User talk:Jenny Hill

	Formal Infobox
Common name:	Wordiness virus
Medical name:	(see notebox below)
Habitat:	here, 19-Dec-2008

Many of the dimensions in a bioretention system are relatively constrained by the performance requirements of the individual 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	Typical Void ratio	Recommended depth (with underdrain pipe)	Recommended depth (no underdrain pipe)
Ponding depth	1	300 mm	See below
Mulch	0.7 for wood based 0.4 for aggregates	75 ± 25 mm
Biofiler media	0.3	300 mm to support turf grass and accept only roof runoff 600 mm to support flowering perennials and decorative grasses 1000 mm to support trees
Choker course	0.4 typical	100 mm
Embedding reservoir	0.4	Is equal to underdrain pipe diameter	Not applicable
Storage reservoir	0.4	See below	See below

Calculate the maximum overall depth[edit]

Step 1: Determine what the planting needs are and assign appropriate depth of media.
Step 2: Decide on the underdrain pipe diameter.
Step 3: Determine maximum possible storage reservoir depth beneath the pipe (d_S):

$d_{S}=f'\times 38.4$

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 (d_P):

$d_{P}=f'\times 19.2$

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 necesary.

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):

$Storage=RVC_{T}\times A_{c}\times 0.095$

Where:

RVC_T is the Runoff volume control target (mm),
A_c is the catchment area (Ha), and
0.095 is the product of a typical runoff coefficient for impermeable surfaces (0.95) and the units correction between m³ and mm.Ha. (0.1)

Step 8. Divide required storage (m³) by the 1 dimensional storage (in m) to find the required footprint area (A_p) for the bioretention in m².
Step 9. Calculate the peak flow rate (Q_p, in L/s) through the filter media:

$Q_{p}=A_{p}\times K_{sat}\times 3.6\times 10^{-3}$

Where:

K_sat is the saturated hydraulic conductivity of the filter media (mm/hr), and
A_p is the area of the practice (m²).

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