Queanbeyan City Council

PO BOX 90

QUEANBEYAN NSW 2620 Job No. CO377

Attn: Ms Eli Ramsland

1 July 2015

Re: Flooding Investigation for Proposed Crossing of the Queanbeyan River as part of

the Ellerton Drive Extension Project

Dear Madam,

This letter sets out the findings of an investigation which was undertaken on behalf of

Queanbeyan City Council (QCC) into the flood related impacts of the proposed bridge crossing of

the Queanbeyan River (Proposed Bridge) as part of the Ellerton Drive Extension Project (EDEP).

Figure 1 attached shows the extent of the EDEP and the location of the Queanbeyan River

Bridge.

The objectives of the investigation were to:

assess the impact of the Queanbeyan River Bridge on present day flood conditions during

a flood with an average recurrence interval (ARI) of 100 years;

undertake a bridge scour assessment for floods with ARI’s of 100 and 2000 years; and

provide a summary of hydraulic inputs for the design of the Queanbeyan River Bridge.

The followings sections of this letter provide background to the development of the hydraulic

model which was relied upon for the purpose of the current investigation, as well as the key

outcomes of the investigation.

1. Background to the Development and Updating of the Molonglo and Queanbeyan

Rivers Hydraulic Model

A one-dimensional unsteady flow hydraulic model of the Molonglo and Queanbeyan Rivers at

Queanbeyan was developed as part of the draft Queanbeyan Floodplain Risk Management Study

and Plan (L&A, 2008). The model was developed using the HEC-RAS software and was used to

define flooding behaviour along the two rivers for floods with ARI’s ranging between 5 and 500

years, together with an Extreme Flood Event (assumed to have a peak flow three times greater

than the 100 year ARI event). The existing hydraulic model has been denoted herein as the MQR

HEC-RAS Model.

The model was updated as part of the present investigation by inserting additional cross sections

in the vicinity of the Proposed Bridge. The additional cross sections were defined using detailed

ground survey along the proposed EDEP corridor. The updated model has been denoted the

pre-EDEP HEC-RAS Model. Figure 2 shows the layout of the pre-EDEP HEC-RAS model. For

the purpose of the present investigation the pre-EDEP HEC-RAS Model was run in steady state

mode.

Page 2

The proposed bridge arrangement was then added to the model to reflect post-EDEP conditions.

Details of the proposed bridge arrangement were based on 15% bridge design drawings prepared

by Opus on behalf of QCC. The updated model has been denoted the post-EDEP HEC-RAS

Model.

Discharge hydrographs used as input to the MQR HEC-RAS Model were generated by a RAFTS

model that was also developed as part of L&A, 2008 (MQR RAFTS Model). The MQR RAFTS

Model was updated as part of the present investigation to include design rainfall intensities for the

2000 year ARI event, procedures for which are set out in Section 3 of Book 6 of Australian

Rainfall and Runoff (IEAust, 1998). Figure 2 shows the discharge hydrographs which were

generated by the MQR RAFTS Model for the Queanbeyan River at Queanbeyan for the 100 and

2000 year ARI flood events.

2. Present Day Flood Behaviour

Figure 3 shows the extent of flooding in the vicinity of the Proposed Bridge, while Figure 4 shows

design water surface profiles extending from a location 490 m downstream of the Proposed

Bridge to the upstream boundary of the pre-EDE HEC-RAS Model for floods with ARI’s of 100 and

2000 years.

The key features of flooding behaviour in the vicinity of the Queanbeyan River Bridge at the

100 year ARI level of flooding are as follows:

i. Flooding along the river occurs to a maximum depth of about 11 m and a maximum width

of about 120 m.

ii. Floodwater is generally confined to undeveloped land along the river corridor. However,

it is shown to extend a short distance into a residential property which is located

immediately upstream of the Proposed Bridge on the southern bank of the river.

iii. The flood slope in the river is relatively flat in the vicinity of the Proposed Bridge, with a

computed slope of about 0.1 per cent.

iv. The average flow velocity in the river is about 2.4 m/s.

The key features of flooding behaviour in the vicinity of the Queanbeyan River Bridge at the

2000 year ARI level of flooding are as follows:

v. Flooding along the river occurs to a maximum depth of about 14 m and a maximum width

of about 180 m.

vi. Floodwater extends across a section of Barracks Flat Drive east of River Drive.

vii. The average flow velocity in the river is about 3.0 m/s.

3. Proposed Bridge Arrangement

As mentioned, the detailed design of the Proposed Bridge is currently being prepared by Opus on

behalf of QCC. Table 1 over the page gives details of the Proposed Bridge arrangement which

were taken from the 15% design drawings. Attachment A of this letter contains a copy of a

select number of the bridge drawings prepared by Opus.

Page 3

TABLE 1

DETAILS OF PROPOSED BRIDGE ARRANGEMENT

ITEM DIMENSION

No. of Spans 6

Length of Spans (m) 25 (Spans 1, 2 & 4), 38 (Spans 3 & 6), 33 (Span 5)

Total Length of Deck (m) 184

Pier Type 2 off 1500 mm diameter bored concrete piles aligned normal to

carriageway

Pier Skew to Direction of Flow 15 degrees

Abutment Type Spill-Through

Abutment Batter Slope 1V:1.5H

Elevation of Bridge Deck (m AHD) 591.8

Elevation of Bridge Soffit (m AHD) 589.6

Elevation of River Bed (m AHD) 567.8

4. Impact of Proposed Bridge on Flood Behaviour

Figures 3 and 4 show the impact the Proposed Bridge will have on the extent of flooding and

water surface elevations in the Queanbeyan River respectively for floods with ARI’s of 100 and

2000 years. Table 2 over the page sets out the difference in peak 100 year ARI flood levels at

the location of the HEC-RAS model cross sections under pre- and post-EDEP conditions (referred

to as “afflux”).

The increase in peak 100 year ARI flood levels upstream of the Proposed Bridge is attributed to

the hydraulic losses associated with flow around the bridge piers. Hydraulic losses due to the

bridge piers were modelled in the post-EDEP HEC-RAS Model using the Momentum Balance

Method, which was found to produce a head loss that was 0.22 m and 0.25 m higher than the

Energy and Yarnell equations, respectively (i.e. the Energy and Yarnell equations gave an afflux

immediately upstream of the Proposed Bridge of 0.07 m and 0.04 m, respectively compared to

0.29 m for the Momentum Balance Method). These results indicate that the computed increases

in peak flood levels attributable to the Proposed Bridge are sensitive to the method which is

adopted for computing pier losses. Further discussion on this issue is contained in Section 6 of

this letter.

Figure 4 shows that the impact of the Proposed Bridge on peak flood levels extends a significant

distance upstream of the road crossing, only reducing to 0.15 m at a location corresponding to the

upstream boundary of the hydraulic model. It is noted that the nearest development adjacent to

the Queanbeyan River is located a further 1.1 km upstream of the model limit.

While the length of river over which the Proposed Bridge will impact flood levels can be attributed

to the flat flood gradient, it is also a function of the higher hydraulic losses which have been

derived using the Momentum Balance Method.

It is also noted that the Proposed Bridge has the potential to increase peak flood levels by up to

0.29 m in the existing residential development which is located on the southern bank of the river

immediately upstream of the road crossing.

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TABLE 2

PEAK FLOOD LEVELS IN THE VICINITY OF QUEANBEYAN RIVER BRIDGE

100 YEAR ARI

River Station Location

Peak Flood Levels (m AHD)

Afflux (m) Pre-EDEP

Conditions

Post-EDEP

Conditions

7375 Upstream limit of post-EDE

HEC-RAS model 580.37 580.52 0.15

7167 580.01 580.19 0.18

6660 579.53 579.77 0.24

6424 579.29 579.55 0.26

6323 Upstream residential

property 579.17 579.45 0.28

6303 579.11 579.39 0.28

6294 Immediately upstream of

Queanbeyan River Bridge 579.09 579.38 0.29

6264 Immediately downstream of

Queanbeyan River Bridge 579.06 579.06 0

6255 579.06 579.06 0

6204 578.79 578.79 0

5772 490 m downstream of

Queanbeyan River Bridge 578.09 578.09 0

5. Bridge Scour Assessment

Damage to bridge approaches due to rare flood events can usually be repaired relatively quickly.

In comparison, major structural damage to a bridge can create safety hazards to motorists and

high social and economic costs to repair. For this reason, Waterway Design – A Guide to the

Hydraulic Design of Bridges, Culverts and Floodways (Austroads, 1994) recommends a greater

assurance that scour will not endanger the foundation of a bridge than is warranted for the design

of its approaches.

In accordance with Austroads, 1994 the following design floods have been adopted in the bridge

scour assessment:

100 year ARI for the design of protection works to the fill around bridge abutments and

bridge approaches; and

2000 year ARI flood event for the design of the bridge foundations under Ultimate Limit

State (ULS) load conditions assuming all stream bed material above the total scour line

has been removed and is not available for bearing or lateral support.

The bridge scour assessment was carried out using the post-EDE HEC-RAS model and the

in-built bridge scour function within the HEC-RAS software. Table 3 over the page provides a

summary of the bridge hydraulic results used in the scour depth assessment.

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TABLE 3

BRIDGE HYDRAULIC SUMMMARY

Hydraulic Characteristic

Design Flood Event

100 year ARI 2000 year ARI

Design Flow Rate (m3/s) 1440 2850

Peak Flood Level (m AHD) 579.4 582.5

Flow Depth

(m)

Northern

Abutment - 0.1

Southern

Abutment - 2.7

Pier 1 0.7 3.8

Pier 2 9.1 12.2

Pier 3 8.3 11.4

Pier 4 3.1 6.3

Pier 5 - 2.7

Flow Velocity(1)

(m/s)

Northern

Abutment - 1.5

Southern

Abutment - 1.5

Pier 1 0.9 1.6

Pier 2 2.4 3.0

Pier 3 2.4 3.0

Pier 4 1.0 1.5

Pier 5 - 1.5

1. Flow velocity is based on an average section velocity within the main channel and overbank areas.

Total scour depths were calculated by adding general contraction scour and local scour at each

pier and abutment. General contraction scour was calculated using the live -bed scour equation.

Local pier scour was calculated using the Colorado State University (CSU) equation, with project

pier widths adjusted to account for the greater of pier skew and debris accumulation. Project pier

widths for debris accumulation were calculated using the procedures set out in Hydraulic

Engineering Circular 18 - Evaluating Scour at Bridges, 5th

Edition (US FHWA, 2012). Local

abutment scour was calculated using the Froehlich’s Abutment Scour Equation.

Parameter assumptions adopted for the bridge scour assessment were based on the proposed

bridge arrangements summarised in Section 3 of this letter and geotechnical information

presented in the report entitled “Ellerton Drive Extension Geotechnical Investigation Report”

(Coffey, 2014). Attachment B of this letter contains a plan and long section of geotechnical

profiles which were extracted from Coffey, 2014.

The available borehole logs show the presence of shale and limestone rock at 2.5 to 3.2 m depth.

The rock is overlain with fill, colluvium and alluvium comprising sand, gravel and clays.

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Table 4 over the page summarises the estimated scour depths at the bridge abutments and piers

floods with ARI’s of 100 and 2000 years. Approximate depths to rock based at each pier and

abutment are also provided in Table 4.

The key features of the 100 year ARI bridge scour results are as follows:

i. The northern and southern bridge abutments are located outside the extent of flooding.

As a result, no scour protection measures are required around the bridge abutments to

protect against scour caused by flooding.

ii. Depth to rock is less than the scour depths computed by the HEC-RAS model. The depth

of the rock layer is therefore the limiting factor in the scour depth assessment.

iii. No scour will occur at the base of Pier 5 as it is located outside the extent of flooding.

iv. Due to the depth and velocity of flow experienced at Piers 1 to 4, rock riprap protection is

recommended to inhibit erosion and reduce maintenance. However, it cannot be relied

upon as a permanent scour counter measure in the design of the bridge piers. The

recommended arrangement is as follows:

o The riprap layer should extend a minimum distance of 3 m out from the base of

each pier.

o At Piers 1 and 4 the riprap layer should comprise a d50 rock size of 400 mm and

have a minimum thickness of 0.8 m, while at Piers 2 and 3 the riprap layer should

have a d50 rock size of 700 mm and a minimum thickness of 1.4 m.

o The riprap shall be graded in accordance with Table 6.2 or Austroads, 1994 and

be hard, dense, durable, resistant to weathering and free of overburden, spoil or

organic matter.

The key features of the 2000 year ARI bridge scour results are as follows:

i. The northern bridge abutment will be inundated to a depth of only 0.1 m during a 2000

year ARI flood, indicating significant scour of the spill-through abutment will not occur

during a flood of this return period.

ii. Depth to rock at the southern abutment, as well as at Piers 1 to 5 is less than the

scour depths computed by the HEC-RAS model. The depth of the rock layer is

therefore the limiting factor in the scour depth assessment.

6. Potential for the EDEP to Exacerbate Flooding Conditions in Existing Development

The present investigation found that depending on which method is used to compute the hydraulic

losses which will be imposed by the bridge piers, peak 100 year ARI flood levels could be

increased by as much as 0.29 m in existing residential development. While this increase is

considered to be an overestimate of the impact the bridge will have on peak flood levels upstream

of the bridge site, it does highlight the potential for the project to exacerbate flooding conditions in

several properties. Because of this, it is recommended that the two-dimensional (in plan)

hydraulic model which we have developed as part of the Kings Highway Upgrade project be

extended upstream to the bridge site to allow a check to be undertaken of the HEC-RAS model

results. It is also recommended that additional ground and floor level survey be commissioned

within the residential properties which are located immediately upstream of the Proposed Bridge

site. The survey would allow the nature of flooding in these properties to be more accurately

defined and for any flood mitigation measures to be scoped.

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TABLE 4

ESTIMATED SCOUR DEPTHS

Location Borehole

Approximate

Depth to

Rock(1)

Design Flood Event

100 year ARI 2000 year ARI

Contraction

Scour

Local Scour

(Abutment or

Pier)

Total Scour

Depth

Contraction

Scour

Local Scour

(Abutment or

Pier)

Total Scour

Depth

Northern Abutment TP46 / B-BH01 1.2 - - - - - -

Southern Abutment B-BH04 / B-BH05 3.2 - - - 1.1 6.9 8.0

Pier 1 B-BH01 0.0 1.0 2.6 3.6 1.7 4.5 6.2

Pier 2 B-BH02 2.8 0.8 6.1 6.9 1.1 6.9 8.0

Pier 3 B-BH02 / B-BH03 2.9 0.8 6.0 6.8 1.1 6.9 8.0

Pier 4 B-BH03 2.9 0.5 4.9 5.4 1.1 5.6 6.7

Pier 5 B-BH04 2.5 - - -(3) 1.1 4.2 5.3

(1) Approximate depth to rock is based on review of geotechnical information presented in Coffey, 2014.

Page 8

We trust that the findings presented in this letter will assist QCC in its understanding of the flood

related impacts of the Proposed Bridge and also Opus in progressing the bridge design. If we can

be of any further assistance in this matter, please do not hesitate the undersigned.

Yours faithfully

Lyall & Associates

Scott Button

Principal

FIGURES

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

TIME (hrs)

0

500

1000

1500

2000

2500

3000

3500

4000

DIS

CH

AR

GE

(m

3/s

)

LEGEND

2000 year ARI

100 year ARI

PROPOSED BRIDGE OVER QUEANBEYAN RIVERFLOODING INVESTIGATION

Figure 2

DESIGN DISCHARGE HYDROGRAPHSQUEANBEYAN RIVER AT QUEANBEYAN

PROPOSED BRIDGE OVER QUEANBEYAN RIVERFLOODING INVESTIGATION

SPA

CE

SPAC

Figure 4

WATER SURFACE PROFILES

5000 5500 6000 6500

570

575

580

585

590

Main Channel Distance (m)

Ele

vatio

n(m

)

Legend

2000 year ARI - Present Day

100 year ARI - Post Upgrade

Invert of River Bed

2000 year ARI - Post Upgrade

100 year ARI - Present Day

5772

5988

6204

6230

6255

6279

6303

6323

6424

6660

7167

7375

Queanbeyan River Upstream of CBD

TOE OF NORTHERN ABUTMENT

TOE OF SOUTHERN ABUTMENT

BRIDGE DECK

ATTACHMENT 1

ATTACHMENT 2