\documentclass{standalone}
\usepackage{luamplib}
\begin{document}
\begin{mplibcode}
input random-torn-edge
beginfig(1);
  path t; numeric x, y; 
  x = 0; y=0; 
  t = (x, -20) -- (x, y) for i=1 upto 288: 
      -- (incr x, walkr y) endfor -- (x, -20) -- cycle;
  fill t withcolor (1, 1, 31/32); draw t withcolor .67 blue;
endfig;
\end{mplibcode}
\end{document}