Support including flood defence infrastructure

[rosepearson]

Add Support for including flood defence infrastructure.

In the first pass, support the inclusion of linear features like:

• Stop banks
• Half tide walls
  This should be applied last (or at least after rivers have been included).

An example of where we might want to do this is Westport and including the half tide walls.

[rosepearson]

Comment for Graeme:
Where there is measured (LiDAR) topography always retain this (blue line above water level on your example).
Use the full width of the flood channel (not the main channel where there are multiple low-flow channels).
Then add central islands back (ideally deduct their area from the flow area).
So your example could end up like the green line below:

One other thing, the assumed cross-section equation goes fairly deep mid channel, - could you fit an equation that is a bit flatter? - not a rectangle but more towards that sort of shape?

@AliceHarang
We also had a further look at the picture and the walls are closing the area upstream (but not downstream) and this second channel seems to be more dead water/lagoon than a "fully open second channel". (That might explain why our second implementation of this part of the river was better).
Rose is looking at using the measured section to get a "reference" bathymetry.

Graeme
The area is only closed up to "half-tide". In a flood it is an open floodway and the half-tide wall was designed to "disappear" during floods.
Maybe the best type of bed fitting would look like the dark red line below:

[rosepearson]

Also inputs from @RegMeasures (also reflected in #231)

• Support provision of a polyline with elevation
• Support provision of a polyline and estimation of elevation from the LiDAR point cloud (similar to waterways estimation)
  • Allow selection of max/mean/median within that region