Surge tanks vs. rain barrels IV

I'm sure I left you dangling in a previous installment with the question of how rainfall intensity affects the usability of surge tanks.  I talked about rainfall intensity without actually answering the question, but we can now get to the answer that I am sure you've been waiting for.

A recent discussion on radio station KPCC asked rain barrel owners what they would do with ALL THAT WATER.  The answer was that their tanks were overflowing, they were too heavy to move, and no one knew what to do with the water.  They were talking as if they were going to hoard it for a hot summer day!  It was a gigantic Duh moment, but they were too blinded by ALL THAT WATER  - all 55 gallons of it - to take a leap to the most logical place which is to reject rain barrels and embrace surge tanks.

Meanwhile, a friend who is an actual meteorologist stopped an earlier post in this blog by to say that most 30 minute southern California rain bands could be accommodated by a surge tank.  That's really the bottom line, isn't it? If you can't flow rain water directly from your gutters to some place where it will infiltrate, then it might pay to have a surge tank type of set up. Let's see if she's right with a little garden engineering. 

It's also nice to make some estimates, since the possibility is that surge tanks needn't be gigantic 55 gallon drums and therefore might be more seemly in the garden.

This post is again mostly stream-of-consciousness garden engineering, which I am pretty sure has a limited appeal.  It may also be wrong.  Therefore you may find this analysis simple or simple-minded.



Recall that I'm writing about the setup pictured below.  I've drawn it a bit differently this time since in the previous post in this series I concluded that a surge tank didn't need to be a 55 gallon drum.  In fact, I noted that tall and skinny has a flow advantage given that the head pressure at the hose spigot will be greater than that of a squat storage vessel.  Higher pressure means higher flow rate, though for this analysis we'll continue to use 2 gal / min which is how fast a column of water 2.5' high will flow through 100' of 5/8" garden hose.  I'll also stick with 600 sq ft. of roof area as a fixed analysis parameter. In the figure below, I've indicated one way to hide a surge tank - as a post in a pergola.

The pergola floor is impermeable, so a surge tank / hose setup is employed to flow rainwater to a remote location where it can infiltrate.

So now the question is how many storms in our neck of the woods could be captured by a surge tank of volume X?  X might be rather small.  For instance, a 4" diameter, 8' pipe re-purposed as a surge tank has a 5 gallon capacity.  To answer the question we need to know about rainfall intensity, which I wrote about in a prior post.  Using the results from that post, we can put some bounds on our expectations.

Using the unit hyetograph, discussed in a the previous post in this series, I concluded that, "Elapsed time for the heaviest period of rainfall ... is about 15 minutes....  Using some estimation, I determined that within this time period about 11% of the total rainfall in the storm occurs. "

Since this is a simple figure of merit (the maximum rate of rainfall for any given storm is 11% of the storm's total and this maximum occurs in 15 minutes) it's suitable for simple garden engineering.

In a stroke of prescient genius or synchronicity, I already figured most of the rest of the puzzle out back in part II when I wrote, "a rainfall rate of only 0.37" / hour for 15 minutes would fill [a 5 gallon surge tank]" and noted that this was a conservative bounding number: "Since it's taller than the typical 55 gallon rain barrel, the pressure head would be greater when it's full, which would drive a greater flow out of the tank (the assumption of 2 gal/min flow rate through the garden hose could be increased) so you'd get a little relief from over-filling that's not counted in this simple analysis."

So if 0.37 " per hour for 0.25 hour = 11% of storm maximum, then a little algebra says that storms of up to 0.84" total would be completely handled by a 5 gallon surge tank with a single garden hose flowing 2 gal/minute.  It's nice to see that my meteorologist friend has accurate intuition.

Folks, that's the majority of our storms!!

Wait, let me write that again.

Folks, that's the majority of our storms!!


In fact, so far this year we've had one day where we accumulated 0.89" (15 Sep 2015) and one day when we accumulated 1.0" (7 Jan 2016).  All the other days where I recorded rain (11 of them) were below 0.84".


Eat Drink that, rain barrel lovers.

FIN