Mudgee Catchment A Vol 2 - Mar 2008 · 2015-07-17 · Mudgee Local Creeks Floodplain Risk...

MUDGEE LOCAL CREEKS FLOODPLAIN RISK MANAGEMENT

STUDY AND PLAN

FLOOD BEHAVIOUR STUDIES

APPENDIX A – CATCHMENT A

MARCH 2008

DRAFT REPORT

Job No: CJ194 File: Mudgee Catchment A Vol 2.doc

Date: March 2008 Rev No.: 2.0

Author: BWL Principal: BWL

Mudgee Local Creeks Floodplain Risk Management Study and Plan

Catchment A - Flood Behaviour Studies

Mudgee Catchment A Vol 2.doc i Lyall & Associates March 2008 Rev 2.0 Consulting Water Engineers

TABLE OF CONTENTS

Page No

1. CATCHMENT AREA..........................................................................................................1

2. HYDROLOGIC MODELLING .............................................................................................3 2.1 Model Setup...........................................................................................................3 2.2 Model Results ........................................................................................................3

3. HYDRAULIC MODELLING ................................................................................................5 3.1 Model Setup...........................................................................................................5 3.2 Cudgegong River Flood Levels ..............................................................................5 3.3 Model Results ........................................................................................................5

3.3.1 General......................................................................................................5 3.3.2 Upstream of Bellevue Road .......................................................................6 3.3.3 Bellevue Road to Railway ..........................................................................6 3.3.4 Railway to Fairy Dale Lane ........................................................................6 3.3.5 Downstream of Fairy Dale Lane.................................................................6

4. IMPACTS OF FLOODING ON EXISTING DEVELOPMENT ..............................................9 4.1 General ..................................................................................................................9 4.2 Impact on Residential Property ..............................................................................9

5. DRAINAGE IMPROVEMENTS.........................................................................................11 5.1 Problem Areas .....................................................................................................11 5.2 Detention Basins..................................................................................................11 5.3 Channel Improvements ........................................................................................15 5.4 Culvert Improvements ..........................................................................................16

6. FUTURE DEVELOPMENT ...............................................................................................17 6.1 West Mudgee Development Control Plan.............................................................17 6.2 Hydrologic Modelling............................................................................................17 6.3 Detention Basins..................................................................................................17

7. SUMMARY AND CONCLUSIONS ...................................................................................19

LIST OF FIGURES 2.1 DRAINS Model Layout 3.1 Water Surface Profiles (Sheets 1 and 2) 3.2 Indicative Extents of Inundation



Mudgee Catchment A Vol 2.doc ii Lyall & Associates March 2008 Rev 2.0 Consulting Water Engineers

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Mudgee Catchment A Vol 2.doc 1 Lyall & Associates March 2008 Rev 2.0 Consulting Water Engineers

1. CATCHMENT AREA The catchment rises in hilly country to the south of the town and flows northwards before discharging into the Cudgegong River. Mortimer Street and Saleyards Lane border the catchment to the north, Henry Bayly Drive to the east, and wooded hills form the southern and western boundaries. Saleyards Lane is the limit of influence on flooding from the Cudgegong River. The total area of Catchment A is 616 ha. Established urban areas are located in the north-east portion of the catchment and areas on the south-western slopes between Bellevue Road and the disused Wallerawang – Gwabegar Railway have more recently been urbanised. The remaining land is currently rural. Most of the flow comes from the hills to the south west and crosses Rifle Range Road in an 825 mm RCP, 550 m west of Henry Bayly Drive. Several hundred metres downstream, the well-defined natural channel is joined by the discharge from the south east part of the catchment. Before crossing Bellevue Road the channel becomes flatter and less well defined. The downstream reaches are quite flat and the watercourse is not well defined. It crosses the railway embankment via two sets of three 1200 mm diameter Armco culverts. The watercourse then flows northwards along the western boundary of playing fields before it flows into a large farm dam several hundred metres upstream of Fairy Dale Lane. Flows from urbanising areas on the north-west of the catchment are controlled by two detention basins. Outflows from the basins cross the railway and join the main channel upstream of the dam. Outflows from a third detention basin from an urbanising area to the north of the other two cross the railway and join the watercourse upstream of Fairy Dale Lane. Just west of the intersection of Gladstone Street and Fairy Dale Lane, a 375 RCP conveys flows under the latter road to a concrete dish drain, which runs along the southern side of Gladstone Street to the intersection with Bell Street. The residential area on the northern side of Gladstone Street discharges its stormwater at Mortimer Street. The main watercourse crosses Saleyards Lane in a 600 mm x 300 mm box culvert and discharges into a large paddock where flows spread out and are conveyed as overland flow to culverts under the Gulgong Road, before discharging into the Cudgegong River.




2. HYDROLOGIC MODELLING

2.1 Model Setup Figure 2.1 shows the arrangement of sub-catchments and drainage links comprising the model. The catchment was divided into 20 sub-catchments to model the generation and movement of flows through the network. The model extended to the Saleyards Lane crossing. From this point, the watercourse flows for a distance of 750 m through the paddocks to the Cudgegong River.

2.2 Model Results Table 2.1 shows peak discharges within the main channel system and tributary branches for the critical storm. Because of the size of the catchment and its large rural component, critical storm duration is between 90 and 120 minutes depending on recurrence interval.

TABLE 2.1 PEAK DISCHARGES

EXISTING (Year 2008) CONDITIONS (m3/s)

Location 5 year

ARI 120 min

20 year ARI

120 min

100 year ARI

90 min

Rifle Range Road (Main Arm) 2.3 4.6 10.9

South-east catchment upstream of confluence with main channel (B1, B2)

0.8 1.6 3.9

Downstream of confluence 3.1 6.3 14.9

Bellevue Road (Main Arm) 3.5 7.10 16.7

Railway Culverts (Main Arm) 4.07 8.03 19.1

Urban catchment south of Gladstone Street 1.6 2.2 3

Fairy Dale Lane (Main Arm) 5.8 11.0 24.5

North-west rural catchments at railway (E1, E2, E11)

2.4 2.6 4.3

Urban catchment south of Mortimer Street (G1, G2) 0.7 1 1.8

Saleyards Lane (Main Arm) 7.5 13.9 31.2

MUDGEE LOCAL CREEKS FLOOD BEHAVIOUR STUDIES - CATCHMENT A

Figure 2.1 DRAINS MODEL LAYOUT

N

0 375 750m

Scale

E3

E2

E11

E1

D2

C1

A2D1

A1

B1

E4

G2

G1

F1 D3C2

B2

EC1

EB1AB2

AB1

GLA

DSTO

NE

ST

MO

RTIMER

ST

MA

RKET

ST

FAIR

Y D

AL

EL

N

BE

LL

EV

UE

RD

RIF

LE

RA

NG

E

RD

PAL

ER

MO

RD

HENRY

BAYLY

DR

RA

ILW

AY

GWABEGAR

COX

ST

REDBANK

CREEK

CUDGEGONG RIV

ER

WALLERAWANG

COURT

DOURO

ST

ST

MA

DE

IRA

RD

SALEYARDS LN

CASTLEREAGH

HWY

MOUNT MISERY RD

LEGEND

E1

Sub-Catchment Boundary

Drainage Reach

Sub-Catchment Number

Existing Detention Basin




3. HYDRAULIC MODELLING

3.1 Model Setup The model starts 100 m upstream of Rifle Range Road and the last cross section is 200 m downstream of Saleyards Lane. The total length of Saleyards Creek modelled is about 2.8 km. Bridges and culverts were incorporated into the model at locations where either road or railway lines crossed the creek.

3.2 Cudgegong River Flood Levels From the Mudgee Floodplain Management Study Report, 2002, the 100 ARI water level at the confluence of the creek and the Cudgegong River is RL 445.6 m. This level approximates the 100 year ARI peak water level from creek flooding at the most downstream cross section below Saleyards Lane, as calculated from the HEC-RAS model and assuming uniform conditions. From these values it can be concluded that a 100 year ARI flood from the Cudgegong River would not influence flooding conditions in developed areas bordering the main arm of Catchment A.

3.3 Model Results

3.3.1 General Water surface profiles for the 5, 20, 100 year ARI and PMF events are shown on Figure 3.1. Indicative extents of inundation are shown on Figure 3.2. Table 3.1 gives details of the bridge/culvert arrangements for each crossing modelled. A broad assessment of the hydrologic capacity of the various bridges modelled is included. For the purpose of this assessment the hydrologic capacity is defined, by the average recurrence interval of the flow required to overtop the roadway.

TABLE 3.1 DETAILS ROAD AND RAILWAY CROSSINGS ON MAIN CHANNEL

Location Opening Waterway

Area (m2)

Approx. Capacity

(ARI years)

Rifle Range Road 825 RCP 0.5 >5

Bellevue Road

Two Openings 3x1.2mx0.6m RCBC, 2x1.2mx0.6m RCBC

3.6 >20

Railway Two Openings each

3x1200 Armco Culverts 6.8 >20

Fairy Dale Lane 375 RCP 0.1 Low Flows Only

Saleyards Lane 600x300 RCBC 0.2 <5




3.3.2 Upstream of Bellevue Road The first road crossed by the main watercourse is Rifle Range Road. Although the upstream grade is about 3%, the water depth above the invert level of the culvert is up to 2 m for the different storm events due to the high level of the road and the resulting backwater. The widths of flow in this area exceed 50 m. Flows greater than the 5 year ARI event flow over the road, with the depth of inundation increasing to 200 mm for the 100 year ARI event. Below the crossing of Rifle Range Road, the watercourse is well defined. It is joined by the flows coming from the south east (sub-catchments B1 and B2). The flow is supercritical in the steeper sections of channel where the bed slope is about 5% and velocities of flow are as high as 2.5 to 3.5 m/s for the 100 year ARI event. About 300 m upstream of Bellevue Road the slope of the watercourse decreases to 1%, which results in smaller velocities and wider flows. Stormwater crosses Bellevue Road in two sets of culverts which have a combined hydrologic capacity greater than the 20 year ARI flood. The width of flow in the vicinity of the road crossing is about 100 m for the 100 year ARI flood.

3.3.3 Bellevue Road to Railway The channel over the 400 m long reach between Bellevue Road and the railway is not very well defined. Due to the flatter bed slope, the velocities decrease and water spreads out. The railway is crossed by two sets of Armco Culverts, which have a total cross sectional area of about 6.8 m2

and conveys flows in excess of 20 year ARI. The crossing causes backwater effects and for major floods a portion of the discharge would be conveyed westwards along the upstream face of the embankment.

3.3.4 Railway to Fairy Dale Lane Downstream of the railway the watercourse flows northwards. For major storms the channel surcharges and a portion of the flow escapes into the playing fields on its western side and is conveyed over the surface of the fields as overland flow. These overland flows are intercepted and conveyed westwards towards the watercourse by the open drain running along the southern side of Lang Street. About 300 m downstream of the railway, the watercourse flows into a farm dam. Overflows are conveyed over by-wash spillways on the eastern and western sides of the embankment. A diversion bank has been constructed between Lang Street and Fairy Dale Lane to protect residential properties on the western side of the creek from surcharges. The crest of this bank is around 100 year ARI flood levels along its extent. The capacity of the culvert under Fairy Dale Lane is very low. For even a 5 year ARI event, the water level is 200 mm above the lowest spot on the centreline of the road.

3.3.5 Downstream of Fairy Dale Lane Immediately downstream of the culvert under Fairy Dale Lane the channel runs along the southern side of Gladstone Street in a concrete lined dish drain. For floods greater than the 20




year ARI the centreline of Gladstone Street would be overtopped and flow would enter residential allotments on the northern side of the street. Downstream of the corner of Bell Street and Gladstone Street, the channel becomes indistinct. The capacity of the culvert under Saleyards Road is low. Downstream of Saleyards Lane, the channel is well defined and is able to convey a 5 year ARI event. For larger floods, the banks will be overtopped and the area along the channel will be inundated by about 200 mm for a 20 year ARI event and about 400 mm for a 100 year ARI event. The last cross section, 200 m downstream of Saleyards Road, does not show any signs of a channel. The stormwater spreads out in the paddock and gradually find its way into the Cudgegong River.

MUDGEE LOCAL CREEKSFLOOD BEHAVIOUR STUDIES - CATCHMENT A

Figure 3.1WATER SURFACE PROFILES (SHEET 1 OF 2)

0 250 500 750 1000 1250 1500440

445

450

455

460

465

470

Main Channel Distance (m)

Legend

WS PMF

WS 100 yr ARI

WS 20 yr ARI

WS 5 yr ARI

Ground

2 C

s 2

- 100

m d

/s S

aley

...

4 C

s 4

- Sal

eyar

ds L

ane

(box

cul

ve...

6.1

Cs

6.1

- Cor

ner G

lads

tone

and

Bel

l Stre

et

7 C

s 7

- 35

m d

/s c

entre

line

Fairy

Dal

e La

ne7.

1 C

s 7

copi

ed to

10

m d

/s F

airy

Dal

e La

ne

8 C

s 8

- Fai

ry D

ale

Lane

(375

mm

RC

P)

10 C

s 10

- B

ase

Larg

e D

am

122.9

Cs

3

11.5

6 C

s 6

- 100

m d

/s B

ell S

treet

Elev

atio

n (m

)

MUDGEE LOCAL CREEKSFLOOD BEHAVIOUR STUDIES - CATCHMENT A

Figure 3.1WATER SURFACE PROFILES (SHEET 2 OF 2 )

1600 1800 2000 2200 2400 2600 2800 3000440

450

460

490

500

Main Channel Distance (m)

Elev

atio

n (m

)

Legend

WS PMF

WS 100 yr ARI

WS 20 yr ARI

WS 5 yr ARI

Ground

13.9

Cs

14 -

0.1

m d

/s R

ailw

ay15

New

CS

15 -

10 m

u/s

Rai

lway

15.5

New

CS1

5.5

70m

d/s

CS1

6

16 N

ew C

S16

150

m d

/s C

S16

.1

16.1

New

CS1

6.1

80 m

d/s

Bel

levu

e R

oad

18 B

elle

vue

Roa

d

20 C

s 20

- en

d Fo

rmed

Roa

d so

uth

of B

elle

vue

Roa

d

21 C

s 21

- 7

0 m

d/s

pow

erlin

es &

170

m d

/s R

ifle

Ran

ge R

oad

23 C

s 23

- R

ifle

Ran

ge R

oad

(825

mm

RC

P)

12.9

Cs

12 2

0 m

d/s

Rai

lway

16.9

New

CS

16.1

10

m d

/s B

elle

vue

Roa

d

21.9

Cs

22 1

0m d

/s R

ifle

Ran

ge R

oad

25

470

480

MUDGEE LOCAL CREEKS FLOOD BEHAVIOUR STUDIES - CATCHMENT A

Figure 3.2INDICATIVE EXTENTS OF INUNDATION

N

0 200 400m

Scale

LEGENDPMF Flood Extent100 Year Flood Extent20 Year Flood Extent

THE EXTENTS OF INUNDATION SHOWN WERE DETERMINED FROM SURVEYED CROSS SECTIONSOF THE CREEK AND FLOODPLAIN AND AVAILABLE DATA AND ARE APPROXIMATE ONLY. THE EXTENTOF INUNDATION OF INDIVIDUAL ALLOTMENTS NEARTHE FLOOD FRINGE SHOULD BE CONFIRMED BY SITE SPECIFIC SURVEY.

HEC-RAS Cross Section 6

Escape of Flows fromCreek at 100 Year ARI

Extent of InundationUncertain

3

645

6.1

78 9

10 11 12

13

14

15

16

17

18

19 20

21

22

23

15.5

16.1

2

1

24

MAR

KET

STREET

MO

RTIMER

STREET

GLA

DSTO

NE

STR

EET

LANG

S

TREET

BELL STREET

FA

IRY

D

ALE

LA

NE

RA

ILW

AY

BE

LLE

VU

E R

OA

D

BE

LLE

VU

E R

OA

D

RIC

HA

RD

S S

TR

EE

T

RIF

LE

R

AN

GE

R

OA

D

RIF

LE

R

AN

GE

R

OA

D

SALEYARDS LANERAILW

AY

RAILWAY




4. IMPACTS OF FLOODING ON EXISTING DEVELOPMENT

4.1 General Existing residential development between Lang Street and Gladstone Street is protected by the diversion bank running along the eastern bank referred to in the previous section. Residential properties on the northern side of Gladstone Street would become flood affected when the dish drain conveying flows along the southern side of the street is surcharged and the road is overtopped. The centreline of the road would function as a broad crested weir and surcharging flows would be conveyed northwards into the allotments. The road would be on the point of overtopping at the 20 year ARI and significant over flows would occur in the event of the 100 year ARI.

4.2 Impact on Residential Property Floor levels of 10 properties in Gladstone Street to the east of the intersection with Bell Street were surveyed. These allotments would be flood affected in the event of floods greater than 20 year ARI. Over the distance between the centreline of the road and the allotments on the northern side of the road there would be a reduction in peak water surface levels as flow is conveyed northwards through these properties. Allowing for a fall of about 0.3 m in water levels between these points, it is considered that three residences would be subject to shallow depths of flooding above their floor levels at the 100 year ARI. Four allotments on the northern side of Lang Street may be flood affected at the 100 year ARI level of flooding, but floodwaters would not overtop floor levels. Table 4.1 summarises the results of this assessment.

TABLE 4.1 IMPACTS OF FLOODING ON

RESIDENTIAL DEVELOPMENT CATCHMENT A

Number of Properties Flood ARI

Flood Affected Flood Damaged

Flood Damages $ x 103

20 0 0 Neg.

100 14 3 225

PMF 35 10 640

Flood Affected = Flooding in Allotments + Residences Flooded above floor level. Flood Damaged = No. of Residences flooded above floor level.




5. DRAINAGE IMPROVEMENTS

5.1 Problem Areas The main problem areas on this catchment are located on the main arm downstream of Lang Street where existing residential development is flood affected, mainly in Gladstone Street. Drainage problems have also been reported in Lang Street, which may be caused by overland flow escaping from the channel of Saleyards Creek and flowing northwards over the playing fields to that street. There are two strategies which could be considered to improve the performance of the drainage system.

Implementation of Detention Basins to reduce downstream flows.

Improvements to channel capacity of Saleyards Creek. These two options are reviewed below.

5.2 Detention Basins 5.2.1 Initial Review of Storage Requirements In assessing storage requirements, it was assumed that a basin would need to have sufficient size to store 20 to 30% of the total volume of runoff to be capable of attenuating peak flows. For the purpose of preliminary screening of basin options, the 100 year ARI 90 minute storm was adopted, as typically these longer storms with their large volume of runoff are critical for the design of detention basins. The basins would be located on the main arm of Saleyards Creek to maximise reductions in flows in the downstream problem areas. Table 5.1 summarises requirements for storage to achieve the above storage criterion. The surface area requirements are based on an average depth of 2 m at peak storage level. These indicative storage requirements show that an excessively large “land take” would be required. Smaller basins at Sites 3 and 4 may be practicable. Preliminary hydrologic analysis was undertaken of several smaller basins. For the purposes of this investigation two basins were assessed:

A basin of 30,000 m3 capacity located on the downstream side of Bellevue Road.

A re-constructed farm dam with 30,000 m3 “air space” for flood mitigation above the conservation storage level.

These basins are smaller than the desired sizes shown in Table 5.1, but appear to be about the maximum storage which could be achieved within site constraints.



Mudgee Catchment A Vol 2.doc Page 13 Lyall & Associates March 2008 Rev 2.0 Consulting Water Engineers

TABLE 5.1 DETENTION BASIN STORAGE REQUIREMENTS

Basin No. Basin Location Description

Indicative Volume Required to Store 20 - 30% of Runoff from 100 year ARI,

storm of 90 minutes duration

(m3)

Basin Dimensions (m) (length x breadth)

1 Main Arm - Wallerawang-Gwabegar Railway

Controls both rural and urbanised runoff generated on the main arm and its tributaries. The basin would be located adjacent to the railway line.

80,000 200 x 200

2 Confluence of Main Arm and Eastern Branch

Controls both rural and urbanised runoff from both the main arm and the Eastern Branch. The basin would be located on the confluence of the two

streams downstream of Rifle Range Road. 67,500 180 x 180

3 Main Arm - Bellevue Road The basin would be located immediately downstream of Bellevue Road. and would be slightly larger Basin 1 above.

100,000 220 x 220

4 Existing Farm Dam d/s Wallerawang-Gwabegar Railway

The existing farm dam would be reconstructed to form a dual purpose water conservation/ flood mitigation storage.

120,000 230 x 230

Note: all basins store to a maximum depth of 2 m




5.2.2 Modelling Basin options Table 5.2 shows the pre- and post- basin peak discharges on Saleyards Creek for the 100 year ARI storm. These results apply for the 90 minute storm duration. The basins would provide significant attenuation of flow in the Fairy Dale Lane – Gladstone Street residential area, with the peak discharge reduced to midway between the 20 and 100 year ARI values. However some complementary channel improvements would be required in the vicinity of the farm dam to prevent overland flows escaping to the playing fields and causing problems in the Lang Street area. It is difficult to provide an accurate estimate of cost given the lack of survey data at the two sites. An order of magnitude costing for the two basins would be:

Basin of 30,000 m3 capacity downstream of Bellevue Road $750,000 (denoted Option A.4 – see Table 8.1 and Figure 8.1 of Flood Behaviour Studies, Volume 2).

Re-Construct Farm Dam to provide 30,000 m3 flood storage $650,000 (denoted Option

A.5)

TABLE 5.2

PEAK DISCHARGES 100 YEAR ARI EXISTING CONDITIONS vs POST-DETENTION BASINS

(m3/s)

Location Existing

Conditions m3/s

With Basins

m3/s

d/s Bellevue Road (Main Arm) 16.7 12.2

Railway Culverts (Main Arm) 19.1 14.0

Fairy Dale Lane (Main Arm) 24.5 14.1

Saleyards Lane (Main Arm) 31.2 18.4

Note: These results apply with two detention basins (1) d/s Bellevue Rd (2) Re-Constructed Farm Dam

5.3 Channel Improvements From Rifle Range Road to 300 m downstream, the grade of the existing channel is up to 6%. Channel improvements over this reach would require the construction of a number of drop structures to dissipate the energy of the water and reduce velocities to non-scouring values. They are probably not warranted as flows are conveyed within the proximity of the channel apart from immediately upstream of Rifle Range Road. Improvements to the hydraulic capacity of the culvert would reduce the incidence of overtopping and are discussed in Section 5.4. The road would presently overtopped in the event of a flood around the 5 year ARI magnitude.




At a location about 300 m downstream of Rifle Range Road, the natural channel becomes less steep. It flows to Bellevue Road at a slope of approximately 1%. No drop structures would be needed along this reach. The crossings at Bellevue Road have recently been upgraded and have a hydrologic standard greater than the 20 year ARI. Over the reach from Bellevue Road to the Railway the channel is indistinct and flows are mainly of an overland flow nature. Creek improvements over this reach are presently not warranted, as the area is in its rural state. Council have proposed a 40 m wide drainage reserve to accommodate future development. Creek improvements will be required at that time. Several drop structures may be required to accommodate the 1% grade. The capacity of the existing Armco culverts beneath the railway is sufficient to discharge the 20 year ARI peak flow. Downstream of the railway improvements in creek capacity are required to contain floodwaters within the confines of the creek and mitigate flooding in Lang and Gladstone Streets. These improvements would be implemented over two reaches as shown on Table 5.3.

From the railway to Lang Street a distance of about 500 m. (Option A.1) For preliminary sizing the channel was sized to convey the 100 year ARI discharge at a maximum depth of 1.5 m with 500 mm of freeboard. The maximum design velocity of flow in the channel would be 2 m/s. Allowance would be made on each side of the channel for landscaping. The indicative cost would be around $250,000.

Along the southern side of Gladstone Street from Fairy Dale Lane to Bell Street. (Option A.2) The existing concrete vee drain would be replaced by a grassed floodway of 5 m bed width containing a new low flow concrete invert. The channel would be sized to prevent the escape of flows over the centreline of Gladstone Road into the residential allotments on the northern side of that street for floods up to the 100 year ARI. The indicative cost would be around $225,000.

TABLE 5.3

PRELIMINARY ASSESSMENT OF CHANNEL IMPROVEMENTS

Reach

From To

Width of Invert of Grassed Floodway

(m)

Length of Improvement

(m)

Railway Fairy Dale Lane 5 500

Along Southern side Gladstone Street

Bell Street 5 150

5.4 Culvert Improvements The culvert under Rifle Range Road presently has a hydrologic standard slightly greater than 5 years ARI and should be upgraded to a higher capacity. Preliminary calculations indicate that a 20 year ARI capacity could be achieved by upgrading the existing 825 mm diameter pipe to a culvert of 3600 x 1200 mm dimensions. The indicative cost of this scheme (denoted Option A.3) would be about $115,000.




6. FUTURE DEVELOPMENT

6.1 West Mudgee Development Control Plan Council’s current zoning policy envisages medium density residential development bordering the main arm of Saleyards Creek and on the eastern fringes of the catchment between Bellevue Road and the railway. Development is expected south of Rifle Range Road but of a rural residential nature which would not be expected to have a major impact on the flood hydrology of the catchment. The remaining areas in the headwaters of the catchment will remain in their rural condition.

6.2 Hydrologic Modelling The effects of urbanisation within Catchment A were modelled by increasing the percentage of impervious area within relevant sub-catchments. The DRAINS model incorporating the ultimate extent of urbanisation was then run for the critical 20 minute storm event. Peak discharges were not significantly higher than for present day conditions although the volume of runoff was increased.

6.3 Detention Basins Table 6.1 compares peak 100 year ARI discharges under developed conditions both with and without the two detention basins modelled in Section 5.2. These results apply for the 120 minute storm duration and indicate that development would marginally increase peak flows in the lower reaches of the catchment. The results are indicative only. With available survey data it is not possible to derive the storage characteristics of the two sites with accuracy, nor is it possible to apportion the costs of mitigating measures (such as increasing the size of the basins) to future development in the catchment.

TABLE 6.1 PEAK DISCHARGES 100 YEAR ARI

FUTURE URBANISATION WITH AND WITHOUT DETENTION BASINS

(m3/s)

Location No

Basins

With Basins

Railway Culverts (Main Arm) 18.6 16.9

Fairy Dale Lane (Main Arm) 23.9 16.2

Saleyards Lane (Main Arm) 30.4 20.4

Note: These results apply with two detention basins (1) d/s Bellevue Rd (2) Re-Constructed Farm Dam




7. SUMMARY AND CONCLUSIONS The major findings of the study were as follows:

The existing culvert conveying Saleyards Creek flows under Rifle Range Road has a capacity slightly greater than a 5 year ARI and needs to be upgraded when the area is developed. A scheme for upgrading the Rifle Range Road culvert is denoted Option A.3 on Table 8.1 and Figure 8.1 of the Flood Behaviour Studies, Volume 2.

The hydraulic capacity of the culverts at the railway crossing is sufficient to convey 20 year ARI flows.

The channel of Saleyards Creek surcharges downstream of the railway causing flooding in residential allotments in Gladstone Street.

Flows in the lower reaches of the creek could be reduced by the implementation of detention basins. Two sites were identified and preliminary modelling undertaken. However the accuracy of modelling and subsequent costing was limited by the lack of survey data. These schemes are denoted Options A.4 and A.5.

Channel improvements were also investigated and indicative sizes and costs presented. On the basis of preliminary costing, this measure would be more cost effective than detention basins. Two schemes have been formulated (Options A.1 and A.2)

Hydrologic analysis showed that future urbanisation of the catchment according to Council’s existing zoning policy would not result in large increases in flood flows.

It was not practicable with existing 2 m contour survey to derive accurate cost estimates for drainage works necessary to mitigate existing flooding problems. Nor was it possible to assign costs required to mitigate the impacts of future development on peak flows.

It may be more cost effective for Council to mitigate the impacts of future development on peak flows through application of its on site detention storage policy rather than via reserving additional storage in “community” detention basins. (This conclusion is subject to confirmation in the Investigation/Concept Design Study recommended in Section 8.2 of the Flood Behaviour Studies, Volume 2).

Mudgee Catchment A Vol 2 - Mar 2008 · 2015-07-17 · Mudgee Local Creeks Floodplain Risk...

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