The EPA reported the following statement on August 1 of this year: “In recent years, a larger percentage of precipitation has come in the form of intense single-day events,” which has occurred since 1996 (EPA “Climate Change Indicators: Heavy Precipitation, August 1, 2022.) This is crucial in civil engineering design since drainage design is based on a pre-determined event, such as the 5-, 10-, 25- or 100-year event. Therefore, “heavy precipitation,” which equates to higher than normal rainfall, will adversely impact the drainage system since it was not designed for heavier precipitation, leading to flooding and excessive loads on a combined sewer system treatment. Furthermore, as we have all witnessed over the years, the overall rainfall in our state (California) and others in the southwest keeps experiencing long-term and persistent drought conditions.

Let’s take a step back and revisit how a drainage system is designed. In simple terms, three factors determine the amount of drainage that a system must accommodate: rainfall intensity (“I”); the size of the drainage basin (“A”); and the coefficient of runoff (“C”), which is related to the area’s surface and how much water it will absorb. For example, concrete will not absorb hardly any water, while sand will. Therefore, the amount of runoff a system has to capture is a product of all three factors: CxIxA. All three factors are important and will provide a result that is used to locate drainage inlets, determine their quantity, and size the pipes. And if any of these three factors change, the amount of runoff will change. Therefore, if the rainfall intensity changes, as reported in the EPA above report, the amount of runoff will increase, and the system will be overburdened, which is why climate change is so important to address.

Recently I wrote an article about porous pavement, roof gardens, and rain gardens, which will decrease the value of “C” and, therefore, counteract the effects of a higher “I” value. Still, ideally, we would like to see rain intensities remain as consistent as possible, which is where environmental protection comes in. Still, decreasing the value of “C” is a great way to address this issue in the short term, but in the long run, we need to impact our global ecosystem as minimally as possible negatively. Otherwise, changing rainfall intensities will continue to cause flooding and overwhelm combined sewer systems, such as the one we have in San Francisco. Additionally, flooding causes property damage, health concerns from stagnant water, and injuries and loss of life. Also, flooding often occurs in lower-income zones, which only worsens the economic and health situation of the residents.

At Sustainable Civil Engineering (SCE), we strive from the start to impact our natural environment as minimally as possible when we design the built environment because we understand that whatever we design and build today will have a long-lasting effect on the future. Therefore, it is up to us to ensure that said “effect: will be positive rather than negative.

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