Wind setup explained

Wind setup, also known as wind effect or storm effect, refers to the rise in water level in seas or lakes caused by winds pushing the water in a specific direction. As the wind moves across the water's surface, it applies a shear stress to the water, prompting the formation of a wind-driven current. When this current encounters a shoreline, the water level along the shore increases, generating a hydrostatic counterforce in equilibrium with the shear force.[1] [2]

During a storm, wind setup is a component of the overall storm surge. For instance, in the Netherlands, the wind setup during a storm surge can elevate water levels by approximately 3 metres above the normal tide. In the case of cyclones, the wind setup can reach up to 5 metres. This can result in a significant rise in water levels, particularly when the water is forced into a shallow, funnel-shaped area.[3]

Observation

In lakes, water level fluctuations are typically attributed to wind setup. This effect is particularly noticeable in lakes with well-regulated water levels, where the wind setup can be clearly observed. By comparing this with the wind over the lake, the relationship between wind speed, water depth, and fetch length can be accurately determined. This is especially feasible in lakes where water depth remains fairly consistent, such as the IJsselmeer.

At sea, wind setup is usually not directly observable, as the observed water level is a combination of both the tide and the wind setup. To isolate the wind setup, the (calculated) astronomical tide must be subtracted from the observed water level. For example, during the North Sea flood of 1953 at the Vlissingen tidal station (see image), the highest water level along the Dutch coast was recorded at 2.79 metres, but this was not the location of the highest wind setup, which was observed at Scheveningen with a measurement of 3.52 metres.

Notably, the highest wind setup ever recorded in the Netherlands (3.63 metres) was in Dintelsas, Steenbergen in 1953, a location approximately 40 km from the sea along the Haringvliet estuary.

Calculation of wind setup

Based on the equilibrium between the shear stress due to the wind on the water and the hydrostatic back pressure, the following equation is used:[4]

dh=
dx
\kappau2\cos\phi
gh
in which:

h = water depth

x = distance

u= wind speed

\kappa=cw

\rhoair
\rhowater
, Ippen [4] suggests

\kappa

= 3.3*10−6

\phi

= angle of the wind relative to the coast

g = acceleration of gravity

cw has a value between 0.8*10−3 and 3.0*10−3

Application at open coasts

For an open coast, the equation becomes:

\Deltah=\sqrt{2\kappa

u2
g

F\cos\phi+h2}-h

in which

Δh = wind setup

F = fetch length, this is the distance the wind blows over the water

However, this formula is not always applicable, particularly when dealing with open coasts or varying water depths. In such cases, a more complex approach is needed, which involves solving the differential equation using a one- or two-dimensional grid. This method, combined with real-world data, is used in countries like the Netherlands to predict wind setup along the coast during potential storms.[5]

Application at (shallow) lakes

To calculate the wind setup in a lake, the following solution for the differential equation is used:

\Deltah=0.5\kappa

u2
gh

F\cos\phi

In 1966 the Delta Works Committee recommended using a value of 3.8*10−6 for

\kappa

under Dutch conditions. However, an analysis of measurement data from the IJsselmeer between 2002 and 2013 led to a more reliable value for

\kappa

, specifically

\kappa

= 2.2*10−6.[6]

This study also found that the formula underestimated wind setup at higher wind speeds. As a result, it has been suggested to increase the exponent of the wind speed from 2 to 3 and to further adjust

\kappa

to

\kappa

=1.7*10−7. This modified formula can predict the wind setup on the IJsselmeer with an accuracy of approximately 15 centimetres.

Note

Wind setup should not be mistaken for wave run-up, which refers to the height which a wave reaches on a slope, or wave setup which is the increase in water level caused by breaking waves.[7]

See also

Notes and References

  1. Smith . S.D. . 1988 . 26 June 2023 . Coefficients for sea surface wind stress, heat flux, and wind profiles as a function of wind speed and temperature . Journal of Geophysical Research: Oceans . 93 . C12 . 15467–15472 . 10.1029/JC093iC12p15467. 1988JGR....9315467S .
  2. Garvine . R.W. . 1985 . 26 June 2023 . A simple model of estuarine subtidal fluctuations forced by local and remote wind stress . Journal of Geophysical Research: Oceans . 90 . C6 . 11945–11948 . 10.1029/JC090iC06p11945. 1985JGR....9011945G .
  3. A model of the European Continental Shelf for wind setup and water quality calculations . Een model van het Europese Kontinentale Plat voor windopzet en waterkwaliteitsberekeningen . 26 June 2023 . Deltares (WL) . Verboom . G.K. . van Dijk . R.P. . deRonde . J.G. . Z0096 . 1 November 1987 . nl .
  4. Book: Ippen , Arthur T. . Estuary and coastline hydrodynamics. 1966. 245. McGraw Hill, New York.
  5. Walton . T.L. . Dean . R.G. . 2009 . Landward limit of wind setup on beaches . Ocean Engineering . en . 36 . 9-10 . 763–766 . 10.1016/j.oceaneng.2009.03.004 . 28 July 2024.
  6. Accuracy of formulas for wind setup based on measurement data from the IJsselmeer. Nauwkeurigheid van formules voor windopzet aan de hand van meetgegevens van het IJsselmeer . 26 June 2023 . 2015 . nl . 10.4121/uuid:4b0483fe-b258-4c1a-900f-8adb030bb42f . Feij . C.C.L . Verhagen . H.J.. TU Delft, department hydraulic engineering .
  7. Choi . B.H. . Kim . K.O. . Yuk . J.H. . Lee . H.S. . 2018 . Simulation of the 1953 storm surge in the North Sea . Ocean Dynamics . en . 68 . 12 . 1759–1777 . 10.1007/s10236-018-1223-z . 1616-7341 . 28 July 2024.