Percent Routed proportion of impermeable runoff flows through the permeable area before being discharged, while the remaining impermeable runoff is directly discharged. If LID facilities are set up at this time, it is necessary to ensure that their processing range does not overlap with the Subarea Routing path, otherwise LID may fail. IMPSERVIOUS mode: Percent Routed proportion of permeable runoff flows through the impermeable area before being discharged, while the remaining permeable runoff is directly discharged. In this mode, if LID facilities handle permeable areas, they need to avoid forming a cycle with Subarea Routing (such as permeable → impermeable → LID → return permeable), otherwise it will cause all runoff to infiltrate and distort the model. Priority conflict handling: Subarea Routing has a higher priority than LID control. For example, if Percent Routed is set to 100% and in PERVIOUS mode, all impermeable runoff will preferentially flow to the permeable area, and the% of Imperfect
2. Key points for setting LID parameters: Definition of% of Imperfect Area Processed: The proportion of impermeable area treated by LID facilities (if set to 30%, only 30% of impermeable runoff enters LID).
. Version difference: Before SWMM 5.2, only non permeable runoff was processed, while after 5.2, permeable runoff treatment is supported (corresponding options need to be checked in LID control). Example: If the impermeable area of the sub catchment area is 1000m2 and the parameter is set to 20%, then 200m2 of impermeable runoff enters the LID facility, and the remaining 800m2 flows along the sub area routing path. Return all outflow to the permeable area: Return the LID treated water back to the permeable area to enhance the infiltration effect. Risk: If combined with the IMPSERVIOUS model, it may form a cycle of "infiltration → non infiltration → LID → return infiltration", resulting in all runoff infiltrating and no runoff discharge from the model. Avoid this combination or break the loop by adjusting the Subarea Routing path. The impact mechanism of characteristic width on peak runoff: The larger the characteristic width, the shorter the central collection channel, and the shorter the time for runoff to reach the discharge outlet, resulting in a more concentrated peak (which may exacerbate waterlogging). Solution: Reduce the feature width to prolong the flow time and disperse peak values. For example, adjusting the feature width from 50m to 30m can reduce
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