ChannelFlow: Qscs

This is a basic implementation of the SCS TR-55 Graphical Peak Discharge method. The graphical check provided from the "view" button in the program shows the area as a bar graph, with acres incremented, and the color faded out in relation to the "CN" value.

Input Parameters:
name Text of up to 10 characters. Blanks are not allowed. This is in order to preserve the tabular printout, which can be read back into the program.
A Drainage or watershed area, in acres.
CN coefficient. Suggested values from the VSMH, Table 4-6a:
Cover Type soil ==> A B C D
%imperv. ....... ....... ....... .......
Open space . 39 61 74 80
parking lots or streets . 98 98 98 98
rural roads, ditches & row . 83 89 92 93
gravel roads & row . 76 85 89 91
urban commercial 85 89 92 94 95
urban industrial 72 81 88 91 93
residential 1/8 acre lots (townhouses) 65 77 85 90 92
residential 1/4 acre lots 38 61 75 83 87
residential 1/3 acre lots 30 57 72 81 86
residential 1/2 acre lots 25 54 70 80 85
residential 1 acre lots 20 51 68 79 84
residential 2 acre lots 12 46 65 77 82
newly graded . 77 86 91 94
+CNA# This is a value to add to the computed CNA for this line. (It is not the computed CNA for this line.) Another "Qscs" row can be referenced, and it will add the CNA value from that row.
tc Time of concentration.
P The rainfall value from the charts in Appendix B of TR-55. For Charlottesville Virginia, these are 3.3(1yr), 4.0(2yr), 6.0(10yr), and 8.5(100yr).
Output:
sum CNA The sum of computed and additional or given CNA values.
S The potential maximum retention after runoff begins, from equation 2-4 of TR-55; [ (1000/CN)-10 ].
Ia The initial abstraction; [ 0.2(S) ]
Q The total runoff, from TR-55 equation 2-1; [ (P-Ia)2/(P-Ia+S) ]
qu The unit peak discharge from exhibit 4-II. This applies only to Type II runoff distributions in the Eastern U.S.

The flow using the TR-55 method is q = qu(A)Q. Please refer to the source for a more detailed description of this method.

(return)