Now sump pump rain gardens are an interesting animal.
Sump pumps offer measured water at a consistent velocity, but are not as straight forward to quantify as your other water sources.
They can contribute water long after a rain storm is done, creating a wetter rain garden for a longer time.
Or a sump pump might not put anything out at all in the middle of summer after a 2″ rain.
Despite these challenges, with more and more experimentation, I believe sump pumps offer a great opportunity for rain garden applications.
But before you even begin estimating your sump pump output you will have to verify that there are no chemicals polluting your potential water source.
Check to make sure that your water softener or laundry machine is not releasing into the sump pump pit.
If either one is, try to reroute the drain lines into a wash basin or floor drain that runs to the sanitary or septic line.
If you can not do this for your water softener, you probably don’t want to use this source as I don’t know of any practical way yet to remove the salt from the water.
The salt destroys the plants and the soil.
If you have a laundry machine that you can not reroute, it may be possible to use the water.
These are called “greywater” gardens.
I have not put a greywater garden in yet, but it is something that is very interesting and I would like to try working on one in the future.
So after verifying your sump pump water is chemical free, you’ll want to look at the conditions at the current sump pump outlet.
First you will want to check if the outlet pushes water 10’ away currently or at least it is flowing away from the house before pooling at least 10’ away.
If the water is pooling close to the house it may be recirculating the same water over and over again.
Meaning the rain is traveling through the soil into the drain tile around the basement foundation and into the sump pump pit where it is being pumped up and onto the soil that is still within the reach of the drain tile around the basement foundation.
Another source of recirculating water are from the downspouts.
Check to see if all the downspouts are connected and pushing water away from the house properly.
If they are not, fix them.
If there is little to no water recirculating, then I move on to calculating the sump pump’s output.
The calculations for my first few rain gardens for a sump pump consisted only of observations of the water conditions around the sump pump outlet contrasted with the recent weather and season.
I believe that this is still a very reliable and important step in calculating your sump pump rain garden design volume and with some practice may be all that is needed for your guesstimation.
I have since then created a little scoring system to help me with my sump pump estimation.
Caution: Below is a somewhat arbitrary scoring system I made up to help with sump pump estimating and it should be used with discretion.
How soaked is the area at the output compared to how much rain we have gotten recently and the current season?
– Very Wet = +3
– Moderate = +2
– Dry = +1
Are there standing water indicators?
– A lot = +3
– Some = +2
– None = +1
Does the sump pump run even in the really dry times of summer?
– *Yes = +3
– Sometimes = +2
– No = +1
*Yes = May potentially have a High Permanent Water Table
Where is the High Seasonal Water Table?
– Within 3′ for over 4 months = +3
– Within 3′ for 3 months = +2
– Within 3′ for 2 months or less = +1
How large is the footprint of the house?
– Large = +3
– Medium = +2
– Small = +1
Does the terrain slope into the house on any sides?
– More than one side and a big hillside = +3
– One side and Medium hillside = +2
– No sides = +1
Do you have any photos of the water conditions after a big rain?
– A lot of water = +3
– Some water = +2
– Little water = +1
Total Points – Sump Pump Design Volume
7-11 Points= 25 Cubic Feet
12-16 Points = 50 Cubic Feet
17-21 Points = 75 Cubic Feet
I am still working on different ways to estimate a sump pump outlet, but this is my working model for now.
If you try it, let me know how it worked for you and if you have any additional tips!