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 7

 

As shown in Table 1, the average developable area available within each lot is approximately
1005 m2, with an average building envelope of approximately 575 m?.

Consequently, for the stormwater analysis of the subject site, an average lot size of 1,000 m? was
adopted, with a fraction impervious of 60% (i.e. building envelope of 600 m’).

A single basin has been modelled for each lot. If the lot is graded such that it falls to the east and the

west, the total basin size (100 m?) can be split proportionally according to the size of the two
catchments.

2.3 Rational Method

The peak discharge from one of the proposed new residential allotments was estimated using the
Rational Method.

A time of concentration of 5 minutes was adopted for a developed lot, in accordance with the
Queensland Urban Drainage Manual (QUDM, 2017) recommended roof drainage system travel times.

The 1% Annual Exceedance Probability (AEP) rainfall intensity for Castaways Beach for a duration of
5 minutes is 319 mm/h.

The 10% AEP runoff coefficient (C10) for a residential allotment assuming 60% fraction impervious is
0.82.

The peak 1% AEP discharge from an average allotment using the Rational Method was therefore
calculated to be 0.087 m/s.

2.4 WBNM Model
A WBNM hydrologic model of a typical allotment was established. The design rainfall data for the
catchment was determined in accordance with Australian Rainfall & Runoff. The information used is
as follows (Source: Intensity-Frequency-Duration application on the Bureau of Meteorology website):

2 Year ARI, 1 hour Intensity 51.5 mm/h

2 Year ARI, 12 hour Intensity 10.4 mm/h

2 Year ARI, 72 hour Intensity 3.65 mm/h

50 Year ARI, 1 hour Intensity 94.2 mm/h

50 Year ARI, 12 hour Intensity 22.1 mm/h

50 Year ARI, 72 hour Intensity 8.71 mm/h

Regional Skewness 0.10
Geographical Factor F2 4.40
Geographical Factor F50 17.58

SLR Consulting Australia Pty Ltd