Does HydroCAD have capability to internally route impervious area over pervious area?
Does HydroCAD have capability to internally route impervious area over pervious area?
(OP)
Does HydroCAD have capability to internally route impervious area over pervious area (i.e, two-step technique) for subcatchments with greater than 30% total imperviousness?





RE: Does HydroCAD have capability to internally route impervious area over pervious area?
For details on this and other options for handling impervious surfaces please see www.hydrocad.net/impervious.htm The procedure is also described in HydroCAD help and the TR-55 manual, where the procedure originates.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Does HydroCAD have capability to internally route impervious area over pervious area?
What about offering internal impervious/pervious routing (as in EPA SWMM) that could handle all situations of unconnected impervious, 1)the situation mentioned above, and 2) for situations that do not conform to that TR 55 criteria, as in greater than 30% total impervious, and smaller pervious receiving area ratios?
RE: Does HydroCAD have capability to internally route impervious area over pervious area?
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Does HydroCAD have capability to internally route impervious area over pervious area?
I actually believe trying to keep track of the assumptions, and very "gray area" weighting procedures of the TR 55 manual is at the core of problem of generating confusion for the staff engineer modeler. It gets too disconnected from the good research in NEH 630, and Mockus' originally observed non-linear runoff responses of basins in the field, and the non-linear curve fitting equations, satisfying conservation of mass, he derived to replicate the response behavior.
We can handle all this easily now with the PC of course. No need to try and save time and paperwork, because we don't use paper anymore (advent PDF file!)..
If HydroCAD offered the Weighted-Volume approach and impervious routing over pervious, it would be accurate in all conditions, and there would never be any more concerns if the CN numbers were close enough in value to be able to use the Area-Weighted approach. The Help section on dealing with impervious and pervious would be vastly simplified for the modeler.
I don't see it as a methodology vs. another methodology, or software vs. software issue. We definitely need good Weighted-Volume method SCS software, that also has the capability to route impervious over pervious for our land development soil cover configurations.
The simplified approach TR 55 takes with unconnected impervious is not accurate enough in today's world of LID based development configurations and awareness.
RE: Does HydroCAD have capability to internally route impervious area over pervious area?
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Does HydroCAD have capability to internally route impervious area over pervious area?
Excerpt:
"We are developing a new field tool that will include features of three of our hydrology programs (EFH2, WinTR-55, and WinTR-20), and we will certainly consider your suggestions, especially those to include an option for using the weighted-volume as well as the weighted-area CN method for calculating runoff estimates and to provide the ability to internally route impervious runoff over pervious area within a
watershed. Your suggestions are very timely as we are just beginning this project."
RE: Does HydroCAD have capability to internally route impervious area over pervious area?
RE: Does HydroCAD have capability to internally route impervious area over pervious area?
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Does HydroCAD have capability to internally route impervious area over pervious area?
http://www.hydrocad.net/curvenumber.htm#average
Scroll down to "Curve number weighting" near the end of the page.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Does HydroCAD have capability to internally route impervious area over pervious area?
I would suggest mentioning the largest runoff computational errors introduced with the Weighted-Area approach are realized when there is a significant difference in CN values within a watershed catchment (basin). This is when the modeler is attempting to weight impervious areas along with pervious areas to obtain a weighted CN to be used in the SCS calculation.
As the CN values become closer to each other in value, such as in many agricultural scenarios, the differences in computed runoff become less.
RE: Does HydroCAD have capability to internally route impervious area over pervious area?
Peter Smart
HydroCAD Software
www.hydrocad.net