Sea Level Rise - How to mask on relief plot

Question

I want to make a diagram which corresponds to sea level rise in the UK. I'm having trouble with relief plot.

So, corresponding to a 30 metre height increase of the sea, we would get the following diagram

sealevelrise = 30;

ReliefPlot[GeoElevationData[Interpreter["Country"]["UK"], UnitSystem -> "Metric"], DataReversed -> True, PlotRange -> {Full, Full, {sealevelrise, All}}]

Giving,

But I want a colour change at the new sea level instead, e.g from green to blue at the sea line. i.e. we could still see the relief in the white/"underwater". Is there any way to do this?

Clarification: I would like for the relief to be green, and for the sea to be blue and for the relief of both to still be seen. I just would like the point where the sea becomes land to have a colour change.

Side (non-important) question: Is there any way to overlay cities?

Answer 1

I'm not completely sure of what you need, but you might try this (where you can change the color schemes as you prefer):

ImageCompose[
 ColorReplace[
  ReliefPlot[
   GeoElevationData[Interpreter["Country"]["UK"], 
    UnitSystem -> "Metric"], DataReversed -> True, 
   PlotRange -> {Full, Full, {0, All}}
   , ColorFunction -> "SandyTerrain"]
  , White -> Transparent]
 ,
 ColorReplace[
  ReliefPlot[
   GeoElevationData[Interpreter["Country"]["UK"], 
    UnitSystem -> "Metric"], DataReversed -> True, 
   PlotRange -> {Full, Full, {sealevelrise, All}}], 
  White -> Transparent]
 ]

The orange-ish parts in the image are the parts that are now underwater.