Hydraflow Seepage Bed Design
Hydraflow Seepage Bed Design
(OP)
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?
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?





RE: Hydraflow Seepage Bed Design
Whether you're allowed to count infiltration through the sides of your pond or only your bottom varies by which water management district you're going under, if you're in Florida. I've done some designs in Georgia as well, but our soils aren't near as ideal for it as Florida soils, and our review agencies aren't that sophisticated with reviewing them, typically, so they leave it up to the engineer to establish his design assumptions. I've never done SWM north of the Mason Dixon line.
Protip 1: Get a perc test. Do not under any circumstances trust the soils maps to get you the right infiltration rate. Get several tests done, and get them in the location of your prospective pond.
2: If you're in an area with a high water table, get the geotech to identify what the seasonal high water table is, and take that into account in your design.
3: Over excavating and back filling with clean sand will often help.
4: Make sure you account for what happens if the pond loses its infiltrative capacity over time, when you design your outlet control structure. It also wouldn't hurt to start your peak storm detention routing with the pond full instead of empty, as a factor of safety.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Hydraflow Seepage Bed Design
I do not use Hydraflow (HydroCAD is my favorite), but I just built a dummy pond in Hydraflow, using contour area as my storage, and exfiltration by contours, everything routed fine.
Is there anything else going on?
RE: Hydraflow Seepage Bed Design
Here is all the data if anyone cares to run the numbers:
Area = 0.81
Runoff Coeff. = 0.40 (Modified Rational)
Seepage Bed Size = 25'x50'x4'
Measured Perc. Rate 15 in/hr (Gravelly Soil)
Design Perc. Rate 5 in/hr
Seepage bed will dewater though exfiltration only. No other discharge is permitted. Exfiltration shall be applied to the bottom contour area only.
Maybe the question should be.... Can I use MRM to route a storm through a seepage bed using exfiltration only?
RE: Hydraflow Seepage Bed Design
RE: Hydraflow Seepage Bed Design
RE: Hydraflow Seepage Bed Design
RE: Hydraflow Seepage Bed Design
What are you using? Stone with a void ratio of 40%
Can you give me the cover breakdown as well? I want to run SCS as well.
I am clueless with vTsumm
RE: Hydraflow Seepage Bed Design
RE: Hydraflow Seepage Bed Design
RE: Hydraflow Seepage Bed Design
RE: Hydraflow Seepage Bed Design
Area = 0.81 Ac.
C=0.40 (0.61 Ac Lawn C=0.22; 0.20 Ac. Imp. C=0.95)
Tc = 5 min.
RE: Hydraflow Seepage Bed Design
RE: Hydraflow Seepage Bed Design
I get for a 100 year event:
Intensity = 8.16 in/hr
Peak Rate = 2.57 cfs
runoff Volume 0.018 Ac.-Ft
Runoff Depth = 0.27"
When I route the storm I only have a max water elevation of 1.44'
RE: Hydraflow Seepage Bed Design
Of course, Rational Method is poorly suited for volume-sensitive calculations such as pond routing. There have been many threads on this issue.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Hydraflow Seepage Bed Design
if you must present a hydrograph to a numb skull reviewer to gain approval, make sure the area under the hydrograph is equal to the volume of runoff you calculated in step 1.
RE: Hydraflow Seepage Bed Design
But as cvg says, you still have to determine the design storm - what I referred to as the rainfall duration. The results posted so far seem to have been done for a 5-minute "design storm", which will produce a much lower volume than a longer event, be it 1, 12, or 24 hours.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Hydraflow Seepage Bed Design
RE: Hydraflow Seepage Bed Design
I can get the 2 to work, nothing else.
Is this an NPDES thing? Why are you trying to route the 100 year storm through infiltration? Can you put an outlet above the 2-year WSE and let that go?
RE: Hydraflow Seepage Bed Design
RE: Hydraflow Seepage Bed Design
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
RE: Hydraflow Seepage Bed Design
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 www.hydrocad.net/rational.htm
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Hydraflow Seepage Bed Design
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
RE: Hydraflow Seepage Bed Design
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Hydraflow Seepage Bed Design
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Hydraflow Seepage Bed Design
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.
RE: Hydraflow Seepage Bed Design
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
www.hydrocad.net
RE: Hydraflow Seepage Bed Design
RE: Hydraflow Seepage Bed Design
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: Hydraflow Seepage Bed Design
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
RE: Hydraflow Seepage Bed Design
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.