Tailwater Boundary Conditions

[stevel88]

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?

 

[gibfrog]

Here in SW Florida, we commonly use the top of culvert pipe for a 25 year tailwater elevation. The top of bank may be a bit conservative depending on the geometry. I have used the seasonal high water level of the ditch and this is field determined by stain lines and/or biological indicators.

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

 

[gbam]

Hopefully, I understand your question correctly, I'll give it a shot. Typically "Fema" will give data for other storms, ie flow rates. The 25-year event discharge may be directly available. Then use the original Fema HEC-2 or -RAS model and run the 25-year. This will give you the answer. If the 25-year discharge is not available and others are you can plot the discharges vs frequency on a log-log plot to extrapolate or interpolate for the discharge. I have done very similar analyses here in Arizona.