RFI - Black & More - Stormwater Management Plan

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 Introduction
 Noosa District Body Corporate Management, via Andrew Bock Architecture, engaged Black & More to prepare a Stormwater
 Management Plan (SMP) for the development at 195 Weyba Road, Noosaville. The SMP is required to meet Noosa Council’s
 development application requirements.
 An architectural drawing set containing existing building plans and proposed development plans was provided to Black & More.
 This SMP assess pre and post development roof water flows and informs the designers of required stormwater controls to meet
 Noosa Council’s Planning requirements.


 Existing Conditions
 Black & More undertook a site inspection to gain an appreciation of the stormwater drainage infrastructure of the site and the
 surrounding area.
 There are two independent structures on the site; a low set blockwork shop frontage and a high set structure which partially
 covers the low set structure. It is believed that these structures were constructed at different times, although their history is
 unknown. A car parking area is also present beneath the highset structure.


 Surrounding Drainage Infrastructure
 The surrounding streets have kerb and channel which
 flow to underground drainage lines, which outlet to the
 adjacent tributary. There is a highpoint at the corner of
 Weyba Road and Swan Street.
 This indicates kerb flows to the south along Swan Street
 and to the west along Weyba Road. These kerb flows
 are collected by drainage pits.
 See Figure 1 which shows the underground stormwater
 drains in green and the assumed highpoint.
 It is intended to discharge roof water to the kerb and the
 minor overland flows will sheet flow to the kerb.




                                                                             Figure 1 - Adjacent Drainage Infrastructure
                                                                            (Source: Noosa Council Interactive Mapping)




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 Existing Roof Water Drainage
 As can be seen in Figure 2 & 3, roof water from the highset structure is collected in downpipes which are then directed beneath
 the highset structure and above the lowset structure to the carparking area (see Figures 5 & 6).
 The roof water from the low set structure also falls to this area. This roof area could not be inspected; however, it is reasonable
 to assume the roof water it collected into the same downpipes eventually. The downpipes then travel below ground and outlet
 to the kerb on Swan Street. This was the only roof water outlet point observed.




                                         Figure 2 – Roof Plan Mark-up of Roof Water Flows




                                 Figure 3 - Ground Floor Mark-up of Existing Roof Water Drainage




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                     Figure 4 - Downpipe flows Directed Between Structures to Car Park




                                     Figure 5 - Downpipes in Car Park




                               Figure 6 - Location of Roof Water Kerb Outlet
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 Allotment Overland Drainage
 There is insignificant ground surface surrounding the building and it generally falls toward the street. There are no defined
 surface drainage channels and it is considered storm water sheet flows toward the street in these locations.
 The car parking area surface was severely damage and had many low spots. A severely clogged PVC pipe was found in the
 carparking surface, see photo below. It could reasonably be assumed that this was once a surface water drainage pit/pipe for
 this carpark. It is also likely that this pipe is connected to the roof water pipe which outlets to the kerb. Although this detail is
 unknown.




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 Stormwater Assessment
 The following design criteria have been adopted after consultation with the Noosa Planning Scheme (PSP05). Please note the
 below are only highlighted for relevance to this project, the entire Noosa Plan must be complied with, to the satisfaction of Noosa
 Council.
 -   For each allotment, two PVC kerb and channel adaptors to be installed in the kerb 1m away from both side boundaries
     (section 4.15)
 -   Design Recurrence Interval of 1 in 10 year for Commercial Development (Section 4.6a). Although a higher immunity will be
     provided.
 -   Allotments shall have a minimum grade of 1% (Section 4.9)
 -   Maximum of 25L/s out of each kerb adaptor (as advised by Client/Council’s Officer)
 It is proposed to discharge all roof water flows via downpipes to the kerb.
 The carparking area should be repaved and re-surfaced. It is proposed to install a surface drainage pit in this carpark, which will
 connect to the kerb adaptor also.
 It is unlikely any stormwater will enter the carpark area. However, this pit will provide a means to drain water from other sources
 (i.e. from cleaning with hose).
 It is also possible, in a major event, that stormwater may enter the car parking area from flooding of the adjacent ground. For
 this reason, we have considered the exterior catchment of the property adjacent the carpark which may contribute to stormwater
 flows as impervious, this is a conservative approach.
 The pre-development impervious area is approximately 375m2. The development increases this impervious area by
 approximately 50m2 (or 13%). The increase in runoff due to this increase is not appreciable and by splitting flows between kerb
 outlets will result in an overall improvement of stormwater drainage.




                                                Figure 7 - Existing Roof Catchments




                                     Figure 8 – Roof Catchments for Post Development Scenario



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 Proposed Stormwater Design
Roof Water Drainage
The architectural drawings show the lowset block structure being demolished and an extension to the highset structure in its place.
This would mean the existing downpipe arrangement would need to be demolished, including existing below ground pipes and
kerb adaptors. This will allow for new guttering and downpipes to be installed.
Please refer to the attached mark-up for the proposed drainage scheme.
 -    The catchment in blue to be directed to a kerb adaptor on Weyba Road
 -    The catchment in red to be direction to a kerb adaptor on Swan Street (including connection of the pit from the car parking
      area.
This proposed scenario will result in:
 -    Q100 = 15L/s out of kerb adaptor 1
 -    Q100 = 23L/s out of kerb adaptor 2
Note these values are for the 1 in 100-year event, therefore are well within limits.
Also, it was assumed that a new guttering and downpipe arrangement was possible due to the extension. Locations of downpipes
have not been considered and are to be confirmed with the architect. The aim of the stormwater arrangements is to split roof
water flows between the 2 kerb adaptors, relatively evenly.


Overland Flow
The existing ground levels provide for adequate surface drainage and will not change. Proposed landscaping works will maintain
the exiting surface drainage outcome, i.e. the development will maintain minimum 1% surface grades to the street(s) to promote
sheet flow.
When the car park is resurfaced, a 450x450mm drainage pit shall be installed and connected to the kerb adaptor on Swan Street.
The sealed carpark surface will grade toward the drainage pit.
Furthermore, when the car park is resurfaced it should be raised above the surrounding area (nominal 100mm), which will prevent
surface run off entering the carpark.




Please do not hesitate to contact our office should you wish to discuss this further.




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                              Kerb Adaptor 1
                              ~15L/sec (Q100)
450x450 Grated Pit
in carpark surface,
location to be                                        Direct Blue roof
confirmed,                                            catchments to Kerb
connected kerb
adaptor 2.                                            Adaptor 1
                             Underground100mm
                             PVC Pipe to Steel Kerb
                             Adaptor                  Direct Red roof
                                                      catchment to Kerb
                                                      Adaptor 2
Underground100mm                                      (incl. carpark pit, 90m2)
PVC Pipe to Steel Kerb
Adaptor                                               Location and size of downpipes to be confirmed
                                                      with architect.

                                                      Downpipes to be 100mm minimum diameter

                                                      Alignment of underground pipes shown is
                                                      indicative only. These can be altered.

           Kerb Adaptor 2                             New car park surface to fall toward the new pit.
           ~23L/sec (Q100)
                                                      Open areas to grade at 1% toward the road(s).

                                                      Kerb Adaptors to be placed 1m offset from lot
                                                      boundary.

                                                      Kerb Adaptors to be steel