You guys are talking crap and please let me explain why.
I am a Dutch Engineer, I am born and raised in the Netherlands.
As you may or may not know the Netherlands is a small country in Europe below sea level.
Keeping water out is one of the biggest national concerns in the Netherlands and therefore, we have a lot of engineers working on that.
We have a lot of rivers, artificial lakes, canals, bridges, pump installations, tunnels, ship locks, sluices and one of the best sewage systems of the world.
All these systems need engineering, maintenance, renovation etc.
To keep water out we have dikes all along the coast line.
Rivers which are naturally around sea level and go through our country also have dikes on both sides.
Bur dikes alone will not help.
The ground water level needs to be monitored and controlled all the time, or else the rainwater alone will drown the whole country. therefore we have underground pumpsystem in almost every street to pump out ground water in artificial canals. most of the rainwater goes via the sewege system to waste water plants, but if there is too much rainfall, the sewege system will flood, and everything will drain directly in the artificial canals, which is not harmfull because the sewege system is flushed relatively clean by the time it floods.
the canals end up in the rivers or directly in the sea.
the rivers are usually around sea level, but the canals are not. to allow ships pass freely through canals and rivers we need a lot of shipslocks.
The siplocks at the coastline to allow ships out to sea serve a special purpose:
to keep the inland free from salt.
We have a lot of agriculture in west-Netherlands who are depending on fresh water and keeping their lands free from salt. the tulip-fields for example.
everytime a ship passes through these locks, tons of seawater comes through these gates and turn our canals into brackish water which is harmfull for the agriculture and eco-system.
These locks have different methods to keep salt water out.
one of them is the air-bubble screen. this works pretty well, but has a few disadvantages. It goes too deep to discuss this here.
One other classic method is to provide these locks with a saltwatercellar. these cellars ar not actually cellars, but are just a part of the locks which are dugged deeper (like pits) on the saltwaterside with a pumpsystem at the bottom. as saltwater rushes in it goes straight to these saltwatercellars, because saltwater is heavier as freshwater and doesn't mix directly in the first contact.
with each "schutcyclus" (a cycle to go though all the steps to get a ship from one side to the other) the water in these cellars are pumped out back to the sea to keep the inland free from salt.
This works pretty well and is used for centuries in the Netherlands.
So go ahead and argue which each other as much as you want, there are a lot of people out there who know how it really works.
Kind regards,
Napoleon