Okay, I need to have this confirmed/clarified. On a hydrograph, if Q=0 at t=0 and the peak runoff is at Q=200 cfs when t=5 then the peak runoff is 200 cfs. But when Q=100 cfs at t=0, and the peak of the graph shows Q=500 cfs at t=5 then the peak runoff is 400 cfs. Is this right? Also, how do I extract data out of a hydrograph to create a unit hydrograph? Thanks for the help.
Hydrographs vs Unit Hydrographs
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EnvEngineer
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The difference between a hydrograph and a unit hydrograph is that the data on the hydrograph has been divided by the inches of rain fall. If there is flow at t=0 that is ground flow and not included as part of the hydrograph, is is subtracted out. If you peat flow at t =400 cfs and 2 inches of rain fell, the peak runnoff on the unit hydrograph would be 200 cfs. If you were asked for the peak runnoff at t=?? for a 3 inch storm you could just multiply the number off your unit hydrograph by 3 to get the runnoff in cfs.
Q=100 cfs at t=0, and the peak of the graph shows Q=500 cfs at t=5 then the peak runoff is 400 cfs. Is this right? yes if you are calculating the contribution of the storm, these measurements are taken in channels and streams that may have flow prior to the storm (100 cfs) and will not be included in your storm calculations.
Q=100 cfs at t=0, and the peak of the graph shows Q=500 cfs at t=5 then the peak runoff is 400 cfs. Is this right? yes if you are calculating the contribution of the storm, these measurements are taken in channels and streams that may have flow prior to the storm (100 cfs) and will not be included in your storm calculations.
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The unit hydrograph essentially gives you a typical runoff curve for a storm in a given setting. It determines when you will first see runoff from upstream, time of peak flow, etc.
It's normalized per inch of rainfall so that you can predict runoff for a storm of any intensity in that system, the 10, 25, or 50 year storm for example. You just plug in the corresponding amount of rainfall.
So if it's 20 cfs/inch at t=8 hours, and you get 3 inches of rain, you'll see 60 cfs at that point. If it was 2 inches, 40 cfs, and so on.
It's normalized per inch of rainfall so that you can predict runoff for a storm of any intensity in that system, the 10, 25, or 50 year storm for example. You just plug in the corresponding amount of rainfall.
So if it's 20 cfs/inch at t=8 hours, and you get 3 inches of rain, you'll see 60 cfs at that point. If it was 2 inches, 40 cfs, and so on.
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