Runoff question 1

[ttuterry]

I am a new engineer working on a runoff calculation. I am to use the 25 year, 24 hour rainfall event and calculate if a retention pond can hold the water. I have calculated the volume of the pond. I think one of the assumptions is that there is 100% runoff, because the water is running off of a salt pile. Hydrology is definitely not my strong point so if someone can give me some ideas i would appreciate it!

 

[cvg]

Not likely that you will have 100%. For 25-year storm, we use a value between .83 and .94 for the C coefficient for pavement and rooftops. Pick a reasonable value for C and then see what your safety factor is in the pond volume. Make sure you have some freeboard to account for uncertainty in amount of precipitation or to account for sediment accumulation in the pond.

 

[francesca]

Well... he shouldn't be using a C (Rational Method) for detention basin design, but rather the TR-55/SCS Curve Number method.

http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models-tr55.html

 

[ttuterry]

I have some calculations that a previous engineer had done for this same project about 6 months ago, and for some reason he assumed 100% runoff. This baffles me because there is a pipe underneath the salt pile which is used for the water that is infiltrating into the salt pile. He did account for freeboard like you mentioned CVG. I guess one of the things I am having trouble with is to calculate the volume of runoff, he multiplied his area times his rainfall event (which is 3.9 inches). To me this seems likes the whole area would have 3.9 inches of water covering it...Does this seem correct?

 

[cvg]

you are correct and that is a conservative assumption. Note that Francesca is correct that if you are not "retaining" but instead "detaining" the storm runoff then rational method can't be used.

 

[ttuterry]

Ok so all of this water is going into an evaporation pond, where it is being held until it all evaporates. So this would be a retaining pond right? and then I can go ahead and use the rational method...

 

[cvg]

sounds like a retention pond to me

 

[ttuterry]

thanks for the help cvg

 

[AegisFPE]

Pond designs I have been involved with included 12" freeboard with a 6-foot wide 6" deep overflow spillway. In the event your pond receives more flow than you anticipated, it is appropriate to plan where it will go, and how it will get there!