Wind tides and surface friction coefficient in semi-enclosed shallow lagoons

Abstract

The present paper is specifically focused on enclosed or semi-enclosed basins where the wind is the dominant driver of water surface tilting, leading to the so-called wind tide contributing to water levels rise. Wind-induced free surface tilting is studied using the 1-D steady form of the depth-averaged shallow water (Saint-Venant) momentum equation which reflects the depth-averaged local balance between surface slope and wind stress. Two contrasted field sites, the Berre and Vaccarès lagoons, have been monitored providing water level data along a reference axis. This study highlighted the occurence of wind tides at the two field sites. The bimodal wind exposure ensured the robustness of the observations, with non-linear but symmetric behaviors patterns observed in winds from opposite directions. It is observed that the higher the wind speed, the steeper the slope of the free surface in accordance with the well known basic trend. In addition, a significant effect of depth is observed, with greater surface tilting in the shallower lagoon. The data analysis confirmed the robustness of such a simple approach in the present context. Using the additional assumption of constant, i.e. wind-independent, drag coefficients (C D ) allowed a good match with the observations for moderate wind speeds for both sites. However, the depth effect required the C D to be increased in the shallower basin. Classical empirical wind-dependent C D parameterizations provide better wind-tide predictions than the constant-C D approach in very strong wind conditions but totally failed in predicting surface tilting in the shallower site, suggesting that physical parameters other than wind speed should be taken into account for the C D parameterization in very shallow lagoons.

Multispecies fish tracking across newly created shallow and deep habitats in a forward-restored lake

“…Secondly, the sheltered 4 m water is a relatively stable environment with little influence of potential water level fluctuations due to changes in wind direction. Like in most large lakes, the long fetch lengths in lake Markermeer can cause water to accumulate downwind and thus decrease or increase water levels within a few hours [ 51 ]. This implies that fish residing in very shallow waters—whether juveniles or adults—have to trade-off rewards of going into risky shallow waters (e.g.…”

Hydro-Saline Dynamics of a Shallow Mediterranean Coastal Lagoon: Complementary Information from Short and Long Term Monitoring

“…Due to their shallow depth, Mediterranean coastal lagoons are generally strongly impacted by the wind in terms of water levels [23]. The duration and intensity of the variations of these levels can be more or less important, depending on the geomorphological characteristics of the lagoons considered, and on the local wind dynamics [26]. The time scales of these tilts generally range from several hours to a few days, requiring continuous monitoring ensured by the use of probes in different locations of the lagoon.…”

“…Preliminary modeling studies [45] suggest that an absence of regulation on the Vaccarès Lagoon System, including the permanent opening of the 13 sluice gates of the Fourcade, would lead to high water levels in the lagoons. In combination with the influence of wind [23,26], these high water levels would cause bank erosion and regular flooding of the surrounding areas. This work also suggests that it would lead to a significant and long-term increase in the dissolved salt mass and overall salinities.…”

“…Although the seashore of the Rhône Delta is subject to a micro-tidal regime (the difference between the lowest and highest value of astronomic tide is 0.42 m, see [25]), the "Vaccarès lagoon system" is not impacted by a tidal regime, [26], due to the small size of the sluice gates in comparison with the area of the Vaccarès Lagoon System, and to their management, with the opening of more than five sluices carried out to decrease water level and mass of salt into the lagoons (flows from the lagoons into the sea).…”

Morphodynamic Behaviour of a Mediterranean Intermittent Estuary with Opening Phases Primarily Dominated by Offshore Winds

“…The impact of wind is rarely considered when describing the multiple and complex mechanisms driving the morphodynamics of intermittent estuaries, even though wind can have a considerable influence on the internal lagoon morphology [42,43]. This is especially important in shallow basins where wind forcing can produce surges of significant amplitude [44][45][46][47][48]. These surges can be comparable in magnitude to the effects produced by river discharges and could thus potentially contribute to controlling an intermittent estuary if the surge is directed toward the barrier.…”