plotting – Find a line fitting in a 2D data field

I’m solvind a slope stability problem whith the finite element method. As an output I get the square root of the second deviatoric invariant of the plastic deformation, which indicates the failure surface for the problem.

The problem output generates a data in the format data={{x1,y1,scalar1},...{xn,yn,scalarn}}. The following code reads this data from dropbox and contourplot it:

data = Import[
   "https://www.dropbox.com/s/vl4sx9adrm4e0d9/plasticsqrtj2462.txt?dl=
1", "Table"];
gph = Graphics[{White, 
    Polygon[{{30, 10}, {50, 10}, {50, 20}, {20, 20}}]}];
plot = ListContourPlot[data, Contours -> 10, ContourStyle -> None, 
   PlotRange -> {0, Max[Transpose[data][[3]]]}];
Show[plot, gph, AspectRatio -> Automatic, Frame -> False]

Here is the output:

enter image description here

I need to find a function that fit’s the detached region, in order to describe the failure surface as a function.

Any ideas on how approach this problem?