Level Spreader design in NC (need for 1 hour storm distributions)

[beej67]

Chewing through a level spreader design in NC right now. They say they want a design flow rate of "1 inch per hour," which is fine if you're going to do a rational peak flow analysis.

Problem is, if your flow is too high, they recommend a detention facility or some such upstream to mitigate peak flows, then don't tell you what sort of hydrograph characteristics to use. Clearly 24 inches in 24 hours on a SCS distribution is not the intent. In Florida I'd just use a FDOT 1 hour distribution. Anyone dealt with this before?

Already sent something to the reviewer, waiting back on that.

 

[TerryScan]

I do not work in NC. However, looking at the Level Spreader Design Guidelines (1/1/2007) for NC, I come to the following:

"Determine the contributing impervious drainage area and the 1”/hour discharge rate using the Rational Method." (first bullet on pg 3 of the guideline)

So, it absolutely intends one to use the Rational Method. One cannot really derive something else from this.

The flow is only "too high" if one does not provide enough level spreader length.

The "Additional Design Options" on page 6 of the guide indicate that if site constraints don't allow a stand-alone level spreader, one may use a form of detention upstream. When one uses a detention measure, the design rate for the spreader (dictating its length) is the discharge rate for the detention practice (drawdown rate for Bioretention or Extended Detention Wetland or maximum release discharge rate for Dry Detention Basin).

 

[cvg]

therein lies the rub - you can't really design and analyze a "detention basin" without routing a hydrograph. Rational method doesn't generate a hydrograph. So you need a method of generating a hydrograph that is consistent with the rational method analysis for the level spreader. good luck finding such an animal

this can't possibly be the first time this has been encountered, your reviewing agency should weigh in on this.

 

[beej67]

"When one uses a detention measure, the design rate for the spreader (dictating its length) is the discharge rate for the detention practice"

For which storm? 1 year discharge? 10 year? 100 year?



As cvg says, that's the rub.

I've got a query in to my reviewer, we'll see what she says back. I'm hopeful, but I know down in Georgia where I do over half my work, the reviewers rarely understand enough hydrology to know the difference between an intensity and a design storm.

I'll let y'all know what she says. I'm pulling for a "1 inch of rainfall over the course of one hour" hyetograph, because it works well with my design.

 

[TerryScan]

I had assumed you were familiary with NC stormwater practices. I am not, but in a few minutes of looking at NC DWQ BMP manual, all of this is described.

"For which storm?" as I cited from the Level Spreader guidance, the flow rate design method depends on whether one uses a bioretention, extetended detention wetland, or dry basin.

Design guidance for these BMPs and the methods for Stormwater calculations are in the DWQ BMP manual. Chapter 3 of the manual states:

"Many stormwater programs have a volume control requirement; that is, capturing the first 1 or 1.5 inches of stormwater and retaining it for 2 to 5 days. There are two primary methods that can be used to determine the volume of runoff from a given design storm: the Simple Method (Schueler, 1987) and the discrete SCS Curve Number Method (NRCS, 1986)."

Once one has chosen a method to determine a volume for the area in question, the manual outlines various methods allowed for routing. Ultimately, it apears one must complete a design that will discharge the volume over 2-5 days. When one does this, there will be a peak rate acheived. ***THIS*** is the discharge rate you must use for the spreader when used after the detention practice.

It also appears programs differ throughout the state as to the specific storm requirements. Figure 2-1 in the BMP manual lists 14 differnt programs. While the manual indicates there are "typical" rainfal depths, some programs differ. So, as cvg mentioned, you need to find the requirements from your review agency.