Background of Macleay River floodplain risk management

Introduction

The configuration of the Macleay catchment is practically suited to the production of high peak flood flows. Following major rain storms the Macleay River at Kempsey may rise from a normal dry weather discharge of about 56m³/sec to 14,160m³/sec (the estimated discharge of the August 1949 flood) within a 30-hour period.

Several significant floods may occur in a short period of time, as was the case in the periods between 1863-75 and 1890-93, and 2009-2013. In 1949 and 1950 Kempsey experienced the two worst floods ever recorded, within eight months of each other.

Aerial view of 1949 flood event Kempsey and floodplain1949 flood event Kempsey & floodplain

The Probable Maximum Flood (PMF) is the largest flood that could conceivably occur at a particular location. An estimate of the PMF at Kempsey (should it occur) would reach a height of 11.2 metres at the Kempsey Traffic Bridge gauge. This flood event would peak more than 3 metres higher than the level reached in the 1949 flood event.

Catchment description

Except for a small area in the south-east which is drained by the Maria River, the entire Kempsey Shire LGA is within the catchment area of the Macleay River. The river rises well to the west of the shire in the Guyra, Dumaresq, Armidale, Uralla and Walcha areas of the Great Dividing Range. Its tributaries extending for a distance of about 160 kilometres from the coast. The whole catchment covers 11,500 square kilometres.

Map of the Lower Macleay FloodplainMap of Lower Macleay Floodplain

Upper catchment

The Upper Macleay River valley consists of two distinct zones:

  1. The New England Tablelands section, where the principal tributaries (the Chandler, Muddy and Apsley rivers) rise. 
  2. The Gorge section, where the rivers leave the tablelands in a series of waterfalls and join to form the Macleay River is a well-defined gorge zone. Here the escarpments are steep sided, stream gradients are steep and flood flow velocities are high. Within this section of the catchment there are several minor tributaries (i.e. Parrabel, Hickeys, Georges and Nulla Nulla creeks and Dykes River). Below the Hickeys Creek confluence the topography becomes less severe as the river emerges from the gorges.

Macleay floodplain

The Lower Macleay River valley consists of primarily one significant landform feature.

The lower Macleay River floodplain covers an area of approximately 400km² which begins at the upper limit of tidal influence. Here there are extensive alluvial flats, and well-defined natural levees along the river and its tributaries (the Belmore River and Christmas, Kinchela and Clybucca creeks). The ground slopes away from the levees to low-lying swamplands (i.e. Doughboy, Cooroobongatti and Belmore Swamps and Swan Pool wetlands) which act as storage areas for flood waters.

The river reaches the sea via the main entrance (New Entrance, South West Rocks) and during floods may do so through Korogoro Creek, Rowes Cut, Ryans Cut, Killick Creek and Big Hill drains.

The lower south-east area of floodplain can also be inundated by floodwaters discharging from the Maria River (Hasting catchment) system.

On some occasions, flooding in the lower valley can be exacerbated by very high tides or by ocean storm surge conditions.

Map of the Kempsey/Lower Macleay Flood Mitigation Scheme

Flood history

The Macleay valley has a history of flooding data from the early days of European settlement. The first recorded flood to cause damage occurred in 1838. Until the flood of 1893, the Macleay River reached the Pacific Ocean at Grassy Head, 3kms north of Stuarts Point. In a major flood that year the river breached the sand dunes just north of South West Rocks. Since that date the entrance has been at South West Rocks assisted by extensive break walls.

Floods which are known to have exceeded 5.5 metres at the Kempsey Traffic Bridge gauge are shown on the following table, along with the heights reached in these events, where known, at Georges Creek and Bellbrook. Some of the earlier values are approximations, but those since 1945 are believed to be accurate.

The table highlights the irregularity of serious flooding on the lower Macleay River.

Winter floods in 1949 where water levels recorded at Kempsey Traffic Bridge reached 8.725m AHD resulted in the loss of six (6) lives and more than 7000 head of stock and 600 building damaged or destroyed. Repeated winter floods again in 1950 recorded water levels at the Kempsey Traffic Bridge reaching 8.295m AHD.

Kempsey 1963 floodKempsey 1963 flood
Peak flood heights at Georges Creek, Bellbrook and Kempsey, 1838-2013
Date Georges Creek
(m)
Bellbrook
(m)
Kempsey
(Traffic Bridge) (m)
AEP (%) at Kempsey
(approx)
ARI (years) at Kempsey
(approx)
  1838     6.7    
  1841     6.7    
August 1848     5.8    
February 1863     6.1    
February 1864     7.4    
July 1864     6.3    
August 1864     7.3    
July 1866     6.3    
April 1867     6.7    
March 1870     6.3    
March 1875     7.6 2.5% 40
June 1879     6.1    
March 1890     6.1    
April 1892   8.8 6.1    
March 1893     6.7    
June 1893   17.1 7.5 3% 30
July 1921   16.16 7.32    
February 1928     5.56    
February 1929   12.20 6.25    
March 1946   12.73 5.99    
August 1949 14.10 17.22 7.92 1% 90
June 1950   18.06 7.77 2% 60
August 1952   13.03 6.02    
February 1954   11.23 5.79 20% 5
November 1959   9.75 5.59    
April 1962   8.15 5.54    
May 1963 13.50 15.54 7.14 7% 15
June 1967   10.24 6.02 16% 6
January 1968   8.84 5.77 20% 5
January 1974 8.63 7.70 5.56 25% 4
March 1974 6.78 7.11 5.69 22% 4.5
February 1976 8.64 7.56 5.54 25% 4
May 1977 7.60 6.75 5.56 25% 4
May 1980 7.00 7.14 5.73 20% 5
April 1989 6.72 6.34 5.57 25% 4
March 2001 11.90 12.80 7.44   13
February 2009     5.98    
April 2009     5.8    
May 2009 10.77 11.40 6.90   9
June 2011 11.00 10.65 6.70   7
February 2013 12.14 10.97 7.23    

Extreme flooding

The configuration of the Macleay catchment is practically suited to the production of high peak flood flows. Severe and damaging floods greater than have been previously seen in the Kempsey Shire must be regarded as inevitable. An estimate of the Probable Maximum Flood (PMF) level at Kempsey could reach a height of 11.2 metres at the Kempsey Traffic Bridge gauge.  This PMF event, would peak more than 3 metres higher than the level reached in 1949. 

River Street, West Kempsey - 1949 floodRiver Street - 1949 flood
Belgrave Street, Kempsey - 1949 floodBelgrave Street - 1949 flood
Kempsey Rail Bridge - 1949 floodKempsey Rail Bridge - 1949 flood
River Street, West Kempsey looking south across the Macleay River - 1949 floodRiver Street - 1949 flood

Flood mitigation history

Following the flood events of 1949 & 1950 the Minister for Conservation, Mr Weir, met with a large number of local organisations and government representative to discuss the impacts on the community (as a whole) of the flood events.

On 30 March 1951 the Minister with the concurrence of the Premier and Minister for Public Works approved the appointment of a committee to enquire into and report upon the problems of flood control and mitigation in the Macleay Valley.

In 1951 the Macleay Valley Flood Mitigation Committee was established under the Chairmanship of C.K. Jacka; Chairman of the NSW Conservation Authority. The Committee’s Report 1953 (aka Jacka Report) on the effects of the 1949 & 50 flood events in the Macleay recommended flood “mitigation” as the only viable option as opposed to flood “control”.

The report indicated that complete flood control within the Macleay River valley was economically impracticable. The Report put forward a proposal for a valley-wide scheme involving extensive drain construction and enlargement of existing drains, construction of headworks and levees, training walls and ocean outlets.

Flood mitigation construction worksFlood mitigation construction works
Flood mitigation construction works - Andersons InletFlood mitigation construction works - Andersons Inlet

Prior to work initiated through the Jacka Report a small levee was constructed at Eden Street in the 1930’s to divert water around the town rather than through it.

Eden Street (Kempsey) embankment being constructed and re-grassedEden Street embankment being constructed and re-grassed

The construction of flood mitigation measures on the Macleay River floodplain began in 1955 with the appointment of an engineer to implement a scheme devised by the Macleay Valley Flood Mitigation Committee.

Subsequently, by July 28 1954 the Macleay River County Council was delegated powers and duties to implement the flood mitigation strategies of the Jacka Report. From May 1955 flood mitigation works were progressively implemented until the mid 1970’s that included the construction of 47 flood-gated structures (of various sizes), 116kms of drainage channels and 180kms of levee. In 1957 the Macleay River County Council established a Flood Advisory Committee.

The stated aims of the scheme were:

  1. Protection of the Kempsey township up to a 1 in 10 year flood event
  2. Protection for the lower Macleay agricultural area from a 1 in 2.5 year flood event
  3. Allow for rapid drainage of floodwaters
  4. Provide for erosion protection
  5. Improve flood mapping, proofing and data, and
  6. Provided for flood warning
  7. Telemetry and forecasting:
Erosion control efforts - Macleay RiverErosion control efforts

Rural areas

Extensive rural flood mitigation works have been initiated since the 1950s on the lower Macleay River floodplain. These comprise levees, barrages, drains, floodgates, floodways, training walls, ocean cuts and river bank stabilisation works. They are designed to reduce (when possible) the frequency, and extent of inundation and through extensive drainage networks reduce the duration of flooding. 

In a number of locations existing drainage channels were enlarged to assist in flood water drainage. Riley’s Drain is one such system.

Riley's Drain, Belmore River, prior to upgrade flood mitigation worksRiley’s Drain, Belmore River, prior to upgrade flood mitigation works
Riley's Drain, Belmore River, after upgrade flood mitigation worksRiley’s Drain, Belmore River, after upgrade flood mitigation works

Kempsey

Kempsey township has levee protection from the Eden Street, First Lane (Cochrane Street) and RSL Club levees. When the Eden Street levee is overtopped, the basin to its immediate north could fill within 90 minutes to a depth of 3-4 metres.

Kempsey - Eden Street levee overtopping during the 2001 flood eventEden Street levee overtopping during the 2001 flood event
Kempsey - First Lane (north) levee overtopping in 2013 flood eventFirst Lane (north) levee overtopping in 2013 flood event

A full explanation of flood behaviour in the Kempsey CBD area and immediate surrounds is available through the Kempsey CBD Flood Study.

Smithtown

Smithtown has levee protection to about the level of the 20% AEP (once-in-five years) flood on the Macleay River.

Aerial view of Smithtown in flood during the 2013 eventSmithtown in flood during the 2013 event

Flood mitigation function & operation

Function

There is a lot of misunderstanding by the general community about the function, capabilities and protection measures afforded by the existing flood mitigation system, with particular emphasis on the operation and capabilities of the flood control structures located on the Belmore River and Kinchela Creek systems.

The Kempsey township area is sited on a natural overbank flow path of the Macleay River in flood and is typically prone to flooding during flood event that exceed the 1 in 10 year flood level.

An extensive series of levees has been constructed around the town together with a major floodway identified through the middle of the commercial district. Currently four main levees protect Kempsey CBD; Eden Street, First lane, RSL Wall and Wide Street.

A full description of the Kempsey flood protection infrastructure is provided in the Kempsey CBD Floodplain Risk Management Study (WMAwater 2017)

The aim of the scheme for areas below Kempsey was to control the passage of an “in bank” flood, which was defined as 2,550m³ per second which equated a river level of 5.705m AHD at the KTB

The scheme design accepted that floods greater than 5.71m ADH at the KTB could not be controlled on the floodplain.

Aerial view of Belmore River Flood Control Structure discharging flood water from the Belmore river into the Belmore floodplainBelmore River Flood Control Structure discharging flood water from the Belmore river into the Belmore floodplain
Aerial view of Kinchela Creek Right Bank Flood Control Structure discharging flood water from Kinchela creek into the Swanpool areaKinchela Creek Right Bank Flood Control Structure discharging flood water from Kinchela creek into the Swanpool area
Aerial view of Kinchela Creek Left Bank Flood Control Structure discharging flood water from Kinchela Creek into the west Kinchela areaKinchela Creek Left Bank Flood Control Structure discharging flood water from Kinchela Creek into the west Kinchela area

The Belmore floodway was designed to discharge flow from the main river and thus lower levels in sections of the Macleay River during small or nuisance river rise events. The Kinchela floodways were designed to prevent overtopping of the natural and artificial levee system along Kinchela Creek.

The Belmore Floodway has a greater effect on levels in the Macleay River than does the Kinchela Floodways. However, the discharge of flood water through both the Belmore & Kinchela control structures has generally very little influence on flood water levels at Kempsey.

The present scheme protects most rural area up to a 1 in 2.5 year flood event.

Aerial view of Ryans Cut - Ocean floodwater outlet discharging flood water from the Belmore area into the oceanRyans Cut - Ocean floodwater outlet discharging flood water from the Belmore area into the ocean

Operation

The flood mitigation scheme consists of a network of levees, drain and passively operated flap gates as well as operating the flood control structures on Right Bank Road, Belmore River and Right and Left Bank Road Kinchela Creek. The system consists of 87 floodgates, major control structures, 116km of drains and 180km of levees.

The Belmore floodway (Flood Control Structure) is designed to accommodate a discharge of 560m³ (NSW Public Works 1994) per second and the Kinchela floodways (Flood Control Structures)140m³ combined - (NSW Public Works 1994) per second. To put this flow discharge rate into perspective, the estimated flow rate at the KTB when water levels reach 6.235m AHD (approx. 1 in 8-year flood event) is approximately 2,549m³ per second.

During non-flood period the floodway gates are kept open to allow water movement and fish passage behind the structure, however the gates are closed when a river rise or flood event is anticipated.

Specific operation procedures (ie when gates are lifted and closed) is directed by Council’s internal flood management procedures and determined by a combination of theoretical considerations and practical imperatives.

Studies & reports

The following is list of reports, studies & plans held by Council.

  • D.C Ref 1 - Lower Macleay Floodplain Management Plan Complete 1999
  • D.C Ref 2 - Lower Macleay Floodplain Management Study 1997
  • D.C Ref 3 - Macleay River Flood Mitigation Feb 1967
  • D.C Ref 4 - Macleay River Flood Study April 1989 Complete
  • D.C Ref 5 - Hydrological Implications of Flood Mitigation Works Aug 67
  • D.C Ref 6 - Final Kempsey To Fredo
  • D.C Ref 7 - Final Kempsey Hydraulic TUFLOW Report Sept 2015
  • D.C Ref 8 - Hydraulic Modelling Report - FULL
  • D.C Ref 9 - Kempsey Evaluation of Options for Flood Protection Complete
  • D.C Ref 10 - Darkwater Drain Report Jan 2016 Project 115002
  • D.C Ref 11 - Jacka Report - complete with attached plates
  • D.C Ref 12 - Kinchela Flood Channel EIS Stage 1 Report July 1994
  • D.C Ref 13 - Kinchela Creek FLood Channel - Stage 2 Oct 1994
  • D.C Ref 14 - Kinchela Creek Flood Mitigation Stage 3 Nov 1995
  • D.C Ref 15 - Kinchela Creek Flood Mit Stage 3 Addenedum Report Feb 1996
  • D.C Ref 16 - Kempsey Levee Sensitivity Assessment Mar 2008
  • D.C Ref 17 - Kempsey CBD Flood Study
  • D.C Ref 17 - Kempsey CBD Flood Plan
  • D.C Ref 18 - Final Draft Kempsey FRM Plan July 2016 (1)
  • D.C Ref 19 - Upper Belmore Management Strategy Jul 2000
  • D.C Ref 20 - Kempsey Shire Local Emergency Man. Plan Jul 2016
  • D.C Ref 21 - Consequence Management Guide
  • D.C Ref 22 - Kempsey Shire Flood Emergency Sub Plan Vol 1 - 2016
  • D.C Ref 23 - Review of Kinchelaa & Belmore Floodway Capacities 1994
  • D.C Ref 24 - Kempsey-Eungai Bypess Flood Study
  • Flood Mitigation Plan - Floodgates and Drains - A0 Landscape