TL;DR: Has anyone with experience using both Carlson Hydrology Module and HydroCAD had an issue with them generating significantly different peak flows? The difference seems to be related to Antecedent Moisture Condition (AMC) used by Carlson. Why would the Carlson AMC value be applied differently from HydroCAD or Hydraflow?
I review stormwater management plans for my municipality. We require the use of the SCS method for stormwater design. I have access to Hydraflow and am a user of HydroCAD. I have successfully convinced most local engineers to adopt HydroCAD, so the majority of plans I review are designed using it. However, the use of HydroCAD is not a requirement. Occasionally, I receive a design that is not based on Hydraflow or HydroCAD. When I do, my first step is to reproduce the submitted model in HydroCAD to compare and check the flows.
I received a stormwater management plan that uses Carlson's Hydrology Module. When I recreated the basins in HydroCAD, I was getting a peak flow of 20% or more. I then reproduced it with Hydraflow as a double check, and Hydraflow's results approximated those of HydroCAD. I set up a Teams meeting with the engineer who submitted the plan. It became apparent that they did not understand the program they were using. I obtained a copy of the Carlson manual to gain a firsthand understanding of how the software worked. I was also able to visit with a local engineer who had the complete Carlson package and play with it some. It appears that the issue is with the selection of the Antecedent Moisture Condition (AMC) value. Using the SCS Curvilinear unit hydrograph in the Carlson Hydrology module, to achieve the same peak flow as HydroCAD, an AMC value of 3 must be selected, rather than 2. Why doesn't Carlson return the same result when using an AMC value of 2? Or is this just a coincidence, and is there something else happening that I'm missing?
Thanks
I review stormwater management plans for my municipality. We require the use of the SCS method for stormwater design. I have access to Hydraflow and am a user of HydroCAD. I have successfully convinced most local engineers to adopt HydroCAD, so the majority of plans I review are designed using it. However, the use of HydroCAD is not a requirement. Occasionally, I receive a design that is not based on Hydraflow or HydroCAD. When I do, my first step is to reproduce the submitted model in HydroCAD to compare and check the flows.
I received a stormwater management plan that uses Carlson's Hydrology Module. When I recreated the basins in HydroCAD, I was getting a peak flow of 20% or more. I then reproduced it with Hydraflow as a double check, and Hydraflow's results approximated those of HydroCAD. I set up a Teams meeting with the engineer who submitted the plan. It became apparent that they did not understand the program they were using. I obtained a copy of the Carlson manual to gain a firsthand understanding of how the software worked. I was also able to visit with a local engineer who had the complete Carlson package and play with it some. It appears that the issue is with the selection of the Antecedent Moisture Condition (AMC) value. Using the SCS Curvilinear unit hydrograph in the Carlson Hydrology module, to achieve the same peak flow as HydroCAD, an AMC value of 3 must be selected, rather than 2. Why doesn't Carlson return the same result when using an AMC value of 2? Or is this just a coincidence, and is there something else happening that I'm missing?
Thanks
