Does HydroCAD have capability to internally route impervious area over pervious area? 1

[LincolnPE]

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?

 

[psmart]

This situation is simulated by using the ground cover option for "unconnected impervious area". This applies a special (reduced) CN weighting to allow for the impervious runoff that is being absorbed by the adjacent pervious surface.

For details on this and other options for handling impervious surfaces please see

http://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

http://www.hydrocad.net

 

[LincolnPE]

Yes, this sounds like the TR 55 method for total imperviousness less than 30%. The pervious receiving area should be at least twice as large as the contributing impervious area for this method also.

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?

 

[psmart]

Those are reasonable suggestions for a future update. However, HydroCAD is focused primarily on the SCS//NRCS methodology, and each option we add is one more opportunity for misunderstanding and misuse. So we're pretty selective about new features.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[LincolnPE]

Thanks Peter.

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.

 

[psmart]

You might want to contact the NRCS WinTR-20 development team about implementing these procedures. They're the ones that really set the standard. Otherwise we're facing an uphill battle to get any local stormwater jurisdiction to accept the results.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[LincolnPE]

I sent this off to the development team. Received a nice response back from them saying it was good timing, as they are getting started developing a new field tool.

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."

 

[LincolnPE]

I haven't been back to HydroCAD for awhile. Has the Weighted-Volume method been implemented yet?

 

[psmart]

The "weighted-Q" approach was added in HydroCAD-10 build 9. You will find this option on the Advanced tab of the Settings|Calculation screen.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

You'll find details on each of the curve number procedures at:

http://www.hydrocad.net/curvenumber.htm#average

Scroll down to "Curve number weighting" near the end of the page.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[LincolnPE]

It is good you mentioned the Weighted-Volume method is the always accurate method.

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.

 

[psmart]

LincolnPE - The page I referenced has always contained precautions regarding the use of a weighted CN, and especially the averaging of pervious and impervious areas. As always, the engineer has a choice of which procedures are appropriate for any given situation and acceptable under their stormwater regulations.

Peter Smart
HydroCAD Software

http://www.hydrocad.net