CT rainfall 3hr totals storm design
CT rainfall 3hr totals storm design
(OP)
Hi guys- I am looking for a chart/authority that
describes what happends when I take the type III 25year 24 hour storm rainfall of 5.7 inches and compress it into 3 hours then it no longer is a 25-year event, but is (if I remember correctly) the 100 year event because of the shortened time and not the 24 hour period. I am sure CT classifies a 5.7 inch rainfall in a 3 hour period not as a 25 year storm but as a higher rain event-- but which event and is it still a type III?
describes what happends when I take the type III 25year 24 hour storm rainfall of 5.7 inches and compress it into 3 hours then it no longer is a 25-year event, but is (if I remember correctly) the 100 year event because of the shortened time and not the 24 hour period. I am sure CT classifies a 5.7 inch rainfall in a 3 hour period not as a 25 year storm but as a higher rain event-- but which event and is it still a type III?
RE: CT rainfall 3hr totals storm design
When you COMPRESS a 24-hour Type III storm to 3 hours, you increase the rainfall intensity 8-fold. (Assuming the same total depth.) All intensities, including the peak, will be eight times greater. This is not an intended use for this rainfall, and the results are not valid. Only a few specific storms (such as the Illinois Huff distributions) are intended to be used at varying durations. In general, if a storm is designated x-hours, it should be used only for that duration.
On the other hand, if you EXTRACT the middle 3 hours (around the peak) you will accurately preserve the intensity information represented by the original curve. Of course, you will need to use the truncated curve with the appropriate 3-hour rainfall depth for your location.
You are correct that a 5.7" rainfall over 3 hours will be much more than a 25-year event. To determine the corresponding return period you would need a table of depth vs. return period for a 3-hour event.
RE: CT rainfall 3hr totals storm design
RE: CT rainfall 3hr totals storm design
http://www.weather.gov/oh/hdsc/
RE: CT rainfall 3hr totals storm design
RE: CT rainfall 3hr totals storm design
RE: CT rainfall 3hr totals storm design
Volume = raincloud duration x average intensity.
You can do a several durations to develop the hydrograph.
For the 3-hr base use 1.6 hrs for the leading edge, 1.4 hrs for the tailing edge, or whatever seems reasonable.
Rainclouds are huge -- a 5-minute duration raincloud moving at 12 mph is one mile long!!
Your site is probably a lot smaller so use the 5-minute duration for all your runoff calcs.