Keith Hall Drainage Options Study

Since the early 1920s successive federal, state, and local governments have constructed floodplain infrastructure, including drains to mitigate the impact of flooding on the Richmond River floodplain. A complex example is the Keith Hall Drainage System located at South Ballina which has been in place for 60 years. As with other historic floodplain drainage systems there are regular concerns raised with the Keith Hall Drainage System:

• the environmental impact, particularly on downstream water quality
• the level of service they provide to landowners, including changes in function over time
• how much maintenance is necessary to meet the level of service required.

Together, in a collaborative initiative with local landowners, Ballina Shire Council and Rous County Council have undertaken a Drainage Options Study on the Keith Hall drainage system.  This is the first detailed, technical examination undertaken by both Councils to identify how a drainage system can be changed to improve water quality, whilst maintaining drainage for local landowners with a reasonable and practical maintenance requirement. The investigation combined the latest scientific understanding of hydrology with the valuable observations and experience of landowners.

The technical investigation was undertaken by the University of NSW Water Research Laboratory in the form of a Drainage Options Study, and the final report provides details on the water quality discharging from the Keith Hall drainage system, along with the benefits and challenges associated with a range of potential changes.

The Keith Hall Drainage Options Study has improved our understanding of the water quality leaving the system and the impact it has on Mobbs Bay and provided detailed, scientific advice on changes that could occur in this system in the future.

Project Aims Reduce any downstream impact on Mobbs Bay and the Richmond River from water quality leaving the Keith Hall flood mitigation drain. Improve drainage efficiency and reduce the impact of floodplain inundation, particularly along the No.1 flood mitigation drain where build-up of sediment and vegetation can occur. Reduce maintenance of the Keith Hall Drainage System for Rous County Council.

The current water quality and six potential options for the drainage system were investigated

The Study investigated the benefits and challenges of six different options for the drainage system. These options were jointly identified by the landowners, Council staff and researchers.  For each of the potential changes, detailed technical information on the impact on drainage, improvement to water quality and future maintenance requirements were provided along with the cost of implementation. These options were:

1. Can cyclic flow occur in the drainage system as it is now?
2. Can cyclic flow occur in the drainage system if it was changed? 
3. Can additional tidal flushing occur in the drainage system?
4. Can we make the floodgates lighter and allow increased drainage? 
5. Can the top half of the Keith Hall No.1 drain be reshaped so it is shallow and wide?
6. Can Keith Hall No.2 drain discharge separately into Mobbs Bay?

Results of the Options Study

The Study included a comprehensive water quality monitoring program which answered many long-held questions including:

Q When is water quality at its worst and why?	A  Monitoring showed water quality is worst after rainfall and flooding, and that it is impacted by acid sulfate soils, deoxygenation caused by rotting vegetation and microbial bacteria (including Enterococci and E.coli).
Q Why is the water often discoloured?	A The discoloured water long associated with the drainage system contains high levels of naturally occurring tannin, but this water can also contain high levels of acidity and iron from acid sulfate soils.

Key findings of the study

1. The Keith Hall drainage system discharges poor water quality that impacts on Mobbs Bay.
2. The system has a localised impact, compared to other floodplain drainage systems which can impact on large stretches of the estuary.
3. Being reliant on landowner goodwill alone reduces the scale of change that is possible, then limits the improvements that can be made to water quality.
4. To improve post-flood deoxygenated (blackwater) conditions, floodwater needs to be retained on land and previous wetlands reinstated.
5. There are few opportunities to reduce the maintenance these systems require if the same level of drainage is expected.
6. Although active floodgate management occurs at the site, the Study confirms further modifications to active floodgate management can improve water quality and is a worthwhile and cost-effective option.
7. Active floodgate management only improves water quality in dry weather. It doesn’t improve water quality after floods when water quality is generally the worst and has the greatest impact downstream.  These limitations must be understood and acknowledged.
8. The Study has confirmed reshaping drains so that they are shallow and wide (rather than deep and narrow), where possible, has potential benefits for water quality and maintenance. 
9. Generally, the worst water quality comes from the lowest lying land.  Draining land that sits below 1mAHD produces the worst water quality and drains located in these areas are the hardest to maintain. Drainage and agriculture on higher parts of the floodplain has less of an impact on water quality. 

Where to from here?

In the immediate term, Rous County Council staff will continue working with local landowners to confirm operations of the actively managed floodgates where Keith Hall Drain No. 1 discharges into Mobbs Bay.

Staff from Rous County Council and Ballina Shire Council will work with individual local landowners to identify options on private property for implementation of recommendations from this study. A staged, incremental approach should be expected, to be determined by:

• landowner support
• regulatory approvals
• funding and resources.

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Resources

• Keith Hall Drainage Options Study - December 2021 (13mb pdf)
• Rous County Council