snowhund
Civil/Environmental
- Apr 16, 2007
- 3
Over the past decade the NRCS CN Method (particularly the selection of a composite CN) has been shown to be fairly inaccurate when modeling smaller storm events (< 2 inches). The problem is an underestimation of runoff depth and this affects designs for water quality and stream channel protection among other issues.
D. Woodward and R. Hawkins wrote an interesting article titled, "Runoff CN Method: Examination of the Initial Abstraction Ratio" that advocates revising the Ia/S ratio to 0.05 AND changing the formula for S. The article may be found on NRCS web site under Hydraulics & Hydrology – Technical References
Implementing the recommendations in the paper combined with using a distributed CN when setting up models would help fix some of the problems. However, there is no model model capable of being adjusted at this time to implement the paper's suggestion (one can't just modify the Ia/S ratio).
Prof. Richard Hawkins told me that the likelyhood of the NRCS revising the TR-20 code anytime soon is remote due to initia of various forms. Peter, are you aware of the paper, the issues with the CN method and do you have any thoughts on providing an alternate runoff methodology modifying the current CN Method or a model using the Green-Ampt infiltration equation?
Erik
D. Woodward and R. Hawkins wrote an interesting article titled, "Runoff CN Method: Examination of the Initial Abstraction Ratio" that advocates revising the Ia/S ratio to 0.05 AND changing the formula for S. The article may be found on NRCS web site under Hydraulics & Hydrology – Technical References
Implementing the recommendations in the paper combined with using a distributed CN when setting up models would help fix some of the problems. However, there is no model model capable of being adjusted at this time to implement the paper's suggestion (one can't just modify the Ia/S ratio).
Prof. Richard Hawkins told me that the likelyhood of the NRCS revising the TR-20 code anytime soon is remote due to initia of various forms. Peter, are you aware of the paper, the issues with the CN method and do you have any thoughts on providing an alternate runoff methodology modifying the current CN Method or a model using the Green-Ampt infiltration equation?
Erik