Storm pond flow 1

[Rob100]

I have been asked to size a pump pit & pumps for a storm water retention pond. I have been dealing in pressure systems for so long that I've forgotten how to calculate gravity flow. How do I determine of the pumps be able to get inlet flow. The retention pond holds approximately 1.25 million gallons. They want to empty this is in a 24 hour period. The pond is approximately 10 feed deep at the the pump station. I will of coarse install a pump station deeper but I will have an opening in the station to get in. The pond is irregular shaped. I'm looking for a quick and dirty estimate to insure the water will get to my pumps. Are there any suggestions?

 

[DMcGrath]

I'll presume you have a concrete pump station and the water enters through an inlet of some sort. You need to compute the capacity of that inlet, be it a pipe inlet or an opening (usually square or round) that lets the water in.

If a square vertical inlet, you can use the weir equation to calculate the depth of water needed to match the intake requirement of your pumps (Q = Cw*L*H^1.5). For US units, Q=ft^3/s, L=ft, H=ft. Assuming a rectangular shaped weir, use Cw=3.3 and reduce your actual L by 0.2*H.
If the depth is such that the entire inlet is submerged, you will need to consider orifice flow also.

Based on a design we did a while ago (round horizontal inlet), a 48-inch diameter intake riser was able to supply over 5,500 GPM with less than six inches of water depth above the rim of the intake.

 

[queque]

DMcGrath
What was the inflow rate and height/volume capacity of the intake?
What was the pumping rate selected and what methodology was used to determine those parameters?

 

[DMcGrath]

The intake was a 48-inch concrete pipe set upright to act as the intake structure. As the water level gets deeper at the rim, the H increases, and the flow increases accordingly. My concern, like yours, was supplying sufficient water to the pumps. Once I verified I could do this with 6" of water, I really wasn't concerned past that point. This intake was part of a pump station in a stormwater pond that was designed to store the bulk of the 25-year storm while pumping it out to another location. The intake is set at a level to maintain a permanent pool of water in the pond for sediment settling (had to meet E&S requirements).

The criteria was the pond could not discharge out the emergency spillway during the 25-year storm and the only other way out was to pump it. I initially decided on a pair of pumps on a initial analysis given head and a reasonable flow (based on a catalog pump curve), then I designed the pond around them. The pumping rate was chosen based on reasonable equipment size, power availability, downstream effects, lots of reasons. Several rounds of hydraulic modeling were required to tune the required pumping rate, storage, "pump on" points, downstream piping and all that.