1. Estuarine Circulation and the Knudsen Relation Puget Sound Oceanography 2011

2. Salinity Sections over a wide range of scales

3. Schematic of Tidally-Averaged salinity and velocity structure

4. A simple “story” for why the exchange flow exists The along- channel salinity (and hence density) gradient is a key factor

5. Thinking more about the physics of the exchange flow The along- channel pressure gradient forces the flow Remember, these are all for tidally- averaged fields

6. Turbulent mixing makes smooth curves out of the 2-layer exchange flow The turbulence is driven by the tidal currents, especially as they rub on the bottom boundary

7. Tidally-averaged velocities Inflow happens during Neap tides because there is less turbulent mixing (which slows down the flow) Admiralty Inlet

8. The Knudsen Relation For many estuaries (like Puget Sound) ds<<S 1 and ds<<S 2 (like 1<<30) Hence Q 1 and Q 2 are VASTLY AMPLIFIED over Q R !!!!!! These relationships are only “diagnostic” – no info about physics If you know the average stratification then you can estimate the exchange flow QRQR Q 1, S 1 = S 2 - dS Q 2, S 2 Volume conservation: Q 1 = Q 2 + Q R Salt conservation: 0 = S 2 Q 2 – S 1 Q 1 Combining these two it is easy to show that: Q 2 = (S 1 /ds)Q R and Q 1 = (S 2 /ds)Q R