Bioretention: Sizing
Many of the dimensions in a bioretention system can be predetermined according to the function of the component. There is greatest flexibility in the ponding depth and the depth of the storage reservoir beneath the optional underdrain pipe. The order of operations in calculating these dimensions depends on whether an underdrain is desired.
Component | Recommended depth (with underdrain pipe) | Recommended depth (no underdrain pipe) | Typical void ratio (Vr) |
---|---|---|---|
Ponding depth | 300 mm | See below | 1 |
Mulch | 75 ± 25 mm |
| |
Bioretention media |
|
0.3 | |
Choker course | 100 mm | 0.4 typical | |
Pipe diameter reservoir | Is equal to underdrain pipe diameter | Not applicable | 0.4 |
Storage reservoir | See below | See below | 0.4 |
Calculate the maximum overall depth[edit]
- Step 1: Determine what the planting needs are and assign appropriate depth of media, using the table above.
- Step 2: Select an underdrain pipe diameter (typically 100 - 200 mm), assign this as an 'embedding' depth.
- Step 3: Calculate the maximum possible storage reservoir depth beneath the pipe (ds):
Where:
- f' = Design infiltration rate in mm/hr, and
- 38.4 comes from multiplying desired drainage time of 96 hours by void ratio of 0.4
Additional step for system without underdrain[edit]
- Step 4: Determine maximum permissible ponding depth (dp):
Where:
- f' = Design infiltration rate in mm/hr, and
- 19.2 comes from multiplying desired drainage time of 48 hours by void ratio of 0.4. Note that conceptually the drainage of the ponded area is limited by ex-filtration at the base of the practice.
- Step 5: Sum total depth of bioretention, and compare to available space above water table and bedrock. Adjust if necessary.
Calculate the remaining dimensions[edit]
- Step 6: Multiply the depth of each separate component by the void ratio and then sum the total to find the 1 dimensional storage (in mm).
- Step 7: Calculate the required total storage (m3):
Where:
- RVCT is the Runoff volume control target (mm),
- Ac is the catchment area (Ha),
- C is the runoff coefficient of the catchment area, and
- 0.1 is the units correction between m3 and mm.Ha.
- Step 8. Divide required storage (m3) by the 1 dimensional storage (in m) to find the required footprint area (Ap) for the bioretention in m2.
- Step 9. Calculate the peak flow rate (Qp, in L/s) through the filter media:
Where:
- Ksat is the saturated hydraulic conductivity of the filter media (mm/hr), and
- Ap is the area of the practice (m2).
Step 1, total volume[edit]
The bioretention facility will comprise three vertical zones:
- a 'bowl' permitting water to pond occasionally (maximum depth, dp = 0.5 m)
- a layer of filter media to support plant growth (minimum depth, dm = 0.6 m) , and
- a reservoir of stone to store and facilitate infiltration of additional water (depth, dR, includes 0.1 m choking layer).
Firstly, all three zones will be sized together within the limited depth to estimate the footprint area of the whole bioretention facility. To do this the mean void ratio (VR) will estimated as ~ 0.4.
To calculate the required practice area (Ap) or footprint where the depth is constrained:
Where:
- V = Required volume of storage in m3
- VR = Void ratio (porosity), as measured (or default to 0.4)
- Ap = Area of the infiltration practice in m2
- dT = Total depth of infiltration practice in m (= db + dm + dR)
Step 1 alternative, accounts for infiltration during a design storm event[edit]
This may be desirable where the practice is located on particularly freely draining soils and/or where the design storm is of longer duration. To calculate the required practice area (Ap) or footprint where the depth is constrained:
Where:
- D = Duration of design storm in hrs
- i = Intensity of design storm in mm/hr
- f' = Design infiltration rate in mm/hr
- VR = Void ratio (porosity), as measured (or default to 0.35 for all aggregates)
- Ap = Area of the infiltration practice in m2
- Ac = Area of the catchment in m2
- dT = Total depth of infiltration practice in m (= db + dm + dR)
Step 2, geometry and perimeter[edit]
Fit the bioretention facility into the site plan. It is necessary to have an idea of the perimeter of the facility.
Narrow, linear cells drain faster than round or blocky footprint geometries.
Divide the area of the practice (Ap) by the perimeter (P).
Step 3, depth of bowl,db[edit]
The 'bowl' area is simply the depressed elevation of the surface compared to the surrounding areas; a space to permit temporary ponding.
The depth of the bowl would not typically exceed 0.5 m due to safety concerns.
Some jurisdictions or particular sites may demand a lower maximum depth of ponding. It is worth bearing in mind that in most cases this ponded water will only occur once or twice per year.
The ponded water within the bowl should drain within 48 hrs. This addresses public concerns about mosquitoes and reduces the probability of losing vegetation due to saturated growing conditions.
The drainage of the bowl is not usually constrained by the surface infiltration (filter media should drain > 25 mm/hr). The infiltration of water from the bowl is determined by the drainage of the filter media and reservoir into the soils.
To estimate the time (t) to fully drain the facility:
Final step[edit]
Check the mounding of groundwater beneath the facility to ensure no incompatibility with nearby sensitive receptors.
Note that this is a minor adaptation (metric units and formatting) from the original tool, written and hosted by the USGS.