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Hydraflow Seepage Bed Design

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?

RE: Hydraflow Seepage Bed Design

It's done routinely in Florida for your first flush, but they don't use Hydraflow to model things down there typically, because (if I recall) Hydraflow can't handle variable tailwater. They commonly use ICPR. I've used XP-SWMM down there. I won't be of much help in coaching you through how to do it in Civil3D, but some other folks around here might. (Francesca maybe?)

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

You aren't going to find this the BMP manual, but if you find Domenic's presentation on infiltration BMPS, (it is on the PADEP ftp), it will tell you to use the wetted perimeter. You can hang your hat on that.

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

(OP)
Normally I design a seepage bed using a spreadsheet. However, the reviewing Engineer is requesting a routed hydrograph though the bed to show that there is no discharge. The problem is that the bed size has decreased in size from the results of the spreadsheet and hydraflow keeps giving me an error that says.... "Warning Discharge values should increase with Stage, Hydraflow may experience errors during routing calculations. Apply exfiltration to wetted areas for better results." The spreadsheet I use is based on the hyetograph for the site area. I have spreadsheets that use both Modified Rational and TR-55 both give a seepage bed that is about the same size. Something doesn't seem right.

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

(OP)
I just ran the same hydrograph using another software program and I need a bed size much larger than what Hydraflow uses. I'm starting to come to the conclusion that HYDRAFLOW SHOULD NOT BE USED WITH MODIFIED RATIONAL

RE: Hydraflow Seepage Bed Design

I use HydroCAD for everything. Try it, I ber you fall in love with the software.

RE: Hydraflow Seepage Bed Design

(OP)
I downloaded a trial version of HydroCAD and ran the same data and got almost the same exact answer as HydraFLOW. I still think its too small. TWINKIE... since you seem to have done work in PA are you familiar with the VTPSUHM software? If I had a choice this is the software I would use for the MRM. Both HydraFLOW and HydroCAD show my 25x50x4 bed filling up a little more than half way. The VTPSUHM software shows it over topping and discharging our the inlet grate. The problem is that the VTPSUHM program is an old old old program that goes back to say Windows V3.11 When I switch to a 64 bit program this program will no longer be available to use sad I have always designed above and below ground systems with this program and have reliable data that shows that seepage beds volumes are accurately predicted with this software. At least for high frequency storms with small drainage areas.

RE: Hydraflow Seepage Bed Design

I will take a look at this in the AM.

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

I mean, you could always do a runoff volume calc by hand for the storm you want to manage (use the DEPs NPDES worksheets), provide a seepage bed that meets that volume, do the dewatering calcs using FOS 3.

RE: Hydraflow Seepage Bed Design

(OP)
Thanks twinkie... Yea .. a void ratio of 40%. Both HydraFLOW and HydroCAD I get a bed filling about 2.5'

RE: Hydraflow Seepage Bed Design

Cover types and HSG?

RE: Hydraflow Seepage Bed Design

(OP)
Drainage area Info:

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

What intensity are you using? I just threw in 2.95 in/hr (PennDOT Region 5 100 yr intensity, 60 minute), I got this to work by increasing the size to 70 x 25, can you do that? Also, do you have any type of perforated pipe running through this? Make sure you count that volume as 100% void, and not 40%.

RE: Hydraflow Seepage Bed Design

(OP)
I must be doing something wrong. When I run it by basin isn't event filling up at 25'x50'x4' For HydroCAD I downloaded the rainfall data from the NOAA for zip code 19081 as per the instructions with HydroCAD.

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

No one seems to have mentioned what rainfall duration they're using. If you're setting the rainfall duration equal to the Tc you're getting a relatively small runoff volume, hence the low WSE. Longer durations, while giving a lower peak flow, will generate a larger volume and hence a higher WSE in the pond.

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

It sounds like you are really not "routing" anything. There is no flow going through an outlet works or spillway, just infiltration. You are simply capturing the entire storm and percolating into the ground. Use the design storm (1-hour, 24-hour or whatever) and the storm rainfall depth (not intensity). Calculate the total runoff volume. Forget about time of concentration.

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

When I say "routing" I'm including the simplified case where there is no outlet and everything is being retained. However, there is an exfiltation "outlet" in this case, which will cause some reduction in the required storage volume, depending on the exfiltration rate. True, you can ignore the exfiltration and just size the storage to hold 100% of the runoff volume for a conservative 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

(OP)
It looks like both HydraFLOW and HydroCAD use a storm duration that is 2 times the Tc. So for a Tc of 5 minutes it uses a duration of 10 minutes. Everyone is right... there's no runoff volume. Does anyone know how to change the storm duration in HydraFLOW or HydroCAD?

RE: Hydraflow Seepage Bed Design

OK...so now that I have a little better grasp on this, I routed your areas via SCS (Used HSG 'C' (CN 74 Lawn; 98 Impervious) because you did not indicate what soils type), used a 24-hour depth of 7.70 inches (Media 100-year NOAA), Tc of 6 minutes...way undersized.

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

FYI...in HydroCAD the storm duration is set by clicking on the calculator button>rainfall. I used 24-hours in the above example.

RE: Hydraflow Seepage Bed Design

(OP)
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

RE: Hydraflow Seepage Bed Design

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 www.hydrocad.net/rational.htm

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Hydraflow Seepage Bed Design

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

RE: Hydraflow Seepage Bed Design

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
www.hydrocad.net

RE: Hydraflow Seepage Bed Design

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

RE: Hydraflow Seepage Bed Design

(OP)
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.

RE: Hydraflow Seepage Bed Design

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
www.hydrocad.net

RE: Hydraflow Seepage Bed Design

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.

RE: Hydraflow Seepage Bed Design

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
www.hydrocad.net

RE: Hydraflow Seepage Bed Design

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

RE: Hydraflow Seepage Bed Design

(OP)
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.

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