Hydraulic Jump Interactive Calculator

Designing a spillway stilling basin or drop structure means you need to know exactly where the flow transitions — and how violently. Supercritical flow hitting a subcritical boundary doesn't slow down gradually; it slams into a hydraulic jump, shedding enormous energy in a short distance. Use this Hydraulic Jump Interactive Calculator to calculate sequent depths, energy loss, Froude numbers, jump length, and jump classification using upstream depth, velocity, and channel geometry. Getting these numbers right matters for spillway design, irrigation drop structures, and stormwater energy dissipation basins — undershoot the sequent depth and you'll scour your channel; overshoot and you'll drown the jump. This page covers the governing equations, a full worked example, jump classification theory, and an FAQ on edge cases like sloped chutes, air entrainment, and scale model effects.

What is a hydraulic jump?

A hydraulic jump is an abrupt transition where fast, shallow (supercritical) water suddenly slows down and deepens into slow, deep (subcritical) flow. The transition is violent and turbulent — it burns off kinetic energy as heat and agitation rather than letting it erode the channel downstream.

Simple Explanation

Think of a hydraulic jump like water hitting the bottom of a kitchen sink — it shoots out fast and thin, then hits a ring where it suddenly piles up into a deeper, calmer ring. That ring is the jump. Engineers deliberately create this effect downstream of dams and spillways to kill flow speed before the water reaches unprotected riverbed. The deeper the upstream flow has to rise, the more energy the jump is destroying.

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Hydraulic Jump Diagram

Hydraulic Jump Calculator

How to Use This Calculator

1. Select a calculation mode from the dropdown — sequent depth, energy loss, jump length, Froude numbers, or upstream depth.
2. Enter the upstream flow depth (y₁) in meters and the upstream velocity (V₁) in m/s, or the downstream depth (y₂) if using the upstream depth mode.
3. Adjust gravity (g) if needed — the default is 9.81 m/s².
4. Click Calculate to see your result.

Governing Equations

Simple Example

Upstream depth y₁ = 0.5 m, upstream velocity V₁ = 6 m/s, g = 9.81 m/s².

Fr₁ = 6 / √(9.81 × 0.5) = 6 / 2.215 = 2.709 — supercritical, jump will form.

Sequent depth: y₂ = (0.5/2) × [√(1 + 8 × 2.709²) − 1] = 0.25 × [√59.77 − 1] = 0.25 × 6.732 = 1.683 m

Energy loss: E₁ = 0.5 + 36/19.62 = 2.335 m; V₂ = (0.5 × 6)/1.683 = 1.783 m/s; E₂ = 1.683 + 3.178/19.62 = 1.845 m; ΔE = 0.490 m (21% dissipated).

Use the formula below to calculate the sequent depth ratio.

Use the formula below to calculate the Froude number.

Use the formula below to calculate energy loss across the jump.

Use the formula below to calculate jump length.

Use the formula below to calculate volumetric flow rate using the continuity equation.

Theory & Practical Applications

The hydraulic jump represents one of the most dramatic and energetically significant phenomena in open channel hydraulics. When supercritical flow — characterized by high velocity and shallow depth (Froude number Fr > 1) — encounters a downstream boundary condition that forces subcritical flow (Fr < 1), the transition occurs through a stationary shock wave analogous to compressible gas dynamics. This abrupt change creates a highly turbulent roller region where kinetic energy converts to potential energy and heat, making hydraulic jumps invaluable for energy dissipation in hydraulic structures.

Momentum Analysis and the Bélanger Equation

The fundamental relationship governing sequent depths derives from the momentum equation applied across the jump. Assuming a horizontal channel bed, negligible boundary friction over the short jump length, and hydrostatic pressure distributions at sections 1 and 2, the specific force (momentum function) remains constant. For a rectangular channel, this yields:

M₁ = M₂ → (ρgA₁²/2Q²) + Q²/(gA₁) = (ρgA₂²/2Q²) + Q²/(gA₂)

Simplifying using the Froude number definition and continuity, this reduces to the Bélanger equation relating sequent depths. The solution demonstrates that for any upstream Froude number greater than unity, a unique downstream depth exists that satisfies momentum conservation. This depth ratio increases nonlinearly with Fr₁ — at Fr₁ = 2.0, the depth ratio is approximately 2.3, while at Fr₁ = 10.0, it reaches about 13.0. This extreme depth change within a few meters creates the violent surface roller characteristic of strong jumps.

Energy Dissipation Characteristics

Unlike gradual flow transitions where energy loss is minimal, the hydraulic jump dissipates substantial energy through turbulent mixing. The energy loss equation reveals that efficiency drops dramatically as Fr₁ increases. For a weak jump at Fr₁ = 1.7, only about 5% of specific energy is lost. At Fr₁ = 5.0 (steady jump range), approximately 45% dissipates. Strong jumps with Fr₁ > 9.0 can destroy over 70% of the incoming energy — making them extraordinarily effective energy dissipators but also potential sources of cavitation damage and structural vibration if not properly designed.

The energy loss occurs primarily through three mechanisms: large-scale vortex formation in the roller, small-scale turbulent eddies cascading to molecular viscosity scales, and air entrainment creating bubbly two-phase flow. The surface roller acts as a powerful mixing zone, with surface velocities near zero while bottom velocities remain substantial. High-speed photography reveals coherent vortex structures periodically shed downstream, creating pressure fluctuations that can fatigue concrete if stilling basins aren't properly reinforced.

Jump Classification and Flow Regimes

Hydraulic engineers classify jumps into five distinct types based on upstream Froude number, each exhibiting unique surface characteristics and requiring different design considerations:

Undular Jump (1.0 < Fr₁ < 1.7): Surface forms standing waves without a pronounced roller. Energy loss minimal (less than 5%). Commonly observed downstream of broad-crested weirs. Design challenge involves preventing wave propagation into downstream channels.

Weak Jump (1.7 < Fr₁ < 2.5): Small rollers develop with smooth downstream surface. Energy loss 5-15%. Used in small irrigation structures. Risk of incomplete jump formation if tail water depth fluctuates.

Oscillating Jump (2.5 < Fr₁ < 4.5): Jet entering at bottom oscillates randomly between channel bed and surface. Energy loss 15-45%. Most problematic for design due to irregular pressure pulsations causing structural vibration. Avoid this range when possible by adjusting upstream conditions or tail water depth.

Steady Jump (4.5 < Fr₁ < 9.0): Well-balanced, stable jump with clearly defined roller. Energy loss 45-70%. Ideal range for stilling basin design. Roller length predictable, allowing optimized basin geometry. Most common target range for spillway design.

Strong Jump (Fr₁ > 9.0): Rough, vigorous surface action with significant spray and wave formation. Energy loss exceeds 70%. Requires additional protective measures like baffle blocks and end sills to stabilize jump position and prevent downstream scour.

Stilling Basin Design Applications

The primary engineering application of hydraulic jumps is energy dissipation downstream of hydraulic structures — spillways, sluice gates, chutes, and drop structures — where high-velocity flow must transition to subcritical conditions without causing erosion. The USBR (U.S. Bureau of Reclamation) developed standardized stilling basin types optimized for different Froude number ranges:

Type I Basin: Simple rectangular basin for Fr₁ < 1.7. No appurtenances required. Length approximately 4 times downstream depth.

Type II Basin: For Fr₁ > 4.5. Incorporates chute blocks at entrance, baffle blocks in middle, and dentated end sill. Reduces required length by 30-40% compared to classical jump. Chute blocks split incoming jet into smaller streams, enhancing mixing. Baffle blocks break up roller into smaller eddies. End sill lifts tail water to help stabilize jump location.

Type III Basin: Designed for Fr₁ = 4.5 to 17 where traditional basins would be excessively long. Uses closely spaced baffle blocks and sloping apron. Compact but requires careful maintenance as blocks subject to cavitation damage.

Critical design parameter is ensuring tail water depth matches sequent depth y₂. If tail water is too shallow, the jump sweeps downstream beyond the basin — defeated protection. If too deep, the jump drowns and moves upstream onto the spillway face — potentially causing cavitation damage. Tail water rating curves must account for flow variation, downstream channel geometry changes, and seasonal water level fluctuations.

Advanced Considerations: Conjugate Depths and Jump Location

The sequent depths y₁ and y₂ are termed "conjugate depths" — depths before and after a hydraulic jump at the same specific force. However, the jump does not form automatically wherever these depths theoretically occur. Jump location is controlled by the intersection of the upstream supercritical profile with the downstream subcritical profile as controlled by tail water conditions. Analyzing jump position requires solving the gradually varied flow equation upstream and downstream, then identifying where the momentum condition is satisfied.

In design practice, engineers often must induce jump formation at a specific location by installing sills, blocks, or abrupt channel expansions. These devices create localized depressions in the specific force function, "anchoring" the jump. Without such controls, jumps can oscillate in position under varying flow conditions, rendering protection ineffective. Modern computational fluid dynamics (CFD) allows three-dimensional modeling of jump behavior, revealing secondary currents and sidewall effects that one-dimensional theory cannot capture.

Worked Example: Spillway Stilling Basin Design

A concrete gravity dam spillway discharges into a rectangular stilling basin 12.0 meters wide. At design flow, the chute delivers water at depth y₁ = 0.85 m with velocity V₁ = 8.5 m/s. The downstream river channel requires subcritical flow at depth 3.2 m or greater to prevent bed scour. Determine whether a hydraulic jump will form, calculate jump characteristics, and assess energy dissipation.

Step 1: Calculate upstream Froude number

Fr₁ = V₁ / √(g·y₁) = 8.5 / √(9.81 × 0.85) = 8.5 / 2.889 = 2.942

Since Fr₁ > 1, flow is supercritical and a jump can potentially form.

Step 2: Calculate required sequent depth using Bélanger equation

y₂ = (y₁/2) × [√(1 + 8Fr₁²) - 1]

y₂ = (0.85/2) × [√(1 + 8(2.942)²) - 1]

y₂ = 0.425 × [√(1 + 69.24) - 1]

y₂ = 0.425 × [√70.24 - 1]

y₂ = 0.425 × [8.381 - 1] = 0.425 × 7.381 = 3.137 m

Step 3: Compare with available tail water depth

Available tail water = 3.2 m > y₂ = 3.137 m

Tail water depth exceeds required sequent depth by 63 mm (2% excess). Jump will form but will be slightly submerged, moving slightly upstream from the theoretical location. This is acceptable — typical design tolerance is ±5%. If excess exceeded 10%, consideration should be given to adjusting apron elevation or installing a tailwater control structure.

Step 4: Calculate downstream flow conditions

Using continuity: V₁·y₁ = V₂·y₂

V₂ = V₁·y₁/y₂ = 8.5 × 0.85 / 3.137 = 2.303 m/s

Fr₂ = V₂ / √(g·y₂) = 2.303 / √(9.81 × 3.137) = 2.303 / 5.548 = 0.415

Downstream flow is subcritical (Fr₂ < 1), confirming successful transition.

Step 5: Calculate energy loss

Upstream specific energy: E₁ = y₁ + V₁²/(2g) = 0.85 + (8.5)²/(2×9.81) = 0.85 + 3.682 = 4.532 m

Downstream specific energy: E₂ = y₂ + V₂²/(2g) = 3.137 + (2.303)²/(2×9.81) = 3.137 + 0.270 = 3.407 m

Energy loss: ΔE = E₁ - E₂ = 4.532 - 3.407 = 1.125 m

Relative loss: ΔE/E₁ = 1.125/4.532 = 0.248 or 24.8%

Step 6: Calculate power dissipation

Discharge: Q = V₁·b·y₁ = 8.5 × 12.0 × 0.85 = 86.7 m³/s

Power dissipated: P = ρ·g·Q·ΔE = 1000 × 9.81 × 86.7 × 1.125 = 956,000 W ≈ 956 kW

This substantial energy dissipation (nearly 1 megawatt) would, without the hydraulic jump, manifest as erosive scour downstream. The turbulent roller converts this kinetic energy to heat and small-scale turbulence that dissipates harmlessly within the protected basin.

Step 7: Estimate jump length and classify type

Jump length: Lⱼ ≈ 6.9 × (y₂ - y₁) = 6.9 × (3.137 - 0.85) = 6.9 × 2.287 = 15.8 m

Based on Fr₁ = 2.942, this is classified as an oscillating jump (2.5 < Fr₁ < 4.5). Design recommendations: Use USBR Type II basin with chute blocks and baffle blocks to stabilize the jump and reduce length to approximately 11 m (30% reduction). Monitor for vibration during operation and consider installing pressure transducers to detect unsteady pulsations that could indicate resonance with natural frequencies of the structure.

This complete analysis demonstrates that the proposed configuration will successfully dissipate energy through a hydraulic jump, but the oscillating nature of the jump at Fr₁ = 2.942 suggests that appurtenances should be installed to enhance stability. An alternative might be to increase upstream depth slightly (if spillway geometry permits) to reduce Fr₁ below 2.5, or ensure adequate freeboard in the basin to accommodate irregular surface waves characteristic of oscillating jumps.

Industry-Specific Applications

Hydroelectric Power: Spillway stilling basins at projects like Grand Coulee Dam (Fr₁ = 5-8) and Hoover Dam use hydraulic jumps to dissipate energy equivalent to several hundred megawatts during flood releases. Modern designs incorporate slotted buckets and pre-aeration to prevent cavitation damage from vapor bubble collapse.

Irrigation Networks: Drop structures in canal systems use weak jumps (Fr₁ = 2-3) to maintain grade while preventing erosion. Compact designs save land and excavation costs. Critical challenge: ensuring jump stability across 50-200% flow variation typical of irrigation delivery schedules.

Wastewater Treatment: Hydraulic jumps in flumes and channels enhance mixing for chemical dosing and aeration. Air entrainment through the roller can increase dissolved oxygen by 20-40%, reducing downstream aeration requirements. Design consideration: avoiding excessive turbulence that could damage biological flocs.

Stormwater Management: Energy dissipation structures at culvert outlets use jumps to reduce flow velocity before discharge to natural channels. Urban development increases peak flows, requiring retrofitted basins. Space constraints often necessitate Type III compact basins despite higher maintenance needs.

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