Sizing the right pump

[DDBR]

I am currently working with a property owner who owns a building in an industrial park. When he bought the building he did not realize that during heavy rains his building will flood. This is because his neighbors filled in the drainage ditches to expand their parking lots, not knowing they were causing his future building to flood. The neighbors are not excepting responsibility and refuse to re establish the ditches.

Now he owns this said building and would like to keep the water from flooding him out. To help come up with a solution I have created a drainage model of the contributing area. The area consists of 5.60 acres of paved surfaces. The calculated runoff to his building (low point) is approximately 20.9 cfs with a rain intensity of 3.89 in/hr and a time of concentration at 20 min.

I would like to install a pump that will collect the water and pump the water to a ditch line that is up hill of his property. The distance to the ditch from the pump is approximately 350 feet with an elevation difference of approximately 5 feet. I would like some assistance in sizing the right pump discharge that will keep up with a storm event and collect the water before it makes it to the building. Can someone give me some advice on the best way engineer this?

Thank you!

 

[DDBR]

I have a storm event that will produce:

Runoff volume = 104,999 Gallons
Runoff Rate = 7,000 gpm

My detention Pond can hold 94,097 gallons
Pond will fill = 13.44 minutes

I believe that all I need to to is pump the difference between the runoff volume and pond volume in 13.44 minutes or less. Right?

If this is true Then all I need to do is Pump 10,902 Gal. in 13.44 minutes or less. This calcs out to 811.16 GPM. Right?

 

[EdStainless]

What part of the world is this? If it is in the US then they have to restore the ditches, screwing with drainage is usually a violation of the terms of the deed.

If the rate of rain is 7,000gpm and you are pumping 800gpm then the pond will fill in 13 min.
I would want to be able to pump more than that.
If you pump 1200gpm then you would be able to last 16 min, I am not sure that is long enough really. But in order to get 30 min you would have to pump 3800gpm.
Perhaps two pumps in the 1,000-1,200gpm range would be a good solution.
4"/hr is one hell of a rain, I hope that it doesn't last very long.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

Your figures are not well explained. I appreciate rainfall is not an exact flow, but you need to say where this data is coming from and your assumptions. Don't mistake number of figures with accuracy.

E.g. it seems the volume above (105000), is a total and the 7000 a peak figure?. 7000 x 20 mins is more than 105000.
Is the pond always empty just before it rains??

Then it depends on how you plan to operate. Do you start pumping as soon as the pond gets say 10% full or do you leave it until it's nearly flooding the building?

If the latter then you will need to match rainfall volume which is a bigger figure.

All depends on how much you really want to save the building. Sounds like you're somewhere where it's a monsoon type rain. Question is what happens when it rains a bit harder or for longer than you've assumed. How long between heavy rains. Do your pumps keep running to empty the pond?

High volume low head pumps are not too expensive compared to a building. Look at 2 x 100% or 3 x50% and them if you need to run all pumps at the same time, preferably with their own discharge pipe or oversize the discharge pipe based on max possible flow.

You're best bet it to try and stem the flow from the 5.6acres into multiple small areas or let the paved areas sit under an inch or two of water for a few minutes and even the flow out a bit.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Can you set up a detour ditch route that will get back to the original discharge point without having to pump ?

 

[jgailla]

Where is groundwater? Can you do underground detention? A pump is usually the most expensive and high maintenance choice and is only used as a last resort.

 

[DDBR]

The top of my pond is at the same elevation of the flow line of the ditch I am discharging to. This site really is a mess and the only way to remove the water from the site is via a pump.

 

[lilliput1]

You should refer the problem to the industrial park owner. He is obligated to provide retention pond for run offs.

 

[Artisi]

DDBR: I would suggest some input on LittleInch's comments might go a long way, we're pretty smart but not mind readers.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[DDBR]

There is no park owner. Each building / lot is privately owned. I have discussed this issue with the county engineer and the owner. The county engineer claims it is not their responsibility and the owner has decided that he would much rather pay the money to fix the issue than pay money for lawyers that MIGHT correct the problem.

 

[LittleInch]

So how big are you planning on making the pumps?

What mode of operation?

How confident are you that you're looking at maximum rain fall loads?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DDBR]

I'm pretty confident with my maximum rainfall loads. I have purchased a HydroCAD Licence and have been making great progress with my design. I have determined two adequate pump sizes (40 hp and 50 hp) for my design based on pond volume, total head and flows from my drainage basin. My design is calculate around a 10 year storm event.

 

[LittleInch]

Good. I would certainly start pumping at 10 -15 % of the pond level / volume until you get to <10% and if you've got a spare pump allowing it to start if you hit 90%.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

DDBR: Quote "I'm pretty confident with my maximum rainfall loads. I have purchased a HydroCAD Licence and have been making great progress with my design. I have determined two adequate pump sizes (40 hp and 50 hp) for my design based on pond volume, total head and flows from my drainage basin. My design is calculate around a 10 year storm event."Unquote.

So, what are you asking us / expecting from the members here if you have it all in hand?





It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[DDBR]

I started this thread before having access to HydroCAD to do the model. Now that I have the model, I have answered my own question.

 

[Artisi]

I would back years of experience on like applications compared to any autocad programme.
Has it thought about the quick rain shower you are likely to experience that doesn't need a 40 or 50 HP pump kicking in every few minutes. No didn't think so.


It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[lilliput1]

You should get a copy of the site drainage drawings submitted to the county for approval to see the road and site drainage system and natural drain routes. It should not be possible for a builder to be allowed to build a building without means to drain rain water run-off. You should locate drains around the building and direct it by underground concrete pipes/manholes to either a retention pond or existing watershed. Having water above the site can lead to problem like water in basement or ground settlement.

 

[DDBR]

Thank you to everyone who responded to my thread. Like most projects it comes down to money and how much risk the owner is willing to take on.

 

[chicopee]

I would particularly heed the first sentence of EdStainless.