Ran a drainage design through SSA, then exported to Hydraflow and ran. The results were markedly different. So I exported the Hydraflow to XML, imported to SSA, configured necessary parameters. Reran. Same problem. Anyone have this happen?
That is to be expected.
Hydraflow performs a standard step method using the Peak Q.
SSA performs a hydrograph routing conserving mass and momentum.
Please explain in more detail. run ana analysis in hydraflow and ssa using the same idf curves and i find that my system floods in hydraflow and i end up having to use larger pipe sizes. However the same system analysed in ssa indicates that the now larger pipe sizes are too big since there is extra capacity. I am trying to optimise my network and the difference between the two software is befuddling.
Hydraflow Storm Sewers follows the procedures outlines in HEC-22 for storm sewer systems. The process starts at the downstream outlet to a system, stepping iteratively up a network in search of a balance of the HGL and time. The rainfall intensity for the each system is overall system intensity for the upstream area. This results in a peak Q for a pipe. It is a very conservative design method.
SSA uses the EPA SWMM hydraulic calculation engine which uses the Saint Venant equations to balance continuity and momentum in the system. This is a time-dependant system and is typically startes from the upstream moving downstream, however, the solutions do allow reverse flow, attenuated system storage, and pumps - items that Hydraflow and the Standard Step method does not support.
The methods using by Hydraflow are still valid, and are simple to the point that many still utilize Excel spreadsheets to accomplish these calculations. In many jurisdictions, this is all that is required.
you may have already answered my question below before, but would like to ask again to get your thoughts:
"if SSA is set to a link routing method of Steady Flow, is that not the same as the standard step method?"
It sounds like you are trying to size pipes using Rational Method hydrographs in a dynamic routing simulation model like SSA. This violates the assumption the Rational Method is based on (rainfall intensity constant over the storm duration, rainfall uniformly distributed over the watershed, etc)..
To size pipes from multiple upstream catchments using the Rational Method, you need to implement a system tc and weighted CA methodology (McCuen) . This is what a storm sewer analysis program like Hydraflow does.
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