Attachment 2 Hydraulic Assessment (SLR Consulting)€¦ · Attachment 2 – Hydraulic Assessment (SLR Consulting) Hydraulic Study 90 Babbage Road Roseville Chase Report Number 610.12418

Ku-ring-gai Council Bushfire, Hydraulic, Riparian and Ecology Studies for 90 Babbage Road, Roseville Chase

27 March 2013 610.112418 90 Babbage Road, Roseville

Chase 20130327.docx

SLR Consulting Australia Pty Ltd

Attachment 2 – Hydraulic Assessment (SLR Consulting)

Hydraulic Study 90 Babbage Road

Roseville Chase

Report Number 610.12418

20 March 2013

Ku-ring-gai Council

818 Pacific Highway,

Gordon NSW 2072

Version: Revision 1

Ku-ring-gai Council Hydraulic Study 90 Babbage Road Roseville Chase

Report Number 610.12418 Revision 1

26 March 2013 Page 2


Hydraulic Study 90 Babbage Road

Roseville Chase

PREPARED BY:

SLR Consulting Australia Pty Ltd ABN 29 001 584 612

2 Lincoln Street Lane Cove NSW 2066 Australia

(PO Box 176 Lane Cove NSW 1595 Australia)

T: 61 2 9428 8100 F: 61 2 9427 8200

E: [email protected] www.slrconsulting.com

This report has been prepared by SLR Consulting Australia Pty Ltd

with all reasonable skill, care and diligence, and taking account of the

timescale and resources allocated to it by agreement with the Client.

Information reported herein is based on the interpretation of data collected,

which has been accepted in good faith as being accurate and valid.

This report is for the exclusive use of Ku-ring-gai Council .

No warranties or guarantees are expressed or should be inferred by any third parties.

This report may not be relied upon by other parties without written consent from SLR Consulting.

SLR Consulting disclaims any responsibility to the Client and others in respect of any matters outside the agreed scope of the work.

DOCUMENT CONTROL

Reference Status Date Prepared Checked Authorised

610.12418 Final 26 March 2013 Peter Gillam Will Legg Peter Gillam

610.12418 Final Draft 20 March 2013 Peter Gillam Will Legg Peter Gillam

610.12418 Draft 1 27 February 2013 Peter Gillam Will Legg Peter Gillam


Report Number 610.12418 Final


Table of Contents


1 INTRODUCTION AND BACKGROUND 5

2 MODELLING APPROACH 6

2.1 Hydrologic Modelling 6

2.2 Hydraulic Modelling 6

2.2.1 TUFLOW Model Set Up 7

2.2.2 Design Flood Standard 8

2.2.3 Hypothetical Development 8

3 90 BABBAGE ROAD FLOOD IMPACTS 9

3.1 Existing Site Characteristics 9

3.2 Design Flood Standard for the Existing Site 9

3.2.1 Flood Mapping for 90 Babbage Road 9

3.3 Flood Impacts of Hypothetical Development 13

3.4 Summary of Findings for 90 Babbage Road 14

APPENDIX A - DRAINS MODEL PARAMETERS 16

APPENDIX B - SITE CONSTRAINTS AND DEVELOPMENT CONTROLS FOR 90 BABBAGE ROAD 17




Table of Contents


FIGURES

Figure 1 Stormwater catchment and flood model domain for Babbage Road 7

Figure 2 Schematic representation of 1D-2D flood modelling domains in TUFLOW 8

Figure 3 Flood hazard definitions as specified in the NSW Floodplain Management Manual 10

Figure 4 Existing 100 year ARI flood extent and flood level contours at 90 Babbage Road 11

Figure 5 Existing flood hazard mapping for 90 Babbage Road after development 12

Figure 6 Existing 100 year ARI flood extent and flood level contours at 90 Babbage Road 13

Figure 7 Existing flood hazard mapping for 90 Babbage Road after development 14





1 INTRODUCTION AND BACKGROUND

Council is seeking to rezone 90 Babbage Road and requires information on drainage, riparian corridor, ecological and bushfire constraints within the sites through rigorous assessments.

Hydraulic modelling has been carried out to determine flood constraints and whether the site is suitable for rezoning.

Modelling has also been used to determine design flood standard as specified in Chapter 7 of Council’s Development Control Plan and to test the impacts development of flood levels in neighbouring lots.





2 MODELLING APPROACH

The floodplain comprises residential development crossed by drainage easements and poorly defined overland flow paths. Several roads intercept and divert overland flows to Babbage Road and these were considered when determining how much overland flow arrives at the site.

The characteristics of flooding in the Babbage Road catchments were determined using DRAINS hydrologic and TUFLOW hydraulic modelling software to determine peak flow rates, stormwater pipe capacities and overland flooding depths and velocities.

Modelling was also used to determine the changes in flood patterns resulting from a hypothetical residential development on 90 Babbage Road.

The adopted modelling approach extends the area of the investigation to the adjacent catchments to ensure that modelling accounts for any flood flows from adjacent overland flow paths.

2.1 Hydrologic Modelling

Hydrologic models were established to determine critical 100 year ARI flood hydrographs across the study catchments.

Catchment boundaries were delineated from 0.5m contour data and the imperviousness of each catchment was determined by inspecting aerial photography.

Flood hydrographs were produced at the hydraulic model boundaries and at critical locations within the floodplain.

DRAINS parameters were selected after reviewing existing flood modelling undertaken for Council and critical storm rainfall intensities, temporal patterns and soil store data are presented in Appendix A.

2.2 Hydraulic Modelling

A combined 1D-2D TUFLOW model was developed for both study areas. The extent of the TUFLOW models developed for each site is shown in Figure 1 below.

This modelling approach is best suited to the study area as it accounts for the capacity of stormwater pipes networks and complex overland flow paths and simulates:

the interchange of stormwater between overland flow paths and stormwater pit and pipe networks;

the timing, arrival and interaction of overland flooding from different regions in the catchment; and

the effect of floodplain storage and altered flow paths resulting from changes in the floodplain.





Figure 1 Stormwater catchment and flood model domain for Babbage Road

2.2.1 TUFLOW Model Set Up

The topography of the study area was defined using ALS data provided by Council. All trees, houses and features other than the ground surface were removed from the data set.

Some minor adjustment was required to improve the definition of floodplain features.

A regular grid was used to represent the topography of the study area in the 2D TUFLOW model adopting a 4m grid.

Stormwater pit and pipe network data was provided in the form of DRAINS models and this was checked in the field by measuring the depths of stormwater drainage pits. Some modifications of the pit inverts was required.





The pit and pipe stormwater drainage network was represented using 1D elements nested within the 2D model domain. This allows the free interchange of water between the 1D and 2D model domains to simulate the interaction between pipes, pits and the floodplain as represented in Figure 3 below.

Figure 2 Schematic representation of 1D-2D flood modelling domains in TUFLOW

Overland flow paths through residential dwellings were simulated using a high manning’s roughness value to account for obstructions created by buildings and fences. Catchment roughness values for other floodplain features were adopted as follows

Table 1 Manning’s n roughness for each floodplain land use

Development Type Catchment Roughness

Residential lots 0.10

Roads 0.025

Grassed reserves 0.035

Bush land reserves 0.08

2.2.2 Design Flood Standard

A design flood standard for each site was established for the 100 year ARI storm event at each site. Stormwater pipes were simulated as operating with 50% blockage. The TUFLOW software does this by modelling each pipe as having 50% of the actual pipe diameter.

Flood extent maps, flood depths and flow velocities for the 100 year ARI storm event at each site were produced in GIS mapping layer formats..

2.2.3 Hypothetical Development

The potential flooding impacts of rezoning the 90 Babbage Road sites was tested by simulating the increased catchment roughness associated with a dwelling and landscaping on each site.

1D

2D





3 90 BABBAGE ROAD FLOOD IMPACTS

3.1 Existing Site Characteristics

The site is covered in a mixture of manicured grasses and rock outcrops. The north and eastern fringes of the site are boarded by native and exotic ground covers, trees and shrubs which would act to obstruct overland flows in a storm event.

Council’s records show that this pipe discharges near the boundary of 90 and 92 Babbage Road, however the pipe headwall was observed within 10m of the boundary of 92 and 94 Babbage Road.

3.2 Design Flood Standard for the Existing Site

A TUFLOW model was established to predict the movement of floodwaters across the study area between the upper and lower sections of Babbage Road as shown in Figure 2 above. The critical 100 year ARI storm duration for the site was found to be the 20 minute duration event.

The model predicts that during this event, a broad overland flow path forms along Babbage Road, with flows approaching the site from the south west and from the west. Flood waters enter the upper portion of the site as overland flow from neighbouring properties. The lower portion of the site is also affected by flows delivered to the 92 Babbage Road by an existing stormwater pipe.

The existing site has been given a Manning’s roughness value of 0.035. A sensitivity assessment was carried out by adopting a Manning’s n o f 0,08 which reflects the natural tendency for vacant sites to revert to bushland if un kept for a period of months. The flood hazard was found to be relatively insensitive to this increase in Manning’s n on account of the steep grades across the site. Therefor the assessment is robust and will reflective of conditions on site whether bush land is re-established or whether the site is manicured and landscaped.

The flood hazard for majority of the site is low with regions of high flood hazard occurring along the northern boundary of the site associated with deeper and swifter flowing waters within the adjacent gully. A region of high flood risk is also associated with water cascading down rock outcrops in the southern corner of the site and an isolated pocket at the front of the site.

Flood modelling predicts the 100 year ARI flood levels at 90 Babbage Road vary from 24.8 mAHD at the rear of the property boundary and 11.6 mAHD at the front of property boundary.

The depth of 100 year ARI flooding varies from 100mm to 300mm in the deepest areas of the site.

The peak 100 year ARI flow entering the upper site is 0.2 m3/s.

3.2.1 Flood Mapping for 90 Babbage Road

The 20 minute 100 year ARI flood extent is shown in Figure 4 below as a solid blue layer over Council’s aerial photo of the site. The extent represents overland flows greater than 30mm in depth.

Under a 50% blockage scenario, the flood hazard for properties to the east of Babbage Road is shown to be a mix of low hazard and high hazard zones as defined by the New South Wales Floodplain Management Manual criteria shown in Figure 4.





0.0

0.5

1.0

1.5

2.0

0.0 0.5 1.0 1.5 2.0

Depth (m)

Ve

loc

ity

(m

/s)

High

Hazard

Low

Hazard

Hazard

Level D

ep

en

den

t on

Site

Co

nd

ition

s

Figure 3 Flood hazard definitions as specified in the NSW Floodplain Management Manual

The flood hazard mapping across the existing site is shown in Figure 5.

Hazards are associated with shallow and fast flowing water due to the steep rock faces and slopes at the upper and lower extents of the site.





Figure 4 Existing 100 year ARI flood extent and flood level contours at 90 Babbage Road





Figure 5 Existing flood hazard mapping for 90 Babbage Road after development





3.3 Flood Impacts of Hypothetical Development

The TUFLOW hydraulic model was adjusted to reflect development on the 90 Babbage Road site by increasing the Manning’s n roughness value from 0.035 to 0.10 to reflect changes in conveyance associated with a dwelling. It was assumed that a dwelling may occupy any part of the site, however it is noted that bush fire safety requirements will most likely result in a potential dwelling being located closer to the front of the property and away from the high flood hazard area associated with the rock outcrops.

Opportunities to intercept and divert overland flow around a dwelling are reasonable due to the relatively low overland flow rate of 0.2 m

3/s across the site. Pipes, however, will most likely encounter

shallow bedrock and significant tree roots if the existing trees are retained.

The modelling showed no increase in the 100 year ARI flood levels along the site boundaries.

Figure 6 Existing 100 year ARI flood extent and flood level contours at 90 Babbage Road





Figure 7 Existing flood hazard mapping for 90 Babbage Road after development

3.4 Summary of Findings for 90 Babbage Road

Flood modelling shows that a dwelling can be constructed on the site without impacting flooding on adjacent properties.

The site is impacted by shallow overland flows in a 100 year event that create high flood hazard conditions in the vicinity of rock outcrops to the rear of the site and near the street frontage. These conditions are isolated and may not necessarily result in damage to property or structures if the site is developed in a manner that accommodates these conditions.





Stormwater pits and pipes can be designed to collect stormwater at the rear of the site and divert flows around steep zones to mitigate high flow velocities. Pits and pipes should be sized to intercept and convey 0.2 m

3/s. These works can be undertaken by the site developer.

Footings should be founded on rock and designed to withstand flow velocities up to 2.6 m/s to mitigate damage if stormwater pipes fail during a design storm.





APPENDIX A - DRAINS MODEL PARAMETERS

ILSAX model soils

100 year ARI 2 hour duration rainfall intensity and temporal pattern

100 year ARI 20 minute rainfall intensity and temporal pattern





APPENDIX B - SITE CONSTRAINTS AND DEVELOPMENT CONTROLS FOR 90 BABBAGE ROAD

29A

610.12418

Ku�ring�gai Council

90 Babbage Road, Roseville ChaseFlooding, Drainage, Riparian and

Bushfire Constraints

Scale Date

THE FLOOD EXTENT AND FLOODDEPTH SHOWN ARE BASED ON

THE STATISTICAL 1% ANNUALEXCEEDENCE PROBABILITY

FLOOD EVENT (OFTENREFERRED TO AS THE 100 YEAR

FLOOD) AND HAVE BEENDETERMINED USING

HYDROLOGIC AND 1D12DHYDRAULIC FLOOD MODELLING

USING A 2M GRID SIZE.

REFER TO STUDY REPORT FORFLOOD LEVELS

¤

ApprovedPeter Gillam

0 4 82

Metres

Prepared for:

Project No:

Project Name:

NOTES

LEGEND

261Mar12013

EGRESS ROUTE TO FRONT OF PROPERTY MUST BE PROVIDED AND SET 300mmABOVE THE 1% AEP FLOOD LEVELS

2 Lincoln StreetLane CoveNSW 2066

AustraliaT: +61 2 6287 0800F: +61 2 6287 0801

www.slrconsulting.com

23m 9 m

INDICATIVE DWELLING FOOTPRINT

THE SITE IS SUBJECT TO BUSH FIRE ATTACK FROM THE GULLY TO THE NORTH AND EMBER ATTACK FROM THE EAST. A DWELLING ON THE SITE WILL ATTRACT A BAL FZ RATING AND MUST BE CONSTRUCTED IN ACCORDANCE WITH AS3959 – 2009 SECTION 5.

FLOOR LEVELS SHALL BE NO LESS THAN 300mm ABOVE 1% AEP FLOOD LEVELS AT ANY LOCATION ON SITE AND IDEALLY 500mm ABOVE THE 1% AEPFLOOD LEVEL

��

Not to scale

High flood hazard

Stormwater pipe

Medium flood hazard

1% AEP Flood Hazard

Gully centreline

Cadastral boundary

92

Babbage Road

90

52

50

GULLY CENTRELINE

94

12m

��

��

DIVERT FLOW AROUNDPROPOSED DWELLING

VIA SWALE OR PIPE

NO DEVELOPMENT PERMITTED WITHIN RIPARIAN CORRIDOR

Low flood hazard

Riparian corridor

��

��

�� DWELLING MUST BE SETBACK 12m FROM FRONTBOUNDARY

HIGH FLOW VELOCITY ASSOCIATED WITH ROCK OUTCROPS SHOULD BE MANAGED BY CONSTRUCTING A SWALE OR PIPE ANDDIVERTING STORMWATER AROUND THE PROPOSED DWELLING AND CONNECTINGTO STORMWATER DRAINAGE IN THE STREET

Documents

Attachment 2 Hydraulic Assessment (SLR Consulting)€¦ · Attachment 2 – Hydraulic Assessment (SLR Consulting) Hydraulic Study 90 Babbage Road Roseville Chase Report Number 610.12418