Understanding Bioretention Facilities: A Clear Creek Solutions Overview
Welcome back to Clear Creek Solutions' Hydrology Education Series! Today, we’re diving into the world of bioretention facilities — exploring how soil layers interact to filter and detain stormwater, and why they are an essential part of sustainable stormwater management.
If you’re interested in other topics related to stormwater modeling and design, be sure to check out more of our educational videos on our YouTube channel.
What Are Bioretention Facilities?
Bioretention facilities are a type of Low Impact Development (LID) practice designed to manage stormwater on-site using natural processes. Surface water infiltrates into engineered soil layers, where it is filtered and treated before either infiltrating into the native soil or exiting through an underdrain system.
The main goals of a bioretention facility are:
Filtering stormwater through natural soil processes.
Detaining and retaining water on-site.
Encouraging infiltration into the native soils.
Managing runoff through underdrain systems if infiltration is limited.
By using natural and engineered soils, bioretention facilities efficiently remove pollutants and reduce the impact of stormwater runoff on the environment.
How Soil Layers Work Together
Natural soil composition — and engineered soil mixes — play a vital role in the effectiveness of bioretention facilities. Soils filter out pollutant particles from stormwater through physical, chemical, and biological processes.
While soil can be overwhelmed by excessive pollution, under normal conditions it provides excellent stormwater treatment. This makes infiltration a preferred method for managing stormwater, especially when pollutant removal is a priority.
Water entering a bioretention facility follows three main pathways:
Infiltration into the bioengineered soil mix.
Evaporation and transpiration back into the atmosphere.
Outflow through an underdrain or overflow to downstream systems.
Designing Effective Bioretention Facilities
Proper design is critical to achieving both water quality improvement and hydrologic modification benefits. In Washington State, the Western Washington Hydrology Model (WWHM) 2012 is the standard tool used to model bioretention facilities.
The process typically involves:
Defining the land use area and pre-development conditions (e.g., forested land).
Updating for post-development land use (e.g., new impervious surfaces).
Adding a bioretention element to the modeled project.
Connecting the bioretention facility to a point of compliance.
Using auto-sizing features to meet the 91% stormwater filtration standard required in Western Washington.
For instance, in a sample project, we might add an extra 1-foot soil layer for a total of 4 feet of engineered soil. Using WWHM 2012, we can automatically size the facility to achieve a 91.4% water quality filtration — meeting local regulatory standards.
The Science Behind Bioretention
Bioretention facilities work through several processes:
Storage: Temporary holding of runoff.
Filtration: Pollutant removal as water percolates through soil.
Infiltration: Recharging groundwater by allowing water to soak into the native soil.
Mitigation: Reducing the volume and flow rate of stormwater runoff.
Water movement through these systems is modeled using sophisticated hydrological equations, including:
Modified Green-Ampt infiltration equation.
van Genuchten equation for relative hydraulic conductivity.
Darcy’s Law for fluid flow through porous media.
These complex equations are solved at 15-minute intervals, modeling detailed moisture movement between soil layers. Because of this complexity, replicating the bioretention algorithm in a basic spreadsheet is not feasible — specialized hydrology models are essential.
Bioretention in Sustainable Communities
Bioretention facilities are key components of Green City initiatives and sustainable urban design. By integrating with other LID practices, they form part of a holistic stormwater management strategy aimed at maintaining natural hydrology, reducing pollution, and enhancing community resilience.
Bioretention is not only a stormwater management tool; it is a building block for greener, more sustainable cities.
Want to learn more?
We offer a free guide on how to model bioretention facilities — find it in the description of our YouTube video!
If you have any questions, feel free to leave a comment on our channel. Thanks for reading, and we’ll see you next time with more hydrology education!
