25 October 2004
Distributed Detention And Retention Storage Requirements. The
previous section demonstrates that significant reductions in runoff volume (and
correspondingly, storage volume) can be achieved by following the LID site design
approach. A conventional detention pond, however, will not normally be used in an LID
design; instead, storage will be provided using distributed retention and detention. The
LID Design Charts (see Appendix D)44 were used to determine the total volume of
storage required to maintain the pre-development 2-year 24-hour peak runoff rate using
retention (Chart 1) and detention (Chart 2). The CN of 63 was used for the pre-
development condition and the CN of 73 was used for the post-development condition.
The depth of storage across the site needed to maintain the pre-development discharge
rate using retention is 12 mm (0.48 in). Equation 10-7 shows that this is equivalent to a
volume of 321 m3 (0.26 acre-feet). Using detention, the required depth of storage
across the site is 8 mm (0.3 in), or using Equation 10-7, a volume of 194 m3 (0.16 acre-
Storage Volume (acre-feet) = Drainage Area (acres) x Depth of Storage (feet)
The soils are HSG B; therefore, they have good potential for infiltration and
the use of retention is appropriate. Hybrid designs that use both retention and detention
are intended for soils with poor infiltration capacity (HSG C and HSG D). The use of
retention will also encourage recharge and maintain the water balance for the site.
A site may be required not only to maintain the pre-development peak
discharge rate, but to maintain the pre-development runoff volume as well. The total
storage volume required to maintain the pre-development runoff volume can be
calculated using Chart 3 in Appendix D, and in this example it equals 11 mm (0.42 in).
This is less than the volume needed to maintain the pre-development peak discharge
rate (13 mm [0.48 in,] calculated above). Although maintaining the pre-development
runoff volume is not a requirement in this case study, this calculation illustrates the
feasibility of maintaining the pre-development recharge and runoff characteristics of the
site (i.e. peak discharge and volume) for frequently occurring storm events up to and
including the 2-year 24-hour storm. Therefore, there is full hydraulic and hydrologic
control of small-scale, frequently-occurring storms.
Selection of Appropriate IMPs. The retention storage volume calculated
above, 321 m3 (0.26 acre-feet,) was used as the total storage volume to be distributed
between the selected IMPs. The selected LID components include bioretention cells,
bioretention swales, and tree box filters. A ponding depth of 305 mm (12 in) was used
to size each of the bioretention devices. Using Equation 10-8, the total area required for
bioretention is approximately 1050 m2 (11,300 sq. ft). Accounting for the volume of
runoff that can be stored in the pore spaces in the bioretention media will further
decrease the required storage area.
Bioretention Area = Storage Volume Bioretention Depth