Question 1

A tidal stream atlas shows rates at springs (3.0kt) and neaps (1.5kt). The actual range today is midway between spring and neap ranges. What tidal stream rate would you use?

Accepted Answer

2.25kt, interpolating proportionally between spring and neap rates using today's actual tidal range.. Estimate the tidal stream rate by interpolating between the spring and neap rates using today's actual tidal range. At the midpoint: rate = neap rate + 0.5 × (spring rate − neap rate) = 1.5 + 0.5 × (3.0 − 1.5) = 2.25kt. Tidal stream atlases show stream directions with arrows and give spring and neap rates in the format specified by their legend. On charts, tidal diamonds identify positions for which stream directions and rates are given in an accompanying table.

Question 2

The tidal stream is said to 'turn' or 'slack' at a certain time. What does this mean?

Accepted Answer

The tidal stream reduces to zero speed and then reverses direction, not necessarily at the same time as HW or LW.. SLACK WATER: the tidal stream slackens to near-zero speed and then reverses. IMPORTANT: slack water does NOT necessarily coincide with HW or LW at that location. In some areas, slack water occurs 1-3 hours before or after HW/LW. Portland Race, for example, has slack water about 1 hour after HW Dover. Always consult local tidal stream data, do not assume slack = HW/LW.

Question 3

What causes a tidal race to form at a headland?

Accepted Answer

Tidal streams accelerate around headlands over shallowing seabeds, meeting opposing eddies and disturbed water to produce the steep, confused seas of a tidal race.. Tidal races can form where tidal flow is concentrated around a headland, particularly over a shallow or uneven seabed. Acceleration through a reduced flow cross-section is explained by continuity: for a given volume flow rate, a smaller cross-section requires a greater mean speed. Strong currents, eddies and seabed irregularities can produce steep, confused or breaking seas, even in calm weather. Wind opposing the stream can worsen conditions. Stream rates vary with location and tidal conditions; consult tidal stream atlases and pilot books for local rates and recommended headland timing.

Tidal Stream, questions and explanations

A tidal stream atlas shows rates at springs (3.0kt) and neaps (1.5kt). The actual range today is midway between spring and neap ranges. What tidal stream rate would you use?

The tidal stream is said to 'turn' or 'slack' at a certain time. What does this mean?

What causes a tidal race to form at a headland?

Practise this properly

Tidal Stream, questions and explanations

Questions

A tidal stream atlas shows rates at springs (3.0kt) and neaps (1.5kt). The actual range today is midway between spring and neap ranges. What tidal stream rate would you use?

The tidal stream is said to 'turn' or 'slack' at a certain time. What does this mean?

What causes a tidal race to form at a headland?

Practise this properly