Rainfall can impact our environment in multiple ways. Stormwater doesn’t just end up back in our waterways only to continually contribute to the water cycle. It can have far-ranging impacts on the health of our water and water ecosystems, and it can cause major headaches here at home in the form of flooding. Let’s take a more in-depth look at what stormwater is, how it can impact us, and how we should manage it.

Watersheds

A watershed is an area of land that drains water into streams, rivers, lakes, and other water bodies. Watersheds drain rainfall and snowmelt into smaller streams and rivers, which eventually end up in larger bodies of water like the Tennessee River or even our oceans. The Mississippi River watershed is the biggest in the United States, and the Tennessee River watershed is a part of it.

Here in Knox County our stormwater flows from smaller creeks and streams into the Tennessee River which then flows to the Mississippi and out into the Gulf of Mexico. The smaller creeks and streams that we have here in Knoxville actually make up their own watersheds. Watersheds can vary in size like the giant Mississippi River all the way down to smaller watersheds like the Beaver Creek watershed here in Knox County. Because all of these waterways eventually connect to larger bodies of water, what ends up in our streams and rivers here can impact the Mississippi River and even our oceans.

You can find out what watershed you live in by looking up your address on KGIS - Knoxville and Knox County’s free online mapping resource.

Stormwater

Stormwater is water that originates from rain, including snow and ice melt. In addition to flowing into our waterways through watersheds, stormwater can seep into the ground and fill up underground reservoirs, or it can be stored on land in ponds and puddles where it will evaporate. Otherwise a lot of the rain we receive, at least in urban areas, lands on hard surfaces like roadways, parking lots, and buildings, and then flows into nearby streams and rivers through storm drains. This flow of stormwater over hard surfaces contributes to surface runoff because it can’t infiltrate into the ground.

Relationship between impervious surfaces and surface runoff.

Whereas in natural landscapes soil and plants absorb much of the surface runoff from stormwater, built landscapes contribute to flooding and water pollution. If you have a heavy rain event and most of the landscape is made of impervious surfaces, then the water is going to continue to build up, potentially outpacing the capacity of the stormwater system, and eventually lead to flooding in certain areas. We have experienced flooding quite often over the last few years here in Knoxville. Additionally, stormwater can pick up pollutants - like fertilizers, disease-causing bacteria, sediments, and litter - from impervious surfaces on the land and convey them to nearby waterways through stormwater infrastructure.

Stormwater Pollution

With less vegetation and more impervious surfaces, developed areas allow less rain to infiltrate into the ground and more runoff to be generated. In a natural environment, if runoff makes its way to a water body it typically travels through a lot more vegetation, infiltrating into the ground along the way, and ends up in our waterways with less pollutants than a built environment because the vegetation has filtered out and prevented these contaminants from reaching the water.

Human activity largely accounts for the pollution that ends up in our rivers and streams. Pollutants can end up on our roads, lawns, roofs, agricultural fields, and other surfaces from human activity. Typical pollutants include pathogens, sediment, litter, nutrients, pesticides, metals, and petroleum byproducts. Here in Knox County pathogens (or disease-causing bacteria) and sediment are the two biggest pollutants that impair our waterways.

The extent of development and the type of stormwater infrastructure in an area contributes to the amount of pollution that ends up in our waterways. If an area is largely made up of impervious surfaces, then stormwater has nowhere to infiltrate and instead will make its way to rivers and streams through storm drains and ditches. If that infrastructure is made up of hard or impervious conveyances or pipes, then surface runoff will enter waterways fully contaminated. These factors contribute to the potential for major flooding events in certain areas, as well.

Flooding

Stormwater is a major cause of urban flooding. Urban flooding happens when stormwater overwhelms the capacity of stormwater infrastructure leading to the build-up of stormwater on a given land surface. Stormwater conveyances such as ditches and storm sewers quickly transport rainwater away from developed areas, which can increase the volume of water in waterways leading to flooding with frequent or heavy rainfall events.

With heavy downpours increasing in frequency and intensity worldwide, the risk of heavy flooding is also likely to increase dramatically. This puts a heavy burden on our aging stormwater infrastructure and will lead to negative economic impacts for individuals, businesses, and governments alike in flood-prone areas of our country. We’ve already experienced massive flooding here in Knoxville, and flooding events are only expected to increase throughout our county.

According to the EPA “the average 100-year floodplain is projected to increase 45 percent by the year 2100, while the annual damages from flooding are predicted to increase by $750 million.” This means that areas that are not currently deemed a flood risk now will be considered flood-prone in the future. While a 100- year flood might not sound scary, the reality is that these types of floods don’t only happen once every 100 years, they actually have a 1% chance of happening every year. With the changing climate, these 1% chance floods are occurring more and more frequently and our definition of the 100-year floodplain changes in a given area continually.

In Knoxville, a new interactive map shows that our flooding chances will increase. Currently about 14% of properties in Knox County are in flood-prone areas, and the future predictions indicate an increase in the number of homes affected by flooding. Luckily, both the city and county are investing money in updating our stormwater infrastructure in flood-prone areas.

We can limit flooding impacts and the impact of pollutants on our bodies of water by incorporating design principles and best management practices that take these negative stormwater effects into consideration. It’s just a matter of implementing these policies and practices in areas that need them most and in time to help mitigate the impacts of climate change on our communities.

Best Management Practices

There are many strategies for managing stormwater, and these days those practices have shifted from traditional stormwater infrastructure, like concrete stormwater conveyances, to practices that allow more stormwater to infiltrate into the ground. This not only benefits our waterways by limiting the amount of pollution, it also limits the likelihood of flooding in our neighborhoods. Some of these stormwater practices include:

Green roofs

Green roofs are roofs covered in vegetation and can include things like rooftop gardens. The rooftop vegetation enables rainwater to infiltrate and evaporate instead of running off buildings and into our streams. Additionally, green roofs can reduce the heat island effect, reduce air pollution, and is a beautiful addition to any building. These benefits have led to their increasing popularity over the years.

Permeable Pavement

Permeable pavement is another stormwater best management practice that helps increase stormwater infiltration and decrease the pollution of our waterways. Permeable pavement works by creating porous surfaces in traditionally impervious areas, such as parking lots and sidewalks. They are built with infiltration in mind, often having multiple layers that offer the most infiltration of stormwater. This infiltration prevents the runoff of pollutant filled stormwater into our streams and also prevents the build up of stormwater thus reducing the likelihood of flooding.

Constructed Wetlands

Constructed wetlands mimic natural wetlands and are a good option for an area that consistently seems wet and retains water after rain events. These can be small, like backyard constructed wetlands, or they can be large like the wetland at Harrell Road Stormwater Park here in Knox County. Constructed wetlands not only capture and filter stormwater, but also create diverse wildlife habitats.

You can find more information about all the different stormwater best management practices, here.

Tune in next time to learn more about Knoxville specific stormwater pollution and how it impacts one of the most biodiverse water ecosystems here in the southeast. So if you’re interested in learning more about all of the cool freshwater organisms we have right here in our own backyards - this is the post for you!

When it rains... it pollutes?