Introduction: Bioretention Cell Design
A bioretention cell, or "rain garden", is a depression in the ground that treats stormwater runoff from developed areas (homes, businesses, paved areas, etc.). Runoff from developed areas can increase flooding and transport fertilizer nutrients, herbicides, and bacteria from pet waste. A bioretention [AL2] cell in a yard can treat this runoff before it enters nearby water bodies. The design requires a simple calculation and a relatively small amount of space and depth into the ground. This Instructable is useful for property owners with experience in stormwater engineering who wish to design a system to treat their stormwater runoff. Another intended audience is practicing stormwater engineers who are looking for an example public outreach program.
Users of this guide should have experience with using the Rational Method for sizing stormwater detention basins. Please consult a professional before constructing a project based on your design.
This guide is mainly for educational purposes, and is primarily for small property owners or homeowners who wish to reduce their contribution to local flooding and pollutant loading to nearby waterways. This guide is not intended for property owners under regulatory requirements to reduce and/or treat their stormwater runoff. The author is not responsible for any legal consequences, property damage, bodily injury, or other consequence which results from the use of this Instructable.
The bioretention design should take about one to two hours. Construction will take at least several days depending on the equipment available.
Step 1: Define Study Area and Land Cover Characteristics
First, define the study area and its characteristics (area, land cover). Go to draftlogic.com, which lets you search for your study area using Google Maps.
Click on the map to plot points and enclose your study area with a series of points. Below the map, you will see the calculated area (Figure 1). Write down the calculated area given in square feet and convert it to acres. You will need the area units to be in acres for later steps.
In this example, the area was determined to be 48,799.95 square feet (1.12 acres). You can easily convert units by Googling: "48,799.95 square feet to acres" or by dividing your area in SF by 43.560 SF/acre.
Step 2: Determine Land Cover and Runoff Coefficient
Use Table 1 to determine the pre-development runoff post-development and coefficients (a ratio of how much rainfall will be translated into stormwater runoff).
For this example, the pre-development condition is Woodland & Forests (C = 0.15); the post-development is Light Residential (C = 0.4).
Step 3: Determine Rainfall Intensity
Use NOAA's Atlas 14 to find the pre-development and post-development rainfall intensities. Once you have selected the location on NOAA's interactive map, make sure that the options for "Rainfall Intensity" and "Annual Maximum" are selected at the top of the page.
For pre-development intensity, choose a duration (T-pre) of 10 minutes, and Annual Exceedance probability of 1/10. The pre-development rainfall intensity in this example is 4.51 inches/hour. For post-development intensity, choose a duration (T-post) of 5 minutes, and Annual Exceedance probability of 1/10. The post-development rainfall intensity is 5.63 inches/hour in this example.
Step 4: Calculate Pre- and Post- Development Peak Runoff Rate
Use the Rational Method to calculate both pre- and post- development peak runoff rates:
Q = CiA
Q = peak flow rate (cubic feet per second, cfs)
C = Runoff Coefficient (unitless, determined in Step 2)
A = Area (acres, determined in Step 1)
Q-pre = (0.15)(4.51 in/hr)(1.12 ac) = 0.758 cfs
Q-post = (0.4)(5.63 in/hr)(1.12 ac) = 2.52 cfs
Note: The post-development peak outflow is over three times greater than the pre-development peak outflow!
Step 5: Calculate Bioretention Cell Size
Calculate the bioretention cell volume using the following formula:
V = (Q-post - Q-pre)/T-post
In other words, to find the volume required, subtract the pre-development peak flow from the post-development peak flow found in Step 4, then divide that result by the post development duration used in Step 3 (5 minutes). As seen in the example above, you will need a conversion factor for the different units of time.
In the example problem, the required storage volume was 21.14 cubic feet, which we will round to 22 cubic feet. This storage volume could be achieved by making a 4' wide by 4' long by 1.375' (or 1' 3/8") deep square basin. You can oversize the basin for more storage or round to whole numbers for easier construct-ability. Also feel free to try trapezoidal, circular, and other basin cross section styles for easier construct-ability and aesthetics.
Step 6: Conclusion
A small basin such as the one in this example can be easily constructed by hand with a shovel. You will need to line the entire basin with small rocks or mulch to prevent erosion. Try to locate the basin at the lowest point on your property so all of the water flows to this point. Or you can put it next to your house and direct your gutter downspouts into the basin.
An overflow spillway will also be needed for when the basin is approaching a certain storage capacity (i.e. half or three quarters full). The spillway is simple cut or hole in the downstream side wall of the basin where water can flow out of when the basin is at a certain capacity. For a basin of this size, a 0.5' high by 1' wide rectangular spillway should be sufficient: be sure to line the spillway with rocks or mulch as well.
Note: Locating the spillway above the bottom of the basin may result in permanent storage (i.e. a permanently wet pond) depending on evaporation and infiltration into the soil. This can further improve water quality and promote the growth of wetland type plants. However, mosquitoes may breed in these permanently wet areas. Mosquito dunks or traps are safe and effective in reducing or eliminating the mosquito population.