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REFERENCED DOCUMENT 100 Year Boundary for the Existing Floodplain Condition - REC13/0095
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Your Reference:
Our Reference: 2013-08
Brisbane Stonnwater Management Pty Ltd
ACN 094 878 734 ABN 75 094 878 734
92 Alkoomie Street, Wynnum Nth, Qld
PO Box 8119, Wynnum Nth, Q 4178
Date: 6 Sept 2013 Ph 07 33963383 Fax 07 39010285
Email: [email protected]
Callaghan & Toth
POBox 2305,Noosa Heads, Q4567
Attn: Mr Lex Toth
Dear Sir,
Re: Proposed 3 Lot Subdivision At 55 Lake MacDonald Drive, Cooroy
In response to your emailed request of 16 August 2013,I offer
my advice in relation to Items 3 and 5 of
SeRC’s 24 July 2013 request for further infonnation about the above proposal; namely-
3 Provide an amendedplan which accurately represents the regionalflood extent on site (Council
flood extent mapping indicates thatpart ofLot 5 is within inundation extents).
5 Provide additional information (or an updatedflood study) which demonstrates the roughness
(manning’s)assumptions adopted in the originalflood study allow for revegetation ofthe main
drainage line. Alternatively provide a sensitivity analysis ofincreased roughness assumptions and
amendflood level extents and lot layout as necessary
The 100 Year Flood Boundary For The Existing Floodplain Condition
Lawson & Treloar Pty Ltd (L&T)undertook a flood study ofthe creeks in the Cooroy area for Council
in July 2002.I understand that some parts ofthat study have recently been updated for Council
by other
consultants, but the updated parts do not include the tributary alongside the proposed subdivision.
Accordingly, I have fonnulated my following advice on the basis that the 2002 L&T flood study is the
latest available study of flood levels in the subject area.
The schematic on the following page of this letter illustrates the floodway / subdivision scenario. One
ofL&T’s model cross-sections abuts the upstream boundary of Lot 5, and that lot extends about 35
percent of the floodway distance between that cross-section and the next downstream cross-section. I
understand that L&T’s predictions of the 100 year flqod levels at those two cross-sections, for their
assessment ofthe existing Manning’s n values, are respectively 101.79 m AHD and 100.62 m AHD,a<;
marked in the last row of data in the schematic. I therefore infer that the 100
year flood level at the
upstream boundary of Lot 5 for the existing floodway condition is 101.79 m AHD,and by arbitrarily
proportioning 35 percent ofthe level differential between the upstream and downstream cross-sections,
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I infer that the corresponding 100 year flood level at the downstream boundary of Lot 5 is 101.38 m
AHD,as also indicated in the last row of the schematic.
I note that Callaghan & Toth have already redrawn the existing 100
year flood boundary on their
drawing number 1225-19 to accord with the above advice, and for the sake of completeness I have
attached a copy ofthat revised drawing to this letter. The redrawn line is based on field survey of the
topography ofLot 5,rather than Council’s Lidar topography,and is transcribed to the schematic below.
The existing 100 year flood boundary is seen to be mostly outside the boundary of Lot 5, except for a
small area at the south-east corner ofthe lot. Please note that the flood boundary on the schematic does
not include the flood boundary alongside the local gully, which runs alongside the eastern boundary of
the lot. Please also note that the proposed filling of Lot 5 extends across the southern boundary ofthe
lot and intrudes slightly into the existing flood 100 year boundary the area offilled flood storage will -
be 12 metre long, with a maximum width of 1.2 metres and a maximum depth of0.08 metre, for which
a prismoidal approximation suggests a displaced floodplain
storage volume ofjust 0.4 cubic metre. This
loss of storage should not adversely affect upstream and downstream interests.
Proposed Min.F Levels: 102.99
102.68
Inferred 100 yr flood boundaries, Pre-fi!ling
’\
For n = 0.15 Post-Filling
\..
’\
0 yr flood boundary
Based on L&T flood levels _
For existing n values
~\.
\\
LOT 5
---______---_
v’, /1
-t:,?f) : Proposed toe of fill :
<;)
f:> 1 1
: 4f"~/1\I :
1 1
1 1
1
1
1
\
\
\
\
\
\
\
100 year Flood levels (mAHD) \
.,
1
1 , 1
85M inference for n value of 0.15
100.89 101.66 102.07
(50%more than 0.10 ~ Add 0.14 m) 1 1 1
1 1 1
BSM inference for n value of 0.10
~ ~ ~
100.75 101.52 101.93
(25%more than 0.08 ~ Add 0.07 m ) I 1 1
I 1 1
I I 1
1 I 1
L&T prediction for existing n values + 25 % I I 1
Adds 0.06 -
0.07 m to the existing flood levels 100.68 101.45 101.86
I I I
(BSM inference ~ n = 1.25 x 0.064 = 0.080 ) I I I
I 1 1
I 1 I
L&T prediction for existing n values of: I I I
1 I 1
Creek bed 0.06 I I I
! ~ ~
Cleared floodplain 0.03 to 0.05
100.62 101.38 101.79
Uncleared floodplain 0.06 to 0.12 I I
I I
(BSM inference ~ average n 0.064 ) I I
,
I
8SM interpolation of flood levels at 35% of the distance from the U/S XSect to the Dls XSect -
The 100 Year Flood Boundary For A Revegetated Floodplain
I have been advised by Council’s officers that they are seeking a sensitivity analysis for Manning’s n
values of 0.1 0 and 0.15.
BSM Doc File 55LkMacD RFI.Doc
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I understand that L&T’s assessment of the existing Manning’s values ranged from 0.06 in the creek
be~ to 0.03 -
0.06 in the cleared floodplain areas, to 0.06 -
0.12 in the uncleared floodplain areas.
Given that the overbank areas in the subject vicinity are mostly grassed paddocks, I infer that L&T’s
model would have simulated an average n value across the floodway ofjust below 0.065 at these cross-
sections; -
or nominally 0.064 as I have indicated in the last row of the preceding schematic.
I understand that L&T also undertook a sensitivity analysis with Manning’s n values 25 percent higher
than the existing values, resulting in the flood levels in the second-last row ofthe preceding schematic.
These flood levels are 0.07 meter higher than the corresponding levels for the existing floodplain
condition. On the basis ofmy nominal inference about average n values in the preceding paragraph, the
L&T model probably simulated an averaged Manning’s n value ofabout 0.064 + 25% =
0.080 for this
sensitivity scenario, still short of Council’s requested values of 0.10 and 0.15.
I have used the abovementioned L&T sensitivity test as a guide to the impacts offurther increases in the
Manning’s n value. An increase in the Manning’s value to 0.10 would represent a 25 percent increase
on the abovementioned value of 0.080, or the same % increase as in L&T’s sensitivity test. This
suggests that a Manning’s n of 0.1 0 would add about another 0.07 metre to L&T’s sensitivity flood
levels, thereby resulting in the flood levels in the third-last row of the preceding schematic. A further
increase,to a Manning’s n 0.15,would represent a 50 percent increase on the value of0.10, or double
the percentage increase in L&T’s sensitivity test, which suggests that this would add about twice 0.07
metre (i.e. 0.14 metre) to the flood levels for the 0.10 value as indicated in the top row of the
-
preceding schematic.
Both ofthe abovementioned increases in Manning’s n value (to 0.10 and 0.15)would, in the absence of
any allotment filling, shift the 100 year flood boundary inside the Lot 5 boundary, as indicated for the
"Pre-Filling"scenario on Callaghan &Toth’s attached drawing and in the schematic on the preceding
page of this letter. However, Lot 5 is proposed to be filled to and beyond the lot boundary, thereby
raising the topography at the lot boundary enough to prevent any additional intrusion of the flood
boundary into the lot, as indicated by the realigned 100 year flood boundaries for the "Post-Filling /
n=0.15" scenario on the attached drawing and in the schematic. The house-pad is proposed to be filled
to a minimum level of 102.68 m AHD, or 0.61 metre higher than the 100 year flood level at the
upstream comer ofthe lot for a Manning’s n of0.15. Given that the 100 year flood depth in this area is
about 1.8 metre, that clearance of 0.61 metre represents a freeboard of about 34 percent of the flood
depth, above the "fully revegetated" flood level. The floor level on Lot 5 is proposed to be even higher,
at 102.99 m AHD,resulting in a freeboard of 0.92 metre (ormore than 50 percent ofthe flood depth)
above the "fully revegetated" flood level. I acknowledge that the abovementioned flood levels for the
revegetated floodway have been derived from some approximate inferences rather than detailed
modelling, but I suggest that the generosity ofthe abovementioned freeboards will be more than enough
to offset any shortcomings in those inferences.
Please do not hesitate to contact me if you require any clarification of the above advice. In the
meantime, I remain,
Yours faithfully,
(?
~".~~
~"-~
Denis Ogle, RPEQ 2405, Director.
8SM Doc File 55LkMacD RFI.Doc
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DRIVEWAY SETOUT fOR LOIS 5 TO
EDGE Of’ CONCRETE AT FINISHED LEVEL
Callaghan & Toth
Civil, Environmental &
SJ
PONT E.ST1NG NORTHING
Hydraulic Engincoel’S
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7 X-491588.519 _...
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IN WIImIt5 .
ASSOC:M1ES
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12 X-491589.683 Y-7079198.040
13 X-491591.869 V-7079211.361
X-491593.076
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14 Y.7079219.395
15 X-491569.805 Y-7079223.023 PROPOSED 3 lOT
16 X-491570.267 Y-7079225.9S7
SUBDMSION AT 55 lAKE
17 X-491593.905 Y-7079222.302
Woc:OOIWD DRM: COOROY
18 X-491599.041 Y-7079220.122
llilIE;
AU. ORMWAYS TO BE CONS11M:ITD TO
STAHOARO DRA_ IPWf-"l SEO R-050
& TO ~PlAH 122~21.
ALSO REFER TO _Y SETOUT TAIIlE
FOR DETAIlEQ SETTIIC OUT.
[:... PlY llD
\I
llilIE;
REFER TO PlAH 1225-23 FOR OO.\llED
[~PlAN1Of2
SETOUT (1f STOR....TER PIPE5 &
STAHOARO DRAWINGS USTEO OH PlAH
1225-17.
lOT 136 on WCH978
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PariIh at 1EWAHTJj
County at YARCH
SUNSIH ~coua.
I.ocaIty at lAKE Ib:llONAIJ)
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=
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LEGEND’ PROPOSED SERViCES I~ $ICU __ "~.-...0 XJU
OOI SCAl[.....1HIS~
RE\1SIONS
\ PROPOSal HOOSE SITE tW dtt<. .....
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\ A Rf.\1:SED lOT LOO.S ,litO 0100 08 13
/’" B RE\1S!D RTW AHO FU lMl.S 08 13
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PROPOSal MNIIIC
PATH
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~ PROPOSED RET~NlHC iI\U. TO ru:s
DETAIL OH PlAH 1225-20
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" .~ PROPOSED f’HSI ~l.E’ " CT ....
lake Woc:OonoId 0I’IYt
~ 1225 30 lob
CWTlnt DWC-Q.\SE
0100 FlOOO lH’ CIR!EHT sruor
n=O.064 (,IPPfiOXlllAll’ W FlOOO) rolAPPKOVAL
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(1f COHCOORAT1OH) POST-FllH:
$Met 3 of 14 IheelI
POINT EAST1NC NORTHINC 0100 FlOOO UHf (,IPPfiOXlllAT1OH --
20 )(-4.915$8.282 Y-7079176.861 VARIOUS n VAlUES) POST-flUNG
21 X-491~S.809 Y.707QI7!t704 Qloo FlOOO UHf (,IPPfiOXlllAT1OH
1225-19
22 X-491570.526 v- 707917tUI14 VARIOUS n V~UES) PRE-FllH:
2J X.49151’.9~ Y.707917S.8.’ dot. !SUI JJM[ lOll
Page 5
No. 55 Lake Macdonald Drive Cooroy Stage 2 Short Duration Culvert
Calculations for Access for Lots 5, 6, & 7.
Lot 7 Culvert: Catchment A (see Plan 1225-25).
Catchment Area A =
1.653 ha
Q 1 00 rainfall event FCIA =
1/360 x 0.94 x 155 x 1.653 0.67 m3/s
Cross Road culvert, 1 X 900x300 RCBC with outlet control.
Maximum HW =
103.475(upstream side of dwy) 102.875(RCBC invert)
- =
0.6m
ke 0.4, A
=
=
0.908 x 0.305 =
0.277 sqm, L =
4.88m
de =
0.467(QIB)2/3 =
0.467 X (0.51/0.9)2/3 =
0.32m
TW =
xsect DIS of2/A (see attached PCDrain Calculation sheet) =
0.232m
(de + 0)/2 =
(0.32 + 0.305)/2 =
0.313m
So ho is greater of 0.232 and 0.313 =
0.313m
HW =
H + ho -
LSo
0.6 =
H + 0.313 -
0.03
H =
0.317
From Figure 9A-5 DTMR Road Drainage Manual, Q =
0.51 m3/s
Q 1 00 weir flow =
Q 1 00 flow 0.67 m3Is -
0.51 m3Is piped
0.16 m3/s
Weir Q100 1.7LH3/2 B =
6m
0.16 1.7 x 6 x H312
H (0.16I (1.7 x 6)i/3
0.063m
Adopt weir capacity for a 0100 of 0.67 m3/s with 6m width, 0.063m upstream depth Hand
900x300 RCBC.
Note culvert more than adequate tor Q5 without overtopping as Q5 =
0.32 m3/s vs culvert
capacity of 0.51 m3Is.
Page 6
Lot 6 Culvert: Catchment A + B (see Plan 1225-25).
Catchment Area A =
1.755 ha
Q 1 00 rainfall event FCIA =
1/360 x 0.94 x 155 x 1.755 =
0.711 m3/s
Cross Road culvert, 1 X 900x300 RCBC with outlet control.
Maximum HW =
1 02.883(upstream side of dwy) -
1 02.364(RCBC invert) =
0.519m
ke=O.4, A =
0.908 x 0.305 =
0.277 sqm,L =
4.88m
de =
0.467(QlBi/3 =
OA67 X (OAI0.9i/3 =
0.272m
TW =
xsect D/S of2/C (see attached PCDrain Calculation sheet) = OA11m
(de + D)/2 =
(0.272+ 0.305)/2 =
0.288m
So ho is greater of 0.411 and 0.288 =
0.411m
HW =
H + ho -
LSo
0.519 =
H + 0.411 -
0.064
H =
0.172
From Figure 9A-5 DTMR Road Drainage Manual, Q =
OAO m3/s
Q 1 00 weir flow Q100 flow 0.711 m3/s -
0.40 m3/s piped
0.311 m3/s
Weir QIOO 1.7LH312 B =
3m
0.311 l.7 x 3 x H312
H (0.3111(1.7 x 3))2/3
0.155m
Adopt weir capacity for a 0100 of 0.711 m3/s with 3m width, 0.155m upstream depth H and
900x300 RCBC.
Note culvert more than adequate for Q5 without overtopping as Q5 =
0.34 m3/s vs culvert
capacity of 0.40 m3/s.
X:\JOBS\CT Projects in progrcss\l225 30 lots lake MacDonald Drive\CorrespondencelStage 2 Correspondence\property access,
2
flooding calcs.doc
Page 7
Lot 5 Culvert: Catchment A + B (see Plan 1225-25).
Catchment Area A =
1.755 ha
Q 1 00 rainfall event FCIA =
1/360 x 0.94 x 155 x 1.755 0.711 m3/s
Cross Road culvert, 1 X 1200x300 RCBC with outlet control.
Maximum HW =
102.579(upstream side of dwy) -
102.1 04(RCBC invert) =
OA 75m
ke= OA, A =
1.212 x 0.305 =
0.370 sqm, L =
4.88m
de =
OA67(Q/Bi/3 =
OA67 X (OAll.2i/3 =
0.246m
TW =
xsect D/S of2/C (see attached PCDrain Calculation sheet) =
OAl1 m
(de + D)/2 =
(0.246+ 0.305)/2 =
0.276m
So 110 is greater ofOAl1 and 0.276 =
OAllm
HW =
H + 110 LSo
-
OA75 =
H + OAl1 -
0.069
H =
0.133
From Figure 9A-5 DTMR Road Drainage Manual, Q =
OA6 m3/s
Q 1 00 weir flow Q 1 00 flow 0.711 m3Is -
OA6 m3Is piped
0.251 m3/s
Weir QI00 1.7LH312 B =
4m
0.251 :::::
1.7 x 4 X .H312
H (0.251 I (1.7X 4))2/3
0.111m
Adopt weir capacity for a 0100 of 0.711 m3/s with 4m width, 0.111m upstream depth H and
900x300 RCBC.
Note culvert more than adequate for Q5 without overtopping as Q5 =
0.34 m3/s vs culvert
capacity of 0.46 m3/s.
X:VOBS\CT Projects in progress\l225 30 lots lake MacDonald Drive\Correspondence\Stage 2 Correspondence\property access,
3
flooding eales.doc
Page 8
APPENDIX A -
PCDRAIN CALCULATION FOR CULVERT
TAILWATER LEVELS
X:\JOBS\CT Projects in progress\1225 30 lots lake MacDonald Drive\Correspondence\Stage 2 Correspondence\property access,
4
flooding cales.doc
Page 9
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Used in these records
REFERENCED DOCUMENT 100 Year Boundary for the Existing Floodplain Condition - REC13/0095Related documentApplication case desk ↗
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