3 POTENTIAL FLOODPLAIN MANAGEMENT MEASURES 3 ......Blackmans Swamp Creek Floodplain Risk Management Study and Plan Blackmans Swamp Creek Vol 1.doc Page 21 Lyall & Associates July 2009

Blackmans Swamp Creek Floodplain Risk Management Study and Plan

Blackmans Swamp Creek Vol1.doc Page 18 Lyall & Associates

July 2009 Rev. 5.0 Consulting Water Engineers

3 POTENTIAL FLOODPLAIN MANAGEMENT MEASURES

3.1 Range of Available Measures

A variety of floodplain management measures can be implemented to reduce flood damages.

These measures may be divided into three categories: property modification, response modification

and flood modification.

Flood modification refers to changing the behaviour of floods in regard to discharges and water

surface levels to reduce flood risk. This can be done by the construction of levees, retarding basins

and channel improvements. Such measures are also known as “structural” options as they involve

the construction of engineering works.

Property modification refers to reducing risk to properties through measures such as land use

zoning, minimum floor level requirements, or house raising. Such options are largely planning

measures, as they are aimed at ensuring that the use of floodplains and the design of buildings are

consistent with flood risk. Property modification measures could comprise a mix of structural and non-

structural methods of damage minimisation.

Response modification refers to changing the response of flood affected communities to the flood

risk by increasing flood awareness by the installation of flood warning systems and the development

of emergency management plans for property evacuation. These options are wholly non-structural.

3.2 Community Views

Comments on potential flood management measures were sought from the local community by way of

a Newsletter. The responses are summarised in Appendix B. Question 11 in the Community

Newsletter outlined a range of potential measures. The responses are shown on Table 3.1, together

with initial comments on the feasibility of the options, which are discussed in more detail in later

sections of this chapter.

The Community favoured the following measures:

Maintenance programs to clear creeks of debris.

Detention basins to store floodwaters.

Controls over future development in flood liable areas.

Improved flood warning, evacuation and flood response procedures, including evacuation

and emergency assistance.

Community education and flood awareness.

After examination in Section 3 and testing for feasibility on a range of criteria in Section 4, these

measures have been included in the draft FRMP in Section 5.

Bla

ckm

ans S

wam

p C

reek

Flo

odpla

in R

isk M

anagem

ent S

tudy a

nd P

lan

Bla

ckm

an

s S

wa

mp

Cre

ek

Vo

l1.d

oc

Pa

ge

19

L

ya

ll &

Asso

cia

tes

Ju

ly 2

00

9 R

ev.

5.0

C

on

su

ltin

g W

ate

r E

ng

ine

ers

TA

BL

E 3

.1

CO

MM

UN

ITY

VIE

WS

ON

PO

TE

NT

IAL

FL

OO

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AN

AG

EM

EN

T M

EA

SU

RE

S

Flo

od

Ma

na

gem

en

t O

pti

on

C

lassif

ic

ati

on

Y

es

No

Co

mm

en

ts

a)

Ma

inte

na

nce

pro

gra

ms t

o c

lea

r cre

eks

of

ve

ge

tatio

n a

nd

de

bri

s im

pe

din

g

flo

ws

FM

3

1

4

Th

is o

ptio

n is s

tro

ng

ly f

avo

ure

d b

y t

he

Com

mu

nity a

nd

is r

evie

we

d in

Secti

on

3.3

.1.

It is n

ot

str

ictly

a f

loo

d m

itig

atio

n s

ch

em

e a

s t

he

hyd

rau

lic c

ap

acity

of

the

cre

eks

wo

uld

no

t b

e s

ign

ific

an

tly

incre

ase

d,

bu

t w

ou

ld h

ave

en

vir

on

me

nta

l b

en

efits

.

b)

En

larg

e t

he

Cre

ek C

ha

nn

els

F

M

17

1

3

Co

mm

un

ity

op

inio

n t

o t

his

op

tio

n is e

ve

nly

ba

lan

ce

d.

Th

e f

ea

sib

ility

of

this

op

tio

n is

revie

we

d in

Secti

on

3.3

.2.

Th

e p

ote

ntia

l fo

r a

ug

me

ntin

g t

he

hyd

rau

lic c

ap

acity

of

loca

l ro

ad

cro

ssin

gs o

n E

ast

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ng

e C

ree

k is a

lso

co

nsid

ere

d.

c)

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nstr

uct

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ten

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asin

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o s

tore

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ters

.

FM

2

2

5

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e c

om

mu

nity

str

on

gly

fa

vo

urs

co

ntin

ua

tio

n o

f C

ou

ncil’

s e

xis

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g p

olic

y o

f

imp

lem

en

tin

g d

ete

ntio

n b

asin

s o

n t

he

cre

eks t

o m

itig

ate

exis

tin

g f

loo

din

g p

rob

lem

s.

Th

e f

ea

sib

ility

of

co

nstr

uctin

g a

dd

itio

na

l b

asin

s t

o r

ed

uce

flo

od

pe

aks is c

on

sid

ere

d

inS

ecti

on

3.3

.4.

d)

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nstr

uct

pe

rma

ne

nt

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es

to

co

nta

in f

loo

dw

ate

rs

FM

1

4

11

C

om

mu

nity

op

inio

n t

o t

his

op

tio

n is e

ve

nly

ba

lan

ce

d.

Th

e f

ea

sib

ility

of

pro

vid

ing

leve

es a

lon

g t

he

cre

eks t

o c

on

tain

flo

od

wa

ters

is c

on

sid

ere

d in

Sec

tio

n 3

.3.3

.

e)

Vo

lun

tary

pro

pe

rty p

urc

ha

se

sch

em

e

PM

8

1

2

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e c

om

mu

nity

do

es n

ot

favo

ur

this

op

tio

n,

whic

h is o

fte

n a

do

pte

d t

o r

em

ove

resid

en

tia

l p

rop

ert

y in

hig

h h

aza

rd a

rea

s o

f th

e f

loo

dp

lain

. T

his

op

tio

n is r

evie

we

d in

Secti

on

3.4

.3.

f)

Pro

vid

e f

un

din

g o

r su

bsid

ies t

o r

ais

e

ho

use

s a

bo

ve

1%

AE

P f

loo

d le

ve

l

PM

7

1

9

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e c

om

mu

nity

do

es n

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ur

this

op

tio

n.

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use

ra

isin

g is a

pp

lica

ble

to

tim

be

r

fra

me

d r

esid

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ce

s o

nly

, u

su

ally

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ca

ted

in

lo

w h

aza

rd z

on

es.

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is o

ptio

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revie

we

d in

Secti

on

3.4

.4.

g)

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od

pro

of

ind

ivid

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7

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ivid

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ert

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ay

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ofe

d b

y d

ive

rsio

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ks (

wh

ich

ma

y a

dve

rse

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aff

ect

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w p

att

ern

s),

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r p

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fed d

oo

rs a

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sh

utt

ers

acro

ss e

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an

ce

s.

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is

op

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n is n

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ure

d b

y t

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co

mm

un

ity.

h)

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ntr

ols

on

fu

ture

de

ve

lop

me

nt

in

flo

od

-lia

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are

as.

(eg

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loca

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th

e f

loo

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, m

inim

um

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or

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etc

.)

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2

9

1

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ntr

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ove

r d

eve

lop

me

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in f

loo

d p

ron

e la

nd

are

ve

ry s

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ly s

up

po

rte

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y t

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co

mm

un

ity

an

d w

ou

ld b

e a

n e

sse

ntia

l p

art

of

the

FR

MP

. T

his

issu

e is c

ove

red

in

th

e

dra

ft F

loo

d P

olic

y in

Secti

on

3.4

.2 a

nd

Ap

pen

dix

C.

Le

ge

nd

: F

M =

Flo

od

Mo

dific

atio

n O

ptio

n

P

M =

Pro

pe

rty M

od

ific

atio

n O

ptio

n

R

M =

Re

sp

on

se

Mo

dific

ation

Op

tio

n

Bla

ckm

ans S

wam

p C

reek

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odpla

in R

isk M

anagem

ent S

tudy a

nd P

lan

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mp

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ek

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l1.d

oc

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20

L

ya

ll &

Asso

cia

tes

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ly 2

00

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5.0

C

on

su

ltin

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ate

r E

ng

ine

ers

TA

BL

E 3

.1

CO

MM

UN

ITY

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WS

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FL

OO

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AN

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EA

SU

RE

S

(Co

nti

nu

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)

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na

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en

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lassif

ic

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ts

i)

Pro

hib

it s

ub

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f p

rop

ert

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with

in t

he

flo

od

pla

in

PM

2

1

6

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is o

ptio

n w

ou

ld e

nsu

re t

ha

t th

e e

xis

tin

g f

loo

d r

isk is n

ot

incre

ased

by

furt

he

r

de

ve

lop

me

nt

an

d is s

tro

ng

ly s

up

po

rte

d b

y t

he

co

mm

un

ity.

T

his

issu

e is c

ove

red

in

the

dra

ft F

loo

d P

olic

y.

j)

Pro

hib

it r

ezo

nin

g f

or

ne

w d

eve

lop

me

nt

with

in f

loo

dp

lain

PM

2

0

5

Th

is o

ptio

n is s

tro

ng

ly f

avo

ure

d b

y t

he

co

mm

un

ity a

nd

wo

uld

en

su

re t

ha

t th

e e

xis

tin

g

flo

od

ris

k is n

ot

incre

ase

d b

y in

ap

pro

pri

ate

la

nd

use

s in

flo

od

pro

ne

are

as.

k)

Imp

rove

flo

od

wa

rnin

g a

nd

flo

od

resp

on

se

pro

ce

du

res

RM

2

6

2

Th

ere

is p

rese

ntly n

o f

loo

d w

arn

ing

sys

tem

fo

r th

e c

ree

k s

yste

m,

wh

ere

flo

od

ing

is o

f

a “

fla

sh

flo

od

ing

” n

atu

re,

with

su

dd

en

ris

es in

wate

r le

ve

ls a

fte

r th

e o

nse

t o

f h

ea

vy

rain

fall.

S

uch

a s

yste

m w

ou

ld b

e s

tro

ng

ly s

up

po

rte

d b

y th

e c

om

mu

nity

an

d is

co

nsid

ere

d in

Secti

on

3.5

.1.

l)

Imp

rove

eva

cu

atio

n a

nd

em

erg

en

cy

assis

tan

ce

pla

ns

RM

2

2

4

Em

erg

en

cy

man

ag

em

en

t in

th

e O

ran

ge

are

a is c

ove

red

by t

he

SE

S O

ran

ge

City

Lo

ca

l F

loo

d P

lan

. Im

pro

ve

me

nts

wo

uld

be

str

on

gly

fa

vo

ure

d b

y th

e c

om

mu

nity.

m)

Co

mm

un

ity

ed

uca

tio

n,

pa

rtic

ipa

tio

n

an

d f

loo

d a

wa

ren

ess p

rog

ram

s

RM

2

0

6

Pro

mo

tio

n o

f a

wa

ren

ess o

f th

e f

loo

d r

isk w

ou

ld b

e s

tro

ng

ly f

avo

ure

d a

mo

ng

th

e

co

mm

un

ity.

T

his

op

tio

n is r

evie

we

d b

elo

w.

n)

Pro

vid

e a

ce

rtific

ate

to

all

resid

en

ts

sta

ting w

heth

er

their p

rope

rty is f

lood

aff

ecte

d a

nd

to

wh

at

exte

nt

RM

2

2

6

Pro

vis

ion

of

info

rma

tio

n o

n f

loo

d a

ffe

ctio

n o

f p

rop

ert

ies w

ou

ld b

e s

tro

ng

ly f

avo

ure

d

by

the

co

mm

un

ity.

Th

is c

ou

ld b

e a

ch

ieve

d b

y a

pp

rop

ria

te n

ota

tio

n o

n S

ectio

n 1

49

Ce

rtific

ate

s.

Th

is o

ptio

n is r

evie

we

d in

Secti

on

3.4

.2.

o)

Insta

ll flo

od

ma

rke

rs

RM

1

8

10

T

his

op

tio

n p

rob

ab

ly a

s p

art

of

an

in

teg

rate

d f

loo

d a

wa

ren

ess p

rog

ram

co

mb

inin

g

op

tio

ns m

) a

nd

n)

ab

ove

wo

uld

be

fa

vo

ure

d b

y t

he

co

mm

un

ity.

Le

ge

nd

: F

M =

Flo

od

Mo

dific

atio

n O

ptio

n

P

M =

Pro

pe

rty M

od

ific

atio

n O

ptio

n

R

M =

Re

sp

on

se

Mo

dific

ation

Op

tio

n


Blackmans Swamp Creek Vol 1.doc Page 21 Lyall & Associates


3.3 Flood Modification Measures

3.3.1 Maintenance of Creek Capacity and Debris Control

Projects identified in the Orange Stormwater Management Plan, 2004 (Appendix E) include the

removal of sediment, woody weeds and willows and revegetation of the creek corridors with native

species. Those measures would have a beneficial, but limited, impact on the conveyance capacity of

the streams. They would not fulfil a flood mitigation role but would improve the aesthetics of the

existing channelised waterway, as well as provide water quality benefits and reduce the debris load

likely to be experienced during flooding.

Debris control within the stream corridor would reduce the potential for blockage at the entrance to the

CBD drain at Kite Street, with consequent surcharge of the channel of Blackmans Swamp Creek into

the CBD area. In the 1999 flood, debris was reported to have built up over the gate at the entrance to

the CBD drain. Although the grate was subsequently removed, there would still be the potential for

trees and other large debris to block the entrance. A creek maintenance program would reduce the

risk of blockage and ensure the drain operates at potential hydraulic capacity.

3.3.2 Channel Works on Blackmans Swamp Creek System

The hydraulic capacity of a stream may be increased by widening, deepening or straightening the

channel and by clearing the banks of obstructions. The scope of such improvements can vary from

minor works such as de-snagging and bank clearing, which do not increase the waterway area but

reduce hydraulic roughness, to major channel excavations. Careful attention to design is required to

ensure stability of the channel is maintained and scour or sediment build up is minimised. The

potential for channel improvements to increase downstream flood peaks also needs to be considered.

In general, channel improvements need to be carried out over a substantial stream length to have any

significant effect on flood levels.

Upper Blackmans Swamp Creek and Rifle Range Creek

The implementation of large scale improvements to the hydraulic capacity of Blackmans Swamp

Creek and Rifle Range Creek is not justified. The detention basins on the headwaters of these two

streams reduce flood peaks so that major floods up to the 1% AEP magnitude would not result in

inundation of residential property as far downstream as their confluence.

East Orange Creek

Damaging flooding occurs at the 5% AEP level of flooding in the residential developments bordering

the East Orange Creek in Eyles Street on the upstream side of the Mitchell Highway. Below the

Highway crossing, residential flooding occurs upstream of the Summer Street and Byng Street bridge

crossings, due to the combination of inadequate size of channel and restrictions imposed by the

bridge waterways. The drop in water surface levels across these structures would be in excess of

1 m in the event of a 5% AEP flood and a portion of the flow would be conveyed over the roadways,

which would act as broad crested weirs. In the event of the 1% AEP flood, additional properties would

be flooded at Autumn and McLachlan Streets due to restrictions in bridge waterway and channel

capacity. Flooding of commercial and industrial properties also occurs in the above zones, with

properties in March Street being flooded at the 5% AEP level of flooding.




From the economic assessment of flooding presented in Appendix A*, the present worth values of

damages for all floods up to the 1% AEP magnitude at a 7 per cent discount rate and over an

economic life of 20 years are:

East Orange Creek Upstream of Mitchell Highway $0.5 Million

East Orange Creek Downstream of Mitchell Highway $1.5 Million

In an economic analysis, the damages prevented by a flood mitigation scheme represent its benefits.

Therefore, provided damages up to the 1% AEP level of flooding were eliminated by the proposed

scheme, expenditure of the above amounts for improvements of East Orange Creek could be

economically justified.

Photographs illustrating the following discussion on channel improvements are attached at the end of

this Chapter. Orange City Council has recently upgraded the section of East Orange Creek between

March Street and William Street. The section is rectangular in shape with a full concrete invert and

concrete block retaining walls and has a waterway area of about 20 m2

(Plate 1). The section of

channel between William Street and the junction with the East Orange Creek has not been upgraded.

The hydraulic modelling carried out for the Flood Study, 2005 showed that because of restrictions in

the capacity of the channel of Blackmans Swamp Creek below the junction, there would be a level

pool backwater during major floods extending upstream along East Orange Creek from the junction to

March Street. This backwater would surcharge the coping of an improved section of channel during

major flooding. Consequently, an improved channel on East Orange Creek downstream of William

Street would not function at its full hydraulic capacity until the Blackmans Swamp Creek Channel was

also upgraded. (Plate 7). Improvements to Blackmans Swamp Creek, as discussed, later do not

appear feasible due to constraints in available land on each side of the channel. Therefore, based on

the above discussion, it is not likely improvements in hydraulic performance of the drainage system

would be achieved by upgrading the section of creek between William Street and the junction with

Blackmans Swamp Creek. Accordingly, the following discussion covers improvements on East

Orange Creek commencing at March Street and proceeding upstream.

The average bed slope of the 1.65 km reach of the channel between the Mitchell Highway and the

junction with Blackmans Swamp Creek downstream of William Street is about 0.7 per cent. In order to

convey the 1% AEP design peak discharge of around 55 m3/s, without the escape of flows into the

local street system which presently would occur, a channel waterway 8 m wide by 1.8 to 2 m high

would be required and the various bridge crossings would also have to be upgraded to minimise

restrictions on the flow.

Table 3.2 provides indicative budget costs of a channel improvements scheme to mitigate damages

on the section of East Orange Creek downstream of the Mitchell Highway crossing. The waterway

areas shown are indicative only and are subject to hydraulic modelling at the design stage. All of the

works would be required to achieve a 1% AEP hydraulic capacity along the creek. The total budget

cost is about $3.8 Million, compared with $1.5 Million in terms of flood damages prevented. Whilst the

benefit/cost ratio of the channel improvements scheme is only 0.4 and cannot be justified solely on

economic grounds, it would minimise flooding in the streets which presently occurs during flash

flooding on the East Orange Creek system and therefore has considerable social benefits in terms of

a reduction in flood risk to residents. Because it would not be financially feasible for Council to

construct the scheme as one project, each item of the scheme has been given a priority ranking to

allow construction to be undertaken as funding becomes available.

* Note: Section 8.3 of Appendix A includes a definition of terms used in the economic assessment of flood impacts




TABLE 3.2

CHANNEL IMPROVEMENT SCHEME – EAST ORANGE CREEK

MITCHELL HIGHWAY TO MARCH STREET

Item Priority

Indicative

Cost

$x106

Comments

Upgrade waterway at

March Street. (Plate 2)1 0.5

The results of hydraulic modelling of East Orange Creek in DHI,

2005 gave a head drop of 1.1 m across the existing triple pipe

culvert for the 5% AEP flood. The existing structure would convey

only 14 m3/s of the total 1% AEP discharge of 55 m3/s. In order to

convey 55 m3/s with a head loss no greater than 200 mm, the

waterway would need to be upgraded to 12 m2 area (8 m x 1.5 m

culvert) with inlet and outlet transitions.

Upgrade channel March

Street to McLachlan

Street

1 0.16

The channel would be widened to provide about 14.5 m2 of

waterway area, for example with a rectangular channel 8 m wide by

1.8 m deep.

Upgrade crossing at

McLachlan Street

(Plate 3)

2 0.5

DHI, 2005 results gave a head drop of 1.2 m across the existing twin

pipe culvert for the 5% AEP flood. The existing structure would

convey only 14 m3/s of the 55 m3/s design 1% AEP discharge. A

waterway of 12 m2 area with transitions would be required.

Upgrade channel

McLachlan Street to

Byng Street

5 0.48



1.8 m deep.

Upgrade crossing at

Byng Street 3 0.5

DHI, 2005 results gave a head drop of 1.5 m across the single cell

box culvert for the 5% AEP flood. The existing structure would

convey only 17 m3/s of the total 55 m3/s design 1% AEP discharge.

The road is humped over the existing culvert indicating that it may

not be practicable to provide a new opening 1.5 m in height. A

culvert wider than 8 m may be required to achieve the proposed 12

m2 waterway area.

Upgrade channel Byng

Street to Summer

Street.

(Plate 4)

6 0.56



1.8 m deep.

Upgrade crossing at

Summer Street.

(Plate 5) 4 0.5

DHI, 2005 results gave a head drop of 1.1 m across the culvert for

the 5% AEP flood. The existing structure varies in waterway area

across its length and has hydraulically poor inlet and outlet

transitions to the creek. It presently conveys 26 m3/s of the design

1% AEP flow of 55 m3/s. A 12 m2 waterway with transitions to

minimise hydraulic losses is proposed.

Upgrade channel

Summer Street to Icely

Road (Plate 6)

7 0.61

The channel could be widened to provide about 14.5 m2 of waterway

area, for example with a rectangular channel 8 m wide by 1.8 m

deep.

Total $3.8 Million

Note: Proposed waterway areas are indicative only. Hydraulic modelling would be required at design stage for confirmation.




Blackmans Swamp Creek

Railway to Dalton Street

The channel is a rectangular concrete lined channel about 10 m in width, 2.5 m deep and about

230 m in length. The channel continues as an unlined channel 205 m in length between Dalton Street

and Leeds Parade. Both bridge crossings increase flood levels upstream in the event of major

flooding. At the 1% AEP design flood, which has a peak flow of 115 m3/s, there is a 430 mm head

drop across Leeds Parade and 300 mm head drop across Dalton Street. Because of these controls,

the water surface profile between the railway and Dalton Street is flat and as mentioned previously,

the backwater extends upstream on the East Orange Creek to March Street.

Reduction in peak flood levels on Lower Blackmans Swamp Creek would reduce flood levels in the

Peisley Street area on the upstream side of the railway culvert, as well as flood levels in lower East

Orange Creek.

By inspection of Plate 7, there is little room available for the widening of the lined section of channel

due to the existing development bordering the creek. Any increase in the width of channel would

need to be accompanied by waterway improvements at the bridge crossings and enlargements to the

channel downstream of Dalton Street.

Consequently, major channel works on the lower reaches of Blackmans Swamp Creek would not be

financially feasible due to the large land resumption costs involved in the reach between the railway

and the Dalton Street crossing.

Upgrading CBD Drain

The culvert beneath the CBD surcharges in the event of flows greater than the 5% AEP. In the event

of a 1% AEP, the peak flow approaching the culvert entrance would be 66 m3/s, of which 56 m

3/s

would enter the culvert and the balance would be conveyed along the street system, which would

function as floodways. The major flow paths would be along Lords Place, March Street and the lower

reaches of Peisley Street where ponding would occur in the street system, particularly on the

upstream side of the culvert beneath the railway embankment.

Local stormwater from the CBD catchment cannot enter the culvert system during major floods

thereby increasing flows along the streets.

Improvements in the capacity of the culvert would involve the following works (Figure 3.1):

i) Duplication of the existing culvert in the 300 m long reach between Kite Street and

Summer Street by a 4200 x 1800 RCBC. This drain could convey up to 20 m3/s and

therefore capture flows which could not enter the existing system at Kite Street.

ii) Provide a 4200 x 2464 RCBC along Lords Place between Summer Street and March

Street, a distance of 450 m. This line would capture stormwater runoff from the local CBD

catchment which cannot enter the existing system. There is a major stormwater intake

point at the intersection of Lords Place and March Street and consequently the flow under

1% AEP conditions would increase by about 12 m3/s at the intersection.

iii) Continue the 4200 x 2464 RCBC along March Street to the intersection with Peisley

Street. At that location it would break into the existing culvert which outfalls to an open




channel a short distance upstream of the railway culvert. The capacity of this 220 m long

section would be about 35 m3/s.

iv) Break into the existing culvert a short distance upstream of the intersection of March Street

and Peisley Street and convey flows conveyed by that culvert in a 200 m long section

running along Peisley street to the head of the open channel leading to the culvert beneath

the railway. This section of line would also capture runoff which presently ponds in Peisley

Street and would have a capacity of 50 m3/s.

The combined capacity of the two lines iii) and iv) above amounts to 35 m3/s plus 50 m

3/s, a total of

85 m3/s, which compares with the peak 1% AEP flow at the railway culvert of 77 m

3/s as assessed by

the Flood Study, 2005. The above hydraulic assessment was based on uniform flow calculations and

is indicative only and would be subject to confirmation by a hydraulic model study.

Table 3.3 gives an indicative cost of upgrading the CBD drain and Table 3.4 is an economic

assessment assuming that the scheme has a 1% AEP design standard and would therefore prevent

damages for floods up to that magnitude. The present worth value of those damages therefore

becomes the economic benefits of the scheme. These results show that the scheme is marginally

economically viable at the 7 per cent discount rate. However, the scheme would be financially and

technically difficult for Council to implement without excessive impact on activities within the CBD

during the construction phase.

TABLE 3.3

INDICATIVE COST OF UPGRADING CBD DRAIN

TO 1% AEP STANDARD

ItemCost

$M

Supply and install culvert in Kite Street to Summer Street, duplicating the existing

culvert, including re-instating road surface 1.1

Supply and install new culvert along Lords Place and March Street to connect

with existing CBD drain at intersection with Peisley Street, including re-instating

road surface.

2.7

Supply and install new culvert in Peisley Street to head of channel leading to

Railway culvert, including connection with existing CBD drain on southern side of

March Street and re-installing road surface.

0.8

Intake structure at Kite Street; major junction boxes along upgrade route and

outlet structure u/s Railway culvert. 1.9

Services adjustment (allowance) 1.0

Contingencies; survey, investigation and design 2.5

TOTAL ESTIMATE $10.0 M




TABLE 3.4

ECONOMIC ANALYSIS OF UPGRADING CBD DRAIN

TO 1% AEP STANDARD

Discount Rate % 4 7 10

Present Worth Value of Benefits

(Damages Prevented) $ x 106 13.7 10.7 8.6

Cost of Scheme $ x 106 10.0 10.0 10.0

Benefit/Cost Ratio 1.37 1.07 0.86

3.3.3 Levees

Levees are an effective means of protecting flood affected properties up to the chosen design flood

level. In designing a levee it is necessary to take account of potential redistribution of flood flows, the

requirements for disposal of internal drainage from the protected area and the consequences of

overtopping the levee in floods greater than the design event. Reinforced concrete and concrete

block walls are often used in situations where there is insufficient land available for earth banks. Such

walls are provided with reinforced concrete footings of sufficient width to withstand overturning during

flood events. A recent example of this form of construction is the levee scheme for the town of

Lismore which protected the town from a severe flood a short time after its opening.

A major difficulty with levee schemes is the provision of facilities for the temporary storage and

disposal of runoff derived from the local sub-catchments upstream of the protected area. In some

situations, evacuation of runoff by pumping over the levee has been adopted where there is

insufficient area available to store runoff for later disposal by gravity as the flood recedes.

Potential for Levees Along Blackmans Swamp Creek and Its Tributaries

As was the case for channel improvements, levees would not be economically justified in the

residential areas of Blackmans Swamp Creek and Rifle Range Creek.

The following factors militate against a levee scheme on East Orange Creek:

Unless the levee was constructed to the elevation of the PMF, there would always be the

chance that it would be overtopped. On the East Orange Creek, flooding is of a “flash

flooding” nature with a very short time of rise after the initiation of heavy rainfall.

Consequently, sudden overtopping could take place with no time available for the

evacuation of residents. These considerations would suggest that a flood greater than the

1% AEP event, possibly the PMF, should be adopted for design purposes.

Hydraulic modelling showed that PMF flood levels at William Street were about 2.5 m

higher than the 1% AEP level. Consequently a PMF levee would not be feasible on

environmental or economic grounds.

Consideration was given to constraining flows on the East Orange Creek within low block

wall levees, which would have a lesser hydrologic standard than the PMF. However, they

are not viewed as a feasible mitigation option due to their adverse visual impact, disruption




to the local road crossings, which would need to grade over the top of the levee at

crossings and difficulties associated with the management of stormwater from the local

catchments within the “protected” areas. It is not practicable to re-route the local

stormwater system so that drainage is maintained from those areas without back-flooding

when creek levels rise during flood events.

From the above considerations, protection of the residential area by a levee is not considered feasible

and has not been adopted for further consideration.

It may be practicable to protect one or more of the flooded properties by low earth or block walls

around their entrances. Such localised flood proofing measures would be of a private nature and

outside the ambit of Council funded works discussed in this present study.

3.3.4 Detention Basins

Detention basins provide flood storage additional to that contained in the natural floodplain which can

reduce the flood peak in downstream reaches of the creek. “Offline” basins, remote from the stream

channels, are preferred to maintain the continuity of the creek system. The following discussion

reviews the potential for additional basins in the creek system. Basin locations are shown on

Figure 3.2.

Storages on Blackmans Swamp Creek

Detention basins on Blackmans Swamp Creek have been very successful in reducing downstream

flood peaks. They have mitigated flooding problems in residential areas of Blackmans Swamp Creek

and increased the flood security in the CBD area, which previously would have suffered severe

flooding during comparatively minor flood events. Under present day conditions, there is a “gap” of

about 10 m3/s between the 56 m

3/s capacity of the CBD drain and the 66 m

3/s peak of the 1% AEP

flood at the entrance to the drain at Kite Street.

The basins constructed by Council on Blackmans Swamp Creek upstream of the railway line have

reduced peak flows emerging from the railway culvert to 21 m3/s for the 1% AEP event. It may be

feasible to effect further reductions in peak flow by raising the embankments of those basins.

However for this initial assessment, attention has been given to a scheme involving the construction

of additional basins on Blackmans Swamp Creek at two sites which were identified in a previous

investigation on stormwater management by SKM, 1980. They are described below.

An on-line basin could be constructed across the channel and its overbanks in the Sampson-Lamrock

Street area a short distance downstream of the existing Basin RB7 at Pilcher Park. This basin site

was also denoted Basin RB7 in the 1980 study. The site has not been surveyed. However, on the

basis of available 2 m contour mapping, it appears that a volume of 40,000 m3 could be achieved by

storing water to an average depth of 1 m on the overbank areas. Within the channel, the depth of

ponding would be around 3 m. Flows up to the 1% AEP would be controlled by low level pipes and a

portion of the embankment crest in the vicinity of the channel would be depressed and armoured with

reno-mattress or equivalent to act as a spillway for the conveyance of higher flows. The peak storage

level would need to be no higher than about RL 870 m AHD to prevent backwater effects in

surrounding urban development.

Additional storage could also be achieved by converting Moulder Park into a detention basin, denoted

RB8 in the SKM 1980 study. On the basis of the 2 m contour survey, a storage of around 25,000 m3




could be achieved. This site is located below the confluence of Blackmans Swamp and Rifle Range

Creeks and would therefore attenuate flows from both catchments.

Storages on Rifle Range Creek

Agricultural Research Centre Water Supply Dam

The SKM, 1980 study identified a potential basin site RB1, about 1 km upstream of the railway.

However, a dam was subsequently constructed at this site as a water supply storage for the

Agricultural Research Centre. The dam comprises an earth embankment about 10 m high, with an

18 m wide bywash spillway on the right abutment.

Based on available survey, it is estimated that 60,000 m3 of “air space” storage for flood mitigation

purposes could be achieved by raising the wall of the dam by about 0.8 m and re-modelling the

spillway. Preliminary Details are as follows:

Item Existing Dam Raised Dam

Embankment

Crest Level RL 895.4 m AHD RL 896.2 m AHD

Spillway Details

Crest Level

Width

RL 893.8 m AHD

18 m

RL 894.5 m AHD

65 m

Low Level Outlets

Type Nil 2 off 2.4 m x 0.9 m RCBC

Intake Pit RL 893.8 m AHD

Top Water Level

1% AEP Flood

PMF

RL 895.1 m AHD

Would Surcharge Crest

RL 894.5 m AHD

RL 895.7 m AHD

(500 mm of freeboard provided)

Flows up to the 1% AEP flood would be conveyed by low level outlets comprising twin box culverts.

The existing spillway crest would be raised by about 0.7 m and widened to 65 m to convey the PMF

with 500 mm of freeboard against overtopping the crest. For preliminary planning, a peak discharge

of 150 m3/s, about four times the peak of the 1% AEP flood, was adopted as the PMF. As the head

over the spillway would be 1.2 m, its crest would need to be armoured to prevent scour. A low

concrete wall spillway comprising an inverted tee shaped section bounded by retaining walls was

adopted for costing purposes.

Basin RB2 Upstream of Orange-Broken Hill Railway

Implementation of a formal detention basin (RB2) on Rifle Range Creek, upstream of the railway

would increase the attenuating effect of the existing railway embankment and its outlet and would also

increase public safety, as the railway embankment would not have been designed to impound flood

flows. Under present day conditions, water ponds against the railway embankment and peak levels

are controlled by the two 1,500 mm diameter pipes. A scheme similar to that constructed at Basin

RB4 on Blackmans Swamp Creek could be implemented at this site. It would involve the construction




of a separate embankment in the playing fields upstream of the railway, and extension of the pipes to

a headwall at its upstream face.

Design peak 1% AEP flood level in the storage would be about RL 878.5 m AHD, which results in a

depth of ponding of about 4 m. However, provision would need to be made for the conveyance of

larger floods over the spillway. For preliminary planning, a crest level of RL 880 m AHD was adopted

for the embankment, along with a central spillway with crest level of RL 878.5 m AHD and a width of

140 m. This configuration would allow the extreme flood to be conveyed with 1 m of head over the

spillway and hence give 500 mm of freeboard against surcharging the embankment crest.

Scour protection of the downstream face would be required and, as for the other basins proposed in

this investigation, this would comprise a significant proportion of the overall cost of the basin. At the

design stage, geotechnical and further hydrologic analyses would be required.

The crest of the railway embankment is at an elevation of approximately RL 882 m AHD, about 2 m

higher than the crest of the proposed basin embankment. It may not be practicable to enlarge the

railway culverts to convey the extreme flood event due to the magnitude of the peak discharge.

Consequently, operation of the basin spillway may result in ponding behind the railway embankment

which, in turn, could result in its failure either by internal seepage pressures, or scouring due to

overtopping of the crest.

The consequences of failure of the railway embankment with the upstream basin embankment in

place would not be as severe as its failure under present day conditions. This is because the

presence of the basin embankment would limit the magnitude of the resulting floodwave.

However, a hydraulic investigation of the consequences of a “dambreak” of the railway embankment

and the embankments of the two upstream storages would be required during the design phase.

Table 3.5 shows the results of modelling the response of the basins to the 1% AEP flood.

Considerable attenuation of peak flows would result from implementation of the four basins. At the

entrance to the CBD drain at Kite Street, the peak discharge would be reduced from 66 m3/s under

present day conditions, to 38 m3/s, which is within the inlet capacity of the drain.

Table 3.6 shows indicative costs of construction and Table 3.7 is an economic analysis of the basin

strategy. The benefits of the scheme would be the reduction achieved in downstream flood damages

in the CBD area. Some improvements in the local stormwater system would also be required and an

indicative amount of 1$ Million for those works has been included in the $5.1 Million total cost of the

scheme. The total cost includes an allowance of 10 per cent of capital cost for survey, investigation

and design. No allowance has been made for land acquisition. The basin sites are located on Council

or Government owned land. These costs apply for a single purpose flood mitigation basin. Wetland

areas are often incorporated into the basin inverts. However, the costs of their inclusion would not be

funded by the Government’s Floodplain Management Program.

The results presented are based on available survey and are preliminary only, but show that a

continuation of the detention basin policy implemented by Council over the last 10 years would be a

cost-effective flood management measure. Further investigation with the benefit of more detailed

survey data is warranted and has been recommended in the list of management measures to be

incorporated in the Floodplain Risk Management Plan.




TABLE 3.5

REDUCTION IN PEAK FLOWS ACHIEVED

WITH ADDITIONAL DETENTION BASINS ON

BLACKMANS SWAMP CREEK AND RIFLE RANGE CREEK

Basin Location Inflow

m3/s

Outflow

m3/s

Basin Details


Sampson Street/Lamrock Street

RB7

34.7 10.1 Basin Storage 40,000 m3

Outlet Configuration

2 x 1.2 m diameter pipes

Rifle Range Creek

Agricultural Dam RB1

38.8 14.8 Basin Storage 63,000 m3


2 x 2.4 m x 0.9 m box culverts

Rifle Range Creek u/s Railway

Embankment RB2

19.5 7.5 Basin Storage 62,000 m3




Moulder Park RB8

48.8 31.8 Basin Storage 24,000 m3



Entrance to CBD Drain at

Kite Street

38(1)

– No Basin

Notes:(1)

This flow compares with a peak 1% AEP flow of 66 m3/s under present day conditions

Basin locations are shown on Figure 3.2




TABLE 3.6

INDICATIVE COSTS OF DETENTION BASINS

ON BLACKMANS SWAMP CREEK AND

RIFLE RANGE CREEK

Blackmans Swamp Creek Rifle Range Creek

Item Sampson/Lamrock

StreetMoulder Park

Agricultural

Research Centre

Dam

Storage Area

Upstream

Orange-Broken

Hill Railway

Basin RB7

$

Basin RB8

$

Basin RB1

$

Basin RB2

$

Establishment,

Environmental

Management of

Creeks

35,000 35,000 35,000 35,000

Foundation

Preparation,

Placement of

Embankment and

Landscaping

254,000 257,000 365,000 322,000

Outlet Works and

Spillway,

including Energy

Dissipation

Facilities

225,000 225,000 508,000 604,000

Sundries and

Unestimated

Items

130,000 130,000 225,000 240,000

Survey,

Investigation and

Design

65,000 65,000 170,000 120,000

TOTAL $ 710,000 $720,000 1,300,000 $1,325,000

TABLE 3.7

ECONOMIC ANALYSIS OF ADDITIONAL DETENTION BASINS ON

BLACKMANS SWAMP CREEK AND RIFLE RANGE CREEK




Cost of Basins (with $1 Million

allowance for upgrading local

stormwater system in CBD)

$ x 106

5.1 5.1 5.1





East Orange Channel

In accordance with the recommendations of the LMCE, 1997 stormwater management study, Council

constructed a detention basin on the headwaters of the catchment, upstream of McNeilly Avenue,

denoted Basin RB 6. In the 1997 study, the peak outflow from the low level outlet was assessed at

3 m3/s for the 1% AEP flood and consequently the basin had a powerful effect in mitigation of

downstream flows. At Ridley Park, located upstream of Bathurst Road, the post-basin discharge was

estimated at 22 m3/s. SKM, 1980 had previously proposed the construction of a large detention basin

of 45,000 m3 storage capacity in Ridley Park which when modelled in the LMCE, 1997 study, resulted

in a reduction in downstream 1% AEP peak flow to 5 m3/s. As expected, the two basins would have

reduced peak flows along the downstream reach in the East Orange Creek as far as its junction with

Blackmans Swamp Creek.

However, it appears that the low level outlets at Basin RB6 which were recommended in the

LMCE, 1997 study were increased in size, possibly to reduce peak storage levels and mitigate

backwater flooding in the areas surrounding the basin. This has resulted in a lesser degree of

attenuation in downstream peak flows than previously modelled by LMCE, 1997. The model results

presented in the Flood Study, 2005 show that the peak outflow from Basin RB6 in the event of a 1%

AEP flood would be 17.6 m3/s (compared with 3 m

3/s in 1997), increasing to 42 m

3/s at Ridley Park.

More recently, a Master Plan was prepared by Council for the Ridley Park area which envisaged a

sporting field at the downstream end, linked to a park at the upstream end.

A major basin of the type envisaged in the previous stormwater management studies would require a

large excavation and high embankments up to 4 m in height to achieve the 45,000 m3 envisaged and

would not be compatible with the Master Plan. Also, there is residential development on the northern

bank of the channel where the proposed embankment had been continued upstream to form the

storage in the SKM, 1980 concept. The basin embankment would be quite high and could affect the

visual amenity of these residents.

On environmental grounds, a two - compartment basin with modest re-grading of the surface and low

embankments which do not detract from the visual amenity is all that appears feasible, given the

Master Plan and the adjacent developments. Preliminary calculations using existing survey showed

that a storage of 17,000 m3 in each compartment could be achieved.

Hydrologic modelling of the basin for the 1% AEP flood, using the discharge hydrograph derived from

the Flood Study, 2005 which had a peak of 42 m3/s through the area proposed for the basins, showed

that the basin compartments would be quickly overtopped and there would not be a significant

reduction in peak flows passing downstream.

Accordingly, it is concluded that implementation of a detention basin in Ridley Park would not be a

cost effective flood mitigation measure.

3.4 Property Modification Measures

3.4.1 Considerations for Setting Flood Planning Level

Flood Planning Level – Selection of the Flood Planning Level (FPL) for an area is an important and

fundamental decision as the standard is the reference point for the preparation of floodplain

management plans. It is based on adoption of the peak level reached by a particular flood (the




“Planning Flood”) plus an appropriate allowance for freeboard. It involves balancing social, economic

and ecological considerations against the consequences of flooding, with a view to minimising the

potential for property damage and the risk to life and limb. If the adopted FPL is too low, new

development in areas above the FPL (particularly where the difference in level is not great) may be

inundated relatively frequently and damage to associated public services will be greater.

Alternatively, adoption of an excessively high flood planning level will subject land that is rarely

flooded to unwarranted controls.

In the Blackmans Swamp Creek floodplains, there are a number of land use classes which need to

be considered in terms of setting appropriate FPLs:

Residential

Commercial and industrial

Essential Community Facilities, Critical Utilities and Flood Vulnerable Residential

development, which require special consideration. Examples of these land uses include

schools, hospitals, retirement homes, evacuation centres, telecommunication, electricity

and water supply facilities.

Councils are responsible for determining the appropriate FPL’s within their local government area.

Whilst the flood used to determine the residential FPL is a decision of the Council, the FPM, 2005

highlights that FPL’s for typical residential development would generally be based around the 1% AEP

flood, plus an appropriate freeboard (typically 500 mm).

The circular issued by the Department of Planning on 31 January 2007 contained a package of

changes clarifying flood related development controls to be applied on land in low flood risk areas

(land above the 1 in 100 year flood). The package included an amendment to the Environmental

Planning and Assessment Regulation 2000 in relation to the questions about flooding to be answered

in Section 149 planning certificates, a revised ministerial direction (Direction 15) regarding flood prone

land (issued under Section 117 of the EP&A Act, 1979) and a new Guideline concerning flood-related

development controls in low flood risk areas. The Circular advised that Councils will need to follow

both the Floodplain Development Manual, 2005 as well as the Guideline to gain the legal protection

given by Section 733 of the Local Government Act.

The Department of Planning Guideline confirmed that unless exceptional circumstances applied,

councils should adopt the 1% AEP flood (1 in 100 year flood) with appropriate freeboard as the FPL

for residential development. In proposing a case for exceptional circumstances, a Council would need

to demonstrate that a different FPL was required for the management of residential development due

to local flood behaviour, flood history, associated flood hazards or a particular historic flood. Unless

there were exceptional circumstances, Council should not impose flood-related development controls

on residential development on land with a low probability of flooding, that is land above the residential

FPL.

Nevertheless, the safety of people and associated emergency response management needs to be

considered in low flood risk areas, which may result in:

Restrictions on types of development which are particularly vulnerable to emergency

response, for example, developments for aged care.

Restrictions on critical emergency response and recovery facilities and infrastructure.

These aim to ensure that these facilities and the infrastructure can fulfil their emergency




response and recovery functions during and after a flood event. Examples include

evacuation centres and routes, hospitals and major utility facilities.

Consideration of the data supports retaining the 1% AEP flood plus freeboard, along with a graded set

of controls depending on the location within the floodplain, as the basis for setting the FPL for

residential, commercial and industrial development in the floodplain and for adoption of a higher FPL

for essential services and uses requiring special consideration as identified above.

3.4.2 Outline of Proposed Flood Policy

Features of the proposed Flood Policy for the Blackmans Swamp Creek study area, which is set out in

draft form in Appendix C are:

The floodplain, or extent of flood prone land, has been defined as the area inundated by

the PMF. The flood prone land has been divided into three precincts using:

a) the depth and velocity criteria presented in the Floodplain Development Manual, 2005

for the provisional categorisation of land into High and Low Hazard zones and

consideration of the factors presented in Table 2.1 in confirming the hazard

categorisation

b) the hydraulic categorisation presented in DHI, 2005 Flood Study to define floodway

and flood fringe areas.

The “High Flood Risk Precinct” is the most flood affected land and extends to the

boundary of the High Hazard zone for the 1% AEP flood. The “Medium Flood Risk

Precinct” extends beyond the High Hazard Precinct to the extent of the 1% AEP. The

remainder of land inundated up to the extent of the PMF lies within the “Low Flood Risk

Precinct”.

The Policy nominates the minimum floor level (Flood Planning Level) for new residential

development in flood prone areas which is based on the peak 1% AEP flood level, plus an

allowance of 500 mm for freeboard. The Policy considers that new residential

development is an unsuitable use for land which is located in the High Flood Risk

Precinct. The High Flood Risk Precinct is a narrow strip of land running along the

centreline of each stream and includes several streets in the CBD area which act as

floodways during major flood events.

There is also a commercial and industrial area on the upper reaches of East Orange Creek

between McNeilly Avenue and Little Brunswick Street which functions as an overland flow

path when flows exceed the capacity of the piped trunk drainage system. This area is

located in the High Flood Risk Precinct. The Policy recognises that because overland

flow velocities would be significant, new commercial and industrial development in this

area could result in an adverse re-direction of flows towards existing developments in the

floodplain and that special precautions need to be taken to prevent this occurrence.

Accordingly, the Policy requires new commercial and industrial development to be

designed to minimise obstructions to the passage of floodwaters, as well as providing

minimum floor levels above the peak 1% AEP flood level plus 500 mm of freeboard.

These requirements will ensure that floor levels are above the level of major flooding and

reduce the potential for flood damages both to the development itself as well as

surrounding properties. In addition, development applications will need to be supported by

a Flood Report which includes the results of hydraulic analysis demonstrating that Policy

objectives are met. Requirements in this regard are outlined in Appendix C.




The other High Flood Risk Precinct areas are in several streets of the CBD, which

function as floodways during major floods. There, flood related controls relate to setting

minimum floor levels of commercial and industrial properties fronting those streets (the

building footprints themselves are situated in the low flow velocity Medium Flood Risk

Precinct area) and ensuring that car parks are set at appropriate levels (including the

entrances to below ground car parks).

In the Medium Flood Risk Precinct, the Policy nominates the peak 1% AEP flood level

plus 500 mm freeboard as the Flood Planning Level for new residential and commercial

and industrial development. The policy considers that flow velocities are not likely to be

significant in the Medium Flood Risk Precinct.

There would be no flood related development controls over residential or commercial and

industrial development in the Low Flood Risk Precinct, apart from the minimum floor

level requirement of peak 1% AEP flood level plus 500 mm of freeboard. This requirement

will ensure that floor levels of new developments located on ground slightly outside the

extent of the 1% AEP flood are no lower than equivalent properties within the Medium

Flood Risk Precinct.

Location of essential community facilities, vulnerable development and critical utilities,

schools and housing for aged persons and people with disabilities and other land uses

require a higher level of protection against flooding. The Policy recommends that those

special categories of land use be excluded from the floodplain.

3.4.3 Voluntary Purchase of Flood Prone Residential Property

This flood management measure involves the purchase of properties by Council for subsequent

rezoning for more appropriate land use. These properties which are usually located in high hazard

areas would be purchased at an equitable price and only where voluntarily offered.

Several properties are located in the 1% AEP high hazard zone bordering East Orange Creek.

Details of the depths of flooding in those properties at both the 1% and 5% AEP are shown on

Table 3.8.

TABLE 3.8

DETAILS OF RESIDENCES IN 1% AEP HIGH HAZARD ZONE

FLOODED BY 1% AND 5% AEP FLOODS

Flooded by 1% AEP Flood Flooded by 5% AEP Flood

Location No. of Residences

Flooded

Max Depth of Inundation

– m

No of Residences

Flooded


– m

Autumn Street 4 0.3 2 0.2

Byng Street 3 0.2 1 0.1

Summer Street 1 0.6 1 0.3

Total 8 0.6 4 0.3

Table 3.9 shows the results of an economic analysis of a Voluntary Purchase scheme. The analysis

has been carried out for the three discount rates nominated by NSW Treasury Guidelines for the




economic analysis of public works. The benefits of the scheme comprise the present worth value of

the flood damages for the residential properties which would be saved by their purchase. For the

analysis the costs were based on an average purchase cost of $300,000 per property, typical of

recent sale prices in the area.

TABLE 3.9

ECONOMIC ANALYSIS OF VOLUNTARY

PURPOSE SCHEME FOR BLACKMANS SWAMP CREEK CATCHMENTS




Cost of Scheme $ x 106 2.4 2.4 2.4


It is clear from the above analysis that a voluntary purchase scheme would not be justified on

economic grounds and was not favoured by the community.

Although the area is subject to “flash flooding” with little warning time, flooding in the street system is

relatively shallow, of short duration and there is ready access to high ground. Accordingly, it is

considered that a voluntary purchase scheme would not be justified on social grounds.

3.4.4 Flood Proofing by Raising Floor Levels of Residential Property

This term refers to procedures undertaken, usually on a property by property basis, to protect

structures from damage by floodwaters. The most common process is to raise the affected house by

a convenient amount so that the floor level is at or above the FPL. For weatherboard and similar

buildings this can be achieved by jacking up the house, constructing new supports, stairways and

balconies and reconnecting services. Alternatively, where the house contains high ceilings, floor

levels can be raised within rooms without actually raising the house. It is usually not practical to raise

brick or masonry houses. Most of the costs associated with this measure relate to the disconnection

and reconnection of services. Accordingly, houses may be raised a considerable elevation without

incurring large incremental costs.

The State and Federal Governments have agreed that flood mitigation funds will be available for

house raising, subject to the same economic evaluation and subsidy arrangements that apply to other

structural and non-structural flood mitigation measures. In accepting schemes for eligibility, the

Government has laid down the following conditions:

House raising should be part of an adopted Floodplain Management Plan

The scheme should be administered by the local authority.

The Government also requires that Councils carry out ongoing monitoring in areas where subsidised

voluntary house raising has occurred to ensure that redevelopment does not occur to re-establish

habitable areas below the design floor level. In addition, it is expected that Councils will provide




documentation during the conveyancing process so that subsequent owners are made aware of

restrictions on development below the design floor level.

Council’s principal role in subsidised voluntary house raising would be to:

Define a habitable floor level, which it will have already done in exercising controls over

new house building in the area

Guarantee a payment to the builder after satisfactory completion of the agreed work

Monitor the area of voluntary house raising to ensure that redevelopment does not occur

to re-establish habitable areas below the design floor level

Table 3.10 shows details of residences located in 1% AEP low hazard areas of the floodplain which

would be inundated above floor level. Nine of these properties would be flooded by a 5% AEP flood.

TABLE 3.10

DETAILS OF RESIDENCES IN 1% AEP LOW HAZARD ZONE

FLOODED BY 1% AEP FLOOD

Location No. of Residences

Flooded


– m

Prince Street 2 1.0

Autumn Street 8 0.3

Summer Street 5 0.6

Byng Street 6 0.2

McLachlan Street 5 0.2

Total 26 1.0

Table 3.11 is an economic analysis of a house raising strategy for the three discount rates assuming

that all of the properties could be raised. The benefits of the scheme comprise the present worth value

of the flood damages for the residential properties which would be saved by their raising. If the houses

were raised to at least the 1% AEP flood level plus an appropriate freeboard then the scheme’s

benefits would comprise the damages up to that flood. For the analysis the costs were based on an

average cost of raising of $70,000 per property.

TABLE 3.11

ECONOMIC ANALYSIS OF HOUSE RAISING

SCHEME FOR BLACKMANS SWAMP CREEK



(Damages Saved) $ x 106 0.61 0.48 0.38

Cost of Scheme $ x 106 1.82 1.82 1.82





This strategy is not economically feasible for the study area. Apart from two properties in the low lying

area near the intersection of Prince Street and Peisley Street, most of the residences would be

subject to comparatively shallow inundation. Site inspection showed that these two properties were of

brick construction and therefore would be technically difficult to raise. There is ready access to high

ground for all of these properties. Accordingly, it is considered that a scheme for raising flood prone

houses could not be justified on social grounds and has not been considered further.

3.5 Flood Response Modification Measures

3.5.1 Flood Forecasting, Warning and Evacuation Planning

Flood forecasting and warning can be an effective flood management measure if there is sufficient

warning time for the community to react to the warning. An effective flood warning system has three

key components, i.e. a flood forecasting system, a flood warning broadcast system and an evacuation

plan.

Flood response to rainfall on the Blackmans Swamp Creek catchments is relatively short and is

expected to be between around 30 minutes to 1 hour. (i.e. from the commencement of heavy rainfall

to the occurrence of the flood peak in the catchments).

A recent workshop was sponsored by Bureau of Meteorology to develop guidelines for the NSW

Flood Warning Consultative Committee to co-ordinate funding proposals for local flash flood warning

systems. Three levels of local flash flood warning system were identified:

General System – relies on existing warning services provided by the Bureau of

Meteorology for severe weather and thunderstorms as well as Flood Watches. These

services are typically issued on a regional basis, or for a larger catchment than Blackmans

Swamp Creek. These warnings can be augmented by real time information from local

weather radars, automatic weather stations and existing rainfall and river gauges.

Intermediate System – General system plus additional rain and river gauges within the

targeted flash flood catchment to help local emergency personnel to assist the community

through improved evaluation and management of the flash flood threat.

Total Warning System – Intermediate system plus a targeted warning dissemination

system to people located on the high flood hazard sites where evacuation may be

necessary.

While all systems need to be underpinned by an appropriate public flood awareness program, the

Total Warning System will require a more comprehensive and recurrent public flood awareness

campaign.

The Total Warning System is recommended for further consideration in the FRMP for Blackmans

Swamp Creek. It would be based on the “READY”, “SET”, “GO” warning phases as follows:

READY – flooding is possible in a general area; monitoring of weather is required.

SET – flooding is more likely in a specific area; prepare to act.

GO – flooding is very likely in a specific area; Action required.




The advantages of the Total Warning System over the two lesser systems are:

Enhanced reduction in risk to life and property from flash flooding through precautionary

actions triggered by general warnings, as per the General System (i.e. READY and SET

phases), and targeted Bureau of meteorology Flash Flood Warnings based on the

predicted exceedence of flash flood thresholds (GO phase), being directly communicated

to the affected community.

Reduction (compared with the Intermediate System) in risk to life and property from flash

flooding by better local emergency response and management, through the Bureau

providing forecasts for the exceedence of flood thresholds for the area.

The six components of the Total Warning System are:

1. Predictions

Bureau of Meteorology warnings and information from radar, AWS and rain and river

gauges as per the Intermediate System used to trigger “READY” and “SET” phases.

Targeted Flash Flood Warnings issued by the Bureau of Meteorology for the

exceedence of Flash Flood Thresholds based upon information from the FRMS for

the area to trigger the “GO” phase. Depending on the information from flood

modelling, predictions may be issued for flood/no flood scenarios or for levels of

flooding resulting from floods of various probabilities of occurrence.

2. Interpretation

Areas likely to be flooded determined from flood maps, from the flood modelling

results or studies for the area, and from SES flood intelligence.

3. Warning message Construction

Pre-determined flash flood warning messages for the specific areas.

4. Communication

Warnings broadcast by media and available on the BOM website.

Warnings directly communicated to the affected area either automatically or manually,

depending on the size of the catchment, population size and available SES

resources.

5. Response

Pro-active community and SES response underpinned by local recurrent public flood

awareness/education program.

6. Review

Performance of the system after each major flood.

Regular review of the readiness and maintenance of system components such as

gauges, communications, public education and planning.

An investigation to test the feasibility of the system would be required prior to further consideration

(indicative cost $80,000). Depending on the results of the feasibility study, the cost of implementing




the system would be about $200,000 initially, with recurrent annual maintenance costs between

$10,000 and $30,000.

Funding to establish local flash flood warning systems has traditionally been made available on the

basis of no Council contribution to the initial capital cost in recognition of the high maintenance costs

which Council would have to meet. The costs of maintaining the system would include such items as

rain and river gauges, warning communication systems and ongoing public awareness/education

programs. The maintenance obligations would need to be identified and included in any initial funding

grant. Upon installation of the local flash flood warning system, the SES Flood Plan for the area could

be used to document the operation and maintenance specifications of the system, including public

education/awareness components.

3.5.2 Public Awareness

Community awareness and appreciation of the existing flood hazards in the floodplain would promote

proper land use and development in flood affected areas. A well informed community would be more

receptive to requirements for flood proofing of buildings and general building and development

controls imposed by Council. One aspect of a community’s preparedness for flooding is the “flood

awareness” of individuals. This includes awareness of the flood threat in their area and how to protect

themselves against it. It is fair to assume that the level of awareness drops as individuals’ memories

of previous experience dim with time.

Means by which community awareness of flood risks can be maintained or may be increased include:

1. Teaching about floods in schools.

2. Sending out regular information with rates notices. The information contained in this present

study could be edited and used by SES to prepare a Flood Information Brochure.

3. Displays at Council offices using the information contained in the present study.

4. Educational videos and photographs of historic flooding in the area.

5. Talks by SES officers with participation by Council and longstanding residents with first hand

experience of flooding in the area.

Flood Information Brochure

The Flood Information Brochure which could also form a component of the education process

associated with the Flash Flood Warning system should contain information on:

What steps to take in advance.

Developing procedures for lifting contents above flood level and evacuating property.

The benefits of a regular flood-preparedness campaign would extend to more than just reducing

monetary losses. The campaign would also have social benefits by improving people’s feeling of

control, since they would have a better idea of how to respond to a flood emergency. However, given

the lack of significant flooding in the area since the last major flood in 1999, it may be difficult to

generate the interest and co-operation required. These difficulties will need to be considered in

planning any public awareness exercise.




Business Floodsafe Plans

SES has recently published a Toolkit manual to allow commercial and industrial businesses to

prepare a Business Floodsafe Plan based on their individual risk. As flooding in the business area is

comparatively shallow and of short duration, flood proofing measures to exclude floodwaters from

individual properties should be considered as a measure for mitigating damaging flooding. Following

acceptance of this Floodplain Risk Management Study it would be appropriate for Council and SES to

inform owners of the risk and for SES to introduce the Toolkit at seminars with the business

community.

3.6 Summary

This Chapter has reviewed a number of potential floodplain management measures. Preliminary

analysis of the flood modification measures (i.e. involving the construction of engineering works) has

been undertaken and indicative cost estimates prepared on the basis of available survey data. The

findings are summarised in Table 3.12.

Continuation by Council of the successful strategy of reducing flood peaks by detention

basins is supported. Several promising basin sites have been identified on the upper

reaches of Blackmans Swamp Creek and Rifle Range Creek and subjected to preliminary

hydrologic modelling and cost assessment. The studies have shown that the potential

exists for a considerable reduction in flood affectation in the CBD area. Further

investigation with the benefit of additional survey information is recommended in the

FRMP as a management measure.

On the East Orange Channel, improvement works are supported to increase the

conveyance capacity of the creek. There does not appear to be scope for further

reductions in flood peaks by the implementation of additional detention basins on this

catchment.

Planning controls separately or in combination with the above measures are an essential

component of the Floodplain Risk Management Plan. A draft Flood Policy has been

developed in Appendix C aimed at ensuring that future development in the floodplain is

compatible with the flood risk.

Response modification measures which are supported comprise investigation and

implementation of the Flash Flood Warning System as outlined previously.

The flood awareness programme (which would also form a component of the Flash Flood

Warning System), whilst concentrating on reducing flood risk in residential areas, should

also give attention to reducing potential damages to the commercial and business

community. With input from SES the recently developed Business Floodsafe toolkit could

be used to assist with planning for a flood emergency.




TABLE 3.12

REVIEW OF POTENTIAL FLOOD MANAGEMENT MEASURES

Scheme Comments

Maintenance of Creek Capacity

and Debris Control

This measure was proposed in the Orange Stormwater Management Study, 2004.

Although it would support environmental objectives and would reduce the

possibilities of blockage of culverts in the creek drainage systems, it is not strictly

speaking a flood management measure within the ambit of this present

investigation.

Enlarge creek channels Existing detention basins on the upper reaches of the catchments reduce flows on

Upper Blackmans Swamp Creek and Rifle Range Creek to the extent that major

floods are contained within the proximity of the respective channels. Downstream

of Kite Street, flows are conveyed beneath the CBD by a culvert system which has

the capacity to capture 5% AEP peak flows from Blackmans Swamp Creek,

provided it operates at full hydraulic capacity without any restriction imposed by

blockage of its inlets. There are however, surcharges of the local stormwater

system due to flows not being able to enter the main CBD culverts. These

surcharges result in damaging flooding within the CBD at the 5% AEP level.

Though marginally economical on the basis of Benefit/Cost ratio, it would be

technically difficult and probably not financially feasible to increase hydraulic

capacity with additional culverts beneath the CBD.

Although the East Orange Creek system is controlled by a large detention basin

(RB6) on its upper reaches, flows surcharge the basin spillway at the 5% AEP

level and result in damaging flooding to downstream residential and commercial

properties bordering the creek. Flooding problems are exacerbated because most

of the local road crossings have restricted hydraulic capacity resulting in “heading

up “ of water levels and capture of flows by the streets.

Orange City Council has commenced a program of creek improvements

commencing at the downstream end near the junction with Blackmans Swamp

Creek. Indicative budgets and priorities for continuing these works are presented in

Table 3.2.

The Blackmans Swamp Creek system downstream of its confluence with East

Orange Creek surcharges during major flooding. The surcharges result from

restrictions in the capacity of the channel and heading up of water levels at road

crossings. The resulting backwater influence is felt as far upstream as March

Street on the East Orange Creek and the outlet of the CBD culvert on Blackmans

Swamp Creek. However, channel enlargement would require extensive resumption

of land and is probably not financially feasible.

Construct permanent Levees Due to the “flash flooding” nature of flooding on the creek system, levees would

need to have a very high hydrologic standard to prevent their being suddenly

overtopped in the event of flooding greater than the design event. It is not

technically or environmentally feasible to construct levees to the PMF level on any

of the creeks. In addition, it is not technically feasible to re-route the local

stormwater system on the “protected” side of the levee so that drainage is

maintained without back flooding when creek levels rise during flood events.

Levee systems in the Blackmans Swamp Creek system are not considered

feasible.




TABLE 3.12 Cont’d

Construct additional Detention

Basins

Reduction in peak flows on Blackmans Swamp Creek could be achieved sufficient

to allow the 1% AEP peak discharge to enter the CBD culvert at Kite Street. The

following basins would be required upstream of the CBD:

Basin RB7 on Blackmans Swamp Creek at Sampson Street/Lamrock

Street downstream of the existing basin in Pilcher Park.

Basin RB8 in Moulder Park on Blackmans Swamp Creek downstream

of its confluence with Rifle Range Creek.

Raising the existing Agricultural Research Centre dam (Basin RB1) on

Rifle Range Creek to include a flood mitigation storage component.

Formalising the existing storage area on Rifle Range Creek just

upstream of the Orange Broken Hill Railway embankment with new

Basin RB2. This measure would also reduce the current risk to public

safety imposed by potential failure of the railway embankment during

flooding.

It is not feasible to provide additional basins on East Orange Creek downstream of

Basin RB6 to mitigate flooding. A basin in Ridley Park would not be effective in

reducing downstream flood peaks.

Voluntary Purchase of Residential

Property

This measure is sometimes employed to remove residential development from

high risk areas of the floodplain. There are several properties located in nominal

high hazard areas on East Orange Creek. However, implementation of a voluntary

purchase scheme for the Blackmans Swamp Creek system is not economically

justified. In view of the relatively shallow and short duration of flooding which

would be experienced in these residences and the ready access to high ground

from the flood affected areas, the scheme could not be justified on social grounds.

House Raising This measure is sometimes employed to raise residential development in medium

and low hazard areas of the floodplain. There are flooded properties located on

East Orange Creek and in a low lying area on Blackmans Swamp Creek at the

intersection of Prince and Peisley Streets which could be considered.

Implementation of a house raising scheme for the Blackmans Swamp Creek

system is not economically warranted. In view of the relatively shallow and short

duration of flooding which would be experienced in these residences the scheme

could not be justified on social grounds.

Planning Controls and

Implementation of Flood Policy

(DCP)

This is a low cost and essential component of the Floodplain Risk Management

Plan and will over time reduce damages. A draft Flood Policy DCP has been

developed in Appendix C. The Policy recommends a graded set of controls for

development, which depends on the nature of the development and its location

within the floodplain which has been sub-divided into High, Medium, and Low

Flood Risk Precincts. The 1% AEP plus 500 mm allowance for freeboard is

recommended as the Flood Planning Level for residential and commercial

development, with a higher level of protection for essential facilities, critical

infrastructure and flood vulnerable development.

Flood Warning and Forecasting Because of the flash flooding nature of the catchment it is not technically feasible

to provide extended warning times with a conventional flood warning system.

However a Flash Flood Warning System along the lines of the system identified

by BOM at a recent workshop would reduce the flood risk in the urban centre of

Orange. A feasibility study should be carried out for such a system.

SES and other emergency management authorities should use the flood

information contained in this Study to update their procedures for flood response

and evacuation.

Flood Awareness Continuation of Council’s policy of notifying flood affectation on S149 Certificates

for properties impacted by floods up to 1% AEP is supported.

BLACKMANS SWAMP CREEK

FLOODPLAIN RISK MANAGEMENT STUDY

Figure 3.1

AUGMENTATION OF STORMWATER SYSTEMKITE STREET TO RAILWAY CULVERT

1

a

1

a

March St

Prince St

4200

x2464

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Existing Culvert System

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ScaleN

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BLACKMANS SWAMP CREEKFLOODPLAIN RISK MANAGEMENT STUDY

Figure 3.2

EXISTING AND POTENTIAL DETENTION BASINS

a

a

a

a

Pl

Lord

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STP

RB7

RB4

RB1

RB2

RB5

RB6

RB3b

RB3a

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Piped Section of Creek

Open Channel of Creek

Informal Detention Basin

Existing Formal Detention Basin

Railway Line

Potential Basin

LEGEND

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PilcherPark

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EAB01


PHOTOGRAPHS OFEAST ORANGE CREEK

a

Improved Channel

March Street to William Street

Plate 1p

Triple Pipe CulvertAt March Street

Plate 2P

Twin Pipe CulvertMcLachlan Street

Plate 3P

Channel upstream Byng Street

Plate 4P

CulvertSummer Street

Plate 5P

Channel upstream Summer Street

Plate 6P


PHOTOGRAPHS OFLOWER BLACKMANS SWAMP CREEK

aaaa

Looking Upstream Towards Junction with East Orange Creek

Plate 7P




4 SELECTION OF FLOODPLAIN MANAGEMENT MEASURES

4.1 Background

The Floodplain Development Manual, 2005 requires a Council to develop a Floodplain Risk

Management Plan based on balancing the merits of social, economic and environmental

considerations which are relevant to the community. This chapter sets out a range of factors which

need to be taken into consideration when selecting the mix of works and measures that should be

included in the overall Floodplain Risk Management Plan.

The community will have different priorities and, therefore, each needs to establish its own set of

considerations used to assess the merits of different options. The considerations adopted by a

community must, however, recognise the State Government requirements for floodplain management

as set out in the Floodplain Development Manual, 2005 and other relevant policies. A further

consideration is that some elements of the Plan may be eligible for subsidy from State and Federal

Government sources and the requirements for such funding must, therefore, be taken into account.

Typically, State and Federal Government funding is given on the basis of merit as judged by a range

of criteria:

Degree of flood hazard and number of properties affected.

Damage caused by flooding and the benefit/cost ratio of proposals.

The importance given to strategic planning in the overall Plan.

Compatibility of proposals with TCM and other government policies.

Community involvement in Plan preparation.

Availability of local funding for proposed works.

The issues which need to be considered in developing a Floodplain Risk Management Plan typically

fall under the following broad headings:

Community Expectations and Social Impacts

Natural Resource Management and Environmental Impact

Economic and Financial Feasibility

Technical Merit

A review of the range of considerations under the four headings listed above is presented in

Appendix D which also includes an analysis assessing the performance of the available options

against the factors to be considered.

4.2 Ranking of Options

The considerations discussed in Appendix D must be assessed in terms of their relative importance

to the community as well as the likelihood of attracting government subsidy. Although multi-objective

assessment methods are now well accepted by government for selecting from a range of options, the

decision to provide State funds is still linked closely to economic and financial factors.

A suggested approach to assessing the merits of various options is to use a subjective scoring

system. The chief merits of such a system are that it allows comparisons to be made between

alternatives using a common “currency”. In addition it makes the assessment of alternatives


Blackmans Swamp Creek Vol 1.doc Page 45 Lyall & Associates July 2009 Rev. 5.0 Consulting Water Engineers

“transparent” (i.e. all important factors are included in the analysis). The system does not, however, provide an absolute “right” answer as to what should be included in the plan and what should be left out. Rather, it provides a method by which the Council can re-examine its options and if necessary, debate the relative scoring given to aspects of the plan.

Each option is given a score according to how well the option meets the considerations discussed above. In order to keep the scoring simple the following system is proposed:

+2 Option rates very highly

+1 Option rates well

0 Option is neutral

-1 Option rates poorly

-2 Option rates very poorly

The scores are added to get a total for each option.

Based on considerations outlined in this chapter, Table 4.1 presents a scoring matrix for the options reviewed in Chapter 3. This scoring has been used as the basis for prioritising the components of the draft Floodplain Risk Management Plan. It should be noted that the proposed scoring and weighting shown in Table 4.1 was carefully reviewed by the Committee as part of the process of finalising the overall Floodplain Management Plan.

4.3 Summary

Table 4.1 indicates that there are good reasons to consider including the following elements into the Floodplain Management Plan:

� Detention Basins

� Channel Improvements (on East Orange Creek)

� Planning Controls.

� Flash Flood Warning System incorporating measures for increased Flood Awareness.

� Incorporation of the Catchment Specific information on flooding impacts contained in this Study in Emergency Management Procedures for the study area.

Property modification measures such as voluntary purchase of residential property or house raising schemes are not considered viable.

Rehabilitation of the creek systems as proposed in the Orange Stormwater Management Plan, 2004, whilst not a flood management measure, would be supported by the community on environmental grounds. However, careful attention in the design would be required to ensure that the works do not adversely affect flooding patterns.

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5 DRAFT FLOODPLAIN RISK MANAGEMENT PLAN

5.1 The Floodplain Risk Management Process

A comprehensive draft Floodplain Risk Management Plan (FRMP) has been prepared for the

Blackmans Swamp Creek catchment as part of a Government program to mitigate the impacts of

major floods and reduce the hazards in the floodplain. The Plan has been prepared as part of the

Floodplain Risk Management Process in accordance with NSW Government’s Flood Prone Land

Policy.

Figure 1.1 shows the elements of the Floodplain Risk Management Process in NSW. The first

steps in the process of preparing the FRMP were the collection of Flood Data and the review of

the Flood Study for Blackmans Swamp Creek prepared by DHI in 2005. The Flood Study was the

formal starting process of defining management measures for flood liable land and represented a

detailed technical investigation of flood behaviour.

5.2 Purpose of the Plan

The overall objectives of the Floodplain Risk Management Study and Plan were to assess the

impacts of flooding, review policies and options for management of flood affected land and to

develop an FRMP which:

Sets out the recommended program of works and measures aimed at reducing over

time, the social, environmental and economic impacts of flooding and establishes a

program and funding mechanism for the FRMP.

Proposes modifications to Council’s existing policies to ensure that the future

development of flood affected land is undertaken so as to be compatible with the flood

hazard and risk.

Ensures the FRMP is consistent with local emergency management planning.

Ensures that the FRMP has the support of the community.

5.3 The Study Area

This FRMP deals with the floodplain of the Blackmans Swamp Creek and its main tributaries Rifle

Range Creek and the East Orange Creek. For the purposes of this Plan the study area

comprises the 34 km2 Blackmans Swamp Creek catchment from its headwaters to the south east

of the City, extending downstream to the Sewage Treatment Plant.

5.4 Relevant Investigations

The Study and Plan drew on the results of several recent investigations on flooding in the study

catchments, including:

Blackmans Swamp Creek Stormwater Management Study, 1997, prepared by Lyall and

Macoun Consulting Engineers.

Blackmans Swamp Creek Flood Study, 2005, prepared by Danish Hydraulics Institute.

Orange Local Flood Plan, 2004, prepared by State Emergency Service.




5.5 Community Consultation

The Community Consultation process provided valuable direction over the course of the

investigations, bringing together views from key Council staff, other departments and agencies,

and importantly, the views of the community gained through:

The delivery of a Community Newsletter to property occupiers located in the floodplain,

as well as inclusion of the Newsletter on Council’s web site to allow the wider

community to gain an understanding of the issues being addressed as part of the

study.

Meetings of the Floodplain Management Committee to discuss the findings.

Exhibition of the draft Study Report and the holding of an Open Day at which a

representative of the Consultants was available to discuss the findings with interested

members of the public.

5.6 Structure of Floodplain Risk Management Study and Plan

The Blackmans Swamp Creek Floodplain Risk Management Study and Plan is incorporated in

two Volumes.

Volume 1 comprises the Floodplain Risk Management Study and draft Floodplain Risk

Management Plan and is supported by Appendices which provide additional details of the

investigations undertaken for the preparation of the Study and Plan. These Appendices are

bound in Volume 2.

A summary of the draft Floodplain Risk Management Plan proposed for the study area is shown

on Table 5.1 at the conclusion of this Chapter. The draft Plan is based on:

The continuation of Council’s detention basin strategy for flood mitigation by the

implementation of additional basins on Blackmans Swamp Creek and Rifle Range

Creek,

Improvements to the hydraulic capacity of East Orange Creek,

Planning and development controls for future development in flood prone areas,

Improvements to existing flood preparedness and awareness in the Blackmans Swamp

Creek community and,

Upgrading of flood warning and response procedures by the development of a Flash

Flood Warning System.

5.7 Flooding Pattern and Impact

5.7.1 Flood Pattern

Figure 2.1 shows the indicative extents of flooding for the 1% AEP and the Probable Maximum

Flood. Figure 2.3 shows residential and commercial properties which would be flooded above

floor level in the event of a 1% AEP design flood.




The extent of flooding and inundation of flood affected properties is indicative only, being based

on available contour mapping and the cross sections of the creeks and floodplains comprising the

hydraulic model developed in the Flood Study, 2005. It should not be used to identify the flood

affectation of individual properties, for which a site specific survey would be required.

5.7.2 Impacts of Flooding

Table 5.2 shows the number of properties which would be flooded to above floor level and the

damages experienced for the various classes of property in the Blackmans Swamp Creek study

area.

TABLE 5.2

ECONOMIC IMPACTS OF FLOODING

BLACKMANS SWAMP CREEK STUDY AREA

No. of Properties Flooded and Flood Damages

Residential Commercial

/Industrial Public Buildings

Total

Flood

Damages

Flood

Event

% AEP No. $ x 10

6No. $ x 10

6No. $ x 10

6$ x 10

6

5 26 1.2 68 16.5 6 0.5 18.2

2 31 1.5 83 19.9 6 0.7 22.1

1 38 1.8 104 27.0 8 1.2 30.0

0.5 48 2.2 121 34.5 13 1.6 38.3

PMF 302 20 286 436 23 87 543

5.8 Flood Modification Measures

Engineering works comprising detention basins and channel improvements are recommended

and are briefly reviewed below.

5.8.1 Detention Basins

Detention basins on the headwaters have mitigated flooding in downstream areas on Blackmans

Swamp Creek, Rifle Range Creek and East Orange Creek. However in the event of major

flooding extensive inundation of residential property and commercial and industrial development

could still occur. Several promising basin sites have been identified on the upper reaches of

Blackmans Swamp Creek and Rifle Range Creek and subjected to preliminary hydrologic and cost

assessment. These studies have shown that the potential exists for a considerable reduction in flood

affectation in the CBD area.

Further hydrologic analysis with the benefit of additional survey information, the preparation of

concept designs and refinement of cost estimates is required and has been included as a

recommended measure in the Plan, as the first step in the implementation of the basin strategy.


Blackmans Swamp Creek Vol1.doc Page 50 Lyall & Associates July 2009 Rev. 5.0 Consulting Water Engineers

5.8.2 Channel Works on East Orange Creek

Channel improvements and improvements to the hydraulic capacity of the channel and local road crossings are recommended on East Orange Creek to reduce flooding in residential developments bordering the creek between the Mitchell Highway and the junction with Blackmans Swamp Creek. This project would represent the continuation of Council’s existing programme of works to mitigate flooding on East Orange Creek.

A priority list of improvement works is presented on Table 3.2.

5.9 Property Modification Measures

The results of the Floodplain Risk Management Study indicate that an important measure for Orange City Council to adopt in the floodplain would be strong floodplain management planning applied consistently by all branches of Council. A draft Flood Policy has been prepared for the guidance of Council officers in the evaluation of development proposals and is included in Volume 2 of the Floodplain Risk Management Study as Appendix C.

The building and development controls set out in the policy involve the imposition of measures aimed at flood proofing future developments in flood affected areas. They include the specification of:

� Minimum habitable floor levels for residential and commercial and industrial developments (including appropriate freeboard provision);

� Appropriate flood compatible building materials.

� Egress routes from buildings.

� Controls over car parks, particular basement car parks in the Orange CBD to reduce the likelihood of water ingress from the street system in the event of flash flooding.

The floodplain of the Blackmans Swamp Creek within the City of Orange has been divided into three Flood Risk Precincts according to the level of the flood risk resulting from the flood producing mechanisms. The approximate extents of the various Flood Risk Precincts are shown in Figure 2.1 of Appendix C and comprise:

� High Flood Risk Precinct. This is the area within the envelope of land subject to a High Flood Hazard and Floodway categorisation in a 1% AEP flood, defined in accordance with the criteria outlined in the Flood Study, 2005 and the Floodplain Risk Management Study, 2007. In the High Flood Risk Precinct high flood damages, potential risk to life and evacuation problems may be expected, or development would significantly and adversely affect flooding patterns. In this precinct, there would be significant risk of flood damages without compliance with flood related building and planning controls. The High Flood Risk Precinct is a narrow strip of land running along the centreline of each stream. It includes the overland flow path on the upper reaches of East Orange Creek between McNeilly Avenue and Little Brunswick Streets, as well as several of the streets in the CBD area which convey flows surcharging the trunk drainage system during major flood events.




Medium Flood Risk Precinct. This is defined as the strip of land between the

boundary of the High Flood Risk Precinct and the line defining the 1% AEP flood

extent. In this precinct there would still be a significant risk of flood damages, but

these damages may be minimised by the application of appropriate development

controls.

Low Flood Risk Precinct. This is defined as all other land within the floodplain, i.e.

within the extent of the Probable Maximum Flood but not lying within the High and

Medium Flood Risk Precincts. In this precinct the risk of damages is low and no

flood related controls would apply to residential or commercial and industrial

development, apart from the minimum floor level requirements (see below).

The Policy requires the minimum floor level (Flood Planning Level) for new residential and

commercial and industrial development equal to the 1% AEP flood, plus an allowance of 500 mm for

freeboard. The Policy considers that new residential development is an unsuitable use for land which

is located in the High Flood Risk Precinct. Similarly, the Policy considers that new commercial and

industrial development is an unsuitable use for land which is located in High Flood Risk Precinct

areas within or bordering the open channels of the creek system.

New commercial and industrial development in the High Flood Risk Precinct zone of the overland

flow path on the upper reaches of East Orange Creek between McNeilly Avenue and Little Brunswick

Streets could result in an adverse re-direction of flows towards existing developments in the floodplain

unless special precautions were taken to prevent this occurrence. Accordingly, the Policy requires

new commercial and industrial development in that reach of the creek to be designed to minimise

obstructions to the passage of floodwaters by ensuring that there are adequate overland flow paths

available for the conveyance of flow which surcharges the capacity of the piped drainage system as a

result of the proposed development. These requirements will ensure that floor levels are above the

level of major flooding and will reduce the potential for flood damages to adjacent development. In

addition, development applications for new commercial and industrial development will need to be

supported by hydraulic analysis demonstrating that Policy objectives are met in this regard.

Requirements in this regard are outlined in Appendix C.

Minimum floor levels of garages and open car parks should not be lower than the 1% AEP level.

All openings to basement car parks should be above the FPL (1% AEP level plus 500 mm of

freeboard), in recognition of the “flash flooding” nature of the catchments and the hazardous

nature of these facilities should their entrances be surcharged.

The Policy recommends that Essential Community Facilities (eg schools, hospitals), Critical

Utilities and Flood Vulnerable Development (housing for aged persons and people with

disabilities) be excluded from the floodplain.

The Flood Policy is based on the recognition that individual developments should not be

evaluated in isolation, but rather, should be considered in a strategic sense as if it were one of

several developments in the area. Whilst individual developments in isolation may not have a

measurable impact on flooding, the cumulative impacts of ongoing development could be

significant.

New buildings or additions to existing buildings would be subjected to building controls with the

long term objective of mitigating flood affectation to all buildings in the floodplain. The Policy

recognises that controls need to be imposed on a merit basis, balancing restrictive development

conditions with the impact of development on flood behaviour in the floodplain.




5.10 Indicative Flood Extents

The plans showing the extents of flooding and flooded properties (Figures 5.1 and 2.3) are

indicative only, being based on available 2 m contour mapping and limited cross sections of the

creeks and their floodplains. This level of accuracy in the flood mapping is supported by DECC,

as the costs associated with undertaking detailed ground survey in each flood affected property

presently lies outside the scope of the NSW Government’s floodplain program.

Under the program, it is Council’s responsibility to identify the flood risk within the floodplain and

prepare maps showing indicative flood extents, with the onus being on the property owner to

carry out sufficient survey to allow a more accurate picture of flood affection to be described in

his allotment.

To allow Council to assess individual development proposals, a detailed site survey would be

required to allow the extent of flooding and the flood hazard to be evaluated using the results of

the Blackmans Swamp Creek Flood Study, 2005 and the revision of the PMF undertaken for the

preparation of this FRMP. For this reason, applicants will be required to submit a detailed survey

plan of the site for which development is proposed.

It would, however, assist Council with the operation of the draft Flood Policy and development of

the recommended Flash Flood Warning System if the extent and depths of inundation in flood

prone areas bordering the creeks could be identified with greater accuracy than is presently

possible. This could be achieved at comparatively modest cost by undertaking an Airborne Laser

Survey of the study area (possibly extended at minimal cost to the whole of Orange), which would

achieve accuracies in defining natural surface levels in the range 150-200 mm. This would be a

major improvement on the accuracy of existing mapping sources and would also assist Council in

the planning and design of other engineering and town planning disciplines (roads, stormwater

management, strategy studies and the like). However, as mentioned the cost of the survey would

be outside the scope of the NSW Government’s floodplain program and would therefore be borne

by Council.

5.11 Voluntary Purchase of Residential Property

Removal of housing is a means of correcting previous decisions to allow buildings in high hazard

areas in the floodplain. The voluntary purchase of residential property in hazardous areas has

been part of subsidised floodplain management programs in NSW.

The review undertaken in the Floodplain Risk Management Study showed that implementation of

a large scale voluntary purchase scheme was not economically viable and could not be justified

on social grounds.

5.12 Raising Floor Levels of Residential Property

The analysis undertaken in the Floodplain Risk Management Study showed that the

implementation of a voluntary house raising program was not economically viable and could not

be justified on social grounds.




5.13 Response Modification Measures

5.13.1 Flood Warning and Response

The floor levels of properties potentially affected by flooding have been surveyed, or estimated

from available topographic survey. Plans have been prepared as part of this present study,

showing the indicative extent of flooding, high hazard areas and the locations of flooded

properties (Figures 5.1, 2.3 and 2.4). Consequently there is information available to identify

areas at risk from flooding for the full range of flood events likely to trigger flood response

procedures (Note, however, that this information could be refined with the ALS survey mentioned

at the conclusion of Section 5.10).

The Orange Local Flood Plan, 2004 should be reviewed and further developed by SES so as to

produce a graded response plan involving:

Ranking the threatened houses according to their hazard situation, taking account of

depth and velocity of floodwaters, and means of access, as a flood develops.

Preparing a detailed response plan which focuses on initial evacuations from the most

hazardous locations, followed by further evacuations in descending exposure to

hazardous conditions.

Preparing a plan for traffic management, which takes account of the sequence of road

flooding as a flood develops. This plan would aim to:

maximise opportunities for the community to evacuate,

prevent unnecessary traffic through the affected area,

ensure access for SES operations.

5.13.2 Flash Flood Warning System

Further investigation into and implementation of a Flash Flood Warning system as outlined in

Section 3.5 is recommended, in conjunction with developing the graded response plan described

in the previous section.

5.13.3 Flood Awareness

A number of measures are recommended to maintain awareness in the community of the threat

posed by floods:

The draft Flood Policy of Appendix C of the Study should be reviewed, amended as

required and adopted by the Council as a Development Control Plan.

Council should continue to promote knowledge of the characteristics of flooding among

the affected property owners. These characteristics should include information on the

frequency of flooding and the depths at various locations. This information could be

included in a flood information booklet containing both general and site specific data

and distributed with the rate notices. The community should also be made aware that

a flood greater than historic levels or the planning level can, and will, occur at some

time in the future. The need for a flood response and preparedness plan to address

such an occurrence should be clearly explained.




The Floodplain Risk Management Plan should be publicised and exhibited in Council

offices and at community gathering places to make residents aware of the measures

being proposed.

Commercial and industrial business owners should be encouraged to develop

individual flood response and preparedness plans according to the “Business

Floodsafe” toolkit recently prepared by SES

5.14 Recommended Measures and Funding

Broad funding requirements for the recommended measures to be included in the draft Floodplain

Risk Management Plan are given in Table 5.1 below. These measures comprise a program of

engineering investigations and capital works, preparation of planning documentation by Council,

community education on flooding by SES to improve flood awareness and response and

investigation and implementation of a Flash Flood Warning system (Figure 5.1). They will over

time, achieve the objectives of reducing the flood risk to existing and future development for the

full range of floods.

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se

d o

n t

he

in

form

atio

n p

rese

nte

d in

th

e F

RM

S.

E

nco

ura

ge

pre

pa

ratio

n o

f B

usin

ess F

loo

dsa

fe P

lan

s f

or

ind

ivid

ua

l co

mm

erc

ial a

nd

in

du

str

ial d

eve

lop

me

nts

bo

rde

rin

g t

he

cre

eks,

ba

se

d o

n t

he

re

ce

ntly p

ub

lish

ed

SE

S T

oo

lkit.

This

measure

wo

uld

reduce f

lood losses p

art

icula

rly

in t

he

Ora

nge C

BD

and h

as a

hig

h p

rio

rity

. It d

oes n

ot

requir

e

Govern

ment

fundin

g.

Imp

rove

me

nts

in

hyd

rau

lic c

ap

acity o

f E

ast

Ora

ng

e C

ree

k

$3

.8 M

illio

n

A

sta

ge

d p

rog

ram

of

wo

rks is p

rese

nte

d in

Ta

ble

3.2

of

the

FR

MS

, co

mp

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g c

ha

nn

el im

pro

ve

me

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an

d e

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rge

me

nts

to

wa

terw

ays o

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gs.

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he

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of

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o b

e c

on

str

ucte

d b

y C

ou

ncil

as f

un

din

g p

erm

its.

This

measure

will

re

duce f

loo

din

g in r

esid

ential are

as

bord

erin

g t

he c

reek a

nd h

as a

hig

h p

rio

rity

. I

t re

qu

ire

s

Cou

ncil

and G

overn

ment

fun

din

g.

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asib

ility

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dy o

f A

dd

itio

na

l D

ete

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n

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sin

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n B

lackm

an

s S

wa

mp

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ek a

nd

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Ra

ng

e C

ree

k

$1

20

,00

0

Ele

me

nts

of

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asib

ility

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dy:

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urv

ey o

f P

ote

ntia

l B

asin

Site

s.

H

yd

rolo

gic

an

aly

sis

of

de

ten

tio

n b

asin

s t

o a

sse

ss r

ed

uctio

n in

do

wn

str

ea

m f

low

s.

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rag

e c

ha

racte

ristics t

o b

e b

ase

d o

n s

ite

su

rve

y.

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rep

are

co

nce

pt

de

sig

ns a

nd

re

fin

e c

ostin

gs p

rese

nte

d in

FR

MS

.

P

rep

are

a s

tag

ed

pro

gra

mm

e o

f d

esig

n a

nd

im

ple

me

nta

tio

n o

f re

co

mm

en

de

d b

asin

s.

This

measure

is t

he f

irst

ste

p in im

ple

menting a

dditio

nal

basin

s a

nd h

as a

hig

h p

rio

rity

in v

iew

of

the e

conom

ic

impacts

resultin

g f

rom

the o

ccurr

ence o

f a 1

% A

EP

flo

od.

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requ

ires C

ouncil

an

d G

overn

ment

fundin

g.

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nstr

uctio

n o

f a

dd

itio

na

l D

ete

ntio

n B

asin

s

on

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ckm

an

s S

wa

mp

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ek a

nd

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Ra

ng

e C

ree

k

$5

.1 M

illio

n

A

pro

gra

m o

f w

ork

s is p

rese

nte

d in

Ta

ble

3.6

of

the

FR

MS

, co

mp

risin

g n

ew

ba

sin

s a

nd

ra

isin

g t

he

exis

tin

g d

am

at

Ag

ricu

ltu

ral

Re

se

arc

h C

en

tre

.

C

osts

are

in

dic

ative

on

ly a

nd

are

to

be

re

fin

ed

in

th

e lig

ht

of

the

Fe

asib

ility

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dy.

C

osts

in

clu

de

an

allo

wa

nce

of

$1

Mill

ion

fo

r u

pg

rad

ing

lo

ca

l sto

rmw

ate

r syste

m t

o p

reve

nt

flo

od

ing

in

CB

D t

o 1

% A

EP

.

C

on

str

uctio

n t

o b

e s

tag

ed

b

y C

ou

ncil

as f

un

din

g p

erm

its

Imple

me

nta

tio

n o

f th

is m

easu

re w

ou

ld d

epe

nd o

n a

favoura

ble

outc

om

e f

rom

the F

easib

ility

Stu

dy a

nd t

he

availa

bili

ty o

f C

ouncil

an

d

Govern

me

nt

fundin

g.

To

tal

Es

tim

ate

d C

os

t $

9.3

Mil

lio

n




5.15 Implementation Program

The steps in progressing the floodplain management process from this point onwards are:

Floodplain Management Committee to consider and adopt recommendations of this

study. In particular, the Committee were to review the basis for ranking floodplain

management measures (as set out in Table 4.1 of the Floodplain Risk Management

Study and the proposed works and measures to be included in the proposed plan as

set out in Table 5.1). A meeting to discuss and approve the findings of the draft Study

and Plan was held in December 2008.

Exhibit the draft Study and Plan and seek community comment. The draft Study and

Plan were exhibited in June 2009.

Consider public comment, modify the document if and as required, and submit to

Council. In addition to Council comments, two responses were received from the

community. The draft Study and Plan were amended and submitted as this final

document in July 2009.

Council adopts the Plan and submits an application for funding assistance from the

Floodplain Management Program administered by DECC and/or the Natural Disaster

Mitigation Program administered by the State Emergency Management Committee and

other agencies.

As funds become available from DECC, other Government agencies and/or Council’s

own resources, implement the measures in accordance with the established priorities.

The Plan should be regarded as a dynamic instrument requiring review and modification over

time. The catalysts for change could include new flood events and experiences, legislative

change, alterations in the availability of funding, reviews of Council’s planning strategies and

importantly, the outcome of some of the studies proposed in this report as part of the Plan. In

any event, a thorough review every five years is warranted to ensure the ongoing relevance of the

Plan.

Scale

LE

GE

ND

BL

AC

KM

AN

S S

WA

MP

CR

EE

KF

LO

OD

PL

AIN

RIS

K M

AN

AG

EM

EN

T S

TU

DY

Fig

ure

5.1

FL

OO

DP

LA

IN R

ISK

MA

NA

GE

ME

NT

PL

AN

a aa a

N

BLA

CKM

ANS S

WAM

P CK

BLA

CKM

ANS S

WAM

P CK

RIFLERANGECK

RIFLERANGECK

MA

TT

HE

WS

PA

RK

MA

TT

HE

WS

PA

RK

EASTORANGECK

EASTORANGECK

PIL

CH

ER

PA

RK

PIL

CH

ER

PA

RK

05

00

10

00

m1

% F

LO

OD

EX

TE

NT

S

PM

F

NO

TE

: F

LO

OD

EX

TE

NT

S A

RE

BA

SE

D O

N C

RO

SS

S

EC

TIO

NA

L S

UR

VE

Y O

F T

HE

CR

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KS

AN

D 2

m C

ON

TO

UR

D

ATA

AN

D A

RE

IN

DIC

AT

IVE

ON

LY. T

HE

Y S

HO

ULD

NO

T B

E

US

ED

TO

AS

SE

SS

TH

E F

LO

OD

AF

FE

CTA

TIO

N O

F

IND

IVID

UA

L P

RO

PE

RT

IES

, F

OR

WH

ICH

A S

ITE

SU

RV

EY

W

OU

LD

BE

RE

QU

IRE

D.

Ch

an

ne

l Im

pro

ve

me

nts

an

dB

rid

ge

Wa

terw

ay E

nla

rge

me

nts

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tch

me

nt

Wid

e W

ork

s a

nd

Me

as

ure

s

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od

Mo

dific

atio

n-

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ten

tio

n B

asin

s B

lackm

an

s S

wa

mp

an

d R

ifle

Ra

ng

e C

ree

ks

- C

ha

nn

el Im

pro

ve

me

nts

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st

Ora

ng

e C

ree

k

be

twe

en

Ma

rch

an

d I

ce

ly S

tre

ets

Pro

pe

rty M

od

ific

atio

n-

Flo

od

Po

licy D

CP

(C

atc

hm

en

t W

ide

)

Re

sp

on

se

Mo

dific

atio

n-

Fla

sh

Flo

od

Wa

rnin

g S

yste

m-

Flo

od

Aw

are

ne

ss

- B

usin

ess F

loo

d P

lan

s f

or

Ind

ivid

ua

l

Co

mm

erc

ial/ I

nd

ustr

ial P

rop

ert

ies

BA

SIN

RB

7S

AM

PS

ON

/ L

AM

RO

CK

ST

MO

UL

DE

R P

AR

KB

AS

IN R

B8

BA

SIN

RB

2B

AS

INR

B2

RA

ISE

AG

RIC

UL

TU

RA

LR

ES

EA

RC

H C

EN

TR

E D

AM

(~ 1

KM

UP

ST

RE

AM

)B

AS

IN R

B1

LIT

TLE

BR

UN

SW

ICK

ST

McN

EIL

LY

AV

E




6 DEFINITIONS

Note: For expanded list of definitions, refer to Glossary contained within the NSW Government’s

Floodplain Development Manual, 2005.

TERM DEFINITION

Annual Exceedence

Probability (AEP)

The per cent probability of occurrence of a flood equal to or greater than

a particular magnitude. For example, the 1% AEP flood has a 1% chance

(i.e a one-in-100 chance) of being equalled or exceeded in any one year.

In relation to the economic life of structures, there is a 26% chance of the

1% AEP event being equalled or exceeded in a 30 year period, a 48%

chance of exceedance in a 50 year period and a 99.9% chance within a

100 year period.

Australian Height

Datum (AHD)

A common national surface level datum corresponding approximately to

mean sea level.

Probable Maximum

Flood

The maximum possible flood that could reasonably be expected to occur

at a particular location.

Floodplain The area inundated by the Probable Maximum Flood.

Flood Planning Level

(FPL)

Flood levels selected for planning purposes, as determined in the

Blackmans Swamp Creek Flood Study, 2005 and referenced in the

Floodplain Risk Management Study, 2009 and associated Floodplain

Risk Management Plan. For residential and commercial/industrial,

development in the floodplain, it is the flood levels derived from the 1%

AEP flood event, plus the addition of a 500 mm Freeboard.

Essential Community Facilities (eg. schools, hospitals), Critical

Infrastructure and Flood Vulnerable Development (eg housing for Aged

Persons and people with disabilities) should be excluded from the

floodplain.

Flood Prone/Liable

Land

Land susceptible to flooding up to Probable Maximum Flood.

Floodway Those areas of the floodplain where a significant discharge of water

occurs during floods, they are often aligned with naturally defined

channels. Floodways are areas that, even if only partially blocked,

would cause a significant redistribution of flood flow or a significant

increase in flood levels.

Freeboard The factor of safety usually expressed as a height above the peak level

of the Planning Level flood. Freeboard allows for factors such as wave

action, localised hydraulic effects, greenhouse and climatic change, as

well as accuracy of flood modelling data. The default value for freeboard

is 500 mm unless a site specific freeboard to take account of localised

effects is agreed to by Council.




TERM DEFINITION

Habitable Room In a residential situation: a living or working area, such as a lounge room,

dining room, rumpus room, kitchen, bedroom or workroom.

In an industrial or commercial situation: an area used for offices or to

store valuable possessions susceptible to flood damage in the event of a

flood.

High Flood Risk

Precinct

This is the area within the envelope of land which is subject to a high

Flood Hazard or is a Floodway in a 1% AEP flood, defined in accordance

with the criteria outlined in the Floodplain Risk Management Study and

Plan, 2009. This zone follows the central thread of Blackmans Swamp

Creek and its tributary streams and also comprises several streets in the

CBD area where most of the discharge is conveyed during periods of

flooding, with significant flow velocities. In the High Flood Risk

Precinct, flood damages, potential risk to life and evacuation problems

may be expected and development could significantly and adversely

affect flooding patterns as flow velocities are likely to be high. In this

precinct, there would be significant risk of flood damages without

compliance with flood related building and planning controls.

Medium Flood Risk

Precinct

This is defined as the strip of land on each side of the streams which is

situated between the boundaries of the High Flood Risk Precinct and

the line defining the 1% AEP flood extent. In this precinct there would

still be a significant risk of flood damages, but these damages may be

minimised by the application of appropriate development controls.

Low Flood Risk Precinct This is defined as all other land located along the stream which lies

within the extent of the Probable Maximum Flood but not lying within

the High and Medium Flood Risk Precincts. In this precinct the risk of

damages is low. Residential and Commercial and Industrial uses would

subject to minimum floor level requirements. However, Essential

Community Facilities (eg schools, hospitals), Critical Utilities and

Flood Vulnerable Development (housing for aged persons and people

with disabilities) would not be permitted.




7 REFERENCES

Danish Hydraulics Institute, 2005. “Blackmans Swamp Creek Flood Study”.

Lyall and Associates Consulting Water Engineers, 1997. “Orange Stormwater Drainage Strategy

Study”.

Lyall and Associates Consulting Water Engineers, 2006. “Blackmans Swamp Creek, Property Survey

Report”.

New South Wales Government, 2005. “Floodplain Development Manual: the Management of Flood

Liable Land”.

Orange City Council, “Orange City Local Environmental Plan, 2000”.

Sinclair Knight and Partners, 1980. “Orange Drainage Study”.

State Emergency Service, NSW, 2004. “Orange City Local Flood Plan”.

Documents

3 POTENTIAL FLOODPLAIN MANAGEMENT MEASURES 3 ......Blackmans Swamp Creek Floodplain Risk Management Study and Plan Blackmans Swamp Creek Vol 1.doc Page 21 Lyall & Associates July 2009