Higher HGL in StormCAD than CivilStorm.

Hi Patrick,

The dynamic solvers available in CivilStorm are completely different from the steady state/peak flow-based Rational method solver used in StormCAD. (apples-to-oranges comparison.) The methodology used by the Unit Hydrograph method is also entirely different from the Rational Method, so it is not possible to configure them both to produce the same results. With the Rational Method in StormCAD, you are looking at a peak flow, steady state condition and the hydraulic grade in profile is calculated with the Gradually Varied Flow backwater algorithms. Overflow is not supported with the GVF-Rational solver and the HGL will be reset to the rim elevation as mentioned here. With the Unit Hydrograph method, a hydrograph (flow over time) is produced, where the peak flow occurs for a particular time and might be less than the peak flow from the Rational Method (Q=CIA), which is then routed with one of the dynamic solvers where additional dynamic effects are supported such as overflow and looping which is not possible with StormCAD's solver. With the Rational solver in StormCAD, the profile has essentially settled on a steady state condition with the peak flow, whereas with the dynamic solver and unit hydrograph-based runoff, the peak flow may only occur for a short period of time (the SCS UH peak tends to be sharp - look at a graph) that may not allow the system to settle on a steady state due to storage effects, which could explain hat you noticed. You will also need to ensure that the dynamic results are stable with the Implicit or Explicit solver in CivilStorm and you can find some guidance here and here.

StormCAD's standard Rational Method is well-suited for peak flow-based pipe design, whereas the dynamic solvers are well-suited for assessing problem systems, modeling storage impacts (such as ponds and wetwells) or when more complex situations need to be assessed (overflow, loops, pumps, ponds, control structures, etc).

Additional supporting information can be found in the following articles from our Wiki:

Why don't CivilStorm results for peak flow closely match StormCAD results for peak flow?

Differences between solvers: GVF-Convex vs. GVF-Rational vs. Implicit vs. Explicit (SWMM)

Hi Patrick,

The dynamic solvers available in CivilStorm are completely different from the steady state/peak flow-based Rational method solver used in StormCAD. (apples-to-oranges comparison.) The methodology used by the Unit Hydrograph method is also entirely different from the Rational Method, so it is not possible to configure them both to produce the same results. With the Rational Method in StormCAD, you are looking at a peak flow, steady state condition and the hydraulic grade in profile is calculated with the Gradually Varied Flow backwater algorithms. Overflow is not supported with the GVF-Rational solver and the HGL will be reset to the rim elevation as mentioned here. With the Unit Hydrograph method, a hydrograph (flow over time) is produced, where the peak flow occurs for a particular time and might be less than the peak flow from the Rational Method (Q=CIA), which is then routed with one of the dynamic solvers where additional dynamic effects are supported such as overflow and looping which is not possible with StormCAD's solver. With the Rational solver in StormCAD, the profile has essentially settled on a steady state condition with the peak flow, whereas with the dynamic solver and unit hydrograph-based runoff, the peak flow may only occur for a short period of time (the SCS UH peak tends to be sharp - look at a graph) that may not allow the system to settle on a steady state due to storage effects, which could explain hat you noticed. You will also need to ensure that the dynamic results are stable with the Implicit or Explicit solver in CivilStorm and you can find some guidance here and here.

StormCAD's standard Rational Method is well-suited for peak flow-based pipe design, whereas the dynamic solvers are well-suited for assessing problem systems, modeling storage impacts (such as ponds and wetwells) or when more complex situations need to be assessed (overflow, loops, pumps, ponds, control structures, etc).

Additional supporting information can be found in the following articles from our Wiki:

Why don't CivilStorm results for peak flow closely match StormCAD results for peak flow?

Differences between solvers: GVF-Convex vs. GVF-Rational vs. Implicit vs. Explicit (SWMM)