102255ltr SMP Iss B

                                                  3


however as this portion of the site is existing vegetation/ grassed area any stormwater runoff
from this area will be sheet flow. The detention tank calculations will have to give consideration
to this bypass flow and adjust the outlet arrangement accordingly.

The proposed design is consistent with the recommendations in Tectonics Geotechnical
Investigation Report No. 22511-001-Rev0 provided in Appendix C. In the report they also
determined that the proposed design would not have any impact on slope stability provided
their recommendations in the report were implemented. Further, Tectonic has determined that
the sandy soil encountered on-site was consistent with clean fine to medium grained sand
which have a design coefficient of permeability of 1 x 10-4 m/s. Details of the proposed
infiltration trench are shown in Appendix A.

The flow rate of the infiltration trench for each lot was calculated using the parameters and
equation below:

Infiltration trench parameters:

Trench dimensions: 0.9m long x 0.82m wide x 0.9 m deep
Vertical permeability: 0.0001 m/s
Ratio of vertical to horizontal permeability: 0.5

The infiltration flow rate of the trench was calculated using the equation below

                                                𝑄 = 𝑞𝐴

Where:
Q = Infiltration trench flow rates (m3/s)
q = permeability (m/s)
A = Area of infiltration, base and sides (m2)

In this instance, the infiltration trench flow rate was calculated to be approximately 2 L/s. Refer
to Appendix B for the full calculations.

The peak stormwater discharge for each individual lot (both with identical areas) was
calculated for both the pre-development and post-development cases using the rational
method. The following parameters were used in the calculations:

Single lot discharge to Noosa Drive pre-development site parameters:

Area: 723m2
Fraction impervious: 39%
Average slope: 33%

Single lot discharge to Noosa Drive post-development site parameters:

Area: 723m2
Fraction impervious: adopted 50% (calculated 47%)
Average Slope: 33%

Single lot infiltration trench bypass to Noosa Drive post-development site parameters:

Area: 144m2
Fraction impervious: 0%
Average Slope: 33%