Tyrolean intakes have been a standard solution for mountain hydropower for over a century. They are cheap to build, mechanically simple, and they work: until the day they fail.
The day they fail comes during flood events. The Tyrolean's flat trash rack lies horizontal across a sloped weir, with bar spacing wide enough to pass small debris and sediment. When a flood sends down logs, branches, gravel, or a sustained sediment surge, the rack clogs in minutes. Power generation drops to zero. An operator has to climb down with hand tools and clean the rack: sometimes during the flood, sometimes for days afterwards. Many small hydropower (SHP) operators tell us the same story: they lost more annual power generation to clogging events than to any other operational issue.
This is why retrofitting Tyrolean intakes to Coanda screens has become one of the most common types of ADENCO project. Coanda screens self-clean during the same flood events that disable Tyroleans. They use steeper screen angles (up to 50° versus a Tyrolean's typical 17°) and narrower slot openings to shear water through the screen while debris slides down the screen surface and off the lower edge (discharge edge) under its own weight.
This guide walks through the retrofit decision: when it is the right choice, what civil (concrete) work is needed, three retrofit types we have delivered, and the common mistakes that turn a clean retrofit into an expensive lesson.
Why operators retrofit Tyrolean intakes
The economic justification for a retrofit is almost always the same calculation, just with different numbers.
A hydropower plant with a Tyrolean intake in the Black Sea region tracked its lost power generation over three years. Annual flood-event clogging averaged 11 days of zero output, plus another 19 days of reduced output as the operator cleared partial blockages. At a 4 MW plant with 40% capacity factor and a feed-in tariff around €0.075 per kWh, those 30 days of lost and reduced output amounted to roughly €25,000 of lost revenue per year, before accounting for the labour cost of manual cleaning and the wear on the trash rack itself from operators forcing tools between the bars to break debris loose.
That is the frequent, low-damage type of loss. There is also a rare but severe type: a single major flood event that physically deforms the trash rack. When a tree trunk lodges between the bars at design flow velocity, it can bend the bars permanently. Replacing the rack costs €30,000-80,000 depending on the structure. A bad year combines both types.
For a Coanda retrofit, you trade that ongoing cost against a one-time capital expense. The retrofit screen itself is a fraction of the hydropower plant's capital cost (CAPEX). The concrete work depends on the retrofit type (more on that below). And once installed, the screen operates for up to 25 years in clean-water service (15 to 20 years in flood-prone, high-sediment rivers) alongside the original concrete structure.
The operators who delay retrofit usually do so because they do not realise that a Coanda screen is an option for their existing weir geometry. The good news is that most existing Tyrolean weirs are retrofit-compatible: sometimes with no change to the concrete at all.
When retrofit is the right choice
Five conditions, in order of importance:
1. Clogging is your biggest operational issue. If you lose more power generation to clogging than to any other cause (outages, low-water seasons, grid issues), the retrofit recovers its cost quickly. If clogging is a minor annoyance compared to a different bottleneck, fix that first.
2. The site has at least 0.45 m of hydraulic head (drop height) available below the weir crest. Coanda screens need this drop to maintain supercritical flow (Froude number > 1) across the screen for self-cleaning. Most Tyrolean intakes have 1-2 m of available head, so this is usually a yes. If you're in a marginal-head situation, a review by an ADENCO engineer will tell you whether the geometry will work.
3. Your weir is at least 150 mm wide. That is the smallest standard Coanda unit (ADENCO-45-0.15), and below it we do not recommend a screen. Wider weirs are not a problem: multi-unit arrays handle any larger flow.
4. The concrete intake structure is in serviceable condition. A Coanda retrofit does not fix concrete that is failing. If your weir is cracked, undermined, or has serious erosion, repair that first or budget the rebuild as part of the retrofit project.
5. The hydropower plant is worth keeping in operation for at least 10 more years. Retrofit costs are amortised against future power generation. A plant near the end of its concession with no renewal in sight is a different decision.
If all five conditions hold, the retrofit decision is straightforward. If three or four hold, talk to an engineer. If two or fewer, retrofit probably isn't your highest-leverage move.
What stays, what changes
The pleasant surprise of most Tyrolean-to-Coanda retrofits is how much of the existing infrastructure stays in place.
Stays in place (most retrofits):
- The concrete weir structure (the sill itself).
- The water collection chamber downstream of the screen.
- The bypass spillway, if present.
- The penstock connection (inlet).
- The headwall and approach geometry, in most cases.
Changes (always):
- The screen panel itself. The Tyrolean trash rack is removed and replaced with a Coanda wedge wire screen.
- Often the screen support frame, sized to the new screen geometry.
May need to change (depends on site):
- The weir crest cross-section, if the Tyrolean was set up to deliver subcritical approach flow and the Coanda screen needs supercritical flow.
- The angle of the screen support, if the Tyrolean was at 15-20° and the Coanda screen design requires a steeper angle (40-50°).
- The headwall height, if you need to raise the upstream water level to deliver the right approach geometry.
The amount of concrete work is what determines the retrofit type. Three types cover most sites.
Three retrofit types
Type 1: Direct panel replacement
The cheapest retrofit. The new Coanda screen panel fits into the existing screen support, bolted in place where the old trash rack was. No change to the concrete. Manufacturing is 2-4 weeks for standard orders (up to 6 weeks for multi-unit arrays and up to 8 weeks for arrays of more than ten units); installation is 1-2 days; commissioning is same-day.
When this works: the existing weir geometry already delivers approach flow within the design limits for a Coanda screen (hydraulic head ≥ 0.45 m, sweeping velocity 1.8-3.7 m/s at design flow), and the existing screen support is dimensionally compatible with a Coanda panel.
Real example: the TISKI Trabzon retrofit for TISKI (Trabzon Water and Sewerage Administration): 64 Coanda screens delivered in two stages, replacing the existing Tyrolean intake plates on ageing municipal water-intake structures. No weir rebuild required.
Type 2: Partial weir modification
The middle type. The screen panel is replaced AND the weir crest is reshaped or the screen support angle is changed. Concrete work is limited to a few cubic metres: a change to the weir crest cross-section, or a new screen support set at a steeper angle bolted to the existing weir.
When this works: the existing weir is structurally sound but geometrically wrong: typically a Tyrolean at 15-20° that needs to become a Coanda screen at 40-50° for proper flow shearing. The existing structure provides the foundation; the modification provides the geometry.
Real example: the Seydioğlu HPP (hydropower plant) Trabzon retrofit: original 17° Tyrolean replaced with a 50° Coanda screen, requiring a complete redesign of the weir cross-section. The original concrete base remained; the screen-bearing portion was rebuilt in the right geometry.
Type 3: Full intake rebuild
The heaviest type. The Coanda screen design dictates a new weir, new headwall, new everything. Concrete work is in the order of tens of cubic metres of concrete plus rebar.
When this works: the existing intake is in poor structural condition, OR the existing geometry is so far from being compatible with a Coanda screen that piecemeal modification costs more than rebuilding from scratch.
This type is rarer than the first two. Most Tyrolean retrofits fall into Type 1 or 2.
ADENCO retrofit project walkthrough: TISKI Trabzon
The TISKI Trabzon project is a useful case study because it shows what a clean Type 1 retrofit looks like end-to-end.
Site: Ageing municipal raw-water intakes in the TISKI network. Multiple intake points across a regional system. The original trash racks were Tyrolean-style plates with bar spacing too wide for fine filtration and an angle too shallow for self-cleaning.
Operational problem: Persistent clogging during high-debris events. Manual cleaning was the standard operating procedure. TISKI was spending operator hours every week on cleaning across the network.
Retrofit decision: Direct panel replacement at each intake point. The existing concrete weirs and support structures were sound. Only the screening panels needed to change.
Engineering pass: ADENCO surveyed each intake point, measured and recorded the existing geometry, and designed Coanda screens that matched the existing mounting dimensions. Where dimensions varied between intakes, screens were customised rather than forcing one design onto every intake.
Manufacturing: Two delivery stages: Stage 1 (22 screens), then Stage 2 (42 screens). 64 total. All in 304 stainless steel.
Installation: No concrete work. Existing trash plates were unbolted; new Coanda screens bolted into the same supports. Per intake site, total downtime was less than half a day.
Result: Zero electricity consumption (the screens are passive). Self-cleaning across the full operating flow range. The network's clogging-related operator hours dropped to a level TISKI describes as "negligible."
The total project investment came in an order of magnitude less than rebuilding the affected intake structures. That is the retrofit cost comparison in practice.
Common retrofit mistakes
In 12 years of Coanda retrofits, the same five mistakes show up repeatedly. Avoid these and your project goes smoothly.
Mistake 1: Specifying the slot too fine. The temptation is to choose a narrower slot than the application requires, on the theory that "narrower is better." It isn't. Narrower slots foul faster, increase head loss, and require a shallower wedge wire tilt that limits sweeping velocity. Choose the slot width for the application: typically 1.0 mm for hydropower with moderate debris, narrower only for fish-protection or fine-particle removal. A wider slot only if you have heavy debris and don't need fine filtration.
Mistake 2: Ignoring approach geometry. A Coanda screen is only as good as the flow that hits it. If the approach is turbulent (uneven weir crest, asymmetric pond behind the weir, unintended weir overflows) the screen will pulse-clog instead of self-clean. Measure and record the approach geometry accurately during the engineering pass, not the day before installation.
Mistake 3: Skipping engineer review at unusual sites. Most Tyrolean retrofits are textbook. Some are not: unusual hydraulic head values, unusual weir widths, unusual debris signatures, very high or very low design flows, retrofits combined with fish-protection requirements. These sites are exactly where engineer review recovers its cost. Our Sizing Tool shows a warning for these conditions automatically and sends them to an ADENCO engineer for review before fabrication starts.
Mistake 4: Underspecifying material grade. 304 stainless steel is the standard choice for fresh-water mountain streams; 316L is selected where the water carries chloride or seawater influence. Neither is the right choice for severe high-chloride water (geothermal, coastal estuarine, warm brackish). Choosing an austenitic grade for a water chemistry that requires Duplex 2205 or Super Duplex 2507 turns a screen designed for 25 years into one that lasts 5. Confirm the water chemistry (a measured analysis, not an assumption) before fabrication.
Mistake 5: No protection bars where you need them. "Heavy debris" and "flood-risk" operating environments need upstream protection bars (stainless steel bars mounted in front of the wedge wire that take the impact of rocks and timber). Logs and large gravel can damage wedge wire panels at flood-event flow velocities. Protection bars cost a small fraction of the screen they protect. If your site has any history of large floating debris during flood events, include them.
Request for quotation (RFQ) checklist for a Tyrolean retrofit
When you're ready to request a quote on a Tyrolean retrofit, send these data points to ADENCO engineering. The more complete the information you provide, the more accurate the engineer-reviewed proposal.
- Existing weir geometry: width, sill cross-section, current trash rack angle, available hydraulic head (drop height) below the weir crest.
- Design flow in l/s or m³/h. Both maximum operating flow and minimum operating flow.
- Current trash rack details: bar spacing, total area, support arrangement.
- Type of water body: river / mountain stream / canal / reservoir; seasonal flow regime; ice-formation history if any.
- Debris type and load: what gets caught currently (leaves / branches / gravel / silt / mixed); flood-event history.
- Water chemistry: pH, chloride content (mg/L), total dissolved solids (TDS), sediment loading. Lab analysis if available.
- Fish-protection requirements, if any. Local regulator's compliance criteria.
- Photos: current intake, weir, trash rack detail, approach geometry, downstream collection.
- Tender or delivery schedule: desired commissioning date, contract or subcontract structure.
You can submit this as a free-form RFQ via the Contact form or enter it in the Sizing Tool first to get a preliminary configuration before formal RFQ. The sizing tool's result is engineer-grade and worth the 5 minutes; it removes most back-and-forth from the quoting cycle.
A Tyrolean-to-Coanda retrofit is one of the highest-leverage capital improvements available to a mid-sized small hydropower plant. The amount of concrete work is usually small. The operational gain is usually large. And the retrofit cost comparison is favourable at almost every site where clogging is the main operational problem.
When the Tyrolean intake is costing you output, retrofit. The new screen recovers its cost quickly.