5.4 Tidal Overfalls, Races, Wind-Over-Tide & Shallow Water Effects

5.4 Tidal Overfalls, Races, Wind-Over-Tide & Shallow Water Effects

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Coastal Hydrodynamics: Headlands & Tidal Races

The fundamental law of fluid dynamics is the continuity equation:

$Q = A \times V = \text{Constant}$

Where $Q$ is volumetric flow rate, $A$ is channel cross-sectional area, and $V$ is flow velocity. When an oceanic tidal stream sweeps past a projecting headland or constricted strait, cross-sectional area $A$ contracts. To maintain flow, velocity $V$ surges dramatically.

This generates Tidal Races—strips of accelerated current running far faster than surrounding waters:

• Portland Race (Dorset): Fast Channel streams pass Portland Bill and the shallow Portland Ledge, creating hazardous races and overfalls.
• Alderney Race (Raz Blanchard): Stream rates exceed 9 to 10 knots between Alderney and Cap de la Hague.
• Falls of Lora & Corryvreckan (Scotland): Severe bathymetric narrowing and pinnacles create standing rapids and whirlpools.

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Tidal Overfalls, Tide Rips & Seabed Topography

While a race describes rapid horizontal flow, Overfalls and Tide Rips describe violent vertical turbulence formed when fast streams strike sudden changes in seabed depth.

When a 4-knot stream flowing over a 60-metre seabed strikes an underwater rock ledge or sandbank rising to 10 metres depth:

1. The deep, laminar water column is deflected violently upward toward the surface.
2. Upwelling currents collide with surface water, generating boils, eddies, and whirlpools.
3. The surface forms standing waves—stationary waves that surge, peak, and break in place over the obstacle.
4. Overfalls occur even in a flat calm with zero wind, purely from current colliding with seabed topography.

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The Physics of 'Wind-Over-Tide'

Wind imparts kinetic energy to the sea, generating waves whose propagation speed and wavelength depend on wind strength and fetch. Because the sea moves with the tidal stream, the relative interaction determines wave shape:

1. Wind WITH Tide (Stream & Wind in Same Direction)

• Relative Shear: Wind speed relative to the moving water decreases ($V_{\text{rel}} = V_{\text{wind}} - V_{\text{stream}}$).
• Wave Profile: The stream stretches wave crests forward, lengthening wavelength ($L$). Wave steepness (height-to-length ratio $H/L$) remains low.
• Sea State: Generally less steep than comparable waves meeting an opposing current. Strong winds, existing swell, limited depth and local topography can still produce rough or breaking seas.

2. Wind OVER Tide (Stream Directly Opposes Wind)

• Relative Shear: Wind shear on the water increases ($V_{\text{rel}} = V_{\text{wind}} + V_{\text{stream}}$).
• Wave Compression: Opposing current retards wave ground speed. Wave energy is compressed into a shortened wavelength ($L$), causing wave height ($H$) to rise sharply.
• Breaking Risk: An opposing current can make waves steeper and cause breaking. Shallow water also slows and steepens waves; do not use a single steepness ratio as a coastal safety threshold.
• Sea State: Short, vertical, 'pyramidal' seas with breaking crests, creating severe risks of broaching, knockdowns, and green water over the deck.

Sea State Comparison Matrix

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Dynamic Under-Keel Clearance (UKC) Formulation

Navigating over shallow bars or sills requires computing dynamic Under-Keel Clearance (UKC):

$\text{Dynamic UKC} = \text{Charted Depth} + \text{Height of Tide} - (\text{Static Draught} + \text{Squat Allowance} + \text{Wave Allowance})$ $\text{Surplus Above Required UKC} = \text{Dynamic UKC} - \text{Required UKC}$

Factors Explained

1. Charted Depth (CD): Sounding below Chart Datum (negative for drying banks, e.g., $-1.4\text{m}$).
2. Height of Tide (HoT): Instantaneous water level above Chart Datum from the tidal curve.
3. Static Draught: Vessel draught when stationary in calm water.
4. Hydrodynamic Vessel Squat: As a vessel enters shallow, restricted water, displaced water accelerates beneath the hull. By Bernoulli's principle, accelerated flow drops hydrodynamic pressure, reducing the upward pressure on the hull so that it settles deeper and its effective draught increases. Squat is often approximated as increasing with speed squared ($V^2$), but its magnitude depends on the hull and waterway. Under that approximation, reducing speed from7 to3kn reduces squat by about82%; it does not eliminate it.
5. Wave Motion (Pitch, Roll, Heave): In swell, keel and rudder dip significantly deeper in wave troughs.
6. Required UKC: The minimum clearance selected for the vessel, bottom and conditions, against which the calculated dynamic clearance is compared.

Comprehensive UKC Calculation Scenario

Use the following supplied exercise allowances; they are not universal yacht squat or wave-response values.

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Tactical Seamanship: Inshore Route vs. Offshore Berth

When rounding prominent headlands like Portland Bill, skippers evaluate two tactical options:

1. The Inshore Route ('Inside Passage'): A narrow route may exist between shore dangers and the race. Use current local sailing directions and stream information to choose the timing and clearance for the intended direction. Attempt it only in suitable weather and visibility with precise pilotage and a plan for loss of propulsion. A generic distance from shore or the time of local high water does not establish safety.
2. The Offshore Route ('Outside Passage'): Plan outside the race and all charted shoals, allowing for the forecast, tide and position uncertainty. Race extent and sea state vary, so a fixed 3–5-mile offset is not a clearance guarantee. If neither route has acceptable conditions, delay or divert.