Coleraine Harbour, Co. Londonderry: wind, direction and sea state for navigation

Coleraine Harbour is situated on the River Bann, approximately 5 nautical miles upstream from its mouth at Portstewart Bay. The river approach is exposed to the open sea, particularly from the north-west through to the north-east. The harbour itself offers shelter from many directions once inside, but the entrance and the river channel can be challenging under certain wind and tide conditions.

The River Bann flows northwards into the Atlantic. This means that an incoming tide flows south, and an outgoing tide flows north. When the wind direction is opposite to the tidal flow, particularly in the narrower sections of the river or at the harbour entrance, standing waves and turbulent conditions can develop. This phenomenon, known as wind against tide, can significantly affect vessel handling and increase the risk of grounding or collision.

Skippers commonly monitor wind speed and direction in relation to the tidal stream, especially for vessels with lower power-to-weight ratios or limited manoeuvrability. The Shear Glass instrument can provide height-matched wind data for the approach and within the harbour area, allowing for a more informed assessment of conditions.

The interaction of wind and tide is a primary concern for safe navigation into and out of Coleraine Harbour. The most challenging conditions typically arise when:

• Northerly winds (from 330° to 030°) coincide with an outgoing tide (flowing north). This creates a direct opposition, leading to steep, short waves and potentially breaking seas in the river mouth and channel.
• Southerly winds (from 150° to 210°) coincide with an incoming tide (flowing south). While less common for significant wave generation in the mouth, this can still create turbulent conditions further upriver, particularly around bends or shallower areas.

Typical thresholds for concern vary by vessel type and skipper experience, but guidance suggests that wind speeds exceeding Beaufort Force 4 (11–16 knots) against a strong tidal stream can make navigation difficult for smaller vessels. Specific operational limits are governed by commercial procedures.

Monitoring both the forecast wind direction and speed, alongside the predicted tidal stream, is essential. The Agreement Spine feature of The Wind Agent allows comparison of multiple model forecasts, providing a broader view of potential wind against tide scenarios.

The entrance to Coleraine Harbour, particularly around the pier heads, can experience localised gusting, especially with winds blowing across or directly onto the structures. These gusts can be exacerbated by:

• Turbulence from land features: Buildings and terrain adjacent to the river mouth can create eddies and sudden changes in wind speed and direction as air flows over them.
• Funnel effect: The narrowing of the river channel can sometimes accelerate wind flow, leading to higher speeds than might be observed in open water nearby.

Guidance for safe berthing and manoeuvring suggests that sustained wind speeds above 20 knots (approximately 37 km/h or 23 mph), or gust speeds exceeding 25 knots (approximately 46 km/h or 29 mph), can make precise vessel handling challenging, particularly for larger vessels or those with limited thruster capability. These figures are illustrative; your vessel's operational limits govern.

The Wind Agent's exceedance fan can show the probability of gusts exceeding a user-defined limit at a specified height, assisting in planning critical manoeuvres such as entering or leaving the harbour.

Once inside Coleraine Harbour, vessels generally find more shelter from the prevailing winds due to the surrounding landmass and structures. However, pockets of exposure can still exist, and wind direction remains a factor for safe berthing.

• Easterly and Westerly winds: These directions tend to be more sheltered by the river banks and urban development, leading to reduced wind speeds within the main harbour basin.
• Northerly winds: While the initial approach is exposed, the harbour's orientation and surrounding buildings offer some protection. However, gusts can still funnel down the river or over structures.
• Southerly winds: These are generally the most sheltered within the harbour, as the wind blows from over land and often has reduced fetch.

When berthing, consider the wind direction relative to the quay or pontoon. Wind blowing directly onto the berth can assist in coming alongside, but strong winds blowing off the berth can make departure difficult. Wind blowing parallel to the berth can create shear forces on the vessel. The Wind Agent's Shear Glass can provide height-matched wind data, relevant for mast height or bridge height, which can be critical for assessing wind loads on the vessel during berthing.

While Coleraine Harbour is well up the River Bann, strong northerly or north-westerly gales can generate significant swell and wave action that propagates some distance up the river channel. This is particularly relevant for the outer reaches of the river and the immediate approach to the harbour mouth.

• North Atlantic Swell: During periods of strong northerly or north-westerly gales in the North Atlantic, long-period swell can enter Portstewart Bay and the mouth of the Bann. Even if local winds are moderate, this swell can create challenging conditions for vessels with shallow drafts or those prone to pitching.
• Wind Sea: Locally generated wind waves (wind sea) will be present when strong winds blow over the river or bay. The fetch is limited, but strong winds against tide can quickly build a steep, choppy sea state.

Guidance for small craft suggests that significant wave heights exceeding 1.5 metres in the approach channel can be hazardous. For commercial vessels, specific stability criteria and operational limits apply. The sea state chart provides modelled significant wave height and period for offshore locations, which can indicate the potential for swell entering the river mouth. It is critical to correlate this with local observations.

For real-time operational decisions at Coleraine Harbour, access to current and forecast wind conditions is critical. The Wind Agent provides a detailed meteogram that displays wind speed, gust, and direction for the coming days, specific to the harbour's location.

Key elements to observe on the meteogram:

• Gust Spikes: Pay close attention to the highest gust values, especially when planning manoeuvres near the pier heads or in confined spaces.
• Direction Shifts: Rapid changes in wind direction can affect vessel handling, particularly when berthing or navigating narrow channels. The meteogram shows directional changes clearly.
• Wind Speed Trends: Identify periods of increasing or decreasing wind strength to anticipate changing conditions during your operation.

This live data, when combined with your vessel's characteristics and your operational limits, supports informed decision-making for safe passage and berthing. Always cross-reference with local observations and official navigational warnings.

Understanding the typical wind patterns at Coleraine Harbour throughout the year can aid in long-term planning and risk assessment. Ireland's climate is predominantly influenced by Atlantic weather systems, bringing a prevalence of westerly and south-westerly winds.

However, the local topography and coastal orientation mean that specific directions can be more frequent or impactful at this location. A wind rose generated from reanalysis data for the Coleraine area illustrates the historical distribution of wind directions and speeds.

• Prevailing Directions: Historically, the most frequent strong winds are from the westerly and south-westerly sectors, though these are often somewhat sheltered within the harbour itself. Northerly and north-easterly winds, while less frequent overall, are critical for the approach channel due to their exposure.
• Seasonal Variation: Winter months (October to March) typically experience higher wind speeds and more frequent gales due to deeper Atlantic depressions. Summer months (June to August) generally have lighter winds, though sea breezes can develop, often from a northerly direction along the coast.

This climatological perspective helps in understanding the typical challenges and opportunities presented by the wind environment at Coleraine Harbour over different seasons.