Puget Sound, Washington is an estuary with precisely this type of topographical control at its mouth. A region of sills exist at the northern opening to the estuary and are called the Admiralty Inlet Sills. The region is roughly 30 kilometers long and consists of a Seaward and Landward sill with a series of shoals and deeps between the two. This project was designed to study, in a controlled lab environment, how the flow rate of the intrusion and roughness in the sill's topography effect the amount of background estuarine water that is entrained, or actively mixed, into the intruding water mass. This process is important because deep water intrusions are thought to control the circulation and residence times in the Puget Sound estuary.
Marine water enters the system in discreet, deep water intrusions.
During intrusions, high density, high salinity marine water masses roll over the sill at depth and sink into the Main Basin. By nature, sill areas have higher current velocities, larger shears and more turbulence than other regions of an estuary. In contrast, the Main Basin waters are stratified, with the dense marine water at depth and fresher river runoff at the surface. As of yet, little is known about the transition zone where intruding marine water flows into the stratified Main Basin. Intrusions in Puget Sound have been found to be tidally controlled.