Today I feel brave so I will dare to be politically incorrect and to give my sincere opinion about the analysis of drainage with two main free and open source tools: HEC (this is, HEC-HMS and HEC-RAS) has been proposed by the US Army and SWMM has been developed by the Environmental Protection Agency, this is the same people that released EPANET. So, what are the main differences between both? When should you use one or the other?

Pic 1: HEC models are not fully integrated. Firstly HMS is used for hydrology, then RAS is run for hydraulics.

From my point of view, HEC-HMS is the best choice to compute rainfall-runoff at each hydraulic component in large basins. HEC-HMS can create random rains while SWMM is deterministic. What is more, SWMM cannot analyse properly open channel flows around inline and lateral structures such as dams, bridges, gates, etc... The main disadvantage of HEC models is that they are not utterly integrated (HEC-RAS is one program and HEC-HMS is another) and cannot take into account the effect of minor drainage systems primarily composed of closed conduits (i.e. pipe networks), but SWMM can calculate them well.

Pic 2: The user interface of SWMM has a flavour of EPANET.

Generally speaking, SWMM is better for urban drainage systems with a lot of continuous closed pipes mixed with open channel whereas HEC software behaves better in pure channels (I mean channels that look like rivers) with short culvert lengths.