“Good to the last drop,” for those who do not know, is a slogan used by a certain brand of coffee for marketing purposes. For those readers who may be enjoying a nice healthy glass of water or other non-caffeinated beverage right now as you read this, here’s to your continued good health! For the other 97% of us that are probably having a cup of coffee in order to wake up, I hope it’s “good to the last drop.” 

Recently, our company had a virtual meeting with more than 1,000 people to discuss reintegration back into the office. During the Q&A, one person piped up and asked, “With the new restrictions in the kitchen, will I still have access to coffee throughout the day?” I just want to say to whoever asked that question, “Thank you!” I like my coffee percolated.

Percolation is defined as “the process of a liquid slowly passing through a filter.” In nature, this action occurs as water percolates down through the ground we walk on. Engineers design systems that will recharge the water at a controlled rate. One of the main reasons to design recharge systems is to control runoff, which is water that can carry impurities. 

Water can also percolate up through the ground as gradients in groundwater pressure fluctuate. This can become a factor when working on projects that have a high water table. This is something to look out for when there is an underground parking garage.

Plumbing engineers typically deal with flows from systems that are defined by fixture units, in the case of sanitary flow, or area and hourly rainfall in the case of storm. Almost always, our charge is to remove drainage out of the building. I think it’s inherent in the definition of the word “drainage,” the process of draining something. 

If you want to witness a plumbing engineer exhibit nervous muscle spasms or facial twitches, just tell them you want to bring groundwater or some other sort of drainage into the building. This has happened to me on more than one occasion. It’s as if the project team thinks, “Let’s ask the plumbing engineer to deal with this extra water, they will know what to do with it.” Sometimes my engineering mind will get excited for the opportunity to pump a bunch of water that nobody else wanted to deal with. 

Eventually, reality sets in as I try to translate groundwater infiltration into a flow rate and present a reasonably sized pump system to the project team so that the electrical engineer can upsize the emergency generator, which in turn, may increase the natural gas load. I cannot think of a single time where everyone looked at each other and said, “Wow, this was a great idea.” 

As irrational as it sounds to bring water into a building, whether through the foundation perimeter drain or some other method, there is a formula that can help us. It just happens to be called the “rational method.” The rational method is outlined in the ASPE Plumbing Engineer Design Handbook, Chapter 4. It acknowledges our involvement with stormwater management has expanded, and we should be familiar with the science of hydrology. Equation 4-1 in the design handbook gives us a runoff value in cubic feet per second. The equation is:

equation 1Instead of sizing storm drainage piping using tables and rainfall rates outlined in the plumbing codes, the rational method gives us a tool to obtain a flow rate in gallons per minute. The runoff coefficients range from 0.1 for a forest floor to 0.9 for asphalt pavement. The rainfall intensity can be evaluated using localized “intensity-duration-frequency” curves for the area you are analyzing. Area is a fixed value and will require some conversion to get from acreage to square footage. The power of the rational method is it allows you to analyze a scenario with different runoff coefficients and rainfall intensities. While some of the design parameters are out of our control since we, as plumbing engineers, do not specify grading and back fill, we can offer a different and valuable perspective regarding how the building will operate.

Some of you may have noticed the increased presence of “tree wells” along city sidewalks or been asked to divert stormwater to “bioswales” in some of your projects. The purpose of these features is to help filter out impurities by allowing the runoff to percolate through natural features. Imagine a building on a site with one acre of land surrounding it. The amount of flow being processed can vary from 100 gpm up to 1,000 gpm, depending upon the type of surface surrounding the building. 

Site with forest type surface:

equation 2

Site with paved surface:

equation 3

When you look at Chapter 4 of The Plumbing Engineer Design Handbook, you will notice that there is a wealth of resources referenced. This includes a reference to the ASPE Research Foundations paper on sizing stormwater systems. The paper is titled the “Storm Drainage System Research Project” and claims to offer an alternative way to size storm drain systems. There is also a technical software resource reference that is available from the Department of Agriculture. It is known as “TR-55: Urban Hydrology for Small Watersheds.”

With these tools at hand and a curiosity that percolates within us, we can continue to become more educated plumbing engineers and make sure our designs can handle whatever water flows people send our way, down to the last drop. Just be sure to add a little room for a top off.