name | anything can go in here |
C | rational method coefficient |
A | drainage area in acres |
IDF File | a text file with the curve values of the appropriate Intensity-Duration-Frequency curve for the drainage area. This file uses the same format as the curve files for structures and the generic hydrograph. The IDF curves for Albemarle County Virginia have been coded into the program. Use "a2", "a10", or "a100" for the 2, 10, and 100 years storm IDF curves. If you would like to have the IDF curves for your area incorporated into the code, please contact me. A sample text file is provided below. |
Tc | representative time of concentration for the drainage area, which is used as the time to peak flow |
Fr | receding limb factor |
dt | time interval in minutes to use in the routing of this hydrograph |
Tl | time limit in minutes at which to terminate the routing if not already stopped |
dur | duration of the peak flow. This is usually set to zero, for the triangular hydrograph of the peak intensity storm. |
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. |
crit.dur | This will cause the program to find the hydrograph which results in the greatest storage volume. This is referred to as the critical duration storm, because the duration and value of the peak flow is altered to find the routing with the highest storage volume. |
route | indicates whether this hydrograph will be routed |
This is a popular approximation of a unit hydrograph. If the critical duration is not checked, it will result in a hydrograph using the peak flow and duration given. Otherwise, it will iterate, increasing the duration of the peak (and lowering its intensity) until the routing with the greatest basin storage is reached. The output will generally give the number of routing interations needed to converge on the largest volume. Convergence is accurate to about 1sf, or 0.04cy. The summary output will indicate if the results are not converging. An error may occur if the limits of the IDF curve are exceeded. Some basin structures may result in ever increasing storage volumes with longer hydrographs.
Sample IDF curve text file
IDF Curve Albemarle County 10yr Curve: duration intensity (in/hr) 0 7.1 5 7.1 6 6.7 7 6.4 8 6.1 9 5.83 10 5.62 11 5.44 12 5.26 13 5.09 14 4.94 15 4.79 16 4.65 17 4.52 18 4.40 19 4.28 20 4.17 21 4.07 22 3.97 23 3.87 24 3.78 25 3.70 26 3.61 27 3.53 28 3.46 29 3.39 30 3.32 31 3.25 32 3.19 33 3.12 34 3.06 35 3.01 36 2.95 37 2.90 38 2.85 39 2.80 40 2.75 41 2.71 42 2.66 43 2.62 44 2.58 45 2.54 46 2.50 47 2.46 48 2.42 49 2.39 50 2.35 51 2.32 52 2.28 53 2.25 54 2.22 55 2.19 56 2.16 57 2.13 58 2.11 59 2.08 60 2.05 1440 2.05 from Virginia Stormwater Management Handbook Appendix 4D, and formula of p.5-19(return)