Revised Stormwater Management Plan

                              STORMWATER MANAGEMENT PLAN
Client: Hass Holdings ATF The Quasar Trust                               Location: Taylor Court, Cooroy
6.0 STORMWATER QUANTITY
The proposed stormwater quantity management for this site ensures non-worsening of flows as a
result of the proposed development. Development of the site will result in an increase in
impervious area and a corresponding increase in peak stormwater runoff rates which will be
mitigated through on-site detention. This section of the report encompasses the hydrological and
hydraulic analysis for the proposed development and identifies the stormwater detention measures
required to ensure non-worsening conditions.

6.1 Stormwater Quantity Management

An analysis of the stormwater (quantity) required for the site, involved the following processes:

      Determining the overall catchment areas contributing to flow from the proposed development
       site;
      Calculating the 1%, 10% and 39% Annual Exceedance Probability (AEP) peak discharges
       using XP-RAFTS for existing case;
      Rational method calculations for comparison and calibration of the XP-RAFTS runoff model
       to ensure the accuracy of results; and
      XP-RAFTS detention modelling for the 1%, 10% and 39% AEP design discharges to
       demonstrate that peak discharges are mitigated in the developed case and flows leaving the
       site are ‘non-worsening’.

6.2 Hydrology (XP-RAFTS)

XP-RAFTS was utilised to determine the peak stormwater discharges for the subject site for pre-
development and post-development scenarios.

The proposed development site has an area of 0.824 ha. The site receives a very small upstream
catchment which when combined with the site area gives a total area of 0.924 ha. The majority of
this upstream area is a landscaping buffer between the site and Holts Road.

6.3 XP-RAFTS Data
6.3.1 Model Scenarios

The existing and proposed conditions for the 1%, 10% and 39% AEP events were modelled in XP-
RAFTS to determine the peak stormwater flows for the site.

The three scenarios modelled for this investigation are:

      CSS_E01b - represents the existing characteristics of the subject site (pre-development);
      CSS_D01b - represents the subject site as fully developed (no detention); and
      CSS_M01b - represents the subject site as fully developed and the effects of the proposed
       detention tank.

6.3.2 Rainfall Intensities

The rainfall intensities used in XP-RAFTS are typical to Cooroy and were generated from the
Bureau of Meteorology’s online IFD data website (accessed 30 May 2016). The rainfall intensity
tables are contained in Appendix G.




Covey Associates Pty Ltd                     8                                       Project No: 161968
Consulting Engineers                                                                    100415 Rpt Iss B