Tailwater Boundary Conditions
Tailwater Boundary Conditions
(OP)
What are acceptable methods for estimating 25 year storm
Tailwater Boundary Conditions (say for a detention basin outletting into an existing ditch)?
A regulatory agency has suggested using top of bank of existing ditch to estimate 25 yr storm elevation.
Also, what about tidally influenced areas?
I proposed using the 100yr flood elevation (from F.I.R.M. maps) of 9.0' times 80% to get elevation 7.20' for 25 year storm. For our county the 24hour 100Yr rainfall is 10" and the 24Hr 25Yr rainfall is 8" (thus 8"/10" = 80%).
Is this not a reasonable approach?
Tailwater Boundary Conditions (say for a detention basin outletting into an existing ditch)?
A regulatory agency has suggested using top of bank of existing ditch to estimate 25 yr storm elevation.
Also, what about tidally influenced areas?
I proposed using the 100yr flood elevation (from F.I.R.M. maps) of 9.0' times 80% to get elevation 7.20' for 25 year storm. For our county the 24hour 100Yr rainfall is 10" and the 24Hr 25Yr rainfall is 8" (thus 8"/10" = 80%).
Is this not a reasonable approach?





RE: Tailwater Boundary Conditions
There is a difference between a 25 year rainfall and a 25 year flood level. In tidal areas the winds/low pressure systems may cause tidal surge and flooding. In non-tidal areas, the storm peak flood level usually occurs much after your site has peaked and discharged. Assuming tailwater condition of a 25 year flood peak for a 25 year storm is excessively conservative. I use the mean high water for a tidal boundary condition for a 25 year rainfall analysis.
You can interpolate the log-log to determine various intermediate flood levels if you have the 100 year and the 2.33 year or the mean annual flood level/rainfall.
Ther USGS has peak flood data for many stream systems. You also may create a basic model of the ditch system to estimate its peak stage and the time it takes to peak. Many local jurisdiction is SW Florida have models of many systems and you may extract the required data from the model to create dynamic or static boundary conditions.
Best of luck....
Clifford H Laubstein
FL PE 58662
RE: Tailwater Boundary Conditions