ATTCH 3 - IR Response - Erosion, Coastal Processes and Stormwater (Dr Trevor Johnson)

Kenlynn Pty Ltd Report Number 620.12166-R01

Castaways Beach 3 November 2017
Stormwater Management Plan Version v0.1
Page 8

 

The design rainfall losses adopted for the analysis were:
Pervious Area: Initial Loss = 0 mm
Continuing Loss = 2.5 mm/h
Impervious Area: Initial Loss = 0 mm
Continuing Loss = 0 mm/h

A Lag Parameter of 1.30 was used in the WBNM model. Studies carried out using WBNM have found
that the average value of the Lag Parameter across a wide range of catchments is between 1.30 and
1.80 (ref. WBNM User Manual). Thus, the adopted value of 1.30 is within the accepted bounds.

The WBNM model was run for a range of storm durations, from 10 minutes to 72 hours, with the 15
minute event producing the peak discharge of 0.082 m%/s for the 1% AEP event. This value is
consistent with the flow estimate calculated using the Rational Method (as discussed in Section 2.3).
Consequently, it was considered that the WBNM model could be used to calculate the discharge
hydrographs from a developed allotment.

As discussed in Section 2.1, it is proposed to construct a basin or basins with a total area of 100 m?
within each developed allotment, to receive the stormwater runoff from that lot. If only one basin is
constructed along the eastern boundary, it would be approximately 16.7 metres long (across the lot)
and 6 metres wide. For two basins, appropriate basin sizes can be determined based on the
catchment areas draining to each basin.

The captured stormwater runoff will infiltrate into the existing sand within the basin(s). An infiltration
rate of 1,000 mm/h was adopted for clean beach sand. There is no topsoil present on the site, and no
plan to import any for the proposed development.

The WBNM model, with the inclusion of the detention/infiltration basin, was again run for a range of
storm durations, from 10 minutes to 72 hours. The peak depth in the basin for the 1% AEP event was
achieved with the 60 minute event, producing a maximum depth of 299 mm.

As discussed in Section 2.1, each basin will comprise a detention depth of 300 mm. Consequently,

there would be no overtopping of the basin for all flood events up to and including the 1% AEP flood
event.

2.5 Summary

As discussed above, the proposed stormwater strategy is to grade each allotment to a detention basin
or basins located at the downstream boundary(s) of that lot. The basin(s) will capture and infiltrate the
stormwater runoff from its respective lot for all floods up to and including the 1% AEP event.

Consequently, there is proposed to be no surface discharge from each lot which could affect stability
on landslip areas or cause erosion.

This addresses Items 10 and 11 of Council's Information Request.

SLR Consulting Australia Pty Ltd