Unsteady Ambient Currents

When predictions are desired in an unsteady ambient flow field, information on the tidal cycle must be supplied. In general, estuaries or coastal waters can exhibit considerable complexity with variations in both velocity magnitude, direction and water depth. The image on the right shows a plume advected by a tidally influenced ambient flow. Minimum dilutions generally occur shortly after slack tide, which historic plume material remaining from the previous tidal cycle is re-entrained.

The example image on left shows the time history of tidal velocities and tidal height for a mean tidal cycle at some site in Long Island Sound. The tidal height varies between Mean Low Water (MLW) and Mean High water (MHW).

The tidal velocity changes its direction twice during the tidal cycle at times called slack tide. One of these times occurs near, but is not necessarily coincident with, the time MLW and is referred to as Low Water Slack (LWS). The slack period near MHW is referred to as High Water Slack (HWS).

Hence, CORMIX needs some information on the ambient design conditions relative to any of the two slack tides.

The rate reversal (time gradient of the tidal velocity) near these slack tides is of considerable importance for the concentration build-up in the transient discharge plume. Tidal reversals will reduce the effective dilution of a discharge by re-entraining the discharge plume remaining from the previous tidal cycle. This process is illustrated by the amimation from the laboratory study on the right.