Measuring area of erosion between 2009 and 2015 bank lines

[Hornbydd]

@dsear and @joewheaton ,

We have bank lines for 2009 and 2015, the area between them could represent erosion or deposition an example is shown below.

The challenge is to:

1. Identify the area between these intersecting boundaries
2. Identify if it is eroding or depositing
3. Identify if it is genuine change or simply down to vagaries in digitising.

Step 1

• Intersect the bank lines and feed the intersection points back into a split by points tool to segment the boundaries.

Intersection split points

Step 2

• Extract centroids of bank segments and measure their distance from the river centreline
  
  Green points are centroids of the segments created by the splitting with the intersection red points

Step 3

• The segment centroids are paired, logic is pair a centroid with closest centroid, this allows us to assign a pairing ID to both points which can be passed back to the bank segments.

Step 4

• A temporary "label layer" is created which is the mean centre of the paired centroids, this is used to pass attribution to the polygon created by enclosing the paired segments

Lines enclosed and attributed with a label layer

Step 5

• The minimum of the two segment lengths is passed back to the polygon, the area of an imaginary 2m buffer is computed and this value is compared with the polygon area. If the polygon area is less than the buffer area then it is flagged as dubious.

Red indicates the polygon area is less than the proposed 2m buffer area and is therefore considered a possible digitising error rather than any real change.

Limitations with this tool

• Banks leading up to the ends of the source banklines are dropped from this analysis as they have no intersection point allowing polygon enclosure.
• This tool cannot deal with islands, it is for bank change only. Hope to fix this with a separate tool that requires another line of logic to deal with.

[Hornbydd]

@dsear,

I have been fixing some of the bugs in the island bank tool (not the river bank tool). Got most of them but there are a couple of topologically knotty issues, those pathological cases that exist. These are the law of diminishing return type problems and for the demonstration dataset I think acceptable. It will take some brain effort to solve those! I assume I just need to cookie cut those data to the floodplain partitions so they can be attributed back to the segment, if that is what you want?

[Hornbydd]

I have constructed a tool that takes the partition floodplain, segmented network and the change in area for banks and islands and transfer this data to the segmented network layer.

This transfer 4 fields of data to the segmented network.

Field name	Field Type	Description
Bank_Deposition_Outside2mError	Double	This is the area of deposition within the segments floodplain partition polygon that was coded as outside the 2m error and thus considered real change.
Bank_Erosion_Outside2mError	Double	This is the area of erosion within the segments floodplain partition polygon that was coded as outside the 2m error and thus considered real change.
Bank_Deposition_Within2mError	Double	This is the area of deposition within the segments floodplain partition polygon that was coded as inside the 2m error and thus considered error rather than real change.
Bank_Erosion_Within2mError	Double	This is the area of erosion within the segments floodplain partition polygon that was coded as inside the 2m error and thus considered error rather than real change.