flood impact assessment report for proposed …

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FLOOD IMPACT ASSESSMENT REPORT

FOR

PROPOSED REDEVELOPMENT

No. 1 Gerrish Street, Gladesville

September 2021


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Report Description

Report Name Flood Impact Assessment Report

Address No. 1 Gerrish Street Galdesville

Client Ryan Kenna

Our Reference FAR1881/21

Prepared By Muna Pradhan Flood and Drainage Engineer MIEAust CPEng NER

Revision History

Date Version Author Comments

06.09.2021 V1 MP First Edition


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Contents

1 Introduction 1

2 Site Description 1

3 Proposed Development 3

4 Flooding Characteristics 4

4.1 Existing Condition 4

4.2 Developed Condition 5

5 Flood Hazard Category 6

6 Flood Related Development Control 7

6.1 Floor Level 7

6.2 Building Component and Materials 8

6.3 Structural Soundness 10

6.4 Flood Effects 10

6.5 Car Parking and Driveway Access 10

6.6 Evacuation 10

7 Conclusions 11

Appendix A: 12


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List of Figures

Figure 1 Location of Site (Source: SIX Maps) .......................................................................................................... 1

Figure 2 Survey Plan of Development Site.............................................................................................................. 2

Figure 3 Site Plan of the proposed development ................................................................................................... 3

Figure 4 Property Flood Level Location Map (Source: City of Ryde Council) .......................................................... 4

Figure 5 Flood Extent and Footprint of Proposed dwelling .................................................................................. 5

Figure 6 Provisional Hazard Category ..................................................................................................................... 6

Figure 7 Flood Risk Map at the development site and surrounding (Source: City of Ryde Council) ...................... 7


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List of Tables

Table 1 Flood Level Data (Source: City of Ryde Council) ........................................................................................ 4

Table 2 Flood Compatible Materials ....................................................................................................................... 8


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This page is left intentionally blank.


Page 1

1 Introduction

BMB Engineers was commissioned to prepare the flood impact assessment report for the proposed

redevelopment of the residential dwelling at No. 1 Gerrish Street Gladesville.

This report has been prepared to accompany a Development Application that will address City of Ryde

Council’s requirements for a flood control lot. This report describes the existing characteristics of the

flood, proposed development and quantifies the impact on flooding due to the future development.

This flood assessment is primarily based upon the Parramatta River-Ryde-Sub Catchments Flood Study

and Floodplain Risk Management Study Plan, 2015, prepared by SKM Pty Ltd for the Ryde City Council.

2 Site Description

The site is located to the eastern side of Gerrish Street near the corner of Gerrish Street and Cambridge

Street. A locality plan of the site is provided in Figure 1 below.

Figure 1 Location of Site (Source: SIX Maps)

The development site slopes towards the front boundary of the property. The ground level at front of

property varies from 27.25 to 27.31 m AHD and rear ground level varies from 28.60 to 28.93 m AHD.

A Survey plan of the development site is as given in Figure 2. The site is currently developed with a

single storey residence and garage with metal roof.


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Figure 2 Survey Plan of Development Site


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3 Proposed Development

The proposed development comprises demolition of the existing dwelling and construction of a two

storey residential dwelling at the same location of the existing building. The site plan of the proposed

development is presented in Figure 3.

Figure 3 Site Plan of the proposed development


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4 Flooding Characteristics

4.1 Existing Condition

The council has advised that the subject site is subjected to flooding. Flood level information around

the development site has been provided by the council. This information is based on the Parramatta

River-Ryde-Sub Catchments Flood Study and Floodplain Risk Management Study Plan, 2015 as shown

in Table 1, Figure 4 and Appendix A.

Table 1 Flood Level Data (Source: City of Ryde Council)

Location 100 Year ARI Flood, m AHD PMF, m AHD

A Nil Nil

B Nil Nil

C Nil 28.73

D Nil Nil

E 27.71 28.83

F Nil 29.09

G 27.78 28.95

H 27.84 29.10

The locations mentioned in Table 1 are shown in Figure 4.

Figure 4 Property Flood Level Location Map (Source: City of Ryde Council)


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The estimated maximum depth of flow within the development site varies from 0.46m (at location E)

to 0.59m (at location G) as shown in Figure 4. The flood level information suggests that only front part

of the property is affected by 1% AEP flooding.

4.2 Developed Condition

The footprint of the proposed dwelling is located mostly outside of the 1% AEP flood extent area as

shown in Figure 5. It is recommended to follow the existing ground level while designing and

constructing the proposed driveway within the flood extent area as far as possible. In such condition,

there will not be any adverse impact on the existing flooding behaviours at the development site and

surrounding area. The proposed double storey dwelling will provide a safer refuge area on the first

floor while comparing the existing single storey dwelling during severe flooding events.

Figure 5 Flood Extent and Footprint of Proposed dwelling


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5 Flood Hazard Category

Flood hazard category is a function of flood depth and flow velocity. A method specified in NSW

Government Floodplain Development Manual, 2005 has been used to determine provisional hydraulic

hazard category of the development site. According to this manual, provisional hydraulic hazard has

been categorised based on the relationship between the depth and velocity of the flow as shown in

Figure 6.

Figure 6 Provisional Hazard Category

Based on available flood information, the proposed development site is located on low hydraulic

hazard and medium to high flood risk precincts (Figure 7).


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Figure 7 Flood Risk Map at the development site and surrounding (Source: City of Ryde Council)

6 Flood Related Development Control

A proposal of residential redevelopment for a residential dwelling has been assessed against City of

Ryde Development Control Plan 2014: Part 8.6: Floodplain Management. The assessments for the

flood controls are presented below.

6.1 Floor Level

Based on the flood level information provided by the council, the most relevant 1% AEP flood level for

the proposed dwelling is considered to be 27.71 m AHD (location E, Figure 4). Therefore, the minimum

habitable floor level is to be 28.21 m AHD or higher. The proposed residence has finished floor level

for habitable room as RL 28.26 m AHD which is higher than the minimum requirement. Hence this

complies the requirement of floor level.


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6.2 Building Component and Materials

All structural components should be constructed with flood compatible materials below the habitable

floor level (RL 28.21 m AHD). All electrical equipment and wiring should be water proofed or installed

above the habitable floor level. Table 2 presents the flood compatible materials that can be used for

the proposed dwelling.

Table 2 Flood Compatible Materials

Building Component Flood Compatible Materials

Building Component

Flood Compatible Materials

Flooring and Sub-

floor Structure

- Concrete slab-on ground - Monolith construction - Suspended reinforced concrete slab

Doors

- Solid panel with water proof adhesives - Flush door with marine ply filled with closed cell foam - Painted metal construction - Aluminium or galvanised steel frame

Floor Covering

- Clay tiles - Concrete, precast or in situ - Concrete tiles - Epoxy, form in place - Mastic flooring, formed in-place - Rubber sheets or tiles with chemical-set adhesives - Silicone floors formed in-place - Vinyl sheets or tiles with chemical-set adhesive - Ceramic tiles, fixed with mortar or chemical-set adhesive - Asphalt tiles, fixed with water resistant adhesive

Wall and Ceiling Linings

- Fibro-cement board - Brick, face or glazed - Clay tile, glazed in waterproof mortar - Concrete - Concrete block - Steel with waterproof applications - Stone, natural solid or veneer, waterproof grout - Glass blocks - Glass - Plastic sheeting or wall with waterproof adhesive

Wall Structure

- Solid brickwork, blockwork, reinforced concrete or mass concrete

Insulation Windows

- Foam (closed cell types) - Aluminium frame with stainless steel

- Rollers or similar corrosion and water resistant material


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Roofing Structure

(for Situations

where the Relevant

Flood Level is Above

the Ceiling)

- Reinforced concrete construction - Galvanised metal construction

Nails, Bolts, Hinges and Fittings

- Brass, nylon or stainless steel - Removable pin hinges - Hot dipped galvanised steel wire, nails or similar

Electrical and Mechanical Equipment

For dwellings constructed on land, where this

plan applies, mechanical and electrical

materials, equipment and installation should

conform to the following requirements.

Heating and Air Conditioning Systems

Heating and air conditioning systems should be

installed at levels above the relevant flood level,

to the maximum height possible. If this is not

feasible, care should be taken to minimise the

potential damage caused by submersion

according to the following guidelines.

Main power supply

The main commercial power service

equipment, including metering equipment,

shall be located above the relevant flood level,

subject to the approval of the relevant

authority. A provision for easily disconnecting

the dwelling from the main power supply shall

be supplied.

Fuel

Gas or oil fuelled heating systems should have

a manually operated valve, which is to be

located in the fuel supply line, to enable fuel

cut-off.

Wiring

All wiring, switches and power outlets should

be located above the relevant flood level, to

the maximum height possible. All electrical

wiring, which is installed below the relevant

flood level, should be suitable for continuous

submergence in water, containing no fibrous

components. Earth core linkage systems (or

safety switches) are to be installed. Only

submersible-type splices are to be used below

the relevant flood level. All conducts, located

below the relevant flood level, should be self

draining in the event of flooding.

Installation

Heating equipment and fuel storage tanks

should be mounted on and securely anchored

to a footing of sufficient size, in order to

withstand buoyancy and to prevent

movement capable of damaging the fuel

supply line. All storage tanks should be vented

to a level 600 millimetres above the relevant

flood level.

Equipment

Equipment installed below/partially below the

relevant flood level should contain a method

of disconnection, by a single plug and socket

assembly.

Ducting

All ductwork, located below the relevant flood

level, should have openings for drainage and

cleaning. A grade may be introduced within

ductwork in order to facilitate self draining. In

the case where ductwork passes through a

water tight wall or a floor below the flood level,

the ductwork should be covered by a closure

assembly which is to be operated from above

the flood level.


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6.3 Structural Soundness

All structures must be designed and constructed to ensure structural integrity up to the minimum

habitable floor level. Structural certification from the qualified structure engineer shall be obtained

confirming that the structure can withstand the forces of floodwater debris, wave action, buoyancy

and immersion up to minimum habitable floor level (RL 28.21 m AHD).

6.4 Flood Effects

Based on the flood information provided by City of Ryde Council, the subject site has been affected by

overland flooding. The development site is located in a medium to high flood risk zone. The footprint

of the proposed dwelling is located mostly outside of the flood extent area (1% AEP) and it is

recommended to follow the existing ground level while designing and constructing proposed

driveway. In such condition, there will not be any significant impact on the existing flooding behaviours

at the development site and surrounding area in post development scenario. The proposed double

storey dwelling will provide a safer refuge area on the first floor while comparing the existing single

storey dwelling during severe flooding events.

6.5 Car Parking and Driveway Access

The minimum garage floor level is to be 28.01 m AHD (1% AEP Flood Level at location E + 0.30 m).

6.6 Evacuation

The minimum habitable floor level of the proposed dwelling is 0.5 m above the 1% AEP flood level (i.e.

27.71 m AHD). Therefore, staying within the home particularly in first floor will provide protection for

a wide range of floods. It is very unlikely the floodwater may enter the habitable area based on existing

flood level information. In a severe flooding events, the first floor may be used as a temporary refuse

area.

Reconnection

In the event that an electrical device and/or

part of the wiring is flooded, it should be

thoroughly cleaned or replaced and checked

by an approved electrician before

reconnecting.

Ancillary Structures (steps, pergolas, etc)

Suitable water tolerant materials, such as

masonry sealed hardwood and corrosive

resistant metals, should be used. Copper

Chrome Arsenate (CCA) treated timber is not a

suitable material.


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7 Conclusions

This report analyses the impacts of proposed redevelopment at No. 1 Gerrish Street, Gladesville on

the existing flooding behaviours. This flood assessment is primarily based on the flood information

provided by the Council and Parramatta River-Ryde-Sub Catchments Flood Study and Floodplain Risk

Management Study Plan, 2015 in addition to the other relevant data and documents.

The floodwater from upstream catchment enters the site via front part of the property. The 1% AEP

flood level adjacent to the proposed building is considered as 27.71 m AHD.

In the post development scenario, the footprint of the proposed dwelling is located mostly outside of

the 1% AEP flood extent area and it is recommended to follow the existing ground level while designing

and constructing proposed driveway a far as possible. In such condition, there will not be any

significant impact due to the proposed redevelopment on the existing flooding behaviours at the

development site and surrounding area in post development scenario.

The followings are the recommendations of this study:

The minimum habitable floor level of the proposed dwelling is to be 28.21 m AHD or higher.

The minimum garage floor level is to be 28.01 m AHD or higher;

The proposed driveway to be designed and constructed so that it follows the existing ground

level within the flood extent area as far as possible;

All structural components below the minimum habitable floor level are to be constructed

with flood compatible materials and should withstand the forces of floodwater debris, wave

action, buoyancy and immersion up to minimum habitable floor level;

Structural certification from the qualified structure engineer shall be obtained confirming that

the structure can withstand the forces of floodwater debris, wave action, buoyancy and

immersion up to minimum habitable floor level.

No obstructions (home appliances such as air condition unit, hot water system, rainwater tank

etc.) are to be placed that obstructs overland flow path.

All external power points are to be set above the minimum habitable floor level;

No impervious fencing is to be constructed within the flood extent area;

All the development controls mentioned in section 6 are to be implemented.


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Appendix A:

Flood Information (Source: City of Ryde Council)


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