2.0 STORMWATER CONCEPT DESIGN 2.1 Overland Flow Paths …

SCECGS Redlands, Cremorne 141531 SCECGS Redlands, Cremorne, Concept Proposal and Stage 1 Development Application
Taylor Thomson Whitting (NSW) Pty Ltd Page 9 © 2015 Taylor Thomson Whitting
2.0 STORMWATER CONCEPT DESIGN
2.1 Overland Flow Paths and Flooding
We have reviewed North Sydney Council 1m Contour Maps. The maps indicate that the site is located near the top of a ridge falling at approximately 4% grade from Military Road toward the northwest.
Figure 3 – North Sydney Council 1m Contour Map with Upstream/ Downstream Catchment
Detailed topographic survey of the site prepared for this project indicates a drainage easement in favour of North Sydney Council extending across the site from the north end of Monford Place to the intersection of Waters Road and Gerard Street near the northwest corner of the site as schematically shown in Figure 3. Our initial investigation suggests the existing pipe size is a 375mm diameter reinforced concrete pipe (RCP) starting in Monford Street cul-de-sac.
Our initial analysis shows the existing 375mm pipe has less than the 100-year average recurrence interval (ARI) capacity (approximately 5-year ARI capacity subject to our detailed analysis).
Figure 4 – Topographic Survey and Drainage Easement Location
The proposed development scheme requires this existing easement/ pipe to be relocated.
TTW initially considered four stormwater diversion options sized to cater for the 100-year ARI storm event including inlet capacity in Monford Place due to lack of overland flowpath through the site. These options were reviewed with North Sydney Council at a meeting held on the 20
th January 2015.
2.2 North Sydney Council Stormwater Diversion Consultation
TTW met with Vesna Ristic, Development Engineer Assessments for North Sydney Council on 20 January 2015. Ms. Ristic provided the following comments on the proposed Council stormwater pipe diversion options, overland flow path and easement:
• Pipe to be sized for 1 in 20 year storm event, or minimum 375mm
• Additional pits may be required in Monford Place to accommodate 1 in 20 ARI storm event
• Minimum 1% grade along pipe
• Provide overland flow path to direct water around the building in the 1 in 100 year ARI storm event
• All changes in pipe direction shall occur at junction pits
• All new pits shall be accessible from the surface
• It is preferable that pipes are not suspended
Comments were provided verbally in the meeting and via email dated 3 rd February 2015.
Following the meeting with Council, TTW proposed two additional diversion options to the project team in order to comply with the Council requirements.
See drawings in Appendix A the proposed schematic design layout showing a 2.5m easement over the proposed stormwater pipe diversion (sized for the 1:20 year ARI storm event) and a 1.6m easement over the propose overland flow diversion (sized for the 1:100 year ARI storm event).
2.3 Stormwater Detention
The North Sydney Council DCP (Section 3.5.7.P5) requires On-Site Detention (OSD) to limit the post-development discharge rates to less than pre-development discharge rates. DRAINS modelling was used to demonstrate that the requirement can be met by increasing pervious area on the site from the existing condition. Thus, OSD is not required.
2.3.1 Stage 1
A catchment area for the site (excluding the Adams Centre) was calculated to be 14,930 m 2
based on an architectural scheme with green roof areas. The existing impervious fraction for this area is 87.3%.
The proposed Stage 1 development incorporates green roof and ground level landscaping areas that reduce the site impervious fraction to 81.5%. Therefore, stormwater flows from the site are reduced from existing flows as shown in the table below.
Table 1 Stage 1 Discharge Rates
Average Recurrence
1-year 386 370
2-year 512 497
5-year 685 670
10-year 732 718
20-year 917 903
50-year 977 963
100-year 1090 1080
Considering that the proposed development with green roof is reducing impervious area, site discharge rates will be less than the pre-development peak flow rates for all storms up to and including the 100-year ARI storm events; hence OSD is not required.
2.3.2 Master Plan
A catchment area for the site was calculated to be 14,930 m 2 . The existing impervious
fraction for this area is 87.3% and the proposed impervious fraction following completion of all stages of Master Plan Construction is 85.6%.
With the reduction in impervious area from existing conditions to the Master Plan design, stormwater flows from the site are reduced from existing flows as shown in Table 2.
Table 2 Master Plan Discharge Rates
Average Recurrence
1-year 386 381
2-year 512 508
5-year 685 681
10-year 732 728
20-year 917 913
50-year 977 972
100-year 1090 1090
Considering that the proposed development is not increasing the existing impervious area the site’s discharge rates will be less than the pre-development peak flow rates for all storms up to and including the 100-year ARI storm events hence OSD is not required.
2.3.3 Adams Centre
Similar to the proposed development on the north side of Military Road, the proposed development on the Adams Centre parcel does not significantly increase impervious area. Therefore, on-site detention to serve the Adams Centre is not proposed.
2.4 Stormwater Retention
The DCP (Section 3.5.7.P4) encourages stormwater retention and reuse for appropriate end use including toilet flushing, water features, car washing and garden irrigation.
2.4.1 Stage 1
A 60m 3 rainwater reuse tank is proposed for the Stage 1 redevelopment. Stormwater
collected from the new building roof will be reused for landscape irrigation.
2.4.2 Master Plan
The 60m 3 rainwater reuse tank proposed for Stage 1 will remain in place throughout the
Master Plan development with additional stormwater collection pits and piping as required.
2.4.3 Adams Centre
Rainwater reuse is not proposed for the Adams Centre redevelopment as the proposed improvements do not significantly change the impervious make-up of this portion of the school.
2.5 Water Sensitive Urban Design
In order to comply with the DCP (Section 3.5.7.P11), the development must achieve the stormwater quality targets outlined in Table 3. The DCP encourages green roofs, minimum 50% pervious pavements, grass swales and rainwater tanks to reduce runoff and meet stormwater quality pollutant reduction targets.
Table 3 Stormwater Quality Targets
POLLUTANT RETENTION CRITERIA
Litter and vegetation larger than 5mm 90% reduction compared to the Baseline Annual Pollutant Load
Total suspended solids 85% reduction compared to the Baseline Annual Pollutant Load
Total phosphorus 65% reduction compared to the Baseline Annual Pollutant Load
Total nitrogen 45% reduction compared to the Baseline Annual Pollutant Load
2.5.1 Stage 1
Stormwater quality modelling in the MUSIC software program indicates that a treatment train including 310m
2 green roof, 60m
3 rainwater reuse tank, trash screen, and proprietary
stormwater treatment devices, such as a SPEL Stormceptor and 4 SPEL Filters can be utilized to meet stormwater quality requirements for the Stage 1 redevelopment.
Figure 5 – Stage 1 MUSIC Model Layout and Results
2.5.2 Master Plan
Stormwater quality modelling in the MUSIC software program indicates that a treatment train including 310m
2 green roof in Stage 1, 60m
3 rainwater reuse tank, trash screen, and
proprietary stormwater treatment devices, such as a SPEL Stormceptor and 6 SPEL Filters can be utilized to meet stormwater quality requirements for the Master Plan redevelopment.
Figure 6 – Master Plan MUSIC Model Layout and Results
2.6 Sediment, Erosion and Dust Controls
Construction works are to be carried out in accordance with the “Blue Book” erosion and sediment control requirements. The controls for each Stage will depend on construction staging and methodology, but will most likely include sediment fences, sandbags around pits and a vehicle wash down. An erosion and sediment control plan has been prepared as part of the schematic design drawings for Stage 1. Similar erosion and sediment control measures will be required in subsequent stages of the Master Plan redevelopment.
2.7 Schematic Stormwater Design
We have prepared schematic stormwater design for the redevelopment showing the layout and level of stormwater pits and piping, on-site detention, rainwater reuse and water quality treatment systems. The stormwater schematic design plan is shown in Appendix A of this report.
3.0 CONCLUSION
Stormwater infrastructure has been designed to provide stormwater quantity and quality outcomes to meet the stormwater quality targets outlined in the DCP and North Sydney Council Performance Guide for Engineering Design and Construction.
Prepared by: Authorised by:
Heather Spencer Stephen Brain
P:\2014\1415\141531\Reports\TTW\150514 Redlands Stormwater Report.doc
APPENDIX A
NTS WW SB
SKC210 STAGE 1 - EROSION & SEDIMENT CONTROL PLAN
SKC230 STAGE 1 - SITEWORKS PLAN - GROUND LEVEL
SKC240 STAGE 1 - DETAILS SHEET 1
SKC241 STAGE 1 - DETAILS SHEET 2
NOTES AND LEGENDS SHEET
PIT SCHEDULE
Note:
C241
A1
48 Chandos Street St.Leonards NSW 2065
T: +61 2 9439 7288 F: +61 2 9439 3146 [email protected]
Consulting Engineers
Taylor Thomson Whitting (NSW) Pty Ltd A.C.N. 113 578 377
141531
Architect Sheet SubjectProject
Rev Description Eng DateDraft Rev Description Eng DateDraft
This drawing is copyright and is the property of TAYLOR THOMSON
WHITTING (NSW) Pty Ltd and must not be used without authorisation.
THIS DRAWING TO BE READ IN CONJUNCTION WITH ALL
RELEVANT NOTES ON DRAWING S0001
STAGE 1
RL 79.49 (AHD)
NOTE: UNDERGROUND ELECTRICITY CABLES
NOTE: UNDERGROUND ELECTRICITY CABLES
TOTAL AREA 1.493Ha
SEDIMENTATION TRAP
48 Chandos Street St.Leonards NSW 2065
T: +61 2 9439 7288 F: +61 2 9439 3146 [email protected]
Consulting Engineers
Taylor Thomson Whitting (NSW) Pty Ltd A.C.N. 113 578 377
141531
Architect Sheet SubjectProject
Rev Description Eng DateDraft Rev Description Eng DateDraft
This drawing is copyright and is the property of TAYLOR THOMSON
WHITTING (NSW) Pty Ltd and must not be used without authorisation.
THIS DRAWING TO BE READ IN CONJUNCTION WITH ALL
RELEVANT NOTES ON DRAWING S0001

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2.0 STORMWATER CONCEPT DESIGN 2.1 Overland Flow Paths …