33522 Stormwater report018-01 SWQA issued 180704 · MCU18/0001

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CONTENTS PAGE




LIST OF TABLES

Table 1 - Developed Scenario WBNM Peak Flows ............................................................................................... 4
Table 2 – Required Infiltration Tank Volumes ..................................................................................................... 5
Table 3 – Required Infiltration Tank Size ............................................................................................................. 6



LIST OF FIGURES

Figure 1 – Location of Site .................................................................................................................................... 3




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    2261 david low way, peregian beach, qld 4573       acn: 626671092                                                [email protected]

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           INTRODUCTION

Bax Engineering has been commissioned by Michael and Judy Klobas to assess the existing stormwater
drainage system and provide a stormwater management plan for the proposed residential development as
shown on the Chris Clout Design plans C0.0 to 7.0; located at 8 Stevens Street, Sunshine Beach Queensland
(Lot 696 on SP279875). The plan is in response to the Noosa Council information request (MCU18-0001) which
requires the development to collect and discharge/infiltrate stormwater and ensure sufficient capacity is
provided in the existing infiltration system. The plan will also demonstrate the existing stormwater easement
along the eastern rear boundary is no longer required.

The site (as identified in Figure 1) covers an area of 506m2 and falls to the east at a slope of approximately 7%.
Any existing runoff from the site currently flows to an open swale and engineered grated drain formed as part
of the shared concrete driveway. Catchment flows are then diverted in to an underground infiltration tank
system via a field inlet pit. It is noted that any calculations have been based on the site area only and no
external catchments have been considered.




                                           Figure 1 – Location of Site




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              FLOW CALCULATIONS


The purpose of the management plan is to ensure the current infiltration system designed for the site is
adequate to contain flows generated by the proposed development. It will also assess whether the existing
overland flow path and existing stormwater easement (Easement B SP279875) are required.

It is expected the proposed development will generate up to approximately 90% of the site with an impervious
surface (Refer Appendix A). From the initial stormwater assessment (Aquaintel Report dated 3rd July 2014)
produced for the site; and on which the stormwater infiltration system was designed and constructed, an
impervious fraction of 70% was estimated.

Calculations generated from the WBNM model in the initial stormwater quantity assessment provided peak
flows for design storm event, with the 15-minute storm being noted as the critical duration. These flows have
been reproduced and are tabulated below in Table 1.


Table 1 - Developed Scenario WBNM Peak Flows

 Catchment             1yr ARI   2yr ARI     5yr ARI       10yr ARI      20yr ARI       50yr ARI        100yr ARI
                       (m3/s)    (m3/s)      (m3/s)         (m3/s)        (m3/s)         (m3/s)          (m3/s)
    Lot 696            0.016     0.021        0.027         0.030         0.034          0.037             0.041
(Aquaintel 03.07.14)


Discussions held with Noosa Council (and documented in the Aquaintel report) indicate that any development
needs to demonstrate that all but the most extreme rainfall events would be infiltrated and that any infiltration
tanks are to be sized to cater for all design events up to and including the 1% AEP (1 in 100-year ARI design
event).
With an increase in the impervious fraction to 90%, the original WBNM model was regenerated and provided
an increase in the peak flow for a 1% AEP design storm event from 0.041m3/s to 0.043 m3/s.




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              INFILTRATION TANK SIZING AND BACKGROUND


The Aquaintel report (2014) recommended that underground infiltration tanks be installed within the
property to minimise the required area for infiltration. These tanks were subsequently design and installed as
per the recommendations outlined in the report.

The original infiltration tank sizes were calculated based on geotechnical information provided by Tectonic
Geotechnical Pty Ltd and supplied in the Aquaintel report (2014). This information expected a saturated
hydraulic conductivity of k = 2.6x10-3 m/s (9216 mm/hr) and assumed that a groundwater table would be at
least 1.0m below the base of any infiltration system installed.

Two alternate methods of sizing the original infiltration tanks were adopted and compared. These methods -
The Hydrological Effectiveness Method and the Design Storm Method are outlined in Section 7 of “The WSUD
Technical Design Guidelines for South East Queensland” (2006).

The Hydrological Effectiveness Method sizes an infiltration trench based on the specified proportion of the
mean annual runoff volume that will infiltrate in to the soil profile. It brevity the methodology states that for
the geotechnical information provided, an infiltration tank 1.0m deep with an area equal to 1% of the
catchment will infiltrate 100% of the mean annual runoff.
The Design Storm Method sizes the detention capacity based on the difference between the volume of runoff
from the catchment during the design event and the volume that could be infiltrated during a rainfall event. It
requires the values of the saturated hydraulic conductivity of the insitu soil and the dimensions of the
proposed tank in order to complete the calculations.

The Aquaintel report (2014) adopted the Design Storm Method and generated volumes of proposed runoff
using the WBNM model for the 1% AEP event. This value based on the original 70% fraction impervious has
been reproduced below in Table 2. It is demonstrated that the Design Event Method provides a larger tank
size and was adopted for design and construction.


Table 2 – Required Infiltration Tank Volumes

 Catchment             Area (ha)                                   Volume
                                   Hydrological Effectiveness Method         Design Event Method
                                                      3
    Lot 696             0.0506                  5.06m                                6.36m3
(Aquaintel 03.07.14)


With an increase in the impervious fraction up to 90%, the original WBNM model was regenerated and
provided an increase in the volume for the Design Event Method from 6.36m3 to 6.94 m3.




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The dimensions of the constructed infiltration system using Atlantis Flo-Tank (Matrix Triple Modules) were
also provided in the Aquaintel report. This system comprises of 18 modules connected in 2 rows of 9 modules
formation. The dimension of each cell used is 408mm wide x 685mm long x 1310mm high, this provides a
volume of 0.366m3 per cell.

Using the Atlantis Flo-tank dimensions, the capacity to accommodate the original flows and the increased area
of impervious were calculated using the Design Event Method volume. The results are shown in Table 3.


Table 3 – Required Infiltration Tank Size

          Required Volume                   Existing Tank Properties (18 modules)
                                                                                         Acceptable Capacity
        (Design Event Method)               Surface Area            Volume
         Original
                              6.36m3           4.9m2                6.59m3                          Yes
 (70% impervious fraction)
        Proposed
                              6.94m3           4.9m2                6.59m3                          No
 (90% impervious fraction)



It is demonstrated that the increase in impervious area would increase the runoff volume calculated using the
Design Event Method. This increase in volume is larger than the available capacity of the infiltration system
currently installed.




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           OVERLAND FLOW PATH


An existing stormwater overland flow path is located along the rear eastern boundary of the property. From
the information provided in the Aquaintel report and the plans of the original project design (Greenhaven
Projects No. GH2019/14; Noosa Council Approval REC14/0007 &OPW14/0253) this channel has been designed
to capture overland flows and channel them to the infiltration system via a letterbox opening in an existing
900x600 field inlet pit. It is expected that this drain along with the installed 2/150dia PVC stubs would capture
and direct the design storm events (up to and including the 1% AEP) in to the infiltration system.

With the proposed development reshaping the fall of the lot to numerous localised areas, it is intended to pipe
any runoff from the site via internal roof water systems to the infiltration system or street frontage. This would
negate the need for the cut-off drain along the rear of the property and extinguish the need for an easement
over the same.




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           RESOLUTIONS

                   Increase in Infiltration Volume

With an increase in runoff volume, the infiltration system would need to be increased to provide adequate
capacity. Additional Atlantis Matrix modules could be added to the site and linked to the existing infiltration
to achieve the additional capacity required. This additional volume of 0.35m3 could be achieved by installing
an additional module of 0.366m3 capacity. While the Atlantis Matrix systems are easily retrofitted, the location
of the existing system underneath the shared concrete driveway would need to be cut, removed and replaced
in order to make the change.

                   Diversion of Stormwater

The design of the proposed development provides the opportunity to divert some of the captured stormwater
to the street frontage. The reduction in catchment of 83m2 is shown in Appendix B.
By diverting this runoff via roof water pipes and kerb adaptors to the Stevens Street frontage, the catchment
being diverted to the infiltration system would be dramatically reduced from 433m2 to 350m2. This would
mean that the impervious area would reduce to under 70% (i.e. 350m2/506m2) and sufficient storage capacity
made available within the existing system.

                   Removal of Drainage Swale

With the proposed development reshaping the fall of the lot to numerous localised areas, it is expected that
the existing cut-off drain along the rear of the property is no longer needed. A roof water pipe system could
direct flows from the property to the infiltration system (and Stevens Street). It is expected minor adjustments
would be made to the existing 900x600 field inlet pit to suit the proposed driveway design.




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           CONCLUSION


Based on the calculations and report provided by Aquaintel (2014), the existing infiltration system would need
to be adjusted to accommodate the additional volume generated by the increase in impervious area of the
proposed dwelling. However, if the fraction of impervious was to reduce to a percentage less than 70%, then
the volume calculated would be adequate.
It is recommended as part of the proposed construction to divert a portion of the proposed roof water
catchment to the Stevens Street frontage. This reduction in impervious area will ensure the capacity of the
infiltration system is not exceeded and also reduce the loading on this system in major events.

The design of the proposed dwelling dramatically changes the runoff characteristics of the lot and would
negate the need for an existing catch drain (and easement over the same) along the eastern boundary. It is
recommended that the catch drain and easement (as shown in Appendix C) be removed and replaced with a
roof water pipe system to direct all runoff (other than those being diverted to the Stevens Street frontage) to
the infiltration system. The runoff will enter the system via the existing 900x600 field inlet located in the
driveway. Any adjustments made to this field inlet are not to include the existing invert levels or the connected
grated drain levels.




Glenn Ainsworth
CPEng MIEAust RPEQ (19204)
Bax Engineering Pty Ltd




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          REFERENCES

Aquaintel, 2014. 8 Stevens Street, Sunshine Beach - Stormwater Quantity Assessment, s.l.: Aquaintel Pty Ltd.
Healthy Waterways, 2006. The Water Sensitive Urban Design Technical Design Guidelines for South East
Queensland, Brisbane: Brisbane City Council and Moreton Bay Waterways.




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APPENDIX A – IMPERVIOUS AREA OF PROPOSED DEVELOPMENT

Chris Clout Design – Preliminary Concept C0.0 (Marked up Bax Engineering)

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APPENDIX B – REDUCTION IN IMPERVIOUS AREA

Chris Clout Design – Preliminary Concept C2.0 (Marked up Bax Engineering)

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APPENDIX C – EXISTING CATCH DRAIN AND EASEMENT

Greenhaven Projects – Roadworks and Drainage Detail Plan GH2019/14/02 Rev. A (Marked up Bax
Engineering)

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EXACT LOCATION, SIZE, AND ARRANGEMENT OF
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‘SUBJECT TO AQUAINTEL APPROVAL.

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1: 250-03,
: Greenhaven Prose cuir BRUCE LANGFORD JONES _ COUNCIL REFERENCE:
Fee OT ie AEN OT HO. PROuECT. PROPOSED 3 LOTS INTO 4 LOTS SUBDIVISION RECIA007 & OPW40265
‘ _ P.0. Box 8200, Maroochydore | 2. OLD 4388 2 3 ey an 8 STEVENS STREET, SUNSHINE BEACH SAE e Wee
: Tuas ruauiscony pnorecreD By oneeiauwenphovecrsery..ro. | MENWIEE Chace: <P owe RUE... LOT 676 ON SP128686 & LOTS 6758657 ON RABI | perme UY
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