Wave properties and water movements in Lough Neagh, N. Ireland

Abstract

The wave characteristics and water movements in a large (383 km2) well-mixed eutrophic lake, Lough Neagh in N. Ireland, are related to wind speed, lake depth and fetch. The lake is relatively shallow for its size with a mean depth of 8.9 m and it is isothermal for most of the year. Wind stress on the water surface is the main determinant of water movements so the wind conditions are analysed. The prevailing wind is from the SW and during 1981 nearly half the year had winds at or above Force 4 (S.S-7.9 m s-1 ). As wind direction and speed are very variable and it can take up to 5 hours for steady-state waves to develop, it is likely that the lake is most often in a non-steady state. Wave properties are dependent on fetch as well as wind stress and due to the approximately rectangular shape of the lake effective fetches are nearly the same as the straight line distances for many areas of the lake. Fetches of over 20 km can produce waves of over 2 m height and 30 m wavelength. The area of bottom influence by wave action at different wind speeds is assessed. Also the energy spectra, significant wave heights, dominant wave periods and orbital velocities of waves are estimated. Water movements were measured using both a current meter and drogues. The direction and distances travelled by the drogues was measured by attached lines and by radiotracking. Deflections of near surface drift currents were to the right of the wind but the return gradient currents were to the left. These deflections decreased as wind speed increased. A linear relationship between wind speed and near surface current speed was found up to S m s-1 but a poor correlation (r = 0.51) was due to the effects of wave action and currents generated by seiche oscillations. The type of water circulation varies with wind direction, with a ·conveyer belt' for winds blowing along the long axes, and a 'Livingstone· circulation pattern when blowing across the short axes (E-W)

Date of Award	1986
Original language	English