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Stormwater Sump Pumps

Stormwater Sump Pumps

Stormwater Sump Pumps

I was asked to give some feedback for a single family residence stormwater sump pump located in Los Angeles County. This pump is used to pump the stormwater up to the street where the drainage structures are located.

Area - 0.5 acres
Isohyet - 7.2 inches of rainfall (50-year)
50-year flow (Using LA County ModRat Method) ~ 900gpm

If I size the pump using the 50 year storm, this makes a HUGE pump for a residential applications, which is larger than any of the off the shelf pumps I have found (seems wrong). The pump manufacturer said to use 2" over the entire area to be serviced, but he didn't know why 2".

1. What parameters have you guys used to size sump pumps for small residential applications? (2-year storm event? 2" over the entire area, etc?)
2. Does anyone know of any package stormwater pump system that they like using for small stormwater applications?
3. HOw is drainage in light wells accounted for? Do people typically install sump pumps for lightwells if no gravity outlet on a property can be obtained or maybe fill it with gravel to have a drywell beneath it?

RE: Stormwater Sump Pumps

I Have a project, less than 1/2 acre in the City of Seattle where they require only the flow from a 2 year event, no storage.

I am thinking of using a PACO QDSC, submersible non-clog, dual system.

Regarding light wells for the basement, if the ground does not perc well, put a slab in the bottom and tightline it to the pump sump manhole.

Mike McCann
McCann Engineering

RE: Stormwater Sump Pumps


First find out if the whole half acre is covered or paved with impervious surface such as asphalt or concrete. If not determine the areas of concrete, asphalt, lawn, etc. Use runoff coefficients of 0.95 for the conc/asphalt and 0.15 for lawn. In the eastern part we a 2” rain represents a 5 year storm and that’s what the building dept’s require.

Calculate the volume of the flow by multiplying individual areas by their corresponding runoff coef. And the 2” and then add them all up. You get a total volume in cubic feet. An 8’ leaching ring provides 42.24 ft3 of storage for every one foot of depth. Calculate the required storage depth.

If you wish to use a sump pump, find out the discharge volume per hour and reduce the storage volume by that amount. Don’t forget to check with the Town for their specific requirements, if any. Hope this helps.

RE: Stormwater Sump Pumps

you may want to factor in some storage in the lawn or elsewhere, plus the sump pit storage.   Your pump discharge then does not have to match the peak rainfall discharge and could be considerably less.  50-year storm does seem like a bit much.  5- or 10- might be more reasonable

RE: Stormwater Sump Pumps

Acknowledge that nature can deliver water faster than you can remove it.  Size pump for 2" of rain/hr. When greater rain falls occurs, a minor flood will occur.  Check to see what fractions of the area will be submerged for various storms.  Keep buildings one foot above the hundred year return interval storm water elevation.

RE: Stormwater Sump Pumps

Is this sump pump going to be in the home basement or out in the yard somewhere? It seems crazy to have half an acre drain into the home - there should be diversions and the basement should be constructed to keep water from flowing directly into it. If it's out in the yard, then there should be plenty of storage keeping the surface water out of the building. It is okay if it takes several hours to drain as long as the flood water doesn't get into a building (owners or neighbor's).

You can calculate an outflow hydrograph (straight line at the pump rate) and perform a storage volume calculation.  The 100-year flood elevation would probably have to be recorded on a plat (though in this case, zero outflow would probably have to be assumed) and the home should have the finished floor elevation at least 1' above the 100-year flood elevation.

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