how to remove or hide streamlines outside the boundary

Hi, How can i remove or hide the streamlines outside the boundary of semi circle, the method of interpolation used is griddata. following is the code and the picture showing contour for streamfunction.

% -- Eckpunkte (vertices)------------------------

plot(-1,-1,'w.'), hold on

plot(21,6,'w.'), hold on

for I = 1:size(RAND,2)

   A = [p(1,RAND(1,I));p(1,RAND(2,I))];

   B = [p(2,RAND(1,I));p(2,RAND(2,I))];

   plot(A,B,'r','linewidth',2), hold on

end

X = p(1,:); Y = p(2,:); Z1 = zeros(1,length(X));

xlin = linspace(min(X),max(X),20);

ylin = linspace(min(Y),max(Y),20);

[U1,V1] = meshgrid(xlin,ylin);

W1 = griddata(X,Y,Z1,U1,V1,'cubic');

%trimesh(t(1:3,:)',X,Y,Z1,'edgecolor','y'), hold on

for I = 1:size(RAND,2)

   A = [p(1,RAND(1,I));p(1,RAND(2,I))];

   B = [p(2,RAND(1,I));p(2,RAND(2,I))];

   plot(A,B,'r','linewidth',2,'erasemode','none'), hold on

end

switch bilda

case 1, disp(' Contour for Z ')

   W1 = griddata(X,Y,Z,U1,V1,'v4');

   [C,h] = contour(U1,V1,W1,[4,3,2,1,0.5,0.1,0.05,0.01],'k','linewidth',2);

   %clabel(C,h,'labelspacing',1000)

   %clabel(C,h,'manual','fontsize',15)

   [C,h] = contour(U1,V1,W1,[0,-0.001,-0.005,-0.01,-0.05,-0.1,-0.3,-0.5,],'r');

   %clabel(C,h,'labelspacing',1000)

   %clabel(C,h,'manual','fontsize',10)

   %weisseln(p,RAND,t)