Hydraflow Seepage Bed Design

[Yakman256]

Hey all,

I'm a complete newb to Civil 3D and Hydraflow and I'm trying to design seepage bed and route a storm though it. I put the exfiltration percolation rate in for the contour area only and it gives me a warning that I should be using the wetted perimeter. However, PADEP only allows for infiltration through the bottom of the system only.

Has anybody else been successful at routing a storm through a basin with no surface water discharge? If so do you have any tips or tricks?

 

[Yakman256]

Its only partially for NPDES. There is not a good place for surface water discharge so I'm forced to put everything in the ground. TR-55 will always give a higher required storage volume than MRM but since the drainage area is less than an acre, rational method should be acceptable. However, I don't feel comfortable using MRM with either HydroCAD or Hydraflow.

When I change the duration to 24 hrs (1440 min) the peak flow RATE is less (0.16 cfs). No matter what software program I use, the peak flow rate for a 100 year storm MRM should be:

Q = 2.64 CFS with
C = 0.40
I = 8.16 in/hr (Tc= 5 min)
A = 0.81 Ac.

No Matter What Program

I agree that the storm duration is the problem. I hate to bring up the idea of Runoff Volumes when using MRM, however a hydrograph with a peak of 2.64 cfs and a longer storm duration would yield larger runoff volumes and would require a larger system. I just don't know how to do that

 

[psmart]

If you want to model a 24-hour event, just set the storm duration to 24 hours AND set the corresponding 24-hour intensity. If you're using an IDF curve with HydroCAD the intensity will be set automatically based on the duration.

Using a 24 hour duration will give a much lower peak runoff than using a 5 minute duration, but you'll get the full (larger) 24-hour volume.

BTW: HydroCAD does not set the storm duration - this is up to the user, depending on what you're trying to do. For details see

http://www.hydrocad.net/rational.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

Peak flows from rational method and SCS method should be similar, but not necessarily the same. The difference comes from how the SCS built the SCS unit hydrograph, which is a long and complicated story you can read about in a textbook. The most common error for those mixing these methodologies is probably mistaking intensity for volume.

CVG is right, you're not really "routing" anything, unless the reviewer is concerned with recovery time of the pond. You should by all rights be able to simply calculate the runoff volume and make sure the pond has that amount of volume. Then if they want to see a recovery time calculation, you can do that with Darcy's Law just as easily as you can with software. That's how I've done it routinely on the panhandle.



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

http://www.campbellcivil.com

 

[psmart]

If you've got an outflow, it's a routing calculation. In this case the only outflow may be exfiltration, but at 5 in/hr it produces a significant outflow during the 24-hour storm. If the bed is 4' deep, the exfiltration will empty the entire bed in about 10 hours, so the required volume is much less than the "retain everything" approach, and you need to perform a pond routing in order simulate the behavior and determine the actual volume required.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

In a perfect world I agree with Peter. I've seen enough variation in perc tests to know we don't live in a perfect world, and I've seen how perc rates tend to fall over time in retention ponds. If you have the room, I recommend storing all the required water as a factor of safety like CVG recommends, regardless of what your routing software tells you the peak stage might be.



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

http://www.campbellcivil.com

 

[Yakman256]

Ok after spending some time doing some research I think I have it figured out. WHEN USING MRM FOR BASIN ROUTING, YOU MUST PROCEED WITH CAUTION. The problem is with the storm duration. The duration will be set a 2X the Tc. So if you have a 5 min Tc, the duration will only be 10 min. That duration may be fine when doing pipe flow analysis when you’re looking at the peak rate, however when routing the storm through a Basin, it’s not nearly long enough to provide enough runoff volume for proper basin sizing. If you use longer time of concentrations, you’ll find that although the peak flow rate is less, the runoff volume is greater and the required basin size will also be larger. For a few years now, I’ve noticed different Townships adopting Act 167 Plans that require the use of TR-55 to develop the Tc’s for use with MRM. Now I see that the reason. It’s because TR-55 will provide a longer Tc and runoff volume.

When I started this design, I was using the PaDOT average velocity charts to determine the time of concentration because it always provided me with the highest peak RATE but it also gave me a 5 min Tc. If I have a 0.81 Ac. watershed with C=0.40 and a 5 min Tc and a 100 year intensity of 8.19 in/hr… I have a tough time believing that only 0.27 inches of runoff will be generated from a 100 year storm event.

So the Moral of the story is when using HydraFLOW ….. NEVER EVER use the average velocity charts to determine the time of concentration for a basin routing. Using TR-55 isn't a bad idea but In my opinion it still generates runoff volumes that are too small when using MRM unless there's a way to change the storm duration.

HydroCAD, appears to be a better program because I can use whatever Tc I want and then manually adjust the duration so that that storm is generating the proper amount of runoff VOLUME. I have not finalized my research but it seems that storm durations around 30 minutes work well when using the average velocity charts.

BTW.... I have sized the facility using only 1/3 the perc rate measured with a double ring infiltrometer which excludes any lateral percolation. I'm using HydroCAD with a 5 min Tc and a 30 minute duration. The size of the system seems to be a little smaller than the total runoff volume that I originally calculated. When I route the storm it fills up top the top of the facility. I think I'm good.

Does anyone think I’m missing anything? I think this was a good discussion. Its nice to dust off the grey matter every once in a while.

 

[psmart]

Remember that the rainfall duration is independent of the Tc. The Tc is a physical property of the catchment area, and the rainfall duration is a parameter of the storm to be studied. When using Rational method to determine peak flow, the duration is generally set equal to the Tc. That's fine for a peak flow study, but not for a volume determination, which generally requires a longer rainfall duration. But the Tc remains the same. The two parameters are independent, and must be determined separately for the purposes of volume determination.

All the Tc procedures mentioned here are reasonable. Looking for a longer Tc value is not the issue. Just leave the Tc as-is and determine the appropriate rainfall duration to size your detention volume. Make sure that any software you use respects the difference and doesn't try to always set the duration equal to the Tc.

As beej67 suggests, you should certainly plan for gradual reduction of exfiltration rates and be sure your system will behave in an acceptable manner if it blocks completely. This can be done by providing a larger volume and/or suitable overflow capability. But you've already reduced the measured exfiltration rate to one-third, and this still produces a very significant rate that may be able to handle the entire runoff volume with a much smaller storage volume. Over the years I've seen many sites that are drained strictly by dry-wells and infiltration structures, especially on sites that had no discernible pre-existing runoff and where therefore required to maintain zero runoff in the proposed condition.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

half hour storm seems a bit low to me. typical procedure here is full containment of the 2-hour storm with 100% infiltration through the soil or dry wells. de-rating factor is required depending on the type of soil and can be as high as 5 and "routing" of a hydrograph is not allowed for retention basins.

 

[psmart]

All stormwater is local. Some regulations require exfiltration while others prohibit it. I'm not making any recommendations about *if* you allow for exfiltration. I'm just responding to the initial question about *how* you would generate a "routed hydrograph though the bed to show that there is no discharge".

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

The moral of this story is just don't use rational method to do storage design. Just don't do it. The assumptions aren't valid, and the design case you'd be designing to isn't remotely the worst case that could hit your pond. Use SCS.

If you're going to use Rational at all in Hydraflow Hydrographs, consider using the Dekalb method, which was intentionally developed to use rational parameters to create a synthetic unit hydrograph for basin routing. But even that method isn't even accepted in Dekalb anymore, the place it was built for.



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

http://www.campbellcivil.com

 

[Yakman256]

I use the VTPSUHM (Va. Tech. Penn State University Hydraulic Module) It actually estimates runoff volumes very well using MRM for small watershed less than 5 ac. It creates a more traditional hydrograph with MRM than the standard triangle shaped hydrographs you see with the newer software packages. I have designed a seepage bed using this software for a Township building and have accurate data that tells me that it sized correctly. For hurricane Floyd it just barely overtopped and for hurricane sandy, it didn't even fill up to the top.

I'm going to start a new thread on this topic because I think its important. Occasionally, I review plans for the Townships and so far all of them have been using SCS. I have to develop some comfort level for plans I review using the latest software and MRM.