modified rational bypass runoff

[kingpawn]

For example.....I have a 20 acre site and 10 acres flow to the detention pond. I have determined a pre-developed allowable runoff rate of 50 cfs. The detention pond is designed to release 30 cfs. How would you determine the runoff rate from the 10 acres that bypasses the detention pond? I used the modified rational method to model the detention pond.

Thanks

 

[civilperson]

Q=CIA

 

[kingpawn]

Can you add the bypass runoff rate together with the detention pond release rate when they have different times of concentration? What if the time of concentration for the bypass is 10 minutes and the detention pond time of concentration is 5 minutes? I didn't think you could combine two rates with different times of concentration because the hydrographs peak at different times?????

Thanks

 

[cvg]

rational and modified rational methods do not generate hydrographs. If you want to route and combine hydrographs, you will need to use another method.

 

[kingpawn]

What does the hastead pondpack program use to determine the storage? Can you not generate a hydrograph from IDF curves?

 

[RWF7437]

"What does the hastead pondpack program use to determine the storage?"

I don't know but it probably uses the NRCS method described in Technical Release 55. This, like any other hydrograph method, must use a real or assumed rainfall pattern. The best thing to use is a calibrated model and a range of real world storms. The worst choice is the Rational method for any drainage area over a few acres.

Can you not generate a hydrograph from IDF curves?

No.


good luck

 

[sammyk]

Kingpawn:
Is there another stormwater management guy in your office?
Please ask him for some instruction. I dont know the project specifics, but I will guarantee your pond, in real life, will not see more than a few centimeters of water in it (ever!!) if you design with a 30cfs discharge.

Unless you are being asked specifically to use the rational method, you should use the TR-55 methodology. If someone in your office has either pondpack, TR-20 please ask them to give you some help.

The rational method will "screw" you when it comes to detention as you cannot model the pond appropriately and you will be attenuating a significantly larger cfs. The above programs allow you to properly model a dynamic pond.

 

[kingpawn]

sammyk,

the numbers are just for example the're not real. What I'm looking for is how to determine the bypass runoff rate by using the modified rational method. If the detention pond is in the same watershed as the bypass area how do I determine an overall post-developed runoff rate.

Thanks

 

[sammyk]

Kingpawn,
Using your example, as long as you arent devleoping the bypass area:
Bypass flow = CI*[10 ac. (not trib.)]
Detainable flow = CI*[10 ac. (Developable area)]
Storage = (Detainable flow - Release Rate)*t/12

Post Develop Rate = Bypass flow + Release Rate

 

[kingpawn]

Sammyk,

Thanks again. Can I add the Bypass Flow and the Release Rate together eventhough they might peak at different times?

Again thanks for the help.

 

[sammyk]

Short answer is "Yes"
The rational method is a "peak" method since it doesnt use a distribution.

Just make sure that you add the bypass and release rates for each individual storm duration. Do not mix durations.

ie.
2-hr storm duration Bypass + 2hr storm release rate = Post develop rate for 2-hr storm duration

You should have a table for the 1-hr thru the 24-hr (or more) with different intensities for each duration.

 

[cvg]

you can add them together, but may or may not give you the correct answer - because as you said previously, the peak from the bypass area may not arrive at the same time as the peak from the pond spillway. In other words, you are mixing apples and oranges