Using weighted Curve Numbers for Basin Routing
Using weighted Curve Numbers for Basin Routing
(OP)
Okay guys....
I just got off the Phone with a guy that is the Township Engineers Stormwater reviewer and all stormwater comments in the Engineers review are generated by this reviewer.
He just told me that I'm not allowed to use weighted curve numbers for a Basin Routing using TR-55. I'm told that I have to generate a separate hydrograph for each type of Land cover (lawn, Impervious, Woods and Meadow) ... Then combine the inflow hydrographs and use the combined hydrograph for the routing. I asked what the time of concentration to use when modeling the roads and he told me to use the same time of concentration for all the land covers. He said that a hydrograph is a volume and that runoff volumes are to be calculated separately.
I told Him that I have a copy of TR-55 and it shows the calculation of hydrographs using a weighted Curve number and he told me TR-55 is wrong.
He is citing this section of the Township code:
"Weighted averaging of runoff coefficients shall not be used for manual computations or input data for water quality and runoff volume calculations."
He says that because a hydrograph is a runoff volume, you can't use weighted curve numbers for basin routings.
I'd love to hear your thoughts.....
I just got off the Phone with a guy that is the Township Engineers Stormwater reviewer and all stormwater comments in the Engineers review are generated by this reviewer.
He just told me that I'm not allowed to use weighted curve numbers for a Basin Routing using TR-55. I'm told that I have to generate a separate hydrograph for each type of Land cover (lawn, Impervious, Woods and Meadow) ... Then combine the inflow hydrographs and use the combined hydrograph for the routing. I asked what the time of concentration to use when modeling the roads and he told me to use the same time of concentration for all the land covers. He said that a hydrograph is a volume and that runoff volumes are to be calculated separately.
I told Him that I have a copy of TR-55 and it shows the calculation of hydrographs using a weighted Curve number and he told me TR-55 is wrong.
He is citing this section of the Township code:
"Weighted averaging of runoff coefficients shall not be used for manual computations or input data for water quality and runoff volume calculations."
He says that because a hydrograph is a runoff volume, you can't use weighted curve numbers for basin routings.
I'd love to hear your thoughts.....





RE: Using weighted Curve Numbers for Basin Routing
http://www.eng-tips.com/viewthread.cfm?qid=335186
RE: Using weighted Curve Numbers for Basin Routing
An option to calculate separate runoff for each CN (Weighted-Q) was added in HydroCAD-10 build 8, released on March 1, 2013. Doing this with other programs (TR-55, TR-20, etc) requires that you create a separate subcatchment for each CN.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Using weighted Curve Numbers for Basin Routing
RE: Using weighted Curve Numbers for Basin Routing
TR-55 states: "The watersheds must be hydrologically homogeneous, that is, describable with one CN". If you have multiple land use types, by definition, it is not homogeneous. In a typical situation you can claim that land use CN's are all close enough to be homogeneous, or land uses are uniformly distributed where grass sheet flows to meadow which sheet flows to forest. The problem emerges with impervious area, which is usually parking lots and roads with their own drainage systems that never sheet flow onto a pervious surface (home gutters you might be able to claim do).
If impervious area is a relatively small component, then it can be lumped into an area-weighted CN without much difference, otherwise it should be separately modeled. As comments at the previous post suggests, I've use a volume-weighted CN rather than an area-weighted CN to get around this and still be able to model with a single watershed. Or, if the model you're using has a %impervious parameter it does the same thing (HEC-HMS, SWMM, many others), in which case the CN should only represent pervious surfaces.
RE: Using weighted Curve Numbers for Basin Routing
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Using weighted Curve Numbers for Basin Routing
RE: Using weighted Curve Numbers for Basin Routing
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Using weighted Curve Numbers for Basin Routing
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Using weighted Curve Numbers for Basin Routing
RE: Using weighted Curve Numbers for Basin Routing
RE: Using weighted Curve Numbers for Basin Routing
I did some research to try an understand the method and located the reference mentioned earlier, NEH 4 Chp. 10, and it mentions that it will essentially produce the same results but weighted Q is more difficult to calculate and technically is more correct.