Comparing 2-hr, 100-yr to 24-hr, 100-year storm 4

[CivDevil]

Question Background: I'm working on a rectangular parcel that is bounded on the north and west sides by existing washes. A regional drainage study (HEC-1) has defined the peak flows associated with a 24-hr, 100-yr storm for the area. The existing washes are not large enough to convey these flows and the 24-hr, 100-yr flows would overtop their banks.

As part of our development, the local municipality requires that we install drainage channels in place of these washes to convey the 2-hr, 100-yr flows.

Hydrology Question:

I have peak flows based on a 24-hr storm. Is there an acceptable voodo-based way to directly reduce these flows to reflect a 2-hr duration storm?

OR...

Should I obtain the HEC-1 model and re-run it with 2-hr, 100-yr rainfall (is this even possible as it's a non-SCS type storm)?

Thanks for your insight/replies!

 

[psmart]

I beleive you will have to do your own calcualtions to determine the 2-hour 100-year flows. This will depend on the exact methodology required by the municipality. The 2-hour duration tends to imply the use of the Rational method. You cannot use the SCS/NRCS method unless the municipality provides a specific 2-hour rainfall distribution. (This is not one of the standard SCS distribitions.)


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[msquared48]

The county where I am located accepts a figure of 64% of the 10 year storm as the two year event, but that percentage will vary depending on your locale.

Mike McCann
MMC Engineering

 

[psmart]

msquared48: I assume that's 64% of the rainfall? Due to the non-linear nature of most runoff procedures, the fraction of runoff will be different, thus requiring a separate runoff analysis. Direct scaling would be possible only when using a linear runoff procedure, like the Rational method. So it all comes back to the local regulations.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

since you are not using SCS methods, this should not be a big deal. 2-hour storm does not imply rational method around here. obtain the HEC1 model and replace the 100-year, 24-hour precipitation data with the data for the 2-hour storm. No other adjustments should be necessary unless you are required to change the loss rates etc. You can probably get the required precipitation data from the NOAA website.

 

[RWF7437]

Please read FAQ 194-1462.

By all means, DO get a copy of any and all previous reports. They will save you much time and many arguments.

If the previous reports are indeed based on HEC-I check to see what hydraulic model was used to arrive at the conclusion that the flow would exceed the bankfull capacity of the swales. Was it HEC-II perhaps or HEC-RAS ? Is there a regulatory flood plain associated with either swale ?

You can use the SCS (NRCS) methods to model rainfall events with durations other than 24 hours. That is NOT how they were intended to be used but it can be done PROVIDED you have a rainfall pattern to use as a starting point. This pattern can be one of the five SCS patterns or it can be based on some actual historical storm or it can be completely arbitrary. Be cautious however if you do this. You must make a judgement as to how realistic your chosen storm is and how likely it is to be realized in the life of the project you are designing.

If you use a design storm with a duration less than 24 hours you will almost certainly find that in the U.S. the peak flow ( cfs) for the shorter storm will be higher than the peak flow for the 24 hour storm. Conversely, the total volume of runoff (cf) for the shorter storm will be less than the runoff for the longer storm.

As for candidates for “acceptable Voodoo” I offer the following, in decreasing order of acceptable:

1. Any CALIBRATED hydrology model based on one of the following:
2. The NRCS Unit Hydrograph Method
3. The Santa Barbara Urban Hydrograph Method
4. The Snyder Hydrograph
5. The Clark Hydrograph
6. The Los Angeles County Modified Rational Method
7. Urban and Rural Regional Regression Equations
8. The Rational Method

In the second tier:
9. Any UNCALIBRATED Model from the list above.
10. Any Wild A__ Guess you care to make
11. Incantations, sacrificial virgins, appeals to higher powers and the Patriot Act

Good luck

 

[CivDevil]

Thanks for all the responses! I only wish my local agencies were as responsive.

In response to RWF7437's question about bankfull capacities, I have run some simple Manning's calc's on the existing channels that show the current swales/washes to be incapable of conveying the indicated peak flows. The previous report was done on the scale of miles and didn't seem to concern itself about how water gets from point to point within the model. This is fairly typical for the area, so micro-level drainage is studied out and patchworked together on a development-by-development basis. So, we'll be putting in the first adequate facilities in the area.

Good point on the intensity of the 2-hr storm being higher than the 24-hr.

I love this forum; it's a great clearinghouse for arcane engineering wisdom! Thanks to everyone for your prompt responses!