1909-13367 SRA referral response approved docs

2. 2       Law ful P oint of D is ch ar ge

The site has two identified lawful points of discharge (LPOD). LPOD 1 in the existing case generally
discharges via sheet and concentrated flow along the North West boundary to the Cooroy golf course.
LPOD 2 in the existing case generally discharges via sheet and concentrated flow along the length of
the eastern boundary also to the Cooroy golf course.

The LPOD’s for the developed case are as follows.

LPOD 1, located at the northern side of the development shall discharge via a headwall via an
appropriately designed energy dissipation structure. The second LPOD is deemed to be at the south
eastern side of the development within the Cooroy golf course via a headwall and energy dissipation
structure.
It is proposed to split the internal developed catchments between the two LPOD’s (as detailed later in
the report).

2. 3       E xte rn a l Cat ch me nt

The development site has a significant external catchment which flows through the existing channel
from Myall St through the golf course from South to North. The size of the external catchment is
approximately 222ha with a percentage impervious of approximately 9.6%. This external catchment
has been assessed as part of the separate flood study as prepared by Westera Partners and is provided
separately. The existing channel is not proposed to be changed from the existing case. A bridge is
proposed to span the existing channel with a span such that creek will flow unimpeded underneath in
all design events up to the 1% AEP (Annual Exceedance Probability) design event. Please refer
separate Flood study and Appendix AC for details.



2. 4       Floo din g

The proposed site has had a detailed flood assessment conducted to assess the suitability of the
development for use and to ensure no adverse impact due to the development to neighbouring
properties. This report has been prepared by Westera Partners and is provided separately.


3      ST O RM W AT E R Q U AN T IT Y M AN AG EM E NT


The stormwater quantity analysis of the existing and developed site conditions has been undertaken
with reference to the requirements and procedure outlined by:

           Queensland Urban Drainage Manual (QUDM)

           Australian Rainfall and Runoff 2016 (AR&R 2016)

The following sections of the report discuss the proposed developments’ impact on stormwater flows
and compare them with the existing case to ensure a no worsening effect to downstream properties.

The XPStorm modelling program, utilising the “Laurenson” runoff routing method, has been used to
model the catchment characteristics to determine the stormwater flow discharge from the site for various
storm durations (5 minutes up to 3 hours typically) and Annual Exceedance Probabilities (AEP’s) from
1% to 1EY (Exceedances Per Year).

3. 1       E xi st i ng C at c hm ent a nd Si t e D r ain ag e

The existing catchment is undeveloped and has been considered pervious. The site is crested with
portions of the site falling north and south (to LPOD 1 and 2 respectively). The existing site catchment
characteristics are detailed in Table 1. Refer Appendix AC for extents of existing catchment.
                                       Stormwater Management Report                                   6
S19-007                                     122 & 144 Myall St, Cooroy