Choosing an instream structure
The best instream structure for a particular location will depend on the purpose of the structure and the local conditions.
Order of preference for road crossing design
An instream structure should:
- allow all aquatic organisms and life stages to move safely and efficiently up and downstream
- have diverse physical habitats and hydraulic conditions (rapids, backwaters and areas of slow flow for example)
- not impede fish movement any more than adjacent stream reaches
- allow natural processes like the movement of sediment and debris to continue
- be durable and require minimal maintenance.
The order of preference for road crossing structures up to 4 m high is generally:
- culvert that simulates a stream
- single barrier circular or box culvert (hydraulic design)
- multi-barrel culvert
The diagram on the right shows the order of preference for road crossing design, based on the degree of connectivity each design facilitates.
- Fords are the least preferred type of river crossing. The bed and banks are typically artificial, water is shallow and fast flowing, and there is often a vertical barrier at the downstream end.
- Different types of culverts have differing levels of impact on fish passage.
- Bridges are the most preferred type of river crossing. They preserve the natural stream bed and banks, and allow natural water depths and velocities to be maintained.
See section 3 of the New Zealand Fish Passage Guidelines and the summary factsheet Considering fish passage in the design of new instream structures (PDF, 452K).
Designing large structures
Any structures that are higher than 4 m are likely to require a specific design. Although no standard guidance is available at this time, much of the available guidance can be used as a starting point.
See fish passage resources for more information.
A process to guide the design and construction of new structures in streams is set out below.
- Assess the site to understand the requirements and risks.
- Set the ecological objectives and the performance standards for biology and water flow.
- Carry out a detailed site survey to inform the design.
- Design a concept structure that meets the objectives and standards set out in the Fish Passage guidelines, then complete the structural drawings and specifications.
- Obtain the approvals required
- Build the structure to specification, using an expert like a fish ecologist to oversee the process.
- Monitor and maintain the structure regularly to make sure it is in good order and continues to meet the objectives and standards.
See section 3 of the New Zealand Fish Passage Guidelines.
Culverts are commonly used to allow a stream to flow under a road or other structure. In most locations they can be designed, installed and maintained to provide for fish passage. Culverts must not impede fish passage unless an exemption has been approved by DOC.
Two recommended approaches to culvert design can provide for fish passage.
- Stream simulation design: this best practice approach creates a natural and dynamic channel through the culvert that is similar in size and character to the stream upstream and downstream. A simulated stream approach enables a wide range of species and life stages to pass, and allows for extremes of flows, longevity and future climate changes.
- Hydraulic design: sets out the minimum design standards for culverts that will provide passage for a typical fish community by allowing for the needs of the weakest swimming organisms. (Note that not all species and life stages may be allowed for in this approach).
See section 4.2 of the New Zealand Fish Passage Guidelines.
Weirs are used for a variety of purposes, such as managing water intakes and gauging flows, but they often impede the movement of fish. Where possible, weirs should be built as full river width with a natural rock-ramp fishway, instead of a conventional solid weir structure.
If it is not practical to build a rock-ramp fishway, a V-shaped broad crested weir with a baffled surface or a weir with a bypass channel should be considered.
Good design principles
- The top or crest of a weir should be round and broad.
- The weir should have a V-shaped lateral profile that has shallow, slow-flowing wetted margins on the weir face across the fish passage design flow range.
- Diverse flow environments should be provided, including wetted margins.
- Make the slope of the downstream weir face as gentle as possible.
- Use roughness on the face of the weir.
- A fish facility should be included if the weir face does not provide passage.
Design features to avoid:
- steep hydraulic drops and undershot weirs
- vertical wing walls
- back-watered upstream habitats.
See section 4.3 of the New Zealand Fish Passage Guidelines.
Fords are the least preferred type of river or stream modification because they often hinder or block fish passage. Fords combine many of the negative aspects of culverts and weirs (like fast-flowing, shallow water, a sharp crest and a steep downstream face). Fords also modify the stream bed and allow vehicles and animals to enter a waterway.
It is best practice to avoid using fords for stream crossings if possible. But if a ford is the only viable option, it should be designed with features that allow fish to pass. These include ensuring a continuous pathway for fish passage is maintained across the structure at all flows.
Causeway ford designs that incorporate culverts are the minimum standard for fords – these should follow the guidance for hydraulic culvert design. Low profile and standard raised roadway ford designs should be avoided. A ford must not impede fish passage unless an exemption has been approved by DOC.
See section 4.4 of the New Zealand Fish Passage Guidelines.
Flood and tide gate design
Flood gates and tide gates are used to control fluctuations in water flow during floods and tides. They can significantly disrupt movements of freshwater fish and invertebrates as well as the habitats upstream.
When flood or tide gates are required, best practice is to install automated gates that close only when the water level reaches a critical height. If operational constraints mean that the use of automated gate systems is not possible, the minimum standard is to install self-regulating ‘fish friendly’ gates. These gates should be kept open as far as possible for as long as possible, particularly on the incoming tide as this is when most juvenile fish are moving upstream.
As well as improving fish passage, fish friendly gates allow better water movement up and downstream, and help to reduce the impacts on habitats (like salt marshes) upstream of the gate.
See section 4.5 of the New Zealand Fish Passage Guidelines.
Monitoring is the only way to understand how well a structure is working and ensure that any reduction in fish passage is not harming upstream or downstream communities. Monitoring is especially important when:
- rare or high value fish communities or ecosystems are upstream of the structure
- more than one structure is in a waterway, and could be causing cumulative effects
- unproven designs or fixes are being used
- a proven design is being used in a new situation
- barriers are being used to manage the movement of invasive species.
See Chapter 7 of the New Zealand Fish Passage Guidelines for more information about monitoring techniques and methods for evaluating the success of fish passage in different circumstances. The fish passage assessment tool can also be used to monitor the structure and the risk to fish passage.