I am new to using SWMM. I have a task to design a sewer network using SWMM. The given information includes the conduit lengths, the contributing area for each conduit, and the upper and lower ground elevation for each conduit.

1- I am not sure how to find the invert elevation of each manhole, is it automatically calculated or should I calculate it myself and then enter the numbers into the program?

2- Should I divide the system into subcatchments?

Hi Halima,

1. I'm not sure what you mean by the "upper and lower ground elevation". Do you mean the ground elevation at the start and finish of each conduit? If you are analysing an existing system, you will need the ground level and the invert level of the sewer conduit. If you are designing a new system, you have to set the manhole depth yourself. SWMM has to be told the ground level and depth to invert or "max depth" at each manhole. The design of new sewers is a topic in itself. It's hard to give general guidelines as each design is very site specific. In general go for a minimum grade of 1:250. Set the minimum manhole depth as the max of (0.6m +D) and (2*D), where D is the pipe ID. If necessary override the grade rule to comply with the manhole depth rule. However, please realise this is just to get you started with something that looks credible; these are not design rules, just first approximations. Your local design authority generally will have more complete and local-specific rules on sewer grade and manhole depth and I recommend you obtain these and follow them unless you have specific reason to deviate.
2. I'm not sure if you mean stormwater or wastewater catchments. I don't do much stormwater hydrology. In stormwater flow, yes, you might need to assign a stormwater catchment to each manhole then assign a rain gage to each sub catchment. However, in sewer modelling, you need to calculate the wastewater (WW) inflow to each manhole. In this case, the catchment might be a land area with a number of houses, for example. You then have to convert the number of houses to an average number of people and assign a wastewater flow rate, typically 175 litres per person per day, to each person. This gives you the average WW flow rate. You can then also assign a time series which simulates the change in WW flow rate during the day. Again your local design authority should have readily available its locally applicable flow determination rules to guide you in selecting the above parameters, as this topic comes up every time you design a sewer.

Hi Halima,

1. I'm not sure what you mean by the "upper and lower ground elevation". Do you mean the ground elevation at the start and finish of each conduit? If you are analysing an existing system, you will need the ground level and the invert level of the sewer conduit. If you are designing a new system, you have to set the manhole depth yourself. SWMM has to be told the ground level and depth to invert or "max depth" at each manhole. The design of new sewers is a topic in itself. It's hard to give general guidelines as each design is very site specific. In general go for a minimum grade of 1:250. Set the minimum manhole depth as the max of (0.6m +D) and (2*D), where D is the pipe ID. If necessary override the grade rule to comply with the manhole depth rule. However, please realise this is just to get you started with something that looks credible; these are not design rules, just first approximations. Your local design authority generally will have more complete and local-specific rules on sewer grade and manhole depth and I recommend you obtain these and follow them unless you have specific reason to deviate.
2. I'm not sure if you mean stormwater or wastewater catchments. I don't do much stormwater hydrology. In stormwater flow, yes, you might need to assign a stormwater catchment to each manhole then assign a rain gage to each sub catchment. However, in sewer modelling, you need to calculate the wastewater (WW) inflow to each manhole. In this case, the catchment might be a land area with a number of houses, for example. You then have to convert the number of houses to an average number of people and assign a wastewater flow rate, typically 175 litres per person per day, to each person. This gives you the average WW flow rate. You can then also assign a time series which simulates the change in WW flow rate during the day. Again your local design authority should have readily available its locally applicable flow determination rules to guide you in selecting the above parameters, as this topic comes up every time you design a sewer.