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

Kentynn Pty Ltd Report Number 620.12166-R01

 

Castaways Beach 3 November 2017

Stormwater Management Plan Version v0.1
Page 6

2 HYDROLOGIC ANALYSIS

2.1 Overview

As shown by the topographic data contained in the AJS Surveys Plan in Appendix A, the existing site
generally drains in a westerly direction towards David Low Way, then in a northerly direction along a
gully (on the eastern side of David Low Way), before discharging into Burgess Creek to the north of
the site.

The proposed layout consists of 6 residential allotments (as shown in Appendix A). It is proposed that
each lot is regraded in one of the following ways:

° Scenario 1 — high point at David Low Way, and lot falls to its eastern boundary, or

¢ Scenario 2 — high point near the middle of the lot, and the lot falls both to its eastern and
western boundaries.

A basin (or basins) will be constructed in each lot to intercept the runoff from that lot, ie. at its eastern
boundary for Scenario 1, or at both the eastern and western boundaries for Scenario 2. The total area
of the basin(s) in each lot will comprise the following characteristics:

e Area = 100 m?
« Detention Depth = 300 mm

Each basin will be vegetated, and will act as both a bioretention and detention basin. Further details
on the bioretention basin analysis are contained in Section 3.

The basins are sized so that they can capture and infiltrate the entire volume of runoff from each lot for
all flood events up to the 1% Annual Exceedance Probability (AEP) event, ie. there will be no
overland flow discharged from the site for events less than or equal to the 1% AEP event.

Thus, there will be no discharge of stormwater runoff or associated pollutants to Burgess Creek
following the development.

2.2 Development Layout
The area of each allotment within the development (refer Appendix A) is summarised in Table 1.

Table 1 Allotment Areas

 

 

 

 

 

 

 

 

Lot No. Lot Area Covenant Area Developable Area Building Envelope

(m?) (m*) (m*) (m*)

711 3336 2142 1194 829
712 1219 170 1049 665
713 4211 173 1038 653
714 1143 176 967 567
715 946 209 737 388
716 1819 773 1046 349
Average 1005 575

 

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