BasinFlow: SCS Type Hydrograph
Input Parameters:
name anything can go in here
A area of watershed in acres
CN Curve Number for the watershed, or drainage area. 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
Type The SCS storm type designation for regions of the United States. Currenly, only the type II (2) storm is available, which covers most of the United States. If you would like to have other types (I, III, etc) added to the program, please contact me.
P Total precipitation for the region, typically from TR55 App. B, an excerpt of which is shown below.
tc representative time of concentration in hours for the drainage area.
dt time interval in hours to use in the routing of this hydrograph
Tl time limit in hours at which to terminate the routing if not already stopped
ff This is a fudge factor to scale the hydrograph. This is for the application of engineering judgement, especially if other hydrologic methods are used, and the result needs to be adjusted to better correlate before routing.
route indicates whether this hydrograph will be routed

This hydrograph is a detailed computation from Section 4 of the SCS National Engineering Handbook, Chapter 16. The runoff equation (Q) and runoff parameters are taken from TR-55. (Although the methodology is that of TR-20.) Below are the parameters used by the program;

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) ]
deltaD   The time increment used to build the flood hydrograph; [ 0.133*tc ] (where tc is time of concentration)
Tp The unit hydrograph time to peak; [ (deltaD/2)+(0.6*tc) ]
qu The unit hydrograph peak discharge; [ 483.432*(A/640)/Tp ] (where A is in acres)

The flood hydrograph is built by adding successive unit hydrographs together, as depicted below.

The unit hydrograph used for this purpose is given in NEH-4. An illustration is given below from the Virginia Stormwater Management Handbook.

The unit hydrograph is scaled by the unit hydrograph peak (qu), and the time to peak (Tp). It is then scaled by each incremental runoff value for the watershed, and added to the flood hydrograph at the corresponding time. The incremental runoff curve used to scale each successive unit hydrograph is obtained by scaling the cumulative runoff curve by Q, and subtracting successive elements.

The cumulative runoff distributions normally used with this method are the Soil Conservation Service 24-hour distributions for the United States, which are shown below (taken from TR55 App. B).  However, any distribution can be used by modifying or creating the text file to be read into the program. It is noted that the final flood hydrograph is very sensitive to the values used in the cumulative distribution curve.

A word of caution: The NRCS methods (TR-55, TR-20) tend to give high results in some cases, and should be balanced or weighted against other methods. The ff input value above can be used to match or adjust results if desired.

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