## AutoCAD Civil 3D - Stormwater

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Posts: 2
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Message 1 of 12 (2,011 Views)

# Hydraflow - Modified Rational Method - Storm Duration

2011 Views, 11 Replies
10-27-2009 12:39 PM
my current case is to turn a nice woodland into a parking lot, therefore C value from 0.1 straight to 0.95
and NJ is only allowing 80% of 100-yr runoff from existing condition to be your post-development runoff.
this leave me to a very strict condition. Then i come to notice that the Hydraflow default SDF is limited to be less than 9.9
by hand i figure the storm duration is going to be like 300+ min with my time of concentration of only 6.
But Hydraflow tells me my SDF is lilmited to 9.9, not even close to what i had by hand calculated ( 300/6 = 50!)
Is there a way i can lift that SDF limit?
thanks
*Matt.Anderson
Message 2 of 12 (2,011 Views)

# Re: Hydraflow - Modified Rational Method - Storm Duration

10-27-2009 02:06 PM in reply to: mhsu
Hydraflow Hydrographs will stop figuring the Intensity out when it hits
60 minutes which is the highest value in the IDF curve it can calculate.

If you storm exceeds the IDF curve, I would suggest SCS methods for such
a small release and extended volume.

Matthew Anderson, PE

mhsu wrote:
> my current case is to turn a nice woodland into a parking lot, therefore C value from 0.1 straight to 0.95
> and NJ is only allowing 80% of 100-yr runoff from existing condition to be your post-development runoff.
> this leave me to a very strict condition. Then i come to notice that the Hydraflow default SDF is limited to be less than 9.9
> by hand i figure the storm duration is going to be like 300+ min with my time of concentration of only 6.
> But Hydraflow tells me my SDF is lilmited to 9.9, not even close to what i had by hand calculated ( 300/6 = 50!)
> Is there a way i can lift that SDF limit?
> thanks
>
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Posts: 1,274
Registered: ‎03-07-2006
Message 3 of 12 (2,011 Views)

# Re: Hydraflow - Modified Rational Method - Storm Duration

10-28-2009 05:24 AM in reply to: mhsu
The other issue is that the modified rational hydrograph is often inappropriate and used incorrectly. It is supposed to be an iterative process adjusting the accending and especially the receding limbs to maximize the storm water volume. HydroCAD is the only software I am aware of that does this correctly and automatically.

I know of a lot of engineers who don't bother and as long as the software prints something out, will use the answer, valid or not.

Make sure you read carefully all the restrictions on the modified rational method before using. It is only useful for a single hydrograph site with 1 inflow and 1 basin. No summing of downstream hydrographs. I don't care if some software packages let you do it.
Ann Wingert, P.E.
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Message 4 of 12 (2,011 Views)

# Re: Hydraflow - Modified Rational Method - Storm Duration

11-21-2009 04:41 PM in reply to: mhsu
C3D12-13
Win7x64 6gb
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Message 5 of 12 (2,011 Views)

# Re: Hydraflow - Modified Rational Method - Storm Duration

11-24-2009 10:18 AM in reply to: mhsu
The COMPLETE Intellisolve package has many more features than the HydroCAD.

HydroCAD is better for modified rational, which is dangerous to use anyway.

It is also better for small underground chambers.

The rest I use Intellisolve.
Ann Wingert, P.E.
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Registered: ‎12-20-2007
Message 6 of 12 (2,011 Views)

# Re: Hydraflow - Modified Rational Method - Storm Duration

11-24-2009 10:46 AM in reply to: mhsu
thanks for everyone's help.
i don't like MRM either, but i was required by agency to use it...
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Message 7 of 12 (2,011 Views)

# Re: Hydraflow - Modified Rational Method - Storm Duration

11-25-2009 05:01 AM in reply to: mhsu
Which agency is that?

I almost always have the oppostie problem. I've tried MRM on a couple small projects and had reviewing agencies kick back as method is only rough kludge compared to SCS.
Ann Wingert, P.E.
*John Mayo
Message 8 of 12 (2,011 Views)

# Re: Hydraflow - Modified Rational Method - Storm Duration

11-27-2009 02:33 PM in reply to: mhsu
Mod rat is allowed by law in NJ on any watershed less than 20 acres as per
RSIS & the NJ Stormwater Rule. Most municipalities were also 'convinced' by
the state to adopt the Stormwater Rule verbatim.

The mod rat is very appropriate for small watersheds it should adjust the
storm duration to find the critical volume not the legs.

--
C3D & RD 2010, W7
Core2Quad @ 2.83 GHz 4 GB Ram
GeForce GTS 250

"mhsu" wrote in message news:6294682@discussion.autodesk.com...
> thanks for everyone's help.
> i don't like MRM either, but i was required by agency to use it...
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Message 9 of 12 (2,011 Views)

# Re: Hydraflow - Modified Rational Method - Storm Duration

11-28-2009 10:18 AM in reply to: mhsu
Allowed or REQUIRED?

We used to get stuck with Penn DOT requiring rational method, (reviewers often didn't understand storm water routing at all, old style use rational to size pipe only.) with the townships & DEP requiring SCS. We would have to have 2 sets of calculations for same site. PennDOT - Best stormwater manual written by structural engineers. Older version used to be excellent for determining load on pipe and horrible for calculations to design required size.

Yes, Rational method is prefered for determing flow value and sizing pipes for small watersheds, but since SCS method is required to determine volume to infiltrate for NPDES permits, it doesn't do any good to use Rational for Routing calculations. The SCS is likely to result in a target infiltration volume above that generated by the rational method site.

So, at least in PA, if you have a site with less than 1 acre disturbance or less than 5 acres with no point discharge (good luck), or a big empty flat site with lots of room for detention basins, go ahead and use MRM, as long as it is done correctly. Everything else, usually >95% of my work, use SCS.
Ann Wingert, P.E.
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Message 10 of 12 (1,166 Views)

# Re: Hydraflow - Modified Rational Method - Storm Duration

12-05-2012 12:19 PM in reply to: mhsu

Yes, I don't know why it is doing that. You should be able to take it out further, to the critical duration,  to get your max required storage.

Consider the two calculations I ran with identical basin parameters: A = 10 ac, C = 0.75, Tc = 20 min, Qtarget = 1.0 cfs below:

I ran one in Hydraflow Hydragraphs shown first below, the second was in Hydraflow Express, shown below that.

• Hydragraphs went out to SDF = 9.9, and came up with 97,245 cf of estimated storage.
• Express took it out to SDF = 20, and came up with 106,923 cf of estimated storage.
• Modified FAA Method software I use, came up with 126,069 cf of estimated storage.

Hydraflow Express has a couple of display issues going on also:

1. It doesn't display the SDF past 9.9, it goes back to displaying 1.0, however it takes the computations out further to the critical duration, as can be seen in the graph and table.
2. It displays minutes in the table but the column heading says "hrs".

• It would help Hydraflow to report a little more estimated storage, if it used the typical Modified Rational Method Receding Limb duration of 1.5xTc, instead of just 1xTc as it does now.

Fred Ernst, PE
C3D 2015
Ernst Engineering
www.ernstengineering.com
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