Restoring the balance - guidelines for managing floodgates and drainage systems on coastal floodplains

Date: 01 Nov 2003  Author: Scott Johnston, Frederieke Kroon, Peter Slavich et al.  

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These guidelines are written for people who actively manage coastal floodplain drainage systems, including local government authorities, landholders, drainage unions, industry groups and community groups.

The guidelines encourage an improved balance between the economic, environmental and social aspects of floodplain drainage.

The guidelines present strategies to reduce the adverse impacts of coastal floodplain drainage systems on fisheries and estuarine water quality. At the same time they are designed to help land managers maintain agricultural production and prevent further degradation.

There is no simple ‘recipe’ to achieve this. Each drain system has to be assessed and solutions devised to fit each unique site. As such the guidelines outline important questions that need to be answered in assessing each drain, and provide a range of management options that can be used to maximise benefits and manage risks.

The guidelines are based on findings from seven years of research by NSW Fisheries and NSW Agriculture on the north coast of NSW. They represent the collaborative effort of many different organisations and individuals.

The full document can be downloaded. The Executive Summary is below.

Executive summary

Many coastal floodplains in Australia have an extensive network of floodgates, constructed drains and modified water courses. These are designed to mitigate the impacts of floods and large rainfall events. Floodgates prevent flood waters and tidal brackish water from inundating low areas of the floodplain. Constructed drains have converted prior wetlands into dryland farming areas. Whilst these developments have enhanced rural settlement and industries they have also caused unintended adverse impacts to fisheries, the ecology of estuaries and downstream water users.

The expanded drainage network has increased the generation and export of acidity from acid sulfate soils. Drainage systems can rapidly transfer acidity and deoxygenated water from backswamp areas to creeks and estuaries after rain. Floodgates and constructed drains have also blocked fish movement to upstream habitat areas and provide conditions that are conducive to the formation of poor water quality, particularly water with low dissolved oxygen.

These guidelines outline principles and strategies which can be employed to improve the environmental performance of coastal floodplain drainage systems, while retaining their benefits for agriculture. They have a particular focus on reducing drainage of acidity from areas with acid sulfate soils. The benefits, limitations and risks associated with management changes are described.

The guidelines emphasise the need to assess key features of coastal floodplain drainage systems before changing their management. Important features include the ranges of salinity and tides in the estuary, the elevation of land, the presence and depth of acid sulfate soils, the acidity of groundwater, the permeability of soils, and the changes that may have occurred in the type of native vegetation.

All stakeholders need to be involved in determining achievable management objectives. While some objectives have conflicting management requirements, many are compatible, enabling multiple objectives to be achieved. Management objectives can include preventing inundation of cropping land, reducing drainage of acidic groundwater, reducing low dissolved oxygen events, enhancing fish passage, enhancing fish habitat, managing aquatic weeds or restoring wetlands to conserve or enhance wildlife.

These objectives can be achieved by integrating three strategies for improved management. The first is to modify floodgates to enable controlled tidal exchange of drain water with fresh or brackish estuarine water. This will enable water quality improvements in the drain, allow fish greater passage, enhance fish habitat and enable the use of salt water to reduce aquatic weeds. Selecting a floodgate opening strategy requires an assessment of the risks of overtopping drain banks and the most suitable opening device to provide the required degree of water level control. It will also require an assessment of subsidiary works (eg levees, penstocks) to prevent or control inundation and limit water movement.

The second strategy is to use water retention structures to reduce the seepage of acidic groundwater to drains in acid sulfate soil backswamps. These structures can also control unwanted intrusion of saline water, or reduce the risk of peat fires. Water retention strategies can also be used to reduce the drainage of acidic or deoxygenated surface water and aid the establishment of wetland pastures or wetland conservation areas.

The third strategy is drain redesign. This can include filling in unnecessary drains, replacing deep drains that intercept groundwater with shallow drains which remove only surface water, and land forming to shed surface water to shallow drains. These guidelines are based on the best scientific understanding of the day. They will need to be applied adaptively given that social, economic and environmental circumstances are continually changing. They will require further development as our understanding of the processes continues to grow.