SWPPP pre and post watershed analisys in HydroCad

[dravn]

I am new to hydrocad and have some basic questions I need help with. Any comments are most welcome.
I have a 7.8 acres site that I am analyzing for pre and post watershed conditions for a SWPPP report.
Basically, the existing site is being modified to include an additional 4.3 acres of impervious areas.
I had an engineer friend do a preliminary hydrocad model for this analysis, but I am struggling to understand how the modeling was done.
The modeling has 2 subcatchments: 1S- pre existing conditions and 2S- proposed conditions. Subcat 2S outflows on to 3P (proposed drywells).
All areas are properly assigned with their respective CN. The final report shows an area of 15.5 acres because I believe it is assuming the pre existing and proposed conditions combined.
Is this the right way to enter the modeling data? Shouldn't it be done separately with one analysis for the pre existing conditions and another for the proposed? I have tried to read the manual and figure it out how it should be done, but I am afraid I'm getting nowhere.

Thank you

 

[peter246]

yes, to correct the final area in the report, split the HCAD file into two files - one for the existing conditions model, and then one for the proposed.

if you are comfortable editing the file by hand, you can edit it in NOTEPAD or any other text editor. otherwise, it would only take but a minute for your engineer friend to do it...

 

[psmart]

Normally your would do pre and post in separate files. Although it's also possible to do them side-by-side in a single file, this renders the various project reports less useful. Total area, percent impervious, CN distributions, and other project-wide statistics will be incorrect.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[dravn]

Thank you very much Peter and psmart! I am glad my instincts were correct.I believe I can separate the files myself or redo them separately.
Thanks again.

 

[psmart]

The easiest way to split the file is to make a copy and delete the appropriate nodes from each file. This will ensure that all the project settings are identical.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[dravn]

Peter,
Precisely what I did. However I had to change the flow length for the post development from 150 ft to 100 ft, as required by the local governing agency. Also, the plan reviewer had a comment on the Tc. He thought 7.5 minutes was too high. He also insists that the analysis has to be done using TR-55, so I printed the HydroCad explanation for using the SCS method based on TR-20 and hope he will understand why the program runs this way. I noticed that if I reduce the Tc to 3 minutes, which is what he suggested, the peak flow is higher than with 7.5 minutes

 

[psmart]

Since HydroCAD includes most of the capabilities of TR-55 and TR-20 it is suitable for most applications that would otherwise use these programs. As far as the Tc calculation, HydroCAD will give you exactly the same result as TR-55, as long as you restrict your calculations to the TR-55 approach (segments of sheet, shallow concentrated, and channel flow.)

As you noted, reducing the Tc will increase the peak runoff, but 3 minutes is pretty short. If you're using a tabular rainfall table with a typical 5 minute time step, shorter Tc values won't make much difference. But a polynomial-based curve (such as the Type II and III in HydroCAD) have more resolution and will continue to produce a higher peak as the Tc is reduced below 5 minutes.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

I find it's common practice to set the minimum Tc for watersheds at 5 minutes or 6 minutes. There's an adjustable setting in HydroCAD for that. Pay attention to your output - it will tell you when it's defaulting to the minimum.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[dravn]

Thank you beej67. I have set the Tc at 5 minutes for the pre and post analysis. I am getting a warning message Hint#37 (longer time span advised for full volumes). Right now I have a 24 hr. time span starting at 0 ending at 24, with a time increment of 0.03. I also set the storm type to Win TR-55 NCRR (site is in NY), which gives me considerably different numbers in my peak discharge, but not in the storage capacity... not sure if I change the settings in storm type I also need to change some other settings to account for it.

 

[psmart]

If you are concerned with reporting the total discharge volume, hint 37 is recommending a longer calculation time span in order to include the trailing flow after 24 hours. The rainfall duration remains at 24-hour, but you can use a calculation time span longer than 24 hours in order to include all of the delayed discharge in the analysis.

Depending on your regulations, you could be using a Type II or III storm in New York, or one of the new distributions which are designated NRCC_A, NRCC_B, NRCC_C, or NRCC_D. Changing the distribution does not necessarily require any other changes. But if you've defined rainfall events, make sure you revise all the events for the new distribution.

Using a different rainfall distribution will change the peak runoff but not the volume. The runoff volume is strictly a function of the curve number.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[dravn]

Hi Peter,
What would be a reasonable exfiltration rate on drywells (setting at constant velocity)for gravelly sands (SW) in HSG A?
The value I had on an example file was 4 inches/ hour.
Thank you.

 

[psmart]

The exfiltration rate will have to be determined by an engineer with appropriate knowledge of the site. In some cases, on-site testing may be required. But unless the exfiltration rate is very high, it won't have much effect on the peak-flow behavior of the site. Conservative designs will also allow for blockage and reduction of the exfiltration rate over time. So the value becomes a matter of policy as well as current site conditions.

If your stormwater regulations permit the use of exfiltration in your drainage system, they should also provide some guidance on specific design values.




Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[dravn]

Thank you Peter. There is no percolation test for this site yet, although the test borings show a consistent SW soil throughout and the water table is around 48ft below grade. Nevertheless, I think perhaps I should uncheck the box "allow exfiltration" . I am mainly concerned about obtaining a model result with the correct storage required, and the exfiltration rate has an impact on the storage capacity results from what I was able to observe.
I also had to change the 100 year storm to the new guideline of 9.5"rain/24hrs, just recently changed from 7.5".

 

[psmart]

Sizing your detention to function without any exfiltration certainly produces a more conservative design. Whatever exfiltration does occur then gives you an extra margin of safety.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[dravn]

I agree. So basically the storage required would equal the total runoff volume from the subcathment (s)?