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.
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). |
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.