AWMA have spent numerous years working with stakeholders and special interest groups to analyse Fish Exclusion Screen designs and identify the specifications required for the protection of native fish in Australian and New Zealand applications.
Based on this knowledge, AWMA offer comprehensive information on all screens available in the market and endeavour to explain pro’s and con’s as well as why AWMA offer our range of products. Should you have further questions or specific site details you’d like to discuss please email Peter, our General Manager of Screens.
A Fish Exclusion Screen (FES) is a physical barrier on a water diversion in a natural water way, specifically designed to meet a set of predetermined parameters effectively preventing the entrainment or impingement of fish at all life cycles as well as other aquatic life. Visit our Fish Exclusion Screen Page for further information.
Proven technology now exists to sustainability filter water entering irrigation and municipal diversion systems. Fish Exclusion Screens prevent the entrainment of fish, larvae and eggs with the added benefit of protecting pumps from damage, clogging and debris. The Fish Exclusion Screens can filter down to 1mm orifices and self-clean to maintain 100% flow with low head loss from the screen. Visit our Fish Exclusion Screen Page for further information.
A well-designed Fish Exclusion Screen, will enhance not impede the efficient diversion of water, whilst protecting aquatic life. Visit our Fish Exclusion Screen Page for further information.
Whilst a Fish Exclusion Screen will act as a debris screen, a debris screen is generally not designed to meet fish exclusion specifications. As a result, will not work or comply with fish protection guidelines. Visit our Fish Exclusion Screen Page for further information.
There have been many types of Fish Exclusion Screens developed, designed and installed over the last 50+ years. A number have proven to operate effectively with reasonable whole of life costs, whilst others have been known to be ineffective, of poor quality and costly.
Since the inception of fish screens, designs and science have improved and there are subsequently a range of screen designs that can be considered best practice to suit most applications for diverting water from natural water ways. Visit Discussion Paper on Fish Exclusion Screens for more information.
A well designed and operated Fish Exclusion Screen will:
- Protect all life cycles of fish and other aquatic life
- Meet local regulation or guidelines in relation to fish protection
- Reduce pumping costs
- Improve diverted water quality
- Assist operators to follow best social and environmental practices
- Improve public image
- Require minimal maintenance
- Have low whole of life costs
Fish Exclusion Screens will secure sustainable results for the native fish strategy across the Murray-Darling Basin (MDB).
Fish screens will significantly leverage and magnify the improvements to native fish populations that proactive initiatives and science have delivered over the last 20+ years. This accelerated action will ensure sustainable native fish populations within the Murray-Darling Basin for the foreseeable future.
Progressive interventions have been improving native fish populations across the MDB. The introduction of Fish Exclusion Screens on gravity and pumped offtakes will retain fish in natural waterways and accelerate the recovery of native fish populations. Visit the Discussion Paper for Fish Exclusion Screens in the Murray-Darling Basin.
A purpose designed Fish Exclusion Screen is a very different screen from a traditional inlet screen.
Water diversions may be pumped or gravity systems and include applications such as irrigation, raw water supplies for towns/cities, commercial and industrial uses such as hydro, cooling towers or manufacturing. Fish Exclusion Screens work by preventing both entrainment and impingement of fish, larvae and eggs during the water diversion process.
- The primary way a fish exclusion screen protects fish is by reducing velocity at the point of diversion.
- The second protection method is by providing a physical barrier.
Visit our Fish Exclusion Screen Page for further information.
The primary way a Fish Exclusion Screen protects fish is by reducing velocity at the point of diversion. This means that the suction force around the screen ‘area of influence’ is significantly reduced.
Fish exclusion screens protect fish by providing a physical barrier at a water diversion point.
The current guidelines indicate that a screen should have 2mm or 3mm orifices. These may be slots, holes or other configurations in the screen media. The idea is that the slots will be small enough to obstruct the entrainment of all stages of fish life.
Whilst the 'Slot Size' is an important screen design criteria, it comes third when weighted against the design criteria of 'Low Velocity' and 'Self-cleaning'.
'Low velocity' keeps both fish, larvae and eggs and debris away from the surface in the first place.
'Self-cleaning' ensures that the slots stay clean so that the velocity doesn’t increase as the screen blocks.
The most common screen material is stainless steel wedge wire. The wedge wire has very well defined slots, the wedge shape assists in cleaning and the wire is a very robust design.
Operationally, keeping the Fish Exclusion Screen clean is the most important factor in sustainably protecting fish populations when using physical screens.
Keeping screens clean not only protects aquatic life it also ensures efficient, high quality water delivery and pump efficiency and therefore reduced maintenance of infrastructure and lower energy costs.
Unless you have crystal clear water or have funds to pay divers, it is good design practice to incorporate a screen retrieval system.
A retrieval system allows safe, efficient, above-water access for viable inspection or physical maintenance if required.
In general all screens should be visually inspected annually, a retrieval system makes this process safer, quicker and cheaper.
Screen performance is impacted by the surrounding hydraulic conditions it works in. In some cases the current in the river, stream or structure going past the screen (referred to as sweeping velocities) may be high or in other locations the water may be still with no sweeping velocity.
The local hydraulic conditions must be understood and considered when selecting and designing a Fish Exclusion Screen.
In most cases it is recommended that the sweeping velocities (current / flow passing a screen) should be less than 1m/sec. Faster sweeping velocities make it difficult to maintain even intake velocities through the screen (a key design and operating criteria) and may require significant increases in screen area to become compliant and costly CFD modelling to prove design compliance.
On the other hand sweeping velocities do assist in enhancing a fish screen operation by ‘sweeping’ cleaned debris from the screen and also assisting in directing fish into the fish bypass, if required.
When designing Fish Exclusion Screen structures with sweeping velocities present, particularly when operating a fish bypass, it is important to ensure that the designed flows are high enough to promote scouring of sediments, but still within the screen performance guidelines. This means that ideally sweeping velocities to scour sediment to keep it moving will be between 0.7m/sec and 1 m/sec.
Be aware that high sweeping velocities can be very beneficial for sediment transfer (as well as debris removal and fish bypass) but will significantly influence the viable area of the screen in most cases. This means that sites with high sweeping velocities should be carefully assessed for complying screen performance including Computational Fluid Dynamic (CFD) modelling. The screen area required to achieve the specified performance will be significantly higher than calculated.
Management of sedimentation becomes more critical in sites that are subject to high suspended solids, such as braided rivers in flood with alluvial beds or rivers with high silt loads. It pays to do a comprehensive study on the operating environment to fully understand the possibility of sediment deposit, particularly if you’re changing the local hydraulic conditions when designing and installing a Fish Exclusion Screen. Sediment deposits in changed hydraulic conditions can be very different to the historical site performance.
Many very expensive fish preservation infrastructure installed on rivers and streams globally have quickly become useless due to inundation and build-up of sediment.
- Typically an existing inlet (suction) (trash) screen is designed to protect a pump from damage and blockages from larger debris in the water way such as sticks, leaves and aquatic vegetation.
- Most traditional screens are not self-cleaning, so are designed with large holes or slots to stop the screen from blocking up. This means that the screen often drags fish of all life stages and other aquatic animals into or onto the screen.
- Traditional trash screens are not designed to reduce approach velocities, just screen, so creates high approach velocities, has a large area of influence’ and impinges and entrains fish and other aquatic life, often killing them; removing them from the natural waterway and future viability. Often inlet screens are so badly designed that they regularly block up and in the end are removed, offering zero protection and increasing fish mortality significantly.
If your existing inlet screen does not meet the three design criteria for Fish Screens, at a minimum it will be killing and removing fish at the diversion.
If a screen is located in a canal or reach that is off the main waterway, the fish excluded by the Fish Exclusion Screen require a path back to the natural waterway, this is facilitated by a fish bypass (channel).
When incorporating a fish bypass usually about 10% of the diverted water does not go through the screens but is used to direct and transport the excluded fish away from the screening area and back into the natural waterway.
A sustainable Fish Exclusion Screen system often involves more than the physical screen. If the screens are located down a dead end river branch or an offtake canal, the excluded fish will require a path back to the natural water way or they will just continue to ‘build up’ in front of the screens with nowhere to go.
There are many papers written on the design of fish bypass systems as the design will vary significantly depending on:
- The location.
- The terrain.
- The distance back to the natural water way.
- The type of fish.
- The amount of by-pass water available etc.
- Suspended sediment loadings.
Typically the bypass;
- Will use about 10% of the diverted flow, (more if the total diversion volume is low).
- The bypass flow will be significantly faster velocity than the (<1m/sec) sweeping in front of the screens. This entices the fish to keep moving down the bypass and not try to return.
- The bypass will have ‘steps’ or other physical barriers to prevent the fish from swimming backwards towards the screens.
- The entry point to the natural water way should consider predatory fish and birds etc.
- The fish bypass return passage should be as short as possible.
There are many types of Fish Exclusion Screens and many applications for Fish Exclusion Screens. There are a variety of screens designs, not all screens are suited to all applications.
Different types of fish and different diversion types require different design considerations.
Many Fish Exclusion Screens will be retrofitted to existing diversions, this can present more challenges with Fish Exclusion Screen design and choice. New installations can and should be designed with the Fish Exclusion Screen in mind throughout the design process.
Other factors to consider when selecting a Fish Exclusion Screen besides the installed application and fish type, include: build quality, support, documentation, safety and whole of life costs.
Fish Exclusion Screens come in different material types particularly the actual screening material but also the frame. The material type may affect capital cost but will also affect life expectancy and ultimately whole of life cost.
Most self-cleaning fish exclusion screens will require mains power supply either single phase (240/220VAC) or more likely three phase (415/ 400VAC). Some will require a high pressure water source or compressed air.
Some innovative screens have been developed that use the flow of water through the diversion to power the self-cleaning mechanism using an impellor or similar.
Plate / Flat Screens are commonly used in many applications both in large and small installations. They require large surface areas and it can be challenging to design effective self-cleaning systems. They typically use either perforated plate or wedge wire as the screening medium. Plate screens have traditionally been used on flooded pump wells and gravity offtakes, they are not suited to direct suction lines.
Drum Screens have been widely used over the history of screening and like all screens have their pros and cons. The screen is only partially submerged, so does not use the full surface area of the screen in the water therefore typically requires a high duty cycle for cleaning which increases maintenance.
Because only part of the screen is in the water the screen has to be very large to achieve the screen surface area required to achieve low approach velocity through the submerged screen area.
The typical design is also high maintenance having many moving and interfacing parts. Drum screens often use woven mesh which has proven to be high maintenance.
The rotation of the exposed drum screen means that it is possible that fish or fish eggs that become impinged on the screen could be ‘carried’ over to the downstream side.
Drum screens can be cleaned using high pressure water, scrapers or brushes.
Cylinder screens (brushed cleaned) are very efficient as the full surface area of the screen is submersed. They can be configured as single or tee configurations. They are typically brushed from both the inside and outside of the cylinder. The cylinder rotates to facilitate the brushed cleaning process either self-powered (impellor in the diverted flow), hydraulic or electric drive (solar DC or mains AC). The brush cleaned cylinder screen is a proprietary design.
The brush cleaned cylinder screen uses wedge wire as the screening medium with a rotary brush cleaning from the inside through the wedge wire slots and a second stationary brush cleaning the outside surface. This combination of wedge wire and dual brushes cleaning the wedge wire is very effective in nearly all applications.
Cylinder Screen (water jet cleaned) are efficient as the full surface area of the screen is being utilised. The screen is cleaned from the inside and /or outside with a water jet system. Often the outside of the screen is not directly cleaned.
High pressure is required and therefore a separate dedicated pump and controls to operate the cleaning system. The water has to be clean, so the feed water is usually filtered. The pump and filter introduce extra maintenance and cost to the screening system.
Water jet cleaned screens often use woven wire or fabric as the screening medium which is not as robust as wedge wire.
Cleaning with water jets from the inside of the screen can be compromised by the diffuser structure that is required to evenly distribute inlet velocities over the entire screen surface.
Water jet cleaned screens without comprehensive diffusers will not be compliant with the even approach velocities over the entire screen surface.
Water jet cleaned screens are primarily debris screens and the design needs to be carefully analysed for suitability for fish exclusion screens compliance, particularly the even spread of velocity over the full screen area.
Cylinder Screen (air burst cleaned) are efficient as the full surface area of the screen is being utilised. The screen is typically manufactured from wedge wire and is cleaned from the inside by the periodic release of large volumes of air. The outside of the screen surface is not physically cleaned. The air is supplied by an air compressor and large accumulator, the air is fed out to the screen in solid large diameter pipes.
This infrastructure increases both the capital cost and the operating cost. There are no moving parts under the water which is a good feature, reducing maintenance costs. Cleaning with large volume air burst from the inside of the screen can be compromised by the diffuser structure that is required to evenly distribute inlet velocities over the entire screen surface.
As the outside of the screen is not physically cleaned, it has been seen that weed matting and other marine growth on the outside of the screen may not be effectively cleaned with the airburst system.
Cleaning with air burst from the inside of the screen can be compromised by the diffuser structure that is required to evenly distribute inlet velocities over the entire screen surface.
Air burst cleaned screens without comprehensive diffusers will not be compliant with the even approach velocities over the entire screen surface.
Air burst cleaned screens are primarily debris screens and the design needs to be careful analysed for suitability for fish exclusion screens compliance, particularly the even spread of velocity over the full screen area.
There is significant disturbance and surface turbulence when the screen is being cleaned that may affect local water users and other aquatic life due to explosive turbulence and increased turbidity.
Traveling screen (polymer) this type of screen uses a moving screen like a conveyer belt. The water passes through the submerged screen area and periodically the screen rotates, bringing the submerged section to the surface carrying any debris with it.
Typically the traveling screen will be cleaned by a scraper or stationary brush. Sometimes water jets are used. The debris collect in a bin or on the bank. The travelling screen is a good design to use for sites that are located in dead end locations without sweeping velocities to take away the debris from the screen post cleaning.
The polymer screen is very low maintenance, it is a proprietary design. Some traveling screen designs use woven wire or other screening mediums, these materials are high maintenance and not recommended.
Traditionally Raked Screens have been used as trash screens. They are usually fairly coarse screens, but it is possible to have a fine raked screen that could be deemed suitable for fish protection. The raked screen is very mechanical like the traveling steel mesh screens and again like the travelling screens the raked screen is an excellent screen for debris removal.
Raked screens would be used on gravity applications either canal offtakes or flooded suctions for pumps.
The Cone Screen is typically used in locations that are shallow, as it is a screen that provides a large surface area in a low profile. The cone screen is a very robust design, the hydraulically powered brushes are powerful and can smash most debris small enough to pass through the screen, so can be suitable for sites that have low or do not have sweeping velocities. The cone screen is made from stainless steel wedge wire. The brushed cleaned Cone Screen is a proprietary design.
Behavioral barriers such as Acoustic Barriers are not a screen but a sensory deterrent. Typical systems use bubble, sound/pressure waves (noise) or high intensity pulses from ‘water guns’ to influence the behavior of fish to deter fish from passing the barrier and redirect them. Acoustic barriers can be subtle or very intense and have been shown to injure aquatic life if not used correctly. There are also electric barriers.
Non-physical behavioral barriers can only be used in conjunction with fish bypass as the fish need to be removed from the barrier area once deterred. Sensory barriers have varying degrees of success and should only be considered when a physical fish exclusion screen is not viable. A sensory barrier will not deter or obstruct passage of fish at all life cycles. Eggs and larvae will pass through without issue. Most legislated consents or best practice guidelines will specify a physical barrier with a defined orifice as a part of the screen solution, therefore a sensory deterrent will not be compliant.
Before Fish Exclusion Screen were commercially available or when a ‘cheap capital’ solution was required, rock bunds were often used to exclude fish from gravity diversions and redirect them through a fish bypass. A rock bund provided a coarse barrier for fish exclusion. Most fish exclusion guidelines will require a defined orifice such as 2mm or 3mm, a rock bund cannot offer this accuracy. Rock bunds also silt up over time, reducing flow through the diversion and need to be pulled apart and reassembled which is an expensive exercise.
Coanda screens are wedge wire wave shaped screens that self-clean using velocity of the water flow. Coanda screens require a degree of head differential to work, significant bypass flows and fish bypass.
Horizontal screen (Farmers screen) is a horizontal, passive fish screen design that uses hydraulics to manage debris and protect fish. The Farmers screen is a proprietary design, requiring a large footprint, a degree of head differential to operate and a fish bypass.
- The first thing that is required is having a good understanding of the hydraulic and operational characteristics of the diversion.
- It is often a good idea to involve a system designer such as pump engineer, hydraulic engineer, irrigation consultant, local fisheries authority or the fish exclusion screen manufacturing company if they offer direct support.
- The more advice and information available to you the better. Do your homework so you understand the advice you are getting.
The investment in a good quality Fish Exclusion Screen will not be a small investment, but if selected correctly it will enhance your operation, protect fish and have a long operating life with low whole of life costs.
Ensure you have a comprehensive understanding of the fish protection local legislation or best practice guidelines for your local area and fish type.
If the fish exclusion screen is selected, designed and installed correctly the ongoing operation of a fish exclusion screen should NOT be onerous on labour requirements or cost.
Screen retrieval systems are used to safely and conveniently raise a screen above the waterline to allow access for inspection and maintenance.
If a screen does not have a retrieval system it would be necessary to engage divers to inspect and maintain a Fish Exclusion Screen or use a crane or similar lifting device to remove the screen and pipe work. Many consents will require periodic visual inspection of the screen.
It is not always possible or economical to install a Retrieval System on a Fish Exclusion Screen. The site has to support the installation of the infrastructure and the capital budget has to support the cost, but it should always be considered at the time of design and when calculating whole of life cost and where possible, implemented.
Drum screens are mostly above water so the screening medium can be visibly inspected as it rotates. However, as the screen interfaces with the structure under water that needs to be inspected and if the screen requires maintenance it would have to be lifted from the water using a crane.
Travelling screens should be installed in slots or guides to allow the screen to be lifted with a crane. A well designed travelling screen will not have serviceable parts under the water line.
A well designed retrieval system should have the same design methodology as the screen in relation to quality material and components.
The pivoting pipe is a simple retrieval system that allows the screen to be ‘rotated’ above the waterline using a winch. The pipe join has a customised pivot fitted allowing the rotation.
The vertical rail and winch system allows very large screens or smaller screens to be raised vertically to the surface. These retrieval systems have been designed to lift screens 60kg in weight with a manual winch and up to 6000kg with a powered winch.
The screen docks into place over the orifice through a wall or directly onto a pipe. The docking interface has to be designed with the same tolerances as the screening medium.
Material selection and coatings specification for the rail system should be given careful consideration as it will be permanently under water and cannot be maintained for its expected service life in in most cases.
Most retrieval systems will incorporate an isolation gate that is closed prior to raising the screen, preventing debris and fish from entering the diversion.
Like the vertical rail system the angled rail system uses a winch to raise or lower the screen on a guided rail usually attached to the pipe. These systems are generally used on axial flow or centrifugal flow pumps direct suction lines. There is a manifold installed over the end of the pipe that the screen docks with when lowered. It is important that the hydraulics are modelled within the manifold to ensure the new arrangement does not affect the pump inlet efficiency. In fact a well-designed manifold could actually reduce head loss at the inlet and improve pump efficiency.
The submersible pontoon is a proprietary design that allows the pontoon to be lowered below the water by filling the pontoons with water and conversely raised for inspection above the water by charging the pontoons with air.