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Runoff science is an important part of managing stormwater in the modern age. We not only need to consider where stormwater runoff goes, but we also need to understand its effects and predict future environmental changes. This is where the physics and chemistry of stormwater runoff are critical.

Stormwater Runoff Science: Physics

With such huge volumes of stormwater runoff produced by urban centers and industrial facilities, physics is an important consideration. The movement of this water, the forces at play, and the physical effects of runoff all need to be taken into account.

Fluid Dynamics

In order to understand where runoff will occur and predict where stormwater will go, we first need to understand fluid dynamics. Fluid dynamics is the study of how liquids and gases move through a system. It can also be applied, in a more limited capacity, to the movement of solid particles that behave like fluids.

Many different factors affect the movement of stormwater through a system. The volume and concentration of the water, its particulate matter (PM) load, its chemical composition, the age and condition of the conduit, and the outflow potential, as well as other factors, all dictate how and where water will flow.

As facility managers and urban planners seek to mitigate the effects of excess stormwater, they need to remove uncertainty wherever possible. This means coming up with reliable predictive models that effectively forecast stormwater patterns so that systems can be optimized. Computational fluid dynamics (CFD) solutions are now on the frontline of stormwater mitigation, providing formidable insight that can be deployed in the optimization of storm drain systems so that these systems do not become overwhelmed.

Mechanical Erosion

Stormwater may contain chemicals that can directly erode or corrode materials, causing damage to stormwater runoff conduits and other drainage solutions. However, from a physics perspective, there are other issues at work. Mechanical erosion, for example, refers to the physical breaking down of materials without altering their chemical properties.

CFD solutions also have a big role to play here. By analyzing the movement of stormwater through the system, engineers will be able to recognize where mechanical erosion is likely to take place first. This insight enables pre-emptive reinforcement and stabilization, increasing both the efficacy and the lifespan of stormwater runoff infrastructure.

Stormwater Runoff Science: Chemistry

Beyond the physical properties of the stormwater runoff, analysts, engineers, managers and other professionals need to be aware of the chemistry of the water itself. As water travels over impervious materials and surfaces such as concrete, it picks up a wide variety of different chemical compounds. Many of these compounds can cause severe damage to local environments, as well as human health. This branch of runoff science is important as engineers and site managers seek to mitigate the harmful effects.

Chemical Composition

The actual chemical composition of stormwater runoff is incredibly complex. In fact, it is this complexity that makes the question of where stormwater goes such a critical one from an ecological perspective. Quite simply, we need to know where runoff ends up because we need to be able to mitigate the chemical contamination it causes.

Stormwater contains a number of different contaminants, which may be dissolved into the water itself, or simply suspended in a mixture. These can be divided into seven main categories.

  • The first category is prescription pharmaceuticalsMetformin and methenamine are found in this category.
  • The second category is plant/animal sterols. Cholesterol and beta-sitosterol are found in 100% and 98% of samples respectively.
  • The pesticide chemicals category is one of the most diverse, with around 20 contaminants that display a detection frequency of 52% or higher. Carbendazim is the most frequently detected of this category and occurs at the highest median concentration level of just under 1,000 nanograms per liter.
  • Fossil fuel and combustion products contribute a number of different contaminants. These contaminants – such as benzo(b)fluoranthene, fluoranthene, pyrene, chrysene, and phenanthrene – tend to have a high median concentration (1,000 nanograms per liter, or above) in stormwater and are detected in more than 82% of samples.
  • Nonprescription pharmaceuticals such as caffeine and nicotine are detected in the majority of samples, demonstrating how human activity influences the content of stormwater. Lidocaine is also present in around two-thirds of samples, albeit at a lower median concentration.
  • The penultimate category is industrial chemicals. While these contaminations occur with less frequency on average than fossil fuel and pesticide contamination events, they still contribute a significant level of pollution to stormwater runoff. Methyl-1H-benzotriazole and p-cresol are the most frequently detected at 92% each, while bis(2-ethylhexyl) phthalate has the highest median concentration at more than 1,000 nanograms per liter.
  • The final category is household chemicals. N-N-diethyl-meta-toluamide, also known as DEET, is a major contaminator in this category, present in 98% of samples. Acetophenone, meanwhile, is present in 76% of samples, but at generally great concentration levels – often around 500 nanograms per liter.

Frequency of Detection vs Median Concentration

As we analyze the chemical composition of stormwater runoff, there are a number of key metrics we need to be aware of. Two of the most important are frequency of detection and median concentration.

For example, we may identify a chemical in more than 90% of samples, but only at a concentration of less than 100 nanograms per liter. On the other hand, we may identify a different chemical in less than 40% of samples, but discover that its concentration is far higher – perhaps 5,000+ nanograms per liter.

This data does not immediately tell us which of these contamination instances is worse. For this, additional study is required to understand the effects of specific toxins and chemical contaminants on local ecosystems and human and animal health. However, bringing frequency and concentration data helps to achieve a more complete picture of what we are dealing with.

An Evolving Understanding of Stormwater Runoff Science

Our understanding of the physical and chemical aspects of stormwater runoff science is developing all the time. While technologies like computational fluid dynamics modeling solutions have been around for many years, they are becoming increasingly refined as data sources and processing methods improve. We still have a long way to go before we can win the battle against stormwater damage and pollution, but an increasingly scientific approach is helping us make significant moves in the right direction.