Mixing Modes of Fresh and Salt Water

Mixing Modes of Fresh and Salt Water
In the Estuarine Environment
By Susan Sobehrad
Fall 97

Summary:

Fresh and salt waters mix in all estuaries to some extent. Because of its higher density, salt water enters the estuary at the bottom of a river, and the fresh water spreads out at or near the surface of the sea. Therefore, salinity in the estuary tends to increase with depth and with distance away from the mouth of the river. Patterns of salinity are also affected by the Coriolis force--that is, the tendency of the earth's rotation to deflect movement. The Coriolis force causes fresh water in the northern hemisphere to move seaward along the right shore (looking out from inland), while water in the southern hemisphere moves seaward along the left shore. While these observations are generally true for estuaries, the specific way in which salt and fresh water mix can vary considerable from one estuary to the next. There are three basic types of mixing modes for the mingling of fresh and salt waters in the estuarine environment.

Salt Wedge Estuaries:

The river is strong, but the tide is weak. These conditions result in a highly stratified estuary, where the line between fresh and salt water layers is strongly marked. At the mouth of the estuary, the salt water wedge takes up almost the whole water depth As you move toward the head of the estuary, the wedge tapers and thins. The water on top of the wedge is quite fresh, while the water at the bottom is quite salty. This narrow zone where the salinity changes very sharpy is the halocline. Because there is little or no tidal movement, the salt water wedge is fairly stationary. The fresh water flows freely over the top of the wedge, and the resulting friction causes underwater waves called submarine waves to occur along the top of the saltwater wedge. When these submarine waves break, minimal mixing of fresh and salt water occurs in an upward process called entrainment.

Partially-Mixed Estuaries:

River flow and tidal flow are balanced. A great deal of turbulence results when the river and tidal flows are closely matched. In this situation, the whole water mass in the estuary moves backwards and forwards, and the friction between the water and the estuary bed causes general turbulence. The turbulence is more effective at mixing than entrainment because it not only mixes salt water upwards, it mixes fresh water downwards. Therefore, the halocline is much less defined than in salt wedge estuaries, and salinity tends to increase with depth and with distance from the estuary head. Even though tidal inflow is much stronger in this type of estuary, there is still water flowing out to sea at the top of the water mass. Due to the enhanced mixing, the outward flow has more salinity than in the salt wedge estuary, and vertical mixing also occurs as the high density, more saline water sinks due to gravitational forces.

Well-Mixed Estuaries:

When the river is weak, but the tide is strong. Strong tides mix waters along vertical boundaries, and the fresh water plunges through and mixes with the advancing salt water. The waters mix so well that the estuary becomes vertically homogeneous, and salinity is equalized from top to bottom. However, the Coriolis force deflects water along one shore, and salinity will therefore vary from side to side of the estuary. These estuaries are likely to be shallow, with a high tidal range. The shallow aspect of these kinds of estuaries facilitates the mixing of the fresh and salt waters. Salt water is trapped in embayments and in shallow areas, and slowly bleeds back into the main body of the flow later in the tidal cycle.

These models are highly simplified, and in any estuary all of these models may occur from time to time. Variations in rainfall and associated river flow many alter mixing patterns, as can topographic restrictions on tidal flow.

The models are helpful in understanding the fertility and variability that occurs in the estuary. The basic pattern of water described in these models is that water at the top of the estuary moves seaward, and water at the bottom moves inland. This pattern sets up a cycle which traps nutrients and enhances the fertility of the estuary. The river currents carry a supply of nutrients like phosphorus and nitrogen toward the sea. As the energy of river waters declines, the nutrients settle to the bottom of the estuary. Bottom waters flow toward land, carrying these nutrient-rich sediments with them, and food for the estuarine producers is assured. However, these same sediments can contribute to the filling in and destruction of the estuary.

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