# Tidal flow in an estuary

I am working on a project. Where this bacteria has been deposited at a port in the Humber Estuary. Where it has been deposited has been marked in red. I am trying to work out the limits of where this bacteria can get moved to. I know that the input of all freshwater by the rivers is 250 m3 per second so I could calculate the movement of the bacteria as it leaves the estuary. However, I want to work out if it can be moved inland by the tide. I have found some information that looks at tide velocities. However, the velocity seems pretty low compared to the amount of freshwater entering the estuary. The thing is the points of tidal incursion are marked on the map so I assume the tide reaches there, especially as the tide can increase the height of the river by metres in some places. I am struggling to know what information I need to calculate this because I am more a biologist by training.

Any help would be appreciated!

Thanks

James

• I would start by looking at the sediment budget and the tidal flows in the estuary. Maybe you can start with sciencedirect.com/science/article/pii/S0048969703000822 Dec 20, 2019 at 15:12
• From personal experience even though you could potentially get the inflow rates, consider that there is a net difference in the flow rates as you move vertically through the water column. The heavier salt water can be flowing upstream during a flood tide while the lighter fresh water flows downstream over the saltwater. I have observed this in smaller volumes but with flows up to 1 m/s.
– user824
Dec 20, 2019 at 18:35
• first off salt water is denser than freshwater so it often comes in under outgoing freshwater. this is called salt wedging . but there is a whole slew of ways they can interact. en.wikipedia.org/wiki/Estuary#Drowned_river_valleys. If you want to work out how the bacteria are moving you are going to have to learn a lot of hydrology.
– John
Dec 21, 2019 at 19:56
• @John isn't it a gravity current phenomenon ? en.wikipedia.org/wiki/Gravity_current. Salt water being heavier and fresh water being lighter. Dec 23, 2019 at 3:54
• @gansub gravity current is a more general term for a wider range of phenomena including things like pyroclastic flows and avalanche as well, salt wedging is specifically about saltwater intruding into rivers.
– John
Dec 23, 2019 at 5:44

The tidal flow velocity is not the same thing as the mass movement. Think of the tidal flow as a pressure pulse that moves up the river. It moves much farther than the actual salt water movement. The extent of the brackish zone is much shorter. The relationship depends on the river flow and the shape of the estuary.

In your case, your source area is within the limit of saline intrusion so there is some mass movement upstream. Given that you are studying bacteria particles rather than dissolved salt, I would say that the limit of saline intrusion marked on your map is an upper limit for the distance it could move upstream. That is assuming that the bacteria haven't established a stable population that can reproduce and grow outward.