Detailed Stormwater Management Practice Design - Tauranga City ...

As = WQV/((fd)(i)(t)-p)
Where:
As = surface area of the trench (m 2 )
WQV = water quality volume (m 3 )
fd = infiltration rate (m/hr) - rate reduced by ½ from measured
i = hydraulic gradient (m/m) - assumed to be 1
t = time to drain from full condition (hours) - maximum time 48 hours
p = rainfall depth for water quality storm (m)
There is a simple test to see how deep an infiltration practice can be to achieve
the discharge of the water quality storm. Any deeper than the amount calculated
will not achieve the two-day draw down period. The equation is the following:
dmax = fd(t/Vr)
Where:
dmax = maximum depth of trench
fd = infiltration rate (m/hr)
t = time to drain from full condition (hours)
Vr = void ratio of reservoir stone (normally 0.35 or 0.5 if scoria is used), 1 for
infiltration basins
Once dmax has been defined, the actual needed depth can be calculated. If the
actual depth exceeds the maximum depth the surface area must be increased to
account
d) Find the practice volume to provide storage for 37% of the volume required to
infiltrate. This allows for storage of excess runoff during those periods when the
runoff exceeds the infiltration rate.
V = 0.37(WQV + pAs/Vr)
Where:
V = practice volume with any aggregate added
NOTE: Permeable paving does not usually have a contributing drainage area
draining to it. As such the volume of storage equals the following:
V = pAs/vr where p is the design rainfall event (at least the water quality storm
but generally will be up to the 10-year rainfall event)
Page 45 Updated 01/11/2012