mudgee catchment e vol 2 - mar · pdf fileat mortimer street flows are conveyed beneath the...

MUDGEE LOCAL CREEKS FLOODPLAIN RISK MANAGEMENT

STUDY AND PLAN

FLOOD BEHAVIOUR STUDIES

APPENDIX E - CATCHMENT E

MARCH 2008

DRAFT REPORT

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

Date: March 2008 Rev No.: 2.0

Author: BWL Principal: BWL

Mudgee Local Creeks Floodplain Risk Management Study and Plan

Catchment E - Flood Behaviour Studies

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

TABLE OF CONTENTS

Page No

1 CATCHMENT AREA.........................................................................................................1 1.1 Introduction ............................................................................................................1

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

3 HYDRAULIC MODELLING ................................................................................................5 3.1 General ..................................................................................................................5 3.2 Downstream Model Boundary Conditions ..............................................................5 3.3 Model Results ........................................................................................................5

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

5 DRAINAGE IMPROVEMENTS...........................................................................................9 5.1 General ..................................................................................................................9

6 FUTURE DEVELOPMENT IN CATCHMENT E ................................................................11 6.1 Development Potential in Catchment E................................................................11

7 SUMMARY AND CONCLUSIONS ...................................................................................13

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



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

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

1 CATCHMENT AREA 1.1 Introduction Catchment E is situated on the eastern side of town and consists of mainly residential areas, with the drainage system flowing in a northerly direction before discharging into the Cudgegong River. Catchment E is roughly bordered by the Wallerawang – Gwabegar Railway to the south, Church/Lewis Streets to the west and Lawson/George Streets to the east. In the upper reaches of Catchment E stormwater flows are conveyed within the road gutter system. Flows are directed in a northerly direction along Lewis Street from the railway before turning west at Gladstone Street. An inlet pit located in the low spot within Gladstone Street drains low flows via a 1000 mm x 700 mm RCBC, draining to an open channel to the east of Lawson Street. During heavy rainfall events stormwater flows pond above the pit, and at times surcharge the roadway and flow across the intersection of Gladstone and Lawson Streets. On the eastern side of Lawson Street the presence of a colour-bond fence at the head of the channel impedes the ingress of water into the head of the channel. At this point stormwater either finds its way beneath the narrow gap beneath the fence or flows to the north where it crosses grassed lawns before entering the open channel, approximately 50 m further downstream. The channel at this point is indistinct and is characterised by a 3 m wide concrete v-drain with minimal banking. Approximately 140 m downstream of the head of the channel, the channel flows between fences before entering the rear of a property fronting Mortimer Street. The concrete channel ends inside the rear of the property and the drainage path becomes indistinct as it passes through the backyard. Low mortared stone walls have been constructed along the western side of the residence which divert low flows away from the house and towards Mortimer Street. At Mortimer Street flows are conveyed beneath the road via a 1500 mm x 600 mm RCBC. During minor flood events stormwater flows pond in the front yard of the above mentioned allotment before surcharging the roadway. Downstream of Mortimer Street a vertical sided concrete channel directs flows towards the Cudgegong River. Approximately 50 m downstream of the road crossing a low concrete weir has been constructed across the channel by Council. The weir promotes ponding at a sag inlet pit which is drained by twin 375 RCP’s. These two pipes run beneath the Cudgegong River floodplain before discharging into the river. In the vicinity of the weir, the right bank of the concrete channel reduces substantially in height. Flows leave the channel and discharge across the grassed Cudgegong River floodplain which slopes away from the channel banks. Flooding from the Cudgegong River extends as far as the concrete weir downstream of Mortimer Street for the 100 year ARI river flood event, which reaches a peak level of RL 450.8 m. However, river flood peaks are not sufficiently high as to influence water levels in the concrete lined channel upstream of the weir.




2 HYDROLOGIC MODELLING 2.1 General Figure 2.1 shows the arrangement of sub-catchments and drainage links used to define the hydrologic model of Catchment E. The catchment was divided into 16 sub-catchments to model the movement of water through the drainage network which extends from the railway to the outlet at the Cudgegong River. Sub-catchment boundaries and definition of flow paths were based on the directions of gutter flows observed during site inspection. In the upper reaches of the catchment, stormwater flows were routed through the catchment as gutter flow. Overflows of the box culvert at the junction of Gladstone Street and Lawson Street were directed to the head of the open channel. 2.2 Model Results DRAINS results showed that for the 5, 20 and 100 year ARI events, the durations of critical storms ranged between 20 and 25 minutes. Peak discharges at various locations within the drainage system for the critical storm are given in Table 2.1.

TABLE 2.1

PEAK DISCHARGES CATCHMENT E EXISTING (2008) CONDITIONS

(m3/s)

Location 5 year ARI

20 min 20 year ARI

25 min 100 year ARI

20 min

Western side of Lewis Street south of Gladstone Street

0.7 1.1 1.7

Eastern side of Lewis Street south of Gladstone Street

0.4 0.5 0.8

Open channel 50 m downstream of Lawson Street

2 2.9 4.4

Mortimer Street 2.6 3.7 5.6

Cudgegong River (Model Outlet) 2.7 3.9 5.9

MUDGEE LOCAL CREEKS FLOOD BEHAVIOUR STUDIES

CATCHMENT EFigure 2.1

DRAINS MODEL LAYOUT

N

0 100 200m

Scale

ST

ST

NIC

HO

LS

ON

ST

LEGEND

E1

Sub-Catchment Boundary

Sub-Catchment Number

EA1

EA2

EA3

EA4

EA5

EB1

EB2

EA7

EA6

EC1

EA8

EA9

ED1

EE1

EA10

EA11

2010

ING

LISH

OR

AT

IOME

AL

EYDE

NIS

ON

GL

AD

ST

ON

EMO

RT

IME

RMA

RK

ETSH

OR

T

CHURCH

LEWIS

ST

GEORGE

ST

ST

ST

ST

ST

ST

ST

Drainage Reach

CUDGEGONG

RIVER

ST

LAWSON

1:4000




3 HYDRAULIC MODELLING 3.1 General The model consisted of a single branch, representing the open channel system downstream of Lawson Street extending to the concrete weir downstream of Mortimer Street. Surveyed cross sections were used to model the characteristics of the channel system. Their locations are shown on Figure 3.2. Several cross sections were created by interpolation of survey data to define constraints on the flow such as fences and buildings. The culvert beneath Mortimer Street was incorporated in the model with details of the culvert based on survey of the road crossing. The weir within the channel downstream of Mortimer Street was also included. The weir has a width of 2 m and a top elevation of RL 450.75 m. The land on the eastern floodplain is at a considerably lower level than the channel. Most of the flow during significant storm events would surcharge the channel and would be conveyed by the floodplain. 3.2 Downstream Model Boundary Conditions Cudgegong River 100 year ARI flooding has a peak water level of RL 450.7 m at the outlet of Catchment E and extends to the concrete weir downstream of Mortimer Street. As a result, the effect of main river flooding upstream of the weir for that event is not significant. 3.3 Model Results Water surface profiles for the 5, 20 and 100 year ARI events are shown on Figure 3.1 and the indicative extents of inundation are shown on Figure 3.2. A broad assessment of the hydrologic standard of the Mortimer Street culvert is included in Table 3.1. For the purpose of this assessment the hydrologic standard is defined as the frequency of the flow required to significantly overtop the roadway.

TABLE 3.1 DETAILS OF MORTIMER STREET

CROSSING CATCHMENT E

Location Opening Waterway Area

(m2)

Approx. Capacity

(ARI years)

Mortimer Street 1500 x 600 RCBC 0.9 <5

The model for catchment E commences downstream of Lawson Street at the upstream end of the vertical sided concrete lined open channel. At the head of the channel the flow within the watercourse is controlled mainly by the capacity of the upstream box culvert and stormwater traverses the channel supercritically at velocities around 3 to 3.5 m/s. Approximately 50 m downstream of the culvert, the channel changes from a vertical sided concrete lined channel to a concrete lined v-drain with minimal banking. Stormwater flows spread out in this region resulting in widths of overland flow of around 20 to 25 m for the 100 year ARI event.




Approximately 140 m downstream of Lawson Street, the drain enters the rear of a property fronting Mortimer Street and becomes indistinct. The capacity of the channel is insufficient to convey flows of 5 year ARI leading to the inundation of the rear of the property. Flooding within the property is exacerbated by backwater flooding caused by the culvert beneath Mortimer Street having insufficient capacity to convey minor flood flows. Surcharging of Mortimer Street occurs for the 5 year ARI discharge, with inundation of the roadway exceeding a width of 30 m for all flood events modelled. Immediately downstream of Mortimer Street the vertical sided concrete lined channel is only 2 m wide and has insufficient capacity to convey the 5 year ARI discharge. Floodwaters which surcharge Mortimer Street are directed towards the channel by sloping overbanks which confine the width of flow for a short distance before the right bank changes grade and slopes away from the channel to the Cudgegong River floodplain. Approximately 120 m from the confluence of the watercourse with the Cudgegong River, at an in-line concrete weir, local catchment flows join flows which have surcharged the river and spread out over the floodplain.

MUDGEE LOCAL CREEKSFLOOD BEHAVIOUR STUDIES - CATCHMENT E

Figure 3.1WATER SURFACE PROFILES

0 100 200 300 400

451

452

453

454

455

457

Main Channel Distance (m)

Ele

vatio

n (m

)

Legend

WS PMF

WS 100 YR

WS 20 YR

WS 5 YR

Ground

10...

12...

30 C

s 3

- 4m

d/s

of M

ortim

er S

treet

...

40 C

s 4

- Mor

timer

Stre

et C

ulve

rt

60 C

s 6

- 85m

d/s

of L

awso

n S

treet

Cul

vert

(at b

end)

70 C

s 7

- d/s

of L

awso

n S

treet

Cul

vert

22

450

25 55 56 57 58 59 63 66

456

MUDGEE LOCAL CREEKS FLOOD BEHAVIOUR STUDIES - CATCHMENT E

Figure 3.2INDICATIVE EXTENTS OF INUNDATION

N

0 25 50m

Scale

LEGEND

PMF 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 River Station RS 6

LAWSON ST

LAWSON ST

MO

RTI

MER S

T

GEORGE ST

GLA

DS

TO

NE

RS

50

RS

40

RS

20

RS

10

RS 60

RS 70

RS 30




4 IMPACTS OF FLOODING ON EXISTING DEVELOPMENT 4.1 Impact on Residential Property Floor levels of residences bordering the creek in Lawson and Mortimer Streets were surveyed. The allotment upstream of Mortimer Street would be the most flood affected. However the floor of the residence would not inundated at the 100 year ARI level of flooding. Table 4.1 shows the impacts of flooding on this catchment.

TABLE 4.1 IMPACTS OF FLOODING ON

RESIDENTIAL DEVELOPMENT CATCHMENT E

Number of Properties Flood ARI

Flood Affected Flood Damaged

Flood Damages $ x 103

20 3 0 25

100 3 0 25

PMF 11 10 430

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




5 DRAINAGE IMPROVEMENTS 5.1 General The open channel section of Catchment E between Lawson Street and the Cudgegong River generally has a capacity less than 5 years ARI. The channel at several locations has minimal banking to contain flows and at the rear of the property fronting Mortimer Street, becomes indistinct. Minor filling within the yards of properties fronting the channel would prevent the spreading out of stormwater flows for the more frequent events but it would be difficult to undertake sufficient filling to contain the larger flood events. The property on the upstream side of Mortimer Street is the most affected as it lies within the backwater of the road crossing. The road level downstream of the property is quite high relative to the property and acts as a dam during major flood events. A lowering of the road level in conjunction with enlargement of the waterway area is required. Preliminary analysis showed that replacement of the existing waterway by a culvert of 3000 x 1200 mm dimensions would convey a flow of 5.2 m3/s , slightly less than the 100 year ARI discharge, with a head drop across the culvert of 300 mm. The improved culvert works would require a lowering of the creek invert on the upstream and downstream sides of the road. A preliminary cost estimate for the work is $80,000. (Option E.1 on Table 8.1 and Figure 8.1 of the Flood Behaviour Studies, Volume 2).




6 FUTURE DEVELOPMENT IN CATCHMENT E 6.1 Development Potential in Catchment E Catchment E is fully urbanised and therefore future development would be restricted to re-development of individual residential allotments or urban consolidation. This form of development would be subject to Council’s On Site Detention Storage Policy, which is aimed at ensuring that development does not increase flood peaks by the implementation of individual on site detention. With continuing development in town there will be pressure for re-development of allotments and urban consolidation. It is recommended that the open channel method of conveyance of creek flows be retained and the development should have adequate freeboard over above-floor inundation. A freeboard of 500 mm is recommended in accordance with Council’s present Flood Policy for Mudgee. Further Council’s existing Flood Policy should be enforced in regard to boundary fencing.




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

Within the upper reaches of the catchment there are no stormwater pipes to convey flows which use the street gutter as a major flow path. Stormwater traverses the street system until it enters an open channel system downstream of Lawson Street.

The open channel is characterised by a concrete invert and low banks which allow stormwater to spread over a wide front for relatively frequent storm events.

There are little practicable opportunities for drainage improvements to the open channel as it is located in the rear of several privately owned properties.

Enlargement of the culvert at Mortimer Street and lowering the road level would reduce flood levels in the upstream allotment which is presently within the backwater influence of the road crossing. (Option E.1)

mudgee catchment e vol 2 - mar · pdf fileat mortimer street flows are conveyed beneath the...

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