H20-DS Hillslopes to Ocean: Decision Support System for Sustainable Drainage Management

This decision support system has been developed to assist waterway management decisions by providing a series of critical Best Management Practices (BMPs), specifically focused on practical drainage management. These BMPs are supported by an analysis of environmental issues within a decision-making framework that encourages that both effects and causes be considered. It has been written to assist landowners, contractors and drain managers. While focusing on environmental benefits it recognises the operational need for maintaining drainage outfall and minimising long term costs.

Management practices are based on the best available science identified in the review of national and international practices. In some cases practices are transposed to New Zealand without local verification, in the belief that the practices have been proven overseas, and that they will be tested locally and refined as necessary.

In these guidelines waterway management problems are identified, causes of these problems are discussed, and management options are reviewed. An operational decision making framework to identify problems and select management measures is presented.

There are a broad range of issues that drainage managers face; some of which are political in nature, such as who pays for maintenance and access and land ownership issues. These issues are not addressed here. Nor do we directly address the issue of overall network efficiency, which is complicated by ownership. The focus is on physical issues related to drainage maintenance, specifically:

• Sedimentation - the loss of outfall due to sediment build up
• Vegetation - loss of outfall due to aquatic weeds and bank vegetation
• Water quality - waterways collect, store and transfer contaminants leading to a deterioration of water quality in receiving waters
• Biodiversity - adverse effects on aquatic, marginal and terrestrial flora and fauna, mahinga kai, cultural and amenity values

While the specific details of many waterway problems can be complicated (e.g. variable causes of excessive sediment supply and poor water quality), the range of management measures used to address these problems is surprisingly small. Based on national and international experience a broad range of problems are addressed with the following measures:

1. Control the generation of sediment and contaminants in the first instance.
2. Control stock access to waterways.
3. Use knowledge of plants to determine an appropriate weed management strategy.
4. Construct or leave a vegetated buffer between cultivated areas and waterways.
5. Provide habitat, food, and shade with streamside planting, and at the same time reduce the need for weed control in small streams.
6. Stabilise eroding stream banks with vegetative control measures, bank re-grading and structural measures.
7. Trap and treat sediment and contaminants in wetlands that are positioned at major sources of contaminants, at tile drain outfalls, and in the stream channel.
8. Provide a corridor for drainage channels and incorporate natural channel features when maintaining or constructing these channels.
9. Identify important flora and fauna and minimise or avoid disturbance of sensitive places at sensitive times (e.g. fish spawning).

It is generally accepted that there is no single ‘silver bullet’ best management option. A system of management practices must be employed. A decision support framework is provided to guide the selection of BMPs. For each BMP the complexity, effectiveness and costs are rated. It is expected that these BMPs will be refined over time, and more practices will be added.

The title of this decision support system “Hillslopes to Oceans” (H20) is intended to convey the message that drainage management can not be sustainably undertaken in isolation:

• Drains are a source, a sink, and a conduit of water, sediment and contaminants
• Events or actions in one part of a catchment might have effects elsewhere
• Local actions are not necessarily enough to solve catchment scale problems.