The document discusses resetting the benchmark for Brisbane River floods based on new information from the 2011 flood. It summarizes that the 2011 flood provided data like Wivenhoe Dam releases and new river flow measurements that indicated previous flood estimates, including for the 1893 and 1974 floods, were too low. The document outlines how Seqwater collected data on almost 50 floods back to 1887 and developed calibrated models to better estimate flood peaks and volumes to help with dam operations, flood studies, and disaster management.
2. Introduction
• Before the January 2011 flood, the January 1974 event was
considered the benchmark for ranking Brisbane River floods
• The peak flow of the ‘74 flood at the Brisbane Port Office gauge
was generally considered to be about 10,000m3/s.
• Highest “reliable” flow measurement of 3,300m3/s at Savages
Crossing in 1968
4. River Works
Brisbane Town to River Bar
• First round concentrated on improving navigation access around mouth of
the river completed in 1867
• Extensive dredging and river improvements work conducted up to 1910
• Improvements seemed to be limited to relatively small navigation
channels
Brisbane-Ipswich
• Some works carried out in 1867 to improve navigation up to Ipswich
• No further works carried out in this reach until Seventeen Mile Rocks
removed in 1965
• River works were estimated to have lowered PO peak records by ~1.5m
5. Centenary Br Gauging – Jan 1974
Peak height = 14.09m AHD @ 28/01/974
Peak flow estimate = 9,514 m3/s
• Current meter
• Measurements of:
• Surface velocity
• Three point velocity
7. Estimates of 1893
• Historical peak level at Brisbane Port Office is 8.35m
• River works were estimated to have lowered the flood level by ~1.5m
• Estimate of 1893 peak flow was13,690m3/s (SKM June 1999)
8. Learnings from 2011
The January 2011 flood changed our knowledge for the
estimation of historical peak flood flows in the Brisbane
River:
• Release data from Wivenhoe Dam
• ADCP gaugings at Centenary Bridge
• MIKE11 model at Mt Crosby
• Revised Savages Crossing rating
9. Wivenhoe Releases
• Wivenhoe releases are considered reasonably accurate as
rating based on physical model
• During drain down phase of 2011, releases were constant
at 3,500m3/s for several days along the river
• Excellent reference points for verifying/establishing ratings:
• Independent of any hydrologic or hydraulic influences
• Equal to previously highest measured flow at Savages Crossing
10. Centenary Br Gauging
Peak height = 12.068m AHD @ 12/01/2011
Peak measurement: Area 3,499m2 with a mean velocity of 2.76m/s
Peak flow estimate = 9,771 m3/s
Four gaugings were done
using a Sontek M9 Acoustic
Doppler Current Profiler at
the peak of the flood
14. Re-assessing Brisbane Floods
• Sufficient information arising from the January 2011 Brisbane River
flood suggests that previous estimates of historical floods are too low
• 1893 – 13,690 increased to 16,000m3/s
• 1974 – 10,000 increased to 11,500m3/s
• Compelling need to re-assess historical estimates as peaks and
volumes have a significant impact on dam operations
• Potential impact on a range of floodplain management issues
Decision made to model every significant flood event where
sufficient data was available
15. Data Sources & Checking
• Rainfall
• Daily - BoM
• Pluvio - BoM
• ALERT – Seqwater, BoM, Councils
• Water Level
• Peak heights - BoM
• Time series – DNRM, BoM
• Dam levels and gate settings – Seqwater
• Extensive data checking!
16. Event Selection
• Selection on basis of magnitude & data
availability
• Some events significant in one part of the basin
but maybe not in the lower Brisbane R
• Events cover good spread of months
• Every event is real, different and bears no
resemblance to idealised design events
• Not all data is found in a database!
17. Floods before 1955
• Largest floods on record occurred in 19th century
• Quantity & quality of data is limited
• Sparse daily rainfall network
• No pluviograph data
• Limited water level data
• No dams
• Large events: 1841, 1844, 1887, 1890, 1893 (3
events), 1898 (2 events), 1908, 1931, 1947
18. 1955 - 1976 Floods
• Quantity & quality of data increases
• Daily rainfall network
• Increasing amount of pluviograph data
• More water level data
• Somerset Dam
• Large Brisbane R floods: 1955, 1968, 1974
• Other events large in parts of the basin
19. 1983 – 1992 Floods
• Quantity & quality of data increases further
• Daily rainfall network
• Increasing amount of pluviograph data
• More water level data
• Significant influence of Somerset &
Wivenhoe Dams
• Large Brisbane R floods: 1983, 1989 (2events)
• Other events large in parts of the basin
20. Post 1992 Floods
• ALERT system installed in early 1990s
• Every ALERT station is a
pluviograph
• More water level data
• Significant mitigating influence of
Somerset & Wivenhoe Dams
• Large Brisbane R floods: 1996, 1999,
2010 (3 events), 2011, 2013
• Other events large in parts of the basin
21. Event Selection
• 48 floods from 1887 to 2013
• Suite of 7 linked URBS hydrologic models
• Calibrated on post 1955 floods
• Tested on pre 1955 floods
• Key advantages of large number of historical floods
• better assess range of routing & loss parameters (e.g. losses in Jan 2013)
• some floods can be used to identify unique routing behaviour
• support better review of rating consistency
• Key challenges: good data management and error checking!
22. Re-setting the Benchmark
• Information from the January 2011 Brisbane River
flood suggests that previous estimates of historical
floods are too low
• Seqwater undertook an extensive data collection
exercise and collated event data sets for nearly 50
floods
• Developed and calibrated floods models suitable for:
– Dam operations - Seqwater
– Brisbane River Catchment Flood Study - DEWS
– Wivenhoe and Somerset Dams Optimisation
Study - Seqwater
– Disaster Management Tool - BCC
• A good understanding of the hazard will lead to a
better approach to managing the risk
