Storm Sewer Design + Tailwaters
Storm Sewer Design + Tailwaters
(OP)
When designing for a 25-year storm network, which discharges into a pond, should we set the tailwater of the the discharging headwall, at the 25 year water surface elevation? My problem occurs when using our design software. (Terramodel)When designing our storm sewer network Terramodel allows us to enter a tail water condition,... and....also assign drainage areas to each inlet. I think I am double counting the amount of rain, b/c common practice in our company, is to enter the 5 minute intensity (8.37 in/hr)of for each inlet, the drainage area and C value. My HGL is going out of the structures by alot..... My thoughts may be that at the 25-year tailwater time (15 hours) the intensity is 0.40 in/hr. Should I be using 0.40 in/hr for data entry into my storm network, when also using the 25 year tailwater?





RE: Storm Sewer Design + Tailwaters
RE: Storm Sewer Design + Tailwaters
RE: Storm Sewer Design + Tailwaters
RE: Storm Sewer Design + Tailwaters
It seems to me that you are presuming that you have a "design storm" type event with a single peak. In reality storms often have several peaks, so there could be another intense 5-minute downpour at some time after the initial peak. It is difficult to get a handle on without a long-term rainfall/hydrologic analysis, however, so it is simpler to take the conservative approach and design for pond-full tailwater.
An adjustment that could be made if you have a long pipe system or a very flat system, however, would be to look at the travel time of water between the most upstream inlet to the pond and use the rainfall intensity that corresponds to that duration. Most times piped storm drain systems on small to medium sites (say 0.1 to 10 acres) are going to be designed using a 5- or 10-minute intensity.
As jgaila says, 0.40 in/hr intensity seems too low for designing a pipe system. Contrasting what jgaila says, for our less-intense climate in the Pacific northwest, we often design piped conveyances for 25-yr and occasionally 100-yr - it all depends on the regulating agencies design criteria. If they don't have any, then a 10- to 25-yr is what we would use, depending on what is at risks when the capacity is exceeded.
RE: Storm Sewer Design + Tailwaters
It seems you are mixing apples and oranges. You are using the rational method to size your storm drain for the peak flow. However, you cannot introduce a unit hydrograph method into the design - rational time of concentration and hydrograph peaks have no relation to one another.
bltseattle is correct in that your design storm may have more than one peak. In addition, the peak flow that your storm drain is designed for does not necessarily occur 10 minutes after it starts raining. It could happen at any time. And unless your pond drains very quickly, you could have back to back storms which would leave your pond partially full before your 25-year rainfall even begins.