Need help with SSA rational method

[drazen-ars]

Need help with SSA rational method

I have to compare results of rational method between SSA and another product. Plain rational method, steady flow, Darcy-Weisbach. Times of concentration (TOC) are manually defined.

I played a bit with the units, formulas, IDF curve is defined as pair of values time/intensity. Seems that I achieved something. Results are more or less similar, but there are some differences. After playing with both programs, comparing in Excel, think that I found what is different. And I am bit confused and need help.

So, I have very simple example, of one channel with catchment areas. All areas have the same runoff coefficient – 0.8. Pipe diameters are constant (500). The whole analysis is restricted between 5 and 10 minutes of rain duration (5 min-397.22 l/s/ha and 10 min – 321.53 l/s/ha). I converted IDF to mm/hr. So I have the next example:

[picture in attachment]

In theory, what I learned about rational method, there is next definition:

Intensity from IDF curve is defined by TOC. TOC is defined by sum of „entrance time“ (which is defined by shape and size of catchment area) and sum of upstream flow times. In my example I defined 5 min for every area.

So, for example for pipe nr. 3, TOC is calculated:

TOC_3 = Entrance time for area 3 + Tflow_1 + Tflow_2 + Tflow_3

For example:

TOC_3 = 5 min + 0.67 + 0.45 + 0.51 = 6.63 min

That TOC_3 (6.63 min) is base for reading appropriate value of intensity (I_3) from IDF curve.

The flow in pipe 3 is then calculated:

Q_3 = I_3 * (A1 * k1 + A2 * k2 + A3 * k3)

This is the way how I learned rational method. But results between SSA and another product (which works in a way explained above) are different. Differencies increase downstream.

After playing with results in Excel, I found out that SSA does not summarize in TOC all flow times upstream. So TOC in SSA is calculated simply by initial entrance time and flow time in specific pipe:

TOC_3 = Entrance time for section 3 + T_flow_3

So, there is no time added from T_flow of pipes 1 and 2. Seems that it is major difference which leads to final results differencies.

I tried to understand all options of SSA to find possibility to add upstream flow times, but without success.

Can somebody help me?

Drazen Galic

D dot galic at studioars dot hr

[Hidden_Brain]

see attached model (rename to remove ".txt" at end of file name), i have two sub-areas set up:

AREA      SIZE (Ac)    Tc (Mins)    C Factor

SUB-1        3.0                  10              0.3

SUB-2        1.5                  5                0.7

I coded in two pipes both 100 ft long.

I entered custom IDF data in the format:

i (Intensity in in/hr) = B/(Tc+D)^E

and I simplified the terms as B = 105, D = 15 and E = 1 to give me the intensity equation as:

i = 105/(Tc+15)

From the model, flow velocity in pipe P1 is 5.45 ft/s, so effective Tc at IN2 is = 10+ (100/(5.45*60)) = 10.3 mins. Per the problem statement, SUB-2 Tc is 5 mins. So, 10.3 mins should be used as Tc to size pipe P2. For Tc as 10.3 mins,

Flow through pipe P2, Q = C'*i'*A'

C' = (0.3*3)+(0.7*1.5)/4.5 = 0.43

i' = 105/(10.3+15) = 4.15 in/hr

A' = (3+1.5) = 4.5 acres

Q = 0.43*4.15*4.5 = 8.1 cfs. This is the flow that pipe P2 should be sized to carry.

SSA calculates the flow in pipe P2 (see attached model in 2013 version) as 8.45 cfs, slightly higher but pretty close to my calculations as above.

Did i understand your issue correctly?

[drazen-ars]

Hi engineer4life1979

Thank you for your answer.

Ok, generally I understand your explanation. Maybe, If you can, to add the third pipe to the model, to understand what will happen with the TOC.

In the second pipe referent TOC is as follows:

TOC2=Tinlet (1 or 2) + Tflow = 10 or 5 + 0.3 = (selected bigger time 10) = 10 + 0.3

But the question is what will be for third pipe. I expect the next

TOC3=Tinlet for 3 + Tflow 1 + Tflow 2

Seems to me that SSA calculates only Tinlet+Tflow of specific section. With the longer system (in my case 6-7 sections), SSA does not add Tflow of all upstream sections(pipes) so at the end difference became quite high.

Did I understand well?

Regards,

Drazen

[Matt.Anderson]

Drazen-ars -

In your original post, you describe the methods typically associated with the Hydraflow Storm Sewer application and the Standard-Step method.

SSA does not calculate the Standard-Step method.  The Rational Methods (including modified, and DeKalb) all compute the runoff to the inlet or junction.  As soon as runoff enters the system, continuity and momentum calculations take over. SInce you are using Steady State Qin = Qout, so no routing occurs on pipes.

[Matt.Anderson · Accepted solution]

Drazen-

In your original post, you describe the methods typically associated with the Hydraflow Storm Sewer application and the Standard-Step method.

SSA does not calculate the Standard-Step method.  The Rational Methods (including modified, and DeKalb) all compute the runoff to the inlet or junction.  As soon as runoff enters the system, continuity and momentum calculations take over. SInce you are using Steady State Qin = Qout, so no routing occurs on pipes.

[drazen-ars]

Hi Matt,

Thanks a lot. You clarified what makes me confused.

Best regards,

Drazen

[Hidden_Brain]

Matt, if i choose to run the simulations with the "Steady State" option, will it not be Qin=Qout? I believe that is what I set for the model I attached previously (cannot confirm at the moment). Please let us know.

[fcernst]

As soon as runoff enters the system, continuity and momentum calculations take over.

Correct Matt, but I see why people are confused if SSA uses system TOC to size pipes.

The SSA manual discusses that a SSA User should be concerned with system TOC for pipe sizing purposes :

[apzalewski]

Hi Matt, I have been experiencing the same problem with SSA as the drazen-ars. It appears that SSA is not using the Rational Method correctly since it is supposed to calculate the rainfall intensity based off the cumulative TOC. Instead it is treating each sub-basin as a separate storm event which is completely wrong. The only work around I have found is to calculate the travel times for the pipes and manually override the TOC at each point that flow enters the system. Is there a hidden button or feature in SSA that I am missing which will make it work correctly because the only work around is not efficient. Thanks in advance. (sorry for the formatting, it does not seem to maintain my formatting when it posts).

[adwc]

Matt
Would you say that the way that the way SSA calculates the flow is a more conservative approach? In other words, does it model higher flows because it is not taking into account the cumulative TOC?

[annw2]

I agree that the SSA not counting the total travel time in the pipes in the system TOC is not correct.

The alternate product, Storm Sewers, has the option to either count total pipe time or not count it.  (I've had ONE reviewing agency request the shorter time in close to 30 years of experience.)

You do have to watch that the calculated flow doesn't drop from the upper pipe to the lower.  That happened once back when I was doing these calculations manually on paper forms.

The shorter time of concentration would be a more conservative calculation with the potential for larger pipes than needed.

[adwc]

Thanks, Ann! This is the first real modeling I've done and the first time using SSA. I'm a little concerned that SSA is doing things behind the scenes that may invalidate the numbers I'm getting. Any chance that I could send you my file for a cursory review?