Beneath the Rain Garden
Anna Logan McClendon
Graduate Student, SEAS

Digital hand-drawn animation


This animation depicts the process of stormwater flowing into a rain garden. While on the surface it may appear as though these gardens are nothing more than beautification, there is actually a complex engineering system lying underneath. Traditional ‘gray’ stormwater infrastructure involves zero treatment and acceleration, leading to poor water quality and erosion. The rain garden, however, filters, stores, and slows down water by passing through a series of layers that treat the water with microbes found in soil. Additionally, the native plants provide a healthy environment for pollinators and an accessible green space for the surrounding community.

This animation depicts the process that occurs when stormwater inflows into a bioretention basin, or better known as a rain garden. While on the surface it may appear as though these gardens are nothing more than beautification for a landscape, there is actually a much more complex engineering system lying underneath. These gardens are a fantastic nature-based alternative for storing, treating, and discharging stormwater compared to traditional ‘gray’ infrastructure like pipes or concrete channels. Piping stormwater involves zero treatment processes and causes rapid acceleration, leading to poor water quality and erosion at the outfall. The rain garden, however, will naturally filter, store, and slow down water flow. First, the water flows over a ‘forebay’, or the two stone mounds seen in the left of the animation. This is the first treatment process that passes water through jagged rocks to catch any large sediment particles before entering into the main basin. Next, the water passes through the basin, where it slowly infiltrates through the soil, sand, and gravel layers. These layers do two important things: 1. store a significant amount of water and 2. filter out contaminants through the gravel/sand and the microbes naturally found in soil. If it is a small storm, these two processes are enough to hold and treat all of the water. However, in the case of large storm events, such as that depicted in my animation, it follows a slightly different path. First, it still follows the initial steps aforementioned until the soil is fully saturated and cannot hold any more water. Then, it begins to overflow into the top of the emergency overflow outlet, shown in the right of the animation with the rectangle bottom and grated dome top. This allows for an immediate escape for excess water the garden cannot store to prevent it from washing out the vegetation and flooding the property. Additionally, right above the deepest layer of the soil, excess water can percolate through a perforated underdrain which connects to the main emergency overflow outlet. This allows some of the water to still get treated through the layers of soil, sand, and rocks that filter out contaminants. After the storm has passed, the water will slowly dissipate through the perforated underdrain, natural infiltration to deep soil, and evapotranspiration into the atmosphere. The animation sequence is timed to accurately represent the different speeds of infiltration after a storm event, initially going somewhat fast and then slowing down as it reaches the deepest layers of soil. On top of the benefits for stormwater flow, it also provides an environment for pollinators with native plants/flowers that improve the health of the overall ecosystem. It also provides social and cultural benefits, particularly for urban areas, by providing an accessible green space for the community.