Attachment C - Stormwater Mangement Plan

Page 2

EXECUTIVE SUMMARY


Marshall Civil Engineering was commissioned by Mr Brian Williams to prepare a Site
Based Stormwater Management Plan (SBSMP) to support the proposed minor change
to an existing approved DA 51987.2966.04 for a Residential Development at Lot 2 on
GTP 1928, 396 Lake Cooroibah Rd, Cooroibah, Qld, 4565


To satisfy alternative stormwater quality management measures, this development will
incorporate a 23kL of rainwater storage per pair (x 2) habitable Glamping Tent
dwellings, for use as both potable water supply and also for landscape irrigation and
other external (common property) uses.


This report outlines the intended stormwater strategy to be adopted on site to achieve
water quality objectives and achieve non-worsening peak flow rates from the proposed
development.


The report is to be read in conjunction with the Engineering and Architect drawings
accompanying the change application.




 Revision: B – 2nd August 2021
                                                                                   2

Page 3

TABLE OF CONTENTS



1.0    INTRODUCTION

2.0    NATURE BASED STORMWATER MANAGEMENT TECHNIQUES TO ENSURE
       NO OFF-SITE STORMWATER RELATED IMPACTS

3.0    PURPOSE AND SCOPE OF THE REPORT

4.0    SITE DESCRIPTION

5.0    PROJECT DESCRIPTION

6.0    PROPOSED INCREASE IN IMPERVIOUS AREA DUE TO DEVELOPMENT:

7.0    WATER HARVESTING TO PROVIDE POTABLE SUPPLY:

8.0    ADDITIONAL DRAINAGE INFORMATION:

9.0    CONCLUSIONS




APPENDICES

APPENDIX A: - Site Locality Plan


 Revision: B – 2nd August 2021
                                                                  3

Page 4

1.0    INTRODUCTION:

This Site Based Stormwater Management Plan (SWMP) has been prepared by
Marshall Civil Engineering in support a minor change to the existing Material Change
of Use which was approved by Noosa Shire Council as DA 51987.2966.04 and issued
in May 1987.

The location and existing details of the land are shown in APPENDIX A.

This document outlines a “nature based” approach to stormwater quantity associated
with the development on this site and provides a Management Plan to ensure that
there will be no off-site stormwater impacts.

2.0    UTILISING NATURE BASED STORMWATER MANAGEMENT TECHNIQUES
       FOR STORMWATER CAPTURE/RE-USE TO ENSURE NO STORMWATER
       RELATED OFF-SITE IMPACTS
       1. Background

       Pollution via stormwater (including the amount of nutrient content) is becoming
       increasingly important as development continues to put increasing pressure on
       our extremely important waterways which are already under stress. The
       importance of sustainable water management, including Water Sensitive Urban
       Design (WSUD), is becoming more widely recognised by both Councils and
       communities alike.
       Over recent years Noosa Council has already embraced a number of WSUD
       based developments and these present to the public as an honest and pleasing
       attempt to introduce nature based stormwater systems to the developed world.

       2. What is Water Sensitive Urban Design?

       Water Sensitive Urban Design (WSUD) integrates urban water cycle
       management with urban planning and design, with the aim of mimicking natural
       systems to minimise negative impacts on the natural water cycle and receiving
       waterway the Noosa River which in turn flows into Laguna bay.
       It also offers an alternative to the traditional capture and conveyance approach
       to stormwater management by acting at the development site (at the source),
       and thereby reducing the required size of any required structural stormwater
       system.
       It seeks to minimise impervious surfaces, re-use water on site, and incorporate
       nature based (Vegetated Infiltration & Level Spreader Swales) treatment



 Revision: B – 2nd August 2021
                                                                                    4

Page 5

      systems to remove pollutants. WSUD also provides the opportunity to achieve
      multiple benefits though sustainable urban water management.
      The use of WSUD principles into new developments helps to protect and
      enhance natural water systems within urban environments by utilising different
      ideas, including:
             Integrating stormwater treatment into the landscape, maximising the
              visual and recreational amenity of developments.
             Improving the quality of water draining from developments into receiving
              environments.
             Reducing runoff and peak flows from rural developments by increasing
              local detention times and minimising impervious areas.
             Minimising drainage infrastructure costs of development due to reduced
              runoff and peak flows.

      3. Why use Water Sensitive Urban Design?

      Stormwater is the water that runs off our urban surfaces following rainfall
      events. It has been identified as a key cause of pollution and declining health
      of our waterways. With increased development, the proportion of impervious
      surfaces in our catchments increases. This increases the velocity and amount
      of water running into our waterways, creating problems of erosion (including
      sediment control) and flooding and changing natural flow regimes, with
      associated ecological damage. It also washes more pollutants into our streams
      and rivers, resulting in a degradation of our river health.
      WSUD has been identified as a means to control flows and filter stormwater to
      remove pollutants. It offers the potential to reduce the costs, infrastructure
      sizing and occupied land area associated with conventional drainage
      approaches whilst treating runoff closer to its source. This more effectively
      mimics a natural system and, as treatment can be located further upstream
      than for conventional approaches, is efficient by providing flow-on effects that
      benefit the entire catchment. WSUD is also an important component of
      integrated water management, and can contribute to multiple benefits such as
      enhancing liveability.

      4. Proposed Nature Based Stormwater Capture/Re-use then Local Landscaping
         Irrigation to Natural Overland Flows for any un-used component.

      All stormwater flowing from Buildings will be harvested into local rainwater tanks
      for re-use. Any overflow from these tanks will flow into a Vegetated Infiltration
      & Level Spreader Swales, which will form part of the designed landscape
      vegetation to beautify the development and helping provide the Nature Based


Revision: B – 2nd August 2021
                                                                                     5

Page 6

          design which is at the heart of the overall design concept for this Nature Based
          Development.
          The only other impervious surface proposed on this Lot is the required on-site
          parking. These parking areas will be graded so that all stormwater flows over
          flush kerbing into the adjacent existing Vegetated swales which form part of the
          Landscaping for the Development.

          5. Conclusion

          High quality urban design must contribute positively to communities, enhance
          liveability, reflect cultural identity, and promote attractive and high amenity
          communities.
          The aim of any sustainable water quality and re use policy is to “protect the
          health of ecological systems and the biodiversity they support” and also to
          “conserve areas with identified environmental and landscape values” and this
          process is enhanced by encouraging the use of alternative water sources.


3.0       PURPOSE AND SCOPE OF THE REPORT:
The purpose of this Nature Based Stormwater Management Plan is to evaluate
existing and potential stormwater impacts of the proposed development against the
requirements of the Noosa Shire Council Panning Scheme.

In particular this Stormwater Management Plan will ensure that:

         Appropriate standards are maintained on all stormwater aspects within the site.
         The surrounding area is not adversely affected nor unduly disrupted by the
          quantity for quality of stormwater during construction, operations and
          maintenance.

4.0       SITE DESCRIPTION:

Our client owns Lot 2 of a 3 Lot subdivision. Lot 0 is the Common Property under the
control of the Body Corporate and Lot 1 is another Lot facing the Lake and contains a
small number of Cottages.

This application proposes a minor change to the approved DA 51987.2966.04 to allow
the approved dwellings (x19) approved for Lot 2 to be changed and become Glamping
Tents (x 18) constructed on permanent elevated timber deck structures.

The proposed Caretakers Residence will be retained and built as a conventional
dwelling structure.



 Revision: B – 2nd August 2021
                                                                                       6

Page 7

This Stormwater Management Plan covers the proposed minor change to the existing
DA 51987.2966.04 approval with the changes limited to Lot 2 and the existing
infrastructure already proposed to be constructed on Lot 0, Body Corporate land will
remain unchanged.

A Locality Plan is attached as APPENDIX A.

The subject land is best described as flat/gently sloping sandy loam cleared rural
residential land having no present drainage issues. Currently stormwater runoff from
the site sheet flows across the site following the contours of the land and flows towards
Lake Cooroibah.

5.0    PROJECT DESCRIPTION:

This minor change to the approved DA 51987.2966.04 for Lot 2, seeks approval for a
revised layout showing 18 x Glamping Tents, a Caretaker residence (including a
separate Shed) and associated out buildings such as the Recreation (BBQ) centre
and distributed rainwater tanks to harvest rainwater from all structures for re-use.

Every building or dwelling will harvest the rainwater to be used as potable water on-
site (to provide the potable water of each dwelling), this stormwater harvesting and
subsequent consumption will act as a balance to the increase in impervious area on
this Lot.

It is proposed that rainwater harvesting tanks will be provided on the basis of one 23kL
tank being shared between 2 Glamping Tent structures and located generally between
the structures within a landscaping area which will also incorporate a Vegetated
Infiltration & Level Spreader Swale which will receive the overflow once the tank is full.
See the Architect Site Plan drawing which shows the proposed detail of Tent/structure
location, the tank location together with the associated landscaping and vegetated
infiltration level spreader swale.

This simple change to the approved use for Lot 2, will not result in the construction of
any new access streets being required as access will be provided via the existing
driveway from Cooroibah Rd to the proposed parking areas located on Lot 2 but
accessed from Lot 0 (Common Property Land).

The total increase in impervious area caused by the development is only 1,823.2m2
which represents only a small 11.4% increase in the total impervious area for Lot 2.




 Revision: B – 2nd August 2021
                                                                                       7

Page 8

6.0    PROPOSED INCREASE IN IMPERVIOUS AREA DUE TO DEVELOPMENT:
       1. Total site area of Lot 2 = 15,997m2

       2. Proposed increase in impervious area
           Car Park A.                                =       110.4m2
           Car Park B.                                =       87.4m2
           Car Park C.                                =       87.4m2
           Caretakers Residence                       =       194m2
           Caretakers Shed = 12 x 10                  =       120m2
           Recreation Shelter = 12 x 12               =       144m2
           18 x Glamping Tents = 18 x 12.5 x 4.8      =       1080m2
           Total Proposed Impervious Area (m2) =              1,823.2m2
           Total Proposed Impervious Area (%) =               1823.2/15997 = 11.4%

7.0    WATER HARVESTING TO PROVIDE POTABLE SUPPLY:
This proposed development in Cooroibah is located just 12.5km North East from the
township of Cooroy. Cooroy enjoys an annual rainfall of 1.5m per year (Source Noosa
Shire Council Water Department June 2007).
For each pair of Glamping Tents, all rainwater from each tent roof will be harvested
and directed into the adjacent 23kL rainwater tank. During the year that 1.5m rainfall
will translate into 2 x 12.5 x 4.8 x 1.5 = 180m3 = 180kL ------------------------------(1)
This equates to 180/23 = 7.8 tank fills of water each year.
From Unity water consumption figures, the average daily consumption for the
Sunshine Coast is 150L/person/day.
Using conservative occupancy estimates we get the water demand created for each
pair of Glamping Tents (1 x 23kL tank)
       Week Ends 90% Occupancy = 0.9 x 52 x 2 x 2 = 187.2 guest days/tent
       School Holidays (mid week) 90% Occupancy = 0.9 x 10 x 5 x 2 = 90.0 guest days/tent
       Bal of mid week @ 40% Occupancy = 0.4 x 42 x 5 x 2 = 168.0 guest days/tent

Total Guest Occupancy Day per Tent = 351.6 so 2 tents = 445.2 x 2 = 890.4 guest days/tank (pair of
tents)
So estimated water consumption becomes 890.4 x 150L/person/day = 133,560L = 133kL ---------(2)

When the annual water consumption of 133kL is subtracted from the harvest of 180kL
this only leaves an excess of 47kL and the overflow Vegetated Infiltration & Level
Spreader Swale has a capacity in excess of 22kL, meaning a small volume will top
and become overland flows.


 Revision: B – 2nd August 2021
                                                                                               8

Page 9

8.0    ADDITIONAL DRAINAGE INFORMATION:
The drainage plan includes the following information:


1.0 Total catchment area.

2.0 Existing drainage services.

3.0 Layout of proposed drainage systems.

4.0 Natural surface contour lines to the AHD.

       1.0 Total catchment area
       The total catchment area for Lot2 is 1.6ha.

       2.0 Existing drainage services
       As shown in Appendix A: Site Locality Plan, the only drainage services currently
       provided are 2 vegetated North-South swales that are located close to the
       boundary on the East and West of Lot 2.
       These swales act to cut off any water external to Lot 2 from entering the Lot
       and the flows collected are directed to a non-formed drainage path within the
       Body Corporate land in Lot 0 to the South of Lot 2.

       3.0 Layout of proposed drainage systems
       Following the construction of the total number of dwellings (Caretakers
       Residence plus 18 Glamping Tents and associated roofed infrastructure such
       as BBQ facility on Lot 2, this will require harvesting of roof water into shared
       rainwater tanks to provide the potable water supply for each dwelling. This
       harvesting activity will also perform a stormwater detention function. Once the
       tanks are full, such as during a sustained rainfall event, the overflow from the
       tanks will discharge into a vegetated stormwater infiltration and level spreader
       swale which will be installed generally between 2 constructed Glamping Tents.
       These swales will provide irrigation to the vegetation and any excess water
       which does not soak into the sandy loam soil during the larger or longer rainfall
       events will overflow the swale and resume sheet/overland flow, similar to what
       occurs pre-development.
       It is contended that the construction of the approved dwellings (including other
       approved buildings) and the associated water harvesting and stormwater
       overflow arrangements described above will in fact result in a "Non-Worsening"


 Revision: B – 2nd August 2021
                                                                                     9

Page 10

       of the currently existing stormwater runoff on each Lot prior to the construction
       of any dwellings.

       4.0 Natural surface contour lines to the AHD
       The contour lines are shown in Appendix A: Site Locality Plan.

9.0    SBSMP CONCLUSIONS:

This proposed development of 18 Glamping Tents (dwellings), the Caretakers
residence (including shed out building), the Recreation area and the 3 associated
Parking areas on this Lot 2 will actually improve the stormwater performance of the
land or will certainly not lead to a worsening of the existing stormwater performance
of Lot 2 when compared to the pre-developed state.

Once the 23kL rainwater tanks shared between 2 Glamping Tent dwellings are full
(such as during major or long rainfall events), the overflow will discharge into the
associated “Vegetated Infiltration & Level Spreader Swale” which is incorporated into
the landscaping for the site. When these vegetated swales overflow, the resultant
stormwater flows will follow the contours of the land, thus replicating the sheet flow
situation similar to how stormwater flows across the land once it is saturated and
before any dwellings or other impervious surfaces were constructed.




Ian J. Marshall JP (Qual), MIEAust, CPEng, NER (Civil), RPEQ

CIVIL ENGINEER

E:             [email protected]
Mobile:        0421 044 884
RPEQ:          15254



APPENDIX A: SITE LOCALITY PLAN




 Revision: B – 2nd August 2021
                                                                                    10