Normally, it is better to have smaller ponding allowed area in SWMM 5 due to considerations outlined in this earlier blog and 2nd Blog but if you have a low lying area with ponding allowed it is better if you have a larger ponding area so that the depth above the rim elevation of the node is small and you do not set up reverse flow and cause flooding at other modes due to the higher HGL. Figure 1 illustrates a typical HGL profile and Figure 2 shows the better flooding graphs as you increase the ponding surface area for the low lying node.
|Figure 1. HGL around a low Lying Node|
|Figure 2. Smoother Flooding Graph due to Higher Surface Ponding Area|