Stormwater Management using Rational Method 1

[mes11]

thread194-120607

I have some questions based on the responses within the referenced thread, particularly when dealing with sites of 5 acres or less. I have been using the Rational Method to calc my storage volumes much in the same way you calc a water quality volume. In Ohio water quality volume is found with the equation WQv=CPA, where C is the runoff coefficient, P is the rainfall depth, and A is acres. When I calc the storage volume for a site I basically use P as the site 100-year 24-hour storm event depth, where I take the difference between post-developed minus pre-developed. In Ohio the general permit requirements by the EPA require that the critical storm method be used as the means to meter runoff control. The problem I run into is that the storage volume I found earlier usually is not enough when using the critical storm method. I use Hydrocad to model the basins I design. The basin will peak based on the type of outlet control structure that is designed. Based on some responses in the referenced thread about the hydrograph, it is comprised of a rising limb of 2Tc, lasts for a duration, and falling limb of 3Tc. Particularly with small sites (5 acres or less) the Tc can be as low as a couple minutes and so essentially you have a rectangular hydrograph. Is this correct and/or usable? Going back to the volume being to small, for small sites you typically get a small outlet orifice (typically 1-3 inches). Because of this small orifice, the basin will peak a long durations because it takes longer for the orifice rate to intersect the falling limb of the hydrograph. So basically I'm just wondering if I'm on the right track with all this and wondering if there are any suggestions to avoid some of these issues. What is typically the smallest site that can be used with TR-55?

 

[psmart]

As others have said, the Rational method is only intended for predicting peak flows, and using it for volume-sensitive calculations (such as pond design) is problematic. I would recommend that you use the SCS/NRCS method for all pond modeling, even for very small watersheds.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[mes11]

When dealing with sites that have an HSG classification as C/D, any concerns with these sites having a retention basin?

 

[psmart]

If you're asking about the feasibility of infiltration/retention basins with C/D soils I would start a new thread for that issue.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

put in dry wells

 

[Ekenny_G]

soils with C/D classification are ideal if doing a retention basin but not for infiltration since its almost like a hard pan.
like others have said, Rational method is recommended for city streets and small drainage areas with high impervious percentage. However, for ponds I would recommend using NCRS/SCS method.

 

[Phidelt218]

Most of the project i have dealt with were small sites (1 acre or so)and i used to rational method to determine the pre and post construction slows. Since typical flows must have pre=post construction for new projects. The critical time was used to calculate the intensities (in/hr)used in the rational method for my flows. Q-C*I*A(acres)

I have used the rational method for these small sites to get a volume, but it was V=C*P (depth in ft)*A(ft^2). Found the difference between the Post Volume-Pre Volume=Storage requirement. The city uses theses equations in their drainage guidelines.

Not sure if this helps or not.

 

[mes11]

Phidelt218,

I have used pretty much the same method that you described. I went back through 5 different projects this week all between 1 to 5 acres to compare the Rational Method basin design results against the SCS Method basin design results and there was minimal difference between any of them. I'm not totally convinced that the Rational Method could not be used to design a basin at least for small sites. However, the SCS method did calculate a slightly larger basin volume in each case. For each storm event on these projects, my peak elevation only changed about 0.1 - 0.25 ft. I do believe that I will continue to use the SCS Method simply because it gives a more conservative result.

 

[mes11]

My question about soils that are classified C/D had to do with the high water table. I didn't do I good job of describing what I meant earlier, but talking about sites where the soil classification is actually C/D, not C or D. The C/D classification means it is typically a type C soil and the /D comes into play because of the water table getting within 60 inches (or less) of grade at times. I just wasn't sure how the occasionally high water table would affect the retention basin, if at all, since the permanent pool would be within the table.