Design fishways that let salmon and trout climb past dams and weirs. Adjust the target species, body length, water temperature and pool geometry to see the inter-pool velocity, flow rate, energy dissipation factor (EDF) and total head drop update in real time, and find a ladder shape that fish can actually ascend.
Parameters
Target species
Sets cruise/burst speeds and jump capability
Body length L_fish
cm
Fishway type
Pool-and-weir, vertical slot, Denil, nature-like
Water depth h_w
m
Channel width W_ch
m
Head drop per pool H
m
Drop between adjacent pools
Number of pools N_pool
Water temperature T_w
°C
Q10 = 2 metabolic correction
Results
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Cruise speed U_crit (m/s)
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Burst speed U_burst (m/s)
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Orifice velocity (m/s)
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Flow rate Q (m³/s)
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EDF energy dissipation (W/m³)
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Total head drop ΣH (m)
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Fishway section — pools & velocity vectors
Stepped pools from downstream (left) to upstream (right). Each orifice generates a Bernoulli velocity. Colour shows passability (green = safe / amber = turbulence warning / red = blocked).
Velocity vs head drop per pool H
Swimming capability by species
Theory & Key Formulas
$$U_{orifice} = \sqrt{2gH},\quad EDF = \frac{\rho g Q H}{V_{pool}},\quad U_{burst} = U_{BL} \cdot L \cdot \sqrt{f(T)}$$
H = head drop per pool [m], Q = flow rate [m³/s], V_pool = pool volume [m³], and U_burst = body length L × BL/s ratio × temperature factor f(T) = Q10^((T−15)/10).
Fish Passage Velocity Barrier — Hydraulic Engineering & River Ecology
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A "fishway" is that staircase-like channel built next to a dam, right? Do fish really climb those things?
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Yes, when they are designed properly fish do climb them. The principle is surprisingly simple: a single long ramp would turn into a torrent that no fish could break, so engineers split the climb into many small steps (pools) and connect them with orifices or slots where the velocity is U = sqrt(2gH). Inside each pool there has to be a calm refuge for the fish to rest. That balance — passable orifice velocity and a quiet resting zone — is the heart of fishway design.
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So just shrink the head drop and any fish can go through?
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Not quite. With the default settings (salmon, 60 cm, H = 0.2 m) the orifice velocity is about 1.98 m/s, far below the salmon burst speed of 5.4 m/s — so the velocity is "passable". But EDF reaches 272 W/m3, above the Larinier salmon limit of 200 W/m3. The pool is too turbulent for the fish to recover even though the velocity itself is fine. Lower H to 0.15 m, or grow the pool volume (depth × width × length), and EDF drops.
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Why does water temperature change the swimming capability?
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Fish are ectotherms — muscle contraction speed scales with body temperature. The Q10 rule (metabolism doubles per 10 C rise) gives a correction factor relative to 15 C. The catch is that the salmon spawning run coincides with cold winter water. So fishways must be checked at the worst case: lowest temperature and highest flow. Drop the slider to 5 C and you will see U_burst fall to about 4.0 m/s.
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There are four ladder types in the menu. How do they differ in practice?
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Pool-and-weir is the classic overflow design, common at small dams. Vertical slot keeps a near-constant velocity at any stage and is the workhorse of the North American salmon programme. Denil ladders cope with steep slopes (up to 1:5) but their turbulence excludes small and benthic species. Nature-like channels use boulders and roughness to create varied refugia and can pass salmon, benthic fish and crustaceans together. Since the EU Water Framework Directive nature-like designs have become the dominant choice, with iconic examples at Chitose (Hokkaido), Laerdalsfoss (Norway) and Bonneville (US).
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When should I use U_crit vs U_burst as the design criterion?
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U_crit (cruise speed, about 4–6 BL/s) can be held for around 30 minutes; U_burst (burst speed, about 8–12 BL/s) only lasts about 20 seconds. Use U_burst for short orifice transits and U_crit for the average pool velocity and long continuous swims. This tool compares U_burst with U_orifice as a conservative first check, but in practice you should look at both depending on pool length and required swim duration.
Frequently asked questions
The velocity through a submerged orifice or slot is given by the Bernoulli equation U = sqrt(2gH), where H is the head drop per pool and g = 9.81 m/s². For H = 0.2 m, U = sqrt(2·9.81·0.2) ~ 1.98 m/s. If this exceeds the burst swimming speed U_burst of the target species, the fish cannot break through and the ladder fails. This tool computes U_burst from species, body length and water temperature, then compares it with U_orifice to judge passability.
EDF = ρgQH / V_pool is the power dissipated per unit pool volume (W/m³). High flow Q, large head H and small pool volume V_pool all increase EDF and intensify turbulence inside each pool. Larinier (2002) recommends 200 W/m³ maximum for salmon and 150 W/m³ for trout. Above these limits the fish are tossed by turbulence and cannot rest, so they fatigue and fail to ascend even when the velocity itself is within their burst capacity.
Each species has a different body-length ratio for cruise and burst speeds (BL/s). Salmon are ~5/10 BL/s, eel 3/5, with jump heights from 1.5 m (salmon) down to 0 m (carp). Water temperature changes metabolism via a Q10 = 2 correction relative to 15 °C. This tool automatically updates U_crit, U_burst and jump capability from the species preset, then compares them against the orifice velocity and EDF.
Pool-and-Weir is simple, with fish leaping or swimming over each weir. Vertical Slot delivers a near-constant velocity at all depths and tolerates stage variation. Denil ladders handle steep slopes (up to 1:5) but produce strong turbulence unsuitable for small or benthic species. Nature-Like fishways use boulders and rough surfaces for diverse refugia and can pass salmonids, benthic fish and crustaceans. This tool uses the same velocity and EDF physics for all four types; type selection is discussed in the application notes.
Real-world applications
Anadromous fish recovery at dams: the Chitose River in Hokkaido, Laerdalsfoss in Norway and Bonneville Dam in the US Pacific Northwest all rely on large pool-and-weir and vertical-slot fishways to maintain salmon and trout runs. Pools are typically sized for H = 0.15–0.25 m and validated so EDF stays below 150–200 W/m³ at the peak spawning flow.
Nature-like fishways and ecological restoration: since the EU Water Framework Directive came into force, boulder-strewn nature-like channels have become the European standard. They pass not only salmonids but benthic species (sculpin) and crustaceans (mitten crab), restoring river connectivity and biodiversity. Adoption is also growing in Japan as part of environmentally conscious public works.
Fisheries management and regulation: where the River Act and the Fisheries Agency require fish passage, simple calculations such as this tool serve as a first-pass screening to confirm that the target species can physically traverse the structure. The economic stakes are large — the US salmon fishery is worth billions of dollars annually — so FERC and NMFS publish detailed Anadromous Fish Passage Design Guidelines for these designs.
Environmental assessment and CAE coupling: for major projects the fishway hydraulics are studied in detail with CFD (OpenFOAM, Flow-3D). The usual workflow is to size the ladder with Bernoulli-based tools like this one, then verify turbulence intensity, vortex structure and resting-zone velocity distribution in CFD. Specialised software such as FishXing and FIPEX is often used alongside.
Common misconceptions and caveats
The biggest trap is assuming that U_orifice ≤ U_burst is enough for passage. Looking only at Bernoulli velocity hides pool turbulence (EDF), attraction flow that guides fish to the ladder entrance, stage variation and peak spawning-season discharge. In practice, more than half of failures occur when the velocity is fine but EDF is too high. Always check EDF against the 200 W/m³ (salmon) / 150 W/m³ (trout) limits in the verdict box.
Next, designing for average temperature and forgetting winter. Salmon and trout runs happen in cold water (5–10 °C), where the temperature factor f(T) = Q10^((T−15)/10) drops to 0.7–0.8. A design calibrated at 15 °C silently creates a "seasonal barrier" that fish cannot break in winter. Re-check the design at the worst case (lowest temperature combined with highest flow).
Finally, ignoring the fishway entrance. Even a perfect ladder is useless if fish never find it. If the entrance is far from the dam release jet, fish are pulled into the main flow and bypass the ladder entirely. NMFS guidelines call for attraction flow of 1–5 % of the main river discharge and entrance velocities of 0.6–2.4 m/s. This tool focuses on the in-ladder hydraulics — entrance design must be assessed separately.
How to Use
Enter fish body length (cm) and select target species—coho salmon typically range 45–65 cm, steelhead trout 40–70 cm—to calculate species-specific burst speed (up to 2.5× body length per second).
Set water depth (feet) and channel width (feet) for your fishway geometry; typical pool-and-weir designs use 1.5–2.5 m depth and 3–6 m width.
Input head drop per pool (m); standard incremental values range 0.3–0.75 m to keep orifice velocity below 2.0 m/s and avoid passage failure.
Review output metrics: cruise speed (sustained swimming), burst speed (short-term sprint), orifice velocity, dissipated power density, and cumulative head drop to verify the fishway meets migration criteria.
Worked Example
Design a fishway for coho salmon (55 cm body length) with water depth 2.0 feet (0.61 m), channel width 4.0 feet (1.22 m), and head drop per pool 0.5 m. Simulator calculates cruise speed ~1.1 m/s, burst speed ~2.8 m/s, orifice velocity 1.85 m/s, flow rate Q = 2.3 m³/s, energy dissipation 18.5 W/m³, and allows passage of six sequential pools (total head ΣH = 3.0 m) before fish fatigue becomes critical. Adjust pool height downward to 0.4 m if burst speed falls below 2.2 m/s.
Practical Notes
Atlantic salmon and Pacific steelhead require different speed thresholds; steelhead tolerate higher velocities (2.8–3.2 m/s) than coho (2.2–2.6 m/s), so tailor pool spacing accordingly.
Energy dissipation below 30 W/m³ reduces turbulence stress; baffle designs (Denil fishways, slot weirs) achieve 12–20 W/m³ and improve passage success from 45% to 85%.
Orifice velocity exceeding 2.5 m/s creates "velocity barriers" that block fish regardless of burst capacity; prioritize maintaining orifice flow under 2.0 m/s for weak swimmers (juvenile trout <25 cm).