sawpit gully catchment app g vol 2 - mar 2008...the hydraulic model consisted of a single branch...

MUDGEE LOCAL CREEKS FLOODPLAIN RISK MANAGEMENT

STUDY AND PLAN

FLOOD BEHAVIOUR STUDIES

APPENDIX G - SAWPIT GULLY

MARCH 2008

DRAFT REPORT

Job No: CJ194 File: Mudgee Sawpit Gully Catchment Vol 2.doc

Date: March 2008 Rev No.: 2.0

Author: BWL Principal: BWL

Mudgee Local Creeks Floodplain Risk Management Study and Plan

Sawpit Gully - Flood Behaviour Studies

Mudgee Sawpit Gully Catchment Vol 2.doc 3 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 Results ........................................................................................................3

3 HYDRAULIC MODELLING ................................................................................................5 3.1 Sawpit Gully – Main Arm........................................................................................5

4 IMPACTS OF FLOODING ON EXISTING DEVELOPMENT ..............................................7 4.1 Impact on Existing Property ...................................................................................7

5 DRAINAGE IMPROVEMENTS...........................................................................................9 5.1 Problem Areas .......................................................................................................9 5.2 Detention Basins....................................................................................................9 5.3 Channel Improvements ........................................................................................10

6 FUTURE DEVELOPMENT ...............................................................................................13 6.1 Catchment Zoning................................................................................................13 6.2 Hydrologic Modelling............................................................................................13

7 SUMMARY AND CONCLUSIONS ...................................................................................15

LIST OF FIGURES 2.1 DRAINS Model Layout 3.1 Water Surface Profiles 3.2 Indicative Extents of Inundation



Mudgee Sawpit Gully Catchment Vol 2.doc 4 Lyall & Associates March 2008 Rev 2.0 Consulting Water Engineers

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

1 CATCHMENT AREA Sawpit Gully is located on the eastern side of town and adjoins the Redbank Creek catchment. Its headwaters are located in the Waterworks Road area and it flows in a general north easterly direction crossing Bruce Road, Lions Drive, Sydney Road and Industrial Avenue before flowing beneath the Wallerawang - Gwabegar Railway to the floodplain of the Cudgegong River. The total catchment area at the railway amounts to 220 ha. Flooding from the Cudgegong River extends upstream along the channel of Sawpit Gully as far as the railway embankment but does not influence the hydraulic capacity of the culverts. The peak 100 year ARI flood level on the Cudgegong River at the junction with Sawpit Gully is about RL 454 m. This level is about 200 mm below the invert of the four 1200 mm diameter culverts conveying flows through the railway embankment. Present day development is mostly located downstream of Lions Drive with the caravan park adjacent to the creek, followed by industrial developments further downstream in the Sydney Road and Industrial Avenue areas, extending to the railway. A recent residential sub-division is located on the western bank of Sawpit Gully just upstream of Lions Drive. Runoff from this development is captured by the recently constructed detention basin just upstream of Lions Drive.




2 HYDROLOGIC MODELLING Figure 2.1 shows the arrangement of sub-catchments and drainage links used to define the hydrologic model of Sawpit Gully. The recently constructed detention basin located on the line of the creek and immediately upstream of Lions Drive was included in the model. The basin is about 4 m high and has a storage volume of 9,600 m3 at spillway level, with outflows controlled by a low level outlet comprising three 900 RCP’s and two high level spillways which would function as broad crested weirs during flood events greater than the 100 year ARI. 2.1 Model Results DRAINS results indicated that for the 5, 20 and 100 year ARI events, the 20 minutes storm duration was critical in terms of generating peak design flows for most frequencies and locations. However, at the detention basin, the 60 minutes storm duration was critical in terms of generating peak storage level and outflow of the 100 year ARI flood. Peak discharges at various locations within the drainage system for the critical storm are given in Table 2.1.

TABLE 2.1 PEAK DISCHARGES SAWPIT GULLY

EXISTING (2008) CONDITIONS (m3/s)

Location 5 year ARI

25 min 20 year ARI

25 min 100 year ARI

20 min

Inflow to Detention Basin 1.8 4.4 9

Outflow from Detention Basin 1.5 3.1 6.5 (1)

Lions Drive 2.1 3.8 6.7

Sydney Road 3.6 6.5 11.2

Industrial Avenue 4.7 8.2 14

Railway Culvert 4.9 8.6 15

Note: (1) The 60 minutes storm duration is critical (i.e. maximises outflows) for the detention basin. The other results in

the table apply for the 20 and 25 minutes storm durations, which are critical for other areas along the creek system.

MUDGEE LOCAL CREEKS FLOOD BEHAVIOUR STUDIES - SAWPIT GULLY

Figure 2.1DRAINS MODEL LAYOUT

N

0 500 1000m

Scale

LEGEND

A1

Sub-Catchment Boundary

Sub-Catchment Number

Drainage Reach

WA

TE

RW

OR

KS

A2

B1

C1

D1

E1

E2

A6

A5

A8

A7

A9

A4

A3

AA1

AA2

RD

SA

WP

IT

RD

RD

BR

UC

E

SPRINGFLAT

RD

BROADHEAD

RD

ROBERTSON

RD

SYDNEY

RD

COX

COURT

DOURO

PERRY

CHURCH

LEWIS

LAWSON

GEORGE

ST

ST

ST

ST

ST

ST

ST

MO

RT

IME

R

GL

AD

ST

ON

E

DE

NIS

ON

HO

RA

TIO

ING

LIS

NIC

HO

LS

ON

ME

AR

ES

MA

DE

IRA

ST

ST

ST

ST

ST

ST

ST

ST

CHURCHCHURCH

OPORTO

HENRYBAYLY

DR

ROBERTSON

GEORGE

RD

REDBANK

CREEK

WALLERAWANG

GWABEGAR

RA

ILW

AY

IND

UST

RIA

L

AV

E

LIO

NS

D

RIV

E

A2A2

250 750

1:15000




3 HYDRAULIC MODELLING 3.1 Sawpit Gully – Main Arm The hydraulic model consisted of a single branch about 1.2 km long, representing the open channel system downstream of the detention basin, continuing to the floodplain of the Cudgegong River downstream of the railway embankment. Figure 3.1 shows peak water surface profiles for floods ranging between the 5 year ARI and PMF. Figure 3.2 shows indicative extents of inundation. Because of the throttling effects of the detention basin flows up to the 100 year ARI magnitude are generally conveyed within the confines of the creek channel in the reach between Lions Drive and Sydney Road. The culverts beneath Lions Drive, the Sydney Road, Industrial Avenue and the railway were included in the model. Table 3.1 provides details of their hydrologic capacity. The culvert beneath Industrial Avenue is frequently overtopped and most of the flow is conveyed across the roadway with considerable backwater effects extending into upstream industrial property on the eastern bank.

TABLE 3.1 DETAILS ROAD AND RAILWAY CROSSINGS ON SAWPIT GULLY

Location Opening Waterway Area

(m2)

Approx. Capacity

(ARI years)

Lions Drive 2 x 1350 RCP 2.9 >100

Sydney Road

2.4m x1.5m RCBC,

3.6 >20

Industrial Avenue 1.5m x 0.75m RCBC 1.125 <5

Railway 4 x 1200 Armco Culverts 4.52 >100

MUDGEE LOCAL CREEKSFLOOD BEHAVIOUR STUDIES - SAWPIT GULLY

Figure 3.1WATER SURFACE PROFILES

0 200 400 600 800 1000 1200 1400460

465

470

475

480

Main Channel Distance (m)

Ele

vatio

n (m

)

Legend

WS 15 Min PMF

WS 100 Yr 20 Min

WS 20 Yr 20 Min

WS 5 Yr 20 Min

Ground

0.8

Rai

lway

1.5

Sur

vey

XS

38

2.5

Indu

stry

Ave

nue

4 S

urve

y X

S42

5.5

Syd

ney

Roa

d

7 S

urve

y X

S46

8 S

urve

y X

S47

9 S

urve

y X

S48

10 L

ions

Driv

e

1.25

1.75

4.75 9.5

MUDGEE LOCAL CREEKS FLOOD BEHAVIOUR STUDIES

SAWPIT GULLYFigure 3.2

INDICATIVE EXTENTS OF INUNDATION

N

0 125 250m

Scale

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.

LEGEND

PMF Flood Extent100 Year Flood Extent20 Year Flood ExtentHEC-RAS River Station RS 3

RS 10

RS 9

RS 8

RS 7

RS 6

RS 5

.5RS 5

RS 4

RS

3

RS

2.5

RS

2

RS 1.5

SYDNEY R

D

SYDNEY RD

IND

US

TR

IAL R

D

IND

USTR

IAL

RD

SYDNEY R

DINDUSTRIAL RD

BU

RR

UN

DU

LLA

RD

BURRUNDULLA R

D

BU

RR

UN

DU

LLA

R

D

SWORDS

CT

DEPO

T RD




4 IMPACTS OF FLOODING ON EXISTING DEVELOPMENT 4.1 Impact on Existing Property No properties would be affected in the caravan park between Lions Drive and Sydney Road. Indicative extents of inundation are within the immediate vicinity of the creek even for the PMF. Floor levels of several industrial properties bordering the creek were surveyed at Sydney Road and Industrial Avenue. Floodwaters would extend into industrial properties on both the upstream and downstream sides of Industrial Avenue. However, no above floor inundation would be experienced to existing property. Flooding extends into two currently vacant blocks on the western overbank of the creek. Properties adjacent to the creek at Sydney Road would not be affected by 100 year ARI flooding. However, the two properties bordering the creek on the downstream side of the road would be flooded to a depth of about 500 mm above floor level for the PMF. Table 4.1 summarises likely flood damages.

TABLE 4.1 IMPACTS OF FLOODING ON

INDUSTRIAL DEVELOPMENT SYDNEY ROAD TO RAILWAY SAWPIT GULLY

Number of Properties

Flood ARI Flood Affected Flood Damaged

Flood Damages $ x 103

20 0 0 0

100 0 0 0

PMF 2 2 300

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 Existing residential development on the catchment is not flood affected. The recent subdivision upstream of Lions Drive is above the top water level of the detention basin storage. The basin mitigates downstream flows in the reach between Lions Drive and Sydney Road to the extent that flows are conveyed within the immediate vicinity of the creek. Industrial properties adjacent to the creek in the reach between Sydney Road and industrial Avenue would not suffer adverse impacts for floods up to the 100 year ARI magnitude, although flooding would extend into currently vacant industrial land. There are two strategies which could be considered to improve the performance of the drainage system and mitigate existing erosion problems in the channel.

Modifications to the existing detention basin to reduce downstream flows.

Improvements to the channel capacity of Sawpit Creek. These two options are reviewed below. The extent to which analysis and concept design may be undertaken in this investigation is limited by survey constraints. Available survey was limited to 2 m contour mapping, together with the creek cross sectional survey undertaken for hydraulic modelling. More detailed survey would assist further investigation of the concepts developed in this investigation. 5.2 Detention Basins The existing detention basin in its present configuration has a powerful effect in mitigating peak flows in downstream areas. Hydrologic modelling was carried out to assess the potential reductions in flow which could be achieved by altering the configuration of the low level outlets. The results are shown on Table 5.1. Two storm durations have been considered: the 20 minutes storm which results in peak flows along most of the creek and the 60 minutes storm which maximises storage levels in the basin sue to the larger volume of runoff associated with this storm. With three 900 RCP outlets functioning, the peak inflow of 9 m3/s would be reduced to 5.5 m3/s (Column (2) of Table 5.1). Although flows further downstream would increase as contributions from sub-areas join Sawpit Creek, the mitigating effect of the basin would be felt as far downstream as the railway. In the short term, further reductions in peak flow could be achieved by temporarily blocking one of the outlet pipes. Column (3) of Table 5.1 shows peak flows with two 900 RCP outlets functioning. At Sydney Road the 100 year ARI peak would be reduced from 11.2 m3/s to 9.9 m3/s and from 14 m3/s to 12.7 m3/s at Industrial Avenue.




The blocking of one of the outlets would be a useful short term measure which could be implemented to mitigate flooding prior to further development of the catchment upstream of the basin. (This scheme is denoted Option SG.1 on Table 8.1 and Figure 8.1 of Flood Behaviour Studies, Volume 2). An indicative allowance of $10,000 has been included for this work. Section 6 includes a discussion on the impacts of future development in the catchment which covers results shown on Columns (4) to (6) of Table 5.1. However, to reduce the extent of inundation at Industrial Avenue, it would be necessary to increase the capacity of the creek and road crossing. This measure is discussed in the following section of the report.

TABLE 5.1

100 YEAR ARI PEAK DISCHARGES – SAWPIT GULLY

MODIFICATIONS TO EXISTING DETENTION BASIN (m3/s)

Location

Present Day Conditions (three low level outlet

pipes)

Present Day (two low level outlet pipes)

Post - Development (three outlet

pipes)

Post - Development

(two outlet pipes

Post - Development

(two outlet pipes + raised

basin)

(1) (2) (3) (4) (5) (6)

Inflow to Detention Basin 9.0 (8.6)

9.0 (8.6)

12.4 (10.9)

12.4 (10.9)

12.4 (10.9)

Outflow from Detention Basin

5.5 (6.5)

4.2 (5.5)

6.5 (7.3)

4.8 (8.1)

4.8 (5.8)

Lions Drive 6.7 (6.7)

5.3 (5.6)

7.3 (7.6)

5.8 (8.4)

5.8 (5.8)

Sydney Road 11.2 (10.5)

9.9 (9.2)

14.4 (13.8)

12.8 (12.1)

12.8 (12.2)

Industrial Avenue 14.0 (12.9)

12.7 (11.6)

17.2 (16.1)

15.7 (14.6)

15.7 (14.6)

Railway Culvert 15.0 (13.7)

13.6 (12.3)

19.2 (17.8)

17.7 (16.2)

17.7 (16.2)

Note: Un-bracketted values of peak discharge apply for 20 minutes storm duration Values in brackets apply for 60 minutes storm duration. 5.3 Channel Improvements

Reductions in the extent of flooding would require an increase in channel capacity in the Industrial Avenue area, together with an augmentation of culvert capacity. For preliminary planning purposes a 20 year ARI capacity was adopted, as this hydrologic standard would eliminate the backwater effects of the Industrial Avenue crossing for all but the very large floods. The surrounding area is industrial rather than residential and flooding does not last long.




Indicative calculations showed that the 20 year ARI discharge of 8 m3/s would be conveyed by a 4200 mm by 1200 mm RCBC, with its invert set about 0.45 m below the invert of the existing RCBC, which is 1.5 m by 0.75 m size. A minor lowering of the culvert invert would give the same clearance between the soffit of the culvert and the road crest as occurs at present. ( It has been assumed that the existing culvert would be replaced, although it may be possible to save the existing culvert and thereby adopt a smaller size for the new culvert to provide the required waterway area.)

It would be necessary to provide inlet and outlet transitions to the culvert from the existing creek, although if the channel is improved as discussed below, any problem matching the culvert width to the channel width would be minimised. In regard to channel improvements, a semi-natural improved creek waterway is recommended, involving a degree of sinuousity (in plan), with pools and riffles and variations in the cross sectional batters to mimic natural conditions. The average bed slope between Sydney Road and Industrial Avenue is 1.4 per cent resulting in excessive flow velocities and erosion of the waterway. The channel gradient should be reduced to about 1 percent to reduce flow velocities.

This could be achieved with one or two drop structures over this 300 m reach, with a total drop of about 1 m. The drops would be rock lined and would therefore provide the riffle effect referred to previously. Council propose to construct a wetland downstream of the railway embankment. The natural creek with pool and riffles would complement the wetland proposal. A preliminary estimate of works (upstream of the railway only) is $200,000. This scheme is denoted Option SG.3 on Table 8.1 and Figure 8.1 of Flood Behaviour Studies, Volume 2.




6 FUTURE DEVELOPMENT 6.1 Catchment Zoning The area downstream of Lions Drive is zoned industrial with developments bordering the creek between Sydney Road and the railway embankment. The area between Bruce Road and Lions Drive is zoned medium density residential (600 m2 lots), portion of which has already been developed and drains to the detention basin. The area upstream of Bruce Road in the catchment headwaters is zoned for large lot residential and rural small holdings. It is expected that these types of development, although causing a modest increase in impervious area, would not have a significant impact on the hydrologic characteristics of the catchment in terms of rates of stormwater runoff. 6.2 Hydrologic Modelling The results of simulations of Post- Development conditions using the DRAINS model are shown in Columns (4) to (6) of Table 5.1. Column (4) shows the situation with the three low level pipes functioning in the detention basin. As previously, the 60 minutes duration storm gave peak outflows from the basin and the 20 minutes storm remained critical in the downstream areas. At Industrial Avenue, the peak 100 year ARI discharge would increase from 14 m3/s (present day) to 17.2 m3/s (post-development). If one of the pipes were intentionally blocked the peak would be reduced to 15.7 m3/s (post- development). A simulation was also carried out for a raised basin and spillway (Column (6) ). In this case both the spillway and dam crest were assumed raised by 1 m. This measure would result in a corresponding increase in peak storage level of 1 m and would reduce the freeboard on floor levels of existing residences bordering the basin storage. It does not achieve a greater reduction in peak flows than blocking one of the pipes. This is due to the fact that most of the increase in flows is caused by development in areas downstream of Lions Drive and therefore outside the influence of the basin. ( Upgrading the basin has been included as a project for further consideration, and is denoted Option B.1 on Table 8.1 and Figure 8.1 of the Flood Behaviour Studies, Volume 2). An indicative allowance for the cost of this work is $150,000. However, with the benefit of existing information, it is considered that the most effective measure for accommodating the increases in flow due to future development would be to increase the capacity of the channel between Sydney Road and the railway, in conjunction to throttling the low level outlet in the basin by blocking one of the pipes.




7 SUMMARY AND CONCLUSIONS Hydrologic and hydraulic models of the drainage system of Sawpit Gully were developed. The objectives were to assess the capacity of the system under present day conditions, to identify areas of low capacity and to undertake preliminary investigation of measures to improve performance. The DRAINS rainfall-runoff model was used to convert design storms to discharge hydrographs which were routed through the catchment. The existing detention basin upstream of Lions Drive was incorporated in the model. Water surface profiles along the main drainage channel were computed using the HEC-RAS steady state backwater program. The model was used to determine the capacity of the channel and estimate the hydrologic standard of the road and railway culverts. The major findings of the study were as follows:

The existing detention basin has a major effect on downstream flood peaks up to the 100 year ARI (Tables 2.1 and 5.1). The basin controls runoff from about half of the total catchment area of about 220 ha.

On the main drainage channel downstream of the detention basin, the locations requiring attention are:

ο The culvert at Industrial Avenue is under capacity. A new culvert arrangement incorporating a waterway of 4200x1200 mm dimensions (sizes preliminary only and to be confirmed by more detailed investigation with the benefit of additional survey) will upgrade this crossing to a 20 year ARI.

ο The improvements at Industrial Avenue should be undertaken in conjunction with measures to mitigate erosion of the creek particularly in the reach between Sydney Road and Industrial Avenue. A scheme is recommended involving rock drop structures to reduce the bed slope, together with pool and riffle zones to mimic a natural creek system. Such a scheme would be compatible with creek improvement works undertaken in the urbanising area upstream of Lions Drive and would complement the proposed wetland on Sawpit Gully downstream of the railway embankment.

ο Increasing the throttling effect of the existing detention basin by blocking one of

the low level outlets could be considered as a means of mitigating the effects of future development on the creek . Because this measure will increase storage levels, confirmation of its feasibility is subject to information on floor levels of existing residential development bordering the storage area.

To complement the above structural measures, Council should also enforce its on-site detention storage policy for future development proposals which result in a “significant” increase in impervious area. Broadly speaking, the objectives of such a policy are to ensure that peak flows from individual developments in the catchment are not increased above present day values.

sawpit gully catchment app g vol 2 - mar 2008...the hydraulic model consisted of a single branch...

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