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?

 

[beej67]

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

 

[Twinkie]

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?

 

[Yakman256]

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?

 

[Yakman256]

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

 

[Twinkie]

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

 

[Yakman256]

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 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.

 

[Twinkie]

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

 

[Twinkie]

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.

 

[Yakman256]

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

 

[Twinkie]

Cover types and HSG?

 

[Yakman256]

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.

 

[Twinkie]

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%.

 

[Yakman256]

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'

 

[psmart]

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

http://www.hydrocad.net

 

[cvg]

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.

 

[psmart]

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

http://www.hydrocad.net

 

[Yakman256]

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?

 

[Twinkie]

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?

 

[Twinkie]

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