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 floodplain

1949 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 Lower Macleay Floodplain

Upper catchment

The Upper Macleay River valley consists of two distinct zones:

The New England Tablelands section, where the principal tributaries (the Chandler, Muddy and Apsley rivers) rise.
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 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 - 1949 flood

Belgrave Street - 1949 flood

Kempsey Rail Bridge - 1949 flood

River Street, West Kempsey looking south across the Macleay River - 1949 flood

River 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 works

Flood 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-grassed

Eden 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:

Protection of the Kempsey township up to a 1 in 10 year flood event
Protection for the lower Macleay agricultural area from a 1 in 2.5 year flood event
Allow for rapid drainage of floodwaters
Provide for erosion protection
Improve flood mapping, proofing and data, and
Provided for flood warning
Telemetry and forecasting:

Erosion 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 works

Riley’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.

Eden Street levee overtopping during the 2001 flood event

First 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.

Smithtown 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.

Belmore River Flood Control Structure discharging flood water from the Belmore river into the Belmore floodplain

Kinchela Creek Right Bank Flood Control Structure discharging flood water from Kinchela creek into the Swanpool area

Kinchela 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.

Ryans 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.