Wind setup and coastal surge

Wind setup, also known as wind stress surge, is the elevation of the mean sea level along a coastline caused by the drag of wind blowing over the surface of the water. When wind blows consistently from the sea towards the shore, it exerts a frictional force on the water, effectively pushing it against the coast. This causes water to accumulate, leading to a temporary rise in sea level.

The magnitude of wind setup depends on several factors:

• Wind speed and duration: Stronger and more prolonged onshore winds generate greater setup.
• Fetch: The distance over which the wind blows across open water towards the shore. A longer fetch allows more energy transfer from wind to water, increasing setup.
• Water depth: Shallower water generally experiences greater setup for a given wind stress, as the volume of water available to be moved is less.
• Coastal geometry: Bays and estuaries can funnel water, amplifying the setup effect.

Typical wind setup values in Ireland can range from a few centimetres in moderate winds to over a metre during severe storms with sustained onshore gales. For example, a sustained 30-knot (approximately 55 km/h) onshore wind over a fetch of 100 km in a shallow bay could contribute 0.5 to 1.0 metre of setup. This effect is distinct from wave height; it is a bulk elevation of the water surface, upon which waves then propagate.

In addition to wind stress, atmospheric pressure also influences sea level. This effect is known as the inverse barometer effect. For every hectopascal (hPa) drop in atmospheric pressure below the average sea level pressure (approximately 1013 hPa), the sea level rises by approximately 1 centimetre. Conversely, higher pressure depresses the sea level.

This is a hydrostatic response: the ocean surface adjusts to balance the pressure exerted by the atmosphere above it. A deep low-pressure system, characteristic of an intense Atlantic depression, can easily have a central pressure of 960 hPa or lower. The pressure anomaly would be 1013 hPa - 960 hPa = 53 hPa.

Applying the inverse barometer effect, this would result in a sea level rise of 53 cm or 0.53 metres. This effect is independent of wind and occurs even in the absence of wind setup. It is a significant component of what is commonly termed 'storm surge', especially in the centre of intense low-pressure systems.

For coastal operations, understanding the pressure tendency is crucial. A rapidly falling barometer indicates the approach of a low-pressure system, which will contribute to elevated sea levels, even before strong winds develop. The 'pressure_tendency' chart on The Wind Agent can provide insight into these changes.

The most severe coastal flooding events often occur when storm surge (the combination of wind setup and inverse barometer effect) coincides with high astronomical tide. Astronomical tides are predictable and are caused by the gravitational pull of the Moon and Sun. They vary in height throughout the lunar cycle, with spring tides (around new and full moons) being higher and neap tides (around quarter moons) being lower.

When a significant storm surge occurs during a spring high tide, the combined water level can exceed normal flood defences and inundate coastal areas. For example, if a storm surge of 1.5 metres coincides with a spring high tide of 4.0 metres above chart datum, the total water level would be 5.5 metres. If the local flood defence level is 5.0 metres, this would result in significant overtopping.

Coastal communities and marine operators must monitor both meteorological forecasts for surge potential and astronomical tide tables. The timing of a storm's arrival relative to the tidal cycle is a critical factor in determining the potential impact. A storm arriving at low tide might cause minimal issues, whereas the same storm at high tide could be catastrophic. This is why official marine forecasts and warnings always include information on predicted high tide times and any expected surge component.

The topography of the seabed and the shape of the coastline play a crucial role in how wind setup manifests. Shallow, gradually sloping seabeds and enclosed or semi-enclosed bays are particularly susceptible to amplified wind setup. As onshore winds push water into shallower areas, the decreasing depth restricts the water's ability to flow back out or disperse, causing it to pile up more significantly.

Consider a scenario where a strong onshore wind blows into a wide, shallow bay. The water depth might decrease from 20 metres at the mouth to 2 metres near the shore over a distance of several kilometres. This bathymetry acts like a ramp, forcing the wind-driven water upwards. The effect is further enhanced if the bay narrows towards its head, creating a funnel effect that concentrates the water mass.

Ireland's indented coastline, with numerous bays, estuaries, and loughs, means many coastal areas are vulnerable to this amplification. Places like Galway Bay, Dublin Bay, and various estuaries along the south and west coasts can experience locally enhanced setup during sustained onshore gales. The specific characteristics of each location dictate the precise amplification factor, which is why localised flood risk assessments are essential.

Once the mean sea level has been elevated by storm surge, the risk of wave overtopping of coastal defences or low-lying land increases significantly. Waves that would normally break offshore or dissipate their energy on a beach can now reach higher up the shore or impact structures that are typically above the waterline.

Wave overtopping is not simply about the height of the waves, but also their period and direction relative to the coastline. Longer period waves carry more energy and can travel further inland. When combined with an elevated water level, even moderate waves can cause substantial damage and pose a risk to life.

For example, if a storm surge raises the water level by 1.0 metre, a sea wall designed to withstand waves up to 2.0 metres above normal high tide might now be exposed to the crests of 3.0 metre waves. The volume of water overtopping the defence can be substantial, leading to erosion, structural damage, and flooding of inland areas. This is a primary concern for coastal infrastructure, port operations, and marine construction during storm events.

Understanding the combined effect of surge and waves requires detailed modelling, which is typically provided by national meteorological and oceanographic agencies in their storm warnings. The Wind Agent's 'sea_state' chart can provide wave height and period, but this must be considered in conjunction with the total water level.

To effectively assess coastal conditions, it is necessary to integrate information from various sources: wind forecasts, atmospheric pressure, astronomical tide predictions, and wave forecasts. The Wind Agent provides detailed wind and wave information, which forms a critical part of this assessment.

When reviewing the 'sea_state' chart, pay attention to the significant wave height (Hs) and the peak wave period (Tp). Hs is the average height of the highest one-third of waves, and Tp indicates the dominant wave period. Longer periods generally correspond to more powerful waves. The wind speed and direction shown on the meteogram will indicate the potential for local wind-sea generation, while swell components are also important, often originating from distant storms.

Crucially, these wave parameters must be considered in the context of the predicted total water level, which is the sum of the astronomical tide and any forecast storm surge. For instance, a 3-metre significant wave height during a normal low tide might be manageable, but the same wave height on top of a 1.5-metre storm surge and a spring high tide could be extremely hazardous. Always use official tide tables for your specific location and consult national meteorological services for storm surge forecasts. The Wind Agent is an instrument for wind and wave conditions, not a water level predictor.

In Ireland, official storm surge warnings and coastal flood advisories are issued by Met Éireann, often in conjunction with the Office of Public Works (OPW) and local authorities. These warnings integrate meteorological models, oceanographic models, and local knowledge to provide the most accurate assessment of potential impacts.

Key sources for surge information include:

• Met Éireann: Provides weather warnings, including marine warnings and specific coastal flood alerts when surge is anticipated. Their website and app are primary sources.
• Office of Public Works (OPW): Responsible for flood risk management in Ireland, the OPW provides information and guidance on flood defence and preparedness.
• Local Authorities: Councils in coastal areas issue localised warnings and advice, often based on national guidance but tailored to specific community vulnerabilities.
• Marine Institute: Conducts oceanographic research and monitoring, contributing to the understanding and forecasting of sea levels and waves around Ireland.

These agencies use sophisticated numerical models that account for complex interactions between atmospheric pressure, wind stress, bathymetry, and astronomical tides. Relying on these official sources is paramount for safety and operational planning in coastal environments. The Wind Agent provides detailed wind and wave forecasts, which are essential inputs for understanding the meteorological conditions contributing to surge, but it is not a direct storm surge forecasting tool.