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
D M
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
Ora
ng
e C
ree
k is a
lso
co
nsid
ere
d.
c)
Co
nstr
uct
de
ten
tio
n b
asin
s t
o s
tore
flo
od
wa
ters
.
FM
2
2
5
Th
e c
om
mu
nity
str
on
gly
fa
vo
urs
co
ntin
ua
tio
n o
f C
ou
ncil’
s e
xis
tin
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)
Co
nstr
uct
pe
rma
ne
nt
leve
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
Th
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
Th
e c
om
mu
nity
do
es n
ot
favo
ur
this
op
tio
n.
Ho
use
ra
isin
g is a
pp
lica
ble
to
tim
be
r
fra
me
d r
esid
en
ce
s o
nly
, u
su
ally
lo
ca
ted
in
lo
w h
aza
rd z
on
es.
Th
is o
ptio
n is
revie
we
d in
Secti
on
3.4
.4.
g)
Flo
od
pro
of
ind
ivid
ua
l p
rop
ert
ies
PM
7
1
8
Ind
ivid
ua
l p
rop
ert
ies m
ay
be
flo
od
pro
ofe
d b
y d
ive
rsio
n b
an
ks (
wh
ich
ma
y a
dve
rse
ly
aff
ect
flo
w p
att
ern
s),
wate
r p
roo
fed d
oo
rs a
nd
sh
utt
ers
acro
ss e
ntr
an
ce
s.
Th
is
op
tio
n is n
ot
favo
ure
d b
y t
he
co
mm
un
ity.
h)
Co
ntr
ols
on
fu
ture
de
ve
lop
me
nt
in
flo
od
-lia
ble
are
as.
(eg
co
ntr
ols
on
loca
tio
n in
th
e f
loo
dp
lain
, m
inim
um
flo
or
leve
ls.
etc
.)
PM
2
9
1
Co
ntr
ols
ove
r d
eve
lop
me
nt
in f
loo
d p
ron
e la
nd
are
ve
ry s
tro
ng
ly s
up
po
rte
d b
y t
he
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
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
20
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
D M
AN
AG
EM
EN
T M
EA
SU
RE
S
(Co
nti
nu
ed
)
Flo
od
Ma
na
gem
en
t O
pti
on
C
lassif
ic
ati
on
Y
es
No
Co
mm
en
ts
i)
Pro
hib
it s
ub
div
isio
n o
f p
rop
ert
ies
with
in t
he
flo
od
pla
in
PM
2
1
6
Th
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 Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol 1.doc Page 21 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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.
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol 1.doc Page 22 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol 1.doc Page 23 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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
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
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
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
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.
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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
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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
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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
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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
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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
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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.
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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
Blackmans Swamp Creek
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
Outlet Configuration
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
Outlet Configuration
1 x 1.3 m diameter pipes
Blackmans Swamp Creek
Moulder Park RB8
48.8 31.8 Basin Storage 24,000 m3
Outlet Configuration
2 x 2.25 m diameter pipes
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
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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
Discount Rate % 4 7 10
Present Worth Value of Benefits
(Damages Prevented) $ x 106 13.7 10.7 8.6
Cost of Basins (with $1 Million
allowance for upgrading local
stormwater system in CBD)
$ x 106
5.1 5.1 5.1
Benefit/Cost Ratio 2.7 2.1 1.7
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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
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“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
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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.
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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
Max Depth of Inundation
– 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
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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
Discount Rate % 4 7 10
Present Worth Value of Benefits
(Damages Prevented) $ x 106 0.29 0.22 0.18
Cost of Scheme $ x 106 2.4 2.4 2.4
Benefit/Cost Ratio 0.12 0.09 0.08
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
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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
Max Depth of Inundation
– 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
Discount Rate % 4 7 10
Present Worth Value of Benefits
(Damages Saved) $ x 106 0.61 0.48 0.38
Cost of Scheme $ x 106 1.82 1.82 1.82
Benefit/Cost Ratio 0.33 0.26 0.21
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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.
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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
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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.
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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.
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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.
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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
RC
BC
4200
x2464
RC
BC
4200 x 2464 RCBC
4200 x 2464 RCBC
4200
x2464
RC
BC
4200
x2464
RC
BC
4200
x1800
RC
BC
4200
x1800
RC
BC
Byng St
Summer St
Kite St
Peis
ley
St
Main
Weste
rnR
ailw
ay
Lord
sP
l
Bla
ckm
ans
Creek
Sw
am
p
East
OrangeCreek
Bla
ckm
ans
Cre
ek
Swam
p
Dalton St
Lord
sP
l
LEGEND
New Culvert System
Existing Culvert System
Storm Water Channel
ScaleN
0 100 200 m
EastO
range
Cre
ek
StChurchill
Gre
atW
este
rn
Blackmans Swamp Creek
Ed
wa
rdS
t
Railw
ay
Rifl
eR
ange
Cre
ek
Black
man
s Sw
amp
Cre
ek
St
St
St
Anson
Byng
March
St
Summer
St
Kite
St
Moulder
StWarrendine
Road
Gardiner
Railway
Orange - Broken Hill
N
BLACKMANS SWAMP CREEKFLOODPLAIN RISK MANAGEMENT STUDY
Figure 3.2
EXISTING AND POTENTIAL DETENTION BASINS
a
a
a
a
Pl
Lord
s
STP
RB7
RB4
RB1
RB2
RB5
RB6
RB3b
RB3a
Peis
ley
St
Piped Section of Creek
Open Channel of Creek
Informal Detention Basin
Existing Formal Detention Basin
Railway Line
Potential Basin
LEGEND
RB8
PilcherPark
AgriculturalDam
RidleyPark
EAB01
BLACKMANS SWAMP CREEKFLOODPLAIN RISK MANAGEMENT STUDY
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
BLACKMANS SWAMP CREEKFLOODPLAIN RISK MANAGEMENT STUDY
PHOTOGRAPHS OFLOWER BLACKMANS SWAMP CREEK
aaaa
Looking Upstream Towards Junction with East Orange Creek
Plate 7P
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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
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“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.
Bla
ckm
ans S
wam
p C
reek
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11
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol1.doc Page 47 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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.
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol1.doc Page 48 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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.
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol1.doc Page 49 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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 Floodplain Risk Management Study and Plan
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.
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol1.doc Page 51 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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.
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol1.doc Page 52 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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.
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol1.doc Page 53 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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.
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol1.doc Page 54 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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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r le
ve
ls o
f d
eve
lop
me
nt
bo
rde
rin
g t
he
cre
eks w
hic
h w
ere
su
rve
ye
d f
or
this
pre
se
nt
stu
dy a
nd
re
su
lts o
f th
e B
lackm
an
s S
wa
mp
Cre
ek F
loo
d S
tud
y,
20
05
wo
uld
be
use
d a
s b
asic
da
ta f
or
the
syste
m.
F
urt
he
r in
ve
stig
atio
n is r
eq
uir
ed
to
re
late
pre
dic
ted
ra
infa
lls t
o t
he
in
cid
en
ce
an
d lo
ca
tio
ns o
f flo
od
ing
pro
ble
ms in
th
e s
tud
y a
rea
.
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
.
Imp
lem
en
tatio
n o
f F
lash
Flo
od
Wa
rnin
g
Syste
m
$2
00
,00
0
C
ost
allo
ws f
or
instr
um
en
tatio
n,
so
ftw
are
, tr
ain
ing
an
d p
ub
lic f
loo
d a
wa
ren
ess p
rog
ram
.
A
llow
an
ad
ditio
na
l a
nn
ua
l co
st
of
$3
0,0
00
fo
r m
ain
ten
an
ce
of
the
syste
m (
Co
un
cil
co
sts
).
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 a
bove F
easib
ility
Stu
dy a
nd t
he
availa
bili
ty o
f C
ouncil
an
d G
overn
me
nt
fundin
g.
En
su
re f
loo
d d
ata
in
th
is F
loo
dp
lain
Ris
k
Ma
na
ge
me
nt
Stu
dy a
nd
Pla
n a
re a
va
ilab
le
to S
ES
fo
r in
clu
sio
n in
flo
od
em
erg
en
cy
resp
on
se
pro
ce
du
res.
SE
S c
osts
SE
S’s
Ora
ng
e L
oca
l F
loo
d P
lan
, 2
00
5 t
o b
e u
pd
ate
d u
sin
g in
form
atio
n o
n lo
ca
tio
ns o
f flo
od
pro
ne
de
ve
lop
me
nt
inco
rpo
rate
d in
th
e
FR
MS
an
d s
ho
wn
in
Fig
ure
2.3
.
This
measure
wo
uld
im
pro
ve e
merg
ency m
anag
em
ent
pro
ce
dure
s a
nd h
as a
hig
h p
rio
rity
. It
do
es n
ot
req
uire
Go
ve
rnm
en
t fu
nd
ing
.
Imp
lem
en
t flo
od
aw
are
ne
ss a
nd
ed
uca
tio
n
pro
gra
m f
or
resid
en
ts b
ord
eri
ng
th
e c
ree
k
syste
m a
nd
ow
ne
rs o
f co
mm
erc
ial a
nd
ind
ustr
ial d
eve
lop
me
nts
in
th
e C
BD
are
a.
SE
S,
Co
un
cil
sta
ff a
nd
Bu
sin
ess
Ow
ne
rs’ co
sts
S
ES
to
pre
pa
re F
loo
d I
nfo
rma
tio
n B
roch
ure
to
ad
vis
e r
esid
en
ts o
f th
e f
loo
d r
isk,
ba
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
risin
g c
ha
nn
el im
pro
ve
me
nts
an
d e
nla
rge
me
nts
to
wa
terw
ays o
f ro
ad
cro
ssin
gs.
T
he
sta
ge
d p
rog
ram
of
wo
rks is t
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.
Fe
asib
ility
Stu
dy o
f A
dd
itio
na
l D
ete
ntio
n
Ba
sin
s o
n B
lackm
an
s S
wa
mp
Cre
ek a
nd
Rifle
Ra
ng
e C
ree
k
$1
20
,00
0
Ele
me
nts
of
Fe
asib
ility
Stu
dy:
S
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.
Sto
rag
e c
ha
racte
ristics t
o b
e b
ase
d o
n s
ite
su
rve
y.
P
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.
It
requ
ires C
ouncil
an
d G
overn
ment
fundin
g.
Co
nstr
uctio
n o
f a
dd
itio
na
l D
ete
ntio
n B
asin
s
on
Bla
ckm
an
s S
wa
mp
Cre
ek a
nd
Rifle
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
Stu
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
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol1.doc Page 56 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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
EE
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
Ca
tch
me
nt
Wid
e W
ork
s a
nd
Me
as
ure
s
Flo
od
Mo
dific
atio
n-
De
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
Ea
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
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol1.doc Page 57 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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.
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol1.doc Page 58 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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.
Blackmans Swamp Creek Floodplain Risk Management Study and Plan
Blackmans Swamp Creek Vol1.doc Page 59 Lyall & Associates
July 2009 Rev. 5.0 Consulting Water Engineers
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”.