MS Bioswale Article Handout v1.1

For Students
Engineering Design in Oregon Science Classrooms
Page 1 of 2
Article Handout for Bioswales
Rainwater might do a number of
different things once it falls, depending on the
environment. If it is a hot day, droplets
evaporate back into the atmosphere. If there is
a lot of vegetation, much of the rainwater is
taken up by plants. However, some rainwater
will soak into the soil and become
groundwater. The space between particles
spaces in soil and other sediments act as a
natural filter because they can trap particles
and allow chemical and biological processes
to take place. This process is referred to as
Figure 1: When there is a lot of vegetation, 50% of storm water will
become groundwater right away, and only 10% will become run-off.
However in urban areas where the majority of the surfaces are
impervious, meaning they do not allow water infiltration, only 15% of
stormwater soaks into the ground, and a whopping 55% becomes run-off.
This is one of the reasons why there is a lot more water pollution in urban
infiltration. It is better for water to infiltrate
slowly, because sediments clean water via chemical attractions as it soaks into the
ground. For example: oil is more attracted to sand than it is to water, so one way
to separate oil from water is to pour it through sand. However, there is a trade-off
when it comes to filtration time. If water takes too long to filter—perhaps because
there are many fine-grained clay particles in the soil—it will begin to pool above
ground and become runoff, a type of surface water that moves across the land.
This is not great because moving water has the energy to pick up and carry more
pollutants from the environment such as oil, fertilizer, pesticide, soap, and animal
waste. Runoff eventually connects to a larger body of water: it may eventually
soak into the ground and collect in an aquifer, or it might flow into a lake or
stream. In urban areas it usually winds up in the stormwater system.
Stormwater, or water created by precipitation such as rain or snowmelt,
ideally soaks into the ground. However, if there are heavy rains and the ground
becomes saturated and does not have space for more water, or if there are a lot of
impervious surfaces like sidewalks, blacktop, and buildings, stormwater becomes
runoff, which is then channeled into storm drains that pipe it to nearby creeks,
rivers and wetlands.
For Students
Engineering Design in Oregon Science Classrooms
for a community is an important job. Environmental
engineers must design systems that minimize flooding and
Managing the quality and quantity of stormwater
Page 2 of 2
pollution. The construction of bioswales is one way to do
this. Bioswales are landscape elements designed to catch
stormwater runoff so it can soak into the ground at a slower
rate instead of flowing across the surface.
This helps clean it of pollutants before it enters an
aquifer or drain system. Bioswales consist of a shallow
ditch with gently sloping sides, and vegetation, as well as
various layers and combinations of sediments beneath.
These filter materials are arranged to maximize the amount
of time water spends in the swale, since slower filtration
makes for more effective cleaning. However, infiltration
cannot be so slow as to create pooling, because overflow
creates runoff which reduces the effectiveness of a
bioswale. It is a tricky trade-off to balance, but as an
environmental engineer who has just been hired by your
city’s Bureau of Environmental Services, it is exactly what
you need to accomplish. Your job is to figure out the right
quantity and combination of filter materials for a bioswale
that minimizes runoff and cleans surface water.
Are you up for the challenge?
Figure 2: Bioswales intercept run-off from
impervious surfaces like streets and parking lots
and filter out pollutants using native vegetation
and natural sediments like soil, sand, and gravel.