Do you have a low-lying path that stays wet and “spongy” when you walk across it? You may have a bioswale.
Your bioswale likely has unique plants, such as sedges, rushes, irises, duck potatoes, etc.
But it is also a well-known stormwater best management practice.
Want to learn more?
Keep reading since we have the top things you need to know about bioswales!
1. What is a bioswale?
A bioswale is a stormwater runoff conveyance system that provides an alternative to storm sewer systems.
It comes in the form of a vegetated swale (or a shallow, landscaped depression).
Bioswales can either absorb low flows or carry runoff from heavy rains to storm sewer inlets or surface waters.
Whichever is the case, a bioswale is designed to capture, treat, and infiltrate stormwater runoff as it moves downstream.
Using bioswales improves water quality as it infiltrates the first flush of stormwater runoff and filters the large sediments that they convey.
As most annual precipitation comes from frequent, small rain events, much of the value of bioswales comes from infiltrating and filtering almost all of this kind water.
Bioswales are the most effective type of green infrastructure facility.
2. How are bioswales designed?
When you design a bioswale, there are several things you must keep in mind.
Here are some core guidelines when designing your bioswale.
Your goal should be to enhance and utilize existing natural drainage swales whenever possible
You can enhance existing swales with native plant species (thicker, heavier grasses, as well as deep-rooted native plants, tend to be better when filtering out contaminants)
You may need to add subgrade drains and amended soils to facilitate infiltration
Your costs may greatly vary when designing or maintaining a bioswale depending on size, plant material, and site considerations (fortunately, bioswales are generally less expensive when used in place of underground piping)
Your soil infiltration rates should be greater than one-half inch per hour
You should strive for a parabolic or trapezoidal shape with side slopes being no steeper than 3:1
You should avoid soil compaction during installation
Your swales should be sized to convey at least a 10-year storm (or about 4.3 inches in 24 hours)
3. What are the primary design constraints for swales?
Bioswales are a conveyance system, and thus, the ditch must be able to transport water of a specific design storm without flooding.
Overall, the purpose of the vegetation is to slow the water down, yet the channel should be hydraulically fast enough to keep water moving during a given storm event.
When it comes to velocity, the slope in the direction of flow is the primary design element that is important.
If the water velocity is too great in a bioswale, then check dams can be installed.
These will allow the water to pool at different terraces.
Furthermore, there are other geometric properties that impact how a bioswale will function, including the cross-sectional slope and area.
Both should be constructed so that any water collected from a more common storm event is relatively shallow.
If the bioswale is shallow, then there will be a higher surface area to volume ratio.
This allows the water to infiltrate the soil at an optimum rate.
Although it isn’t necessary in all scenarios, an underdrain can be installed beneath the ditch.
This underdrain is a perforated pipe that collects the filtered water and moves it away from the ditch, allowing more water to infiltrate.
We recommend using an underdrain if water quality is a larger issue than water quantity.
4. How are bioswales categorized?
Bioswales can be categorized by the type of vegetation that is used.
Here are a few examples.
Grass swales: These swales are planted with turf grass that is mowed to provide a more manicured look.
However, you should note that grass swales are generally less effective in slowing stormwater runoff than swales with taller plants.
Vegetated bioswales: The swales can be planted with ornamental grasses, shrubs, perennials, or a combination of these.
Often, mulch or stone are used to protect the soil in areas that are not covered by turf grass.
Larger stones can reduce velocity and break up concentrated flows of water.
Xeriscape: These swales are useful in areas with hot summers or generally dry conditions because they don’t use a lot of water.
Most bioswales are designed to be dry except just after rain events.
Wet swales: These swales function similarly to stormwater wetlands.
5. How do you maintain a bioswale?
After a bioswale is established, it must be maintained properly.
Luckily, you’re looking at less maintenance than a lawn because bioswales will need comparatively less water and no fertilizer.
In fact, you should actively avoid using fertilizer and herbicides in and around your swale.
Bioswales normally receive the necessary nutrients which makes fertilizers unnecessary.
Bioswales often use native grasses and forbs that are adapted to local rainfall patterns.
Native grasses also resist local pests and disease, which makes them particularly hearty.
Additionally, when a bioswale is well-designed and maintained, it can be very attractive and serve as a landscaping area within site.
When compared to other stormwater management practices, a bioswale can add aesthetic appeal to a residential or commercial site, including (but not limited to) texture, color, and utility.
This allows for more usable space in a development.
We recommend inspecting your bioswale semi-annually as well as after large storm events.
This includes looking for signs of trash and debris, sediment accumulation clogged inlets or outlets, and erosion.
You should try to pick up trash and debris as soon as possible.
Use a shovel to remove accumulated sediment and repair areas that are eroding.
This can be done using sod, rock, or special erosion-control matting to both slow the flow of water and protect from future failure.
6. What’s an example?
Believe it or not, a road ditch can serve as a bioswale.
The rock trench and wetland vegetation in this example serve as a bioswale for residential runoff.
7. How do you choose an appropriate site?
An appropriate site for a bioswale involves several factors, such as soil infiltration rate, slope, and groundwater level.
Here’s more information about each.
Soil infiltration rate – The rate should be half an inch per hour or greater.
Sites with poorly drained soils will require an underdrain system to remove excess water during peak flows.
Soils can also be amended by adding sand to increase infiltration.
Slope – Bioswales are impractical in areas with very flat or very steep topography.
They’re also not recommended in areas where the flow rate may exceed 5 cubic feet per second.
The sides of the bioswale should be constructed with slopes of 5 percent or less.
It’s recommended that the longitudinal slopes range from 1 to 4 percent with 4 percent possibly requiring the use of a weir.
Groundwater level – Bioswales shouldn’t be used in areas with high water tables where groundwater would reach the bottom of the swale.
8. How is a bioswale low impact development (LID)?
Bioswales are part of a lower impact development plan.
This approach emphasizes site design and planning techniques that mimic the natural infiltration-based, groundwater-driven hydrology of our historic landscape.
Low impact development is critical to the environment, residents, developers, and communities for multiple reasons.
Here’s a quick summary of why you may consider a low impact development approach.
The environment benefits from LID because…
It protects sensitive areas
It increases habitat for wildlife by preserving trees and vegetation
It protects local and regional water quality by reducing sediment and nutrient loads
It reduces streambank and channel erosion by lowering the frequent surges and bounces of higher flows from storm sewer discharges
It reduces frequent high and low flows associated with surface runoff and stabilizes stream flow volumes by restoring groundwater discharges into receiving waters
It may reduce the potential for flooding
The residents of an area benefit from LID because…
It increases community character
It improves the quality of life
It provides more access to trails and open spaces
It’s more pedestrian-friendly
The developers of an area benefit from LID because…
It reduces land clearing and grading costs
It lowers infrastructure costs, including those for streets, curbs, gutters, and sidewalks
It increases community marketability
The community benefits from LID because…
It balances growth needs with environmental protection
It reduces infrastructure and utility maintenance costs
9. How much do bioswales cost?
How much bioswales cost depends on both installation and annual maintenance costs.
You can reduce maintenance costs by using native grasses and plants that are already adapted to your area.
This means that they require less water, no fertilizer, and infrequent mowing.
A baseline level of maintenance will be required to ensure that your bioswale functions properly.
Periodically, your bioswale could need to be regraded to restore the proper flow.
To create a bioswale, costs vary.
The estimated construction cost for a bioswale between 9 feet long and 16 feet wide is around $58 per linear foot in length.
Professional installation typically costs about $10-$20 a square foot.
So, you’re in the $1,500 to $3,000 range for more than 150 square feet.
10. What’s the difference between a bioswale and a rain garden?
The main difference is that the bioswale moves water to somewhere else in the garden.
This allows some but not all the water to infiltrate the soil.
On the other hand, a rain garden is specifically meant to increase infiltration.
You’ll often see bioswales used to convey water to a rain garden.
11. Do bioswales attract mosquitos?
No, bioswales can reduce standing water, which attracts mosquitoes.
By introducing bioswales onto your land or community, you reduce water stagnation.
This, in turn, reduces the rate of mosquito multiplication.
Bioswales can help keep homeowners safe from diseases like malaria.
12. What’s the difference between bioswales and bioretention?
In short, bioswales are wet or dry swales made of grasses, rocks, and other types of vegetation.
They are sloped to allow water to efficiently move through the system.
Bioretention pounds are depressed vegetation areas intended to capture and store stormwater runoff.
They can work in combination with bioswales, but have a different core purpose.
Overall, bioswales are beneficial to the land because they treat stormwater and add visually appealing vegetation.
The added greenspace can even serve as a habitat for wildlife species (such as birds) depending on the plants selected.
Both these benefits offer a pleasant change to the traditional gray roadside conveyance systems.
Reach out to an engineer and a landscape architect to see if your land could benefit from a bioswale.
If it could, they can help you design and build it correctly.
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Disclaimer: we are not lawyers, accountants or financial advisors and the information in this article is for informational purposes only. This article is based on our own research and experience and we do our best to keep it accurate and up-to-date, but it may contain errors. Please be sure to consult a legal or financial professional before making any investment decisions.