Floodwall useless?

[Nanny]

So I built canals and placed floodwalls alongside troublesome spots but they don't seem to keep the water out as the nearby areas are continually flooded.

[Bishop]

put some water drainage system (just normal water source for every city), it will help to suck some water in critical areas.

[Sonic]

Water does not accept the floodwalls as an obstacle. ONLY dams are accepted. In all other cases only a terrain difference are an obstacle for the water. You must use FIRST the terrain tool, then make your waterwalls and then correct the terrain - deep for river or lake, high for streets, meadows, fields etc.

[SilvermistInc]

Idk what your problem is. Flood walls work great for me.

[grapplehoeker]

Originally posted by Nanny:

So I built canals and placed floodwalls alongside troublesome spots but they don't seem to keep the water out as the nearby areas are continually flooded.

"troublesome spots"
That's an indication of the real problem.The canals aren't holding the capacity of water.
You haven't provided much information or even a screenshot.
In any case, floodwalls and quaywalls do contain water, as long as the capacity of the water doesn't cause the surface level to rise higher than the wall. Naturally, no 10m high wall is going to hold back an 11m or higher wall of water.
Here you can see clearly that floodwalls can contain water.
http://steamcommunity.com/sharedfiles/filedetails/?id=650491553
Here you can see a closed canal that doesn't quite contain the capacity.
http://steamcommunity.com/sharedfiles/filedetails/?id=654977355
But encircling the same canal with a quaywall is enough to hold it all in.
http://steamcommunity.com/sharedfiles/filedetails/?id=655704224

Floodwalls useless? No, they function as they're supposed to, but I prefer to use them in a different way. I like to use them to create raised reservoirs ;)
http://steamcommunity.com/sharedfiles/filedetails/?id=654978199

[Sonic]

I think, nanny does not mean the floodwalls but the quay walls. Here the problem occurs when the terrain level is not aligned. You can attach a riverside with a quay wall. This works only if the higher level (top of the quai wall) will be adjusted to the environment with the terrain tool.
In another case occurs precisely the described effect, the water goes under the wall and floods the enviroment.

4 steps are necessary to make it work.
1) Lower of ground to the desired level
2) put the quay wall to the transition between the high to the low terrain
3) align the high terrain level at the top of quay wall
4) align the deep terrain - ground of the river or lake - to wall.