Last summer I was sitting on the porch with a gentleman I had met while taking a stand-up paddle boarding break. He had a big, beautiful home on the beach of Balboa Island in Newport Beach, California. My friend and I had paddled up onto the sand and joined him on his patio for coffee. As I admired his home, he explained how he had it built and that the details were the greatest challenge. I replied with, “The devil is in the details,” to which he responded, “No, the money is in the details.”
Ironically, the guy turned out to be a mega-million-dollar slumlord, and his coffee was awful. Turns out it was instant coffee, poorly made. You can’t buy taste.
Anyway, onto plumbing. Here in SoCal and elsewhere, rainwater harvesting is growing in popularity and becoming mandatory in some jurisdictions. The basic system is quite simple: the rainwater goes into a tank from which it is pumped through a filter system and to its points of use, such as irrigation and/or cooling tower make-up.
One of the trickier aspects of the design is sizing the stormwater tank. Often the civil engineer and/or landscape architect will get involved in this effort. Since these tanks can be quite large, you want it to be large enough to suit its purpose, but no larger. The tanks can take up a lot of real estate and often have significant structural requirements, and both of these consume big dollars.
The tank volume is driven by its purpose. If the system is required by Low Impact Development (LID) or Standard Urban Stormwater Mitigation Plan (SUSMP), the usual requirement is to treat the first ¾ inches of rainwater, above which the unit can overflow or be diverted, since the rainwater is considered clean at that point. The ¾-inch volume is a product of the site area and rainfall rate.
Here in SoCal, if the calculated seven-month irrigation volume is greater than the ¾-inch storm water volume, then the storage capacity must be increased to meet the irrigation volume requirement. If there is a cooling tower also supplied by the storm cistern, then a calculation is required to determine how many times a year the cistern would “turn over.” In some jurisdictions, the cistern is required to circulate such that it filters its entire volume every 48 hours. This is a good practice whether it is a local requirement or not.
On one of our projects, there was a relatively small rainwater system required by SUSMP to serve landscape drip irrigation, and those devilish details threw us a bit of a curve ball.
The system was shoehorned into a parking structure, so both real estate and headroom were limited. As the system was coordinated with our detailed pipe routing, the elevation of the inlet pipe to the tanks was forced down, lower than originally intended. When it was realized that this in turn reduced the capacity of the tank, the manufacturer added upturned elbows inside the tank to bring the volume back up to the required amount.
The system was installed and put into operation. All that was required at this point was a rainstorm, and those can take a while here. But you probably know that this past winter provided our first heavy rains in many years, ending our drought. When the first rains came, the project team eagerly went to see the rainwater system in operation, but something was wrong. It wasn’t doing anything.
We opened the tank and looked inside expecting it to be full, but it was dry as a bone. How could that possibly be? It was a very heavy storm!
We scratched our heads and studied the drawings. An elevation of the system is shown in the drawing below. Prior to entering the tank, the stormwater goes through a vortex filter to intercept any large debris. From there it goes to the tanks (noted SSDT). The tanks have emergency overflow outlets above the inlets, but there is an internal overflow in the vortex filter such that the filter should overflow prior to the stormwater reaching the tank overflow. Everything looked just fine.
After studying the problem further, the source of the issue was realized. The top of the upturned elbows inside the tanks that were added as a Band-Aid were higher than the overflow inside the vortex filter such that the stormwater went to overflow before it entered the tanks. Removal of the elbows inside the tank solved the problem, but that raised concern about the tank volume.
Tank volume turned out to be a non-issue. The tank level is determined by the height of the overflow weir in the filter, so the inlet height doesn’t matter — it just means the inlet will be flooded if the tank reaches full volume, but that is of no concern.
The other devilish detail on rainwater systems is the approvals that are required. First, you need your LID or SUSMP approval. Then you will probably need approval from the cross-connection control unit. Then you might need health department approval. All of these approvals can be time-consuming, so don’t wait until the last minute or you might delay the project.