Dear Shraddha Badwar , see attached table. A conduit can also be designated to act as a culvert if a 'Culvert Inlet Geometry' code number is assigned to it. These code numbers are listed in Appendix A.10 of the SWMM manual: https://www.epa.gov/sites/default/files/2019-02/documents/epaswmm5_1_manual_master_8-2-15.pdf.

There are quite a few things to consider in designing and placing culverts to fit the circumstances. Generally a licensed professional engineer specifically trained and insured, reviews and certifies these type decisions for urban areas and public road network, designed for long term access and safety, and also addressing issues associated with aquatic organisms, sediment and debris load, etc.

I had to walk dogs. Anyway, great that you are showing this interest and have determined some degree of design is important. If you can find a similar sized watershed or drainage in your physiographic area, with similar land uses, etc., and can find a culvert that has been in place for 20-30 years or more, run the SWMM model and see how it compares with what has worked. If you know how to identify the bankfull level which is approximately the 1.5 year flood, 2 times the bankfull depth cross section may approximate the 50 year flood (Rosgen training guide). There may be other flood indicators or public knowledge associated with floods. Also if there are any vicinity or physiographic area stream gauging stations with 25 years flood data, that can help, and might also be useful when validating the SWMM model to your area. Culvert guides are available for galvanized corregated and concrete culverts with inlet or outlet control, amount of head or backwater, etc. I have used the one developed by Iowa, but there are probably others that have rated culvert size and flow properties. If you intent to use for example, the 25 year storm, expect that this estimate may be off as one point. Another point is that the 25 year storm has on average occurance of 4% in any year, so if the culvert is in for 20 years, 4 x 20 = 80%. So there is an 80% probability that the estimate will be exceeded. Point 3 is that if you can afford to overdensign the culvert size, there is new technology available to resurface existing structures that may slightly reduce capacity. This can be a huge savings in the future for some circumstances. If the culvert size Is on the marginal size, design measures can sometimes be incorporated for bypass or overflow, possibly with some limits to traffic under some flooding conditions.

I don't understand if the junction nodes are randomly added to the project or are they added according to the current locations of the rainwater drainage nodes, can you help me?