1 / 14

Chapter Two S-curve method

Chapter Two S-curve method. UH Lagging. A UH has a particular duration, D. Unit hydrographs are linear, they can be added, for example, to generate a hydrograph for a storm 2x, 3x, 4x, etc. … longer. 1–hr UH. 2–hr UH.

jprater
Download Presentation

Chapter Two S-curve method

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter TwoS-curve method

  2. UH Lagging A UH has a particular duration, D. Unit hydrographs are linear, they can be added, for example, to generate a hydrograph for a storm 2x, 3x, 4x, etc. … longer 1–hr UH 2–hr UH The linear property of a UH can be used to generate a UH of a larger duration. Here two copies of a 1-hour UH are lagged by 1-hr and added, then the ordinates are divided by 2, to make a 2-hour UH (1 inch over two hours). However this lagging method is restricted to integer multiples of the original duration.

  3. S –curve method S –curve method works for any duration. The first step is to add a series of UH’s of duration D, each lagged by time period D, This corresponds to the runoff hydrograph from a continuous rainfall excess intensity of 1/D inches/hour.

  4. S –curve method works for any duration, D’ By shifting a copy of the S-curve by D’ hours, and subtracting the ordinates, the resulting hydrograph (dashed line - - - - - - ) must be due to rainfall of intensity 1/D inches/hour that lasts for a duration of D’ hours. To convert the hydrograph (dashed line - - - - -) to a UH, multiply ordinates by D/D’ ,resulting in a UH of duration D’. D’ need NOT be an integral multiple of D.

  5. Example 2-4 • Convert the following 2-hr UH to a 3-hr UH using the S-curve Method ____________________________________________________ TIME (hr) 2-hr UH Ordinate (cfs)________ . 0 0 1 75 2 250 3 300 4 275 5 200 6 100 7 75 8 50 9 25 10 0

  6. Make a spreadsheet with the 2-hr UH ordinates, then copy them in the next column lagged by D=2 hours. Keep adding columns until the row sums are fairly constant. The sums are the ordinates of your S-curve.

  7. Draw your S-curve, it looks like the upper curve. Figure E2-4

  8. Copy your S-curve ordinates into a new spreadsheet, then copy the same ordinates D’=3 time units down. Calculate the difference, then multiply by D/D’ ,here = 2/3, to get your 3 hour UH.

  9. Why multiply by a correction factor? • Here the original UH was for a 2-hour storm, and we need a UH for a 3-hour storm. • A 2-hour UH is for a storm with 1/2 in/hr for 2 hours • A 3-hour UH is for a storm with 1/3 in/hr for 3 hours • 1/2 x 2/3 = 1/3 • For each s-curves difference of lagged UHs , multiply the ordinates by D /D’ to get the correct UH

  10. Review of this method • Let’s look at the example again.

  11. Here are the two copies of your S-curve based on a sum of 2-hr UHs. The second copy is lagged by D’=3 hours.

  12. Calculate the difference, and re-scale the difference by D/D’

  13. Plot the new ordinates, and here is your new 3 hour UH, or whatever D’ you need

  14. Prediction • This method provides a hydrograph that predicts the behavior of a flood from a storm of any duration

More Related